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The Colours of Quartz

Sat, 23 Mar 2013 23:57:45 +0100

The latest issue of The Journal of Gemmology has an excellent article by Ulrich Henn and Rainer Schultz-Güttler called ‘Review of some current coloured quartz varieties’. For those who don’t have access to this journal, published by the Gemmological Association of Great Britain, this is a short summary to help you distinguish the different varieties.

Uncut amethyst crystal, prasiolite (11,5 ct), rock crystal (20,5 ct), amethyst (2,5 ct), citrine (natural?) (8 ct), citrine and rose quartz rough

We all know that quartz occurs naturally in various colours with varietal names – colourless rock crystal, yellow citrine, purple amethyst, pink rose quartz, brown smoky quartz, black morion, and the rare natural green prasiolite. Some of these colours can be produced artificially by radiation, sometimes coupled with heating. Artificial heating can also lighten some dark coloured quartz. Given the variations in natural quartz and their different reactions to irradiation and heating, a correspondingly wide range of artificial colours can be produced.

Most colourless natural quartz contains chemical impurities, even if these do not produce visible colour. They are mainly iron, in Fe-bearing quartz, and aluminium, in Al-bearing quartz. Natural or artificial irradiation with subsequent heating causes different colours in these two different quartz varieties.

First let’s consider Fe-bearing quartz. Naturally occurring clear Fe-bearing quartz crystals can experience low-level gamma irradiation from radioactive minerals in the surrounding rock. Over long periods of geological time this produces a change in the bonding of the iron impurity atoms, which results in the violet or purple colour of amethyst. Heating most amethyst to about 450°C causes it to bleach to colourless or pale yellow. Continued heating causes precipitation of iron oxide particles, which causes a deeper yellow. Most citrine on the gemstone market is such heat-treated amethyst. Because the colour is caused by uniformly dispersed iron oxide particles, this citrine is not pleochroic, that is, it does not show different intensities of colour when viewed in different crystallographic directions. Heat treated amethyst usually shows evidence of Brazil law twinning, often with colour zoning.

Some amethyst, when heated, turns green. Natural prasiolite is very rare and probably results from natural heating of amethyst. Most prasiolite on the gemstone market is artificially heated amethyst. This colour is produced by another change in the bonding of the impurity iron atoms, rather than by the precipitation of iron oxides. This material comes from only a few sources, mainly the Montezuma mine in Minas Gerais, Brazil.

Prasiolite itself can be subjected to artificial gamma irradiation and subsequent heat treatment to produce so-called ‘blueberry quartz’. This is a deep violet blue and resembles tanzanite.

Heating amethyst above 500°C not only bleaches it but can produce a milkiness, resulting in so-called ‘neon quartz’. This looks like lilac-coloured rose quartz. Stronger heating bleaches out the lilac colour completely and tiny water droplets form in the quartz. This material resembles adularescent gem materials and may be used as imitation moonstone.

Natural ametrine is bicolour quartz, purple and yellow, mainly from the Anahí mine in Bolivia. The colouring process is complicated. It involves differing concentrations of water in different growth sectors of the crystal and natural irradiation acting on the water to inhibit the formation of the purple colour in those sectors. Artificial heat treatment of ametrine to bleach the amethyst sectors can produce bicoloured citrine/colourless stones, sometimes marketed as ‘Lunasol’.

So much for Fe-bearing quartz. What about Al-bearing quartz? Usually the concentrations of aluminium in quartz are much higher than iron. Low levels of natural irradiation of Al-bearing quartz produces natural coloured citrine – yellow, yellow-green to yellow-orange. The details of the production of colour with irradiation in Al-bearing quartz are not well understood, but with increased levels of gamma irradiation the colour darkens, producing smoky quartz, and eventually black morion. Obviously, these colours also can be produced artificially with gamma irradiation, as is the case with the black Arkansas quartz.

Al-bearing quartz can also contain lithium in significant quantities. If is it lithium-poor, morion can be bleached by heat treatment to produce smoky quartz (and presumably some yellowish smoky quartz could be bleached to citrine). If Al-bearing morion, either naturally or artificially produced, is also lithium-rich, then gentle heat treatment at below 280°C can produce yellowish-green ‘lemon quartz’.

The distinctive characteristics of untreated and treated Al-bearing quartz are that if it is coloured it is pleochroic, and that it does not show evidence of Brazil law twinning. So, naturally coloured citrine will show pleochroism from pale to intense yellow and no Brazil law twinning, while citrine produced by heat treatment of Fe-bearing amethyst will show no pleochroism but Brazil law twinning may be present. Lemon quartz produced by heat treatment of Al-bearing smoky quartz will show yellow to yellow-green pleochroism but no Brazil law twinning.

Other quartz varieties exist too. ‘Greened amethyst’ is produced by artificial gamma irradiation of pale amethyst containing a very high content of water, from southern Brazil. The resulting crystals can be a deep green, with a ‘greasy’ lustre. This green quartz shows red under the Chelsea Colour Filter, while Fe-bearing prasiolite shows green. Heating above 500°C produces a cloudy opalescence due to exsolved water in very fine droplets.

Common pink rose quartz, generally described as ‘massive’ although it is crystalline, is thought to owe its pink colour to tiny included crystals of pink dumortierite. The cloudy appearance is due to scattering of light from these tiny inclusions. (In some material these inclusions must be crystallographically orientated, because they can produce asterism.) The much less common rosettes of pink quartz, usually on a white quartz crystal matrix, are essentially different from massive rose quartz. These clusters of pink single crystals have colour attributed to aluminium and phosphorus. Gamma irradiation can intensify the colour of these rosettes of single crystals to a stronger purplish pink.

The tendency of coloured quartz varieties to change colour on heating means that jewellers need to take care to avoid heating stones when repairing jewellery set with any coloured variety of quartz. Duncan Miller

Jaspers - part 2

Sat, 23 Mar 2013 23:48:31 +0100

Nebula stone. To quote from the Nebula Stone website: “There are companies that are trying to capitalize on the popularity of our stone’s name (Nebula Stone) because they have learned it has become very popular around the World. Some unethical companies have intentionally sold Kambaba/Kambamba/Kabamba Jasper/Crocodile rock/Galaxyite from Madagascar and South Africa falsely calling them Nebula Stone. Kambaba Jasper is not Nebula Stone. Nebula Stone is an igneous stone (from within the Earth). Kambaba jasper is a sedimentary stone of fossilized algae. “Kambaba jasper is an algae (a stromatolite - a clump of algae) that fossilized over time turning the algae into stromatolite Jasper. Stromatolite Kambaba Jasper is from the South African Rift that runs from South Africa to Madagascar an island nation off the east coast of Africa. Whatever names they use Kambaba Jasper is still Kambaba Jasper... a sedimentary fossilized algae. Kambaba is usually colored bluish-gray-green with dark and greenish orbs. Nebula Stone is NOT a fossilized algae. Nebula Stone is of igneous origin (from within the Earth). Nebula Stone is NOT from South Africa nor Madagascar. Nebula Stone is from only one location on the planet…………..North America.
"The technical description of the Nebula Stone may not be much fun to read but at least it would be familiar to a professional geologist if it were in your interest to use it in that manner. In plain English stripped of jargon it means that this stone is a fresh and unusual alkalitic volcanic rock composed of the minerals Quartz, Anorthoclase, Riebeckite, Aegirine, Arfedsonite and Zircon. Quartz and Anorthoclase form the groundmass of the gem, while Riebeckite and Aegirine are an integral part of the spherulites. The darker matrix is richer in Riebeckite and also contains more Quartz and Anorthoclase. The light green spherules you see in the stone composed of radiating fibers are riebeckite needles mantled with fine grained Aegirine." "What this means is that the minerals were once molten and glass-like but cooled very slowly, allowing the discrete minerals to begin to separate out and crystallize so the final product had lost its glass-like condition. This allowed the green Nebula eyes (orbicules or spherulites) to form as the different component minerals cooled and crystallized at various rates. “

Kambaba Jasper is simply a newly invented name (source of the name cannot be found) for "Green Stromatolite Jasper" a sedimentary stone that has been around for a long time. Most mineralogists know it as a fossilized algae. During Precambrian times, bacterial mats formed a platform for trapping and cementation of sediment. For photosynthetic bacteria, depletion of carbon dioxide in the surrounding water could cause precipitation of calcium carbonate that along with grains of sediment were then trapped within the sticky layers of mucilage (that formed a film for UV protection) that surrounded the bacterial colonies. Cyanobacteria are also capable of directly precipitating calcium carbonate, with minimal incorporation of sediment within the structure. The bacteria could repeatedly re-colonize the growing hard sedimentary platform, forming layer upon layer in a cyclic repetitive process. The resulting successive layering can assume a myriad shapes dependent upon microorganism and environment, and if left undisturbed by forces of nature could form huge domes and flat laminar structures that grew upward toward the life-sustaining rays of the sun. Cyanobacteria are found to be a primary organism in the formation of microbial carbonates. Prokaryotic bacteria is blue-green algae owning to its pigmentation involved in photosynthesis.

Kambaba jasper is commonly formed by the trapping, binding, and cementation of sedimentary grains by microorganisms, especially blue-green algae. (Much reduced information from an article posted on http://www.mindat.org/mesg-55-85320.html

Madagascar's Ocean Jasper - A Lost Treasure Found Again
(Source M. McDonough, Yahoo! Contributor Network July 29, 2008)
During the last few years, a stone known as ocean jasper has become a favorite of mineral collectors. This stone is found in only one place in the world, a site along the northwest coast of the African island Madagascar. The location which holds the Madagascar ocean jasper can only be reached by boat when the tide is low. Ocean jasper is a variety of orbicular jasper, a type of jasper named for the spherical shapes that pattern the stone. Various forms of orbicular jasper can be found in many areas around the world, including the United States. However, the ocean jasper of Madagascar is unique due to the beautiful colors and markings it possesses.

Another aspect of ocean jasper that lends to its mysterious aura is the story of how it was found, or perhaps "found again" would be the more accurate phrase. Stories about the stone have existed since the early 1900s, and a few mineral collectors have passed and traded the ocean jasper since that time. However, all that was known about the stone until recently was that it came from someplace in Madagascar and that the original site of the quarry had been lost.

Ocean jasper made its reappearance to the world at the 2000 Tucson Gem and Mineral Show, and the story behind its rediscovery was the talk of the crowd. After 45 days of tirelessly searching along the Madagascar coast, an exploration group from the mining company Madagascar Minerals located the ocean jasper deposit. It turns out that the reason the site was lost for so long is that is only visible at low tide. Even today, the location can only be reached by boat at low tide and miners have to plan their trips carefully.

Leopard skin jasper is an opaque sedimentary rock that occurs in shades of red, yellow, or brown as a result of mineral impurities. This once abundant orbicular jasper comes from Mexico.

The scientist does not study nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful. If nature were not beautiful, it would not be worth knowing, and if nature were not worth knowing, life would not be worth living. — Jules-Henri Poincare (1884–1912)

Jaspers Galore

Sat, 23 Mar 2013 23:31:28 +0100

JASPER – THE HISTORY

The name jasper means "spotted or speckled stone", and is derived via Old French jaspre (variant of Anglo-Norman jaspe) and Latin iaspidem (nom. iaspis)) from Greek ἴασπις iaspis, (feminine noun) from a Semitic language (cf. Hebrew יושפה yushphah, Akkadian yashupu).

Green jasper was used to make bow drills in Mehrgarh between 4th and 5th millennium BC. Jasper is known to have been a favourite gem in the ancient world; its name can be traced back in Arabic, Persian, Hebrew, Assyrian, Greek and Latin. On Minoan Crete, jasper was carved to produce seals circa 1800 BC, as evidenced by archaeological recoveries at the palace of Knossos.

Although the term jasper is now restricted to opaque quartz, the ancient iaspis was a stone of considerable translucency. The jasper of antiquity was in many cases distinctly green, for it is often compared with the emerald and other green objects. Jasper is referred to in the Niebelungenlied as being clear and green. Probably the jasper of the ancients included stones which would now be classed as chalcedony, and the emerald-like jasper may have been akin to the modern chrysoprase. The Hebrew word yushphah may have designated a green jasper. Flinders Petrie suggested that the odem, the first stone on the High Priest's breastplate, was a red jasper, whilst tarshish, the tenth stone, may have been a yellow jasper. (Wikipedia)

Buddstone is an attractive green and white metamorphosed chert that owes its colour to inclusions of chlorite, epidote and fuchsite. It exhibits complex swirling patterns, the result of extensive folding and distortion. The material takes a good finish, making interesting cabochons, beads, eggs and other ornamental objects. It became known as buddstone because it was originally discovered by Billy Budd in the Barberton district of South Africa. (Source “Gemstones – properties, identification and use.” Arthur Thomas)

Dalmatian Stone, also called “Dalmatian Jasper”, is a white to cream-colored material with black spots that is produced in Chihuahua, Mexico. It reminds people of the dalmatian breed of dogs – and that is where it gets its name. It is very easily polished to a bright lustre and is a familiar semi-precious stone that is cut into beads, spheres, cabochons and carvings. It is also commonly seen as tumbled stones. Dalmatian stone is porous and readily accepts dye. Jasper is a microcrystalline variety of quartz but the composition of our dalmatian stone is much more interesting. It is a mixture of minerals and therefore it is a rock. The types of minerals in the rock are those that crystallize from a melt. So, instead of being a jasper, dalmatian stone is an igneous rock. (rocktumbler.com)

Puddingstone Jasper conglomerates are a distinct type of glacial erratic collected mostly from glacial drift. The conglomerates were derived from the North American region of the Canadian Shield that is present on St. Joseph Island and north and northwest of the Bruce Mines of Northern Ontario, approximately a 65 km east of Sault Ste. Marie. This rock is derived from the Precambrian Lorrain Formation, which contains rocks associated with Precambrian (2200-2400 Mya) glaciation and the jasper conglomerates are attributed to sand and pebbles derived by erosion from older rocks and re-deposited as gravity flows in water. The red pebbles or cobbles in the conglomerate are fragments of jasper or banded iron formation. Later these materials were lithified to form conglomerates and transformed by heat and pressure to form quartzite conglomerates. The early British settlers in the Bruce Mines, called the jasper conglomerate “puddingstone”, because it looked like boiled suet pudding with cherries. (source: Wikipedia)

Bloodstone - The martyr's gem Bloodstone, green jasper dotted with bright red spots of iron oxide, was treasured in ancient times and served for a long time as the birthstone for March. This attractive chalcedony quartz is also known as heliotrope because in ancient times polished stones were described as reflecting the sun: perhaps the appearance of the gem reminded the ancients of the red setting sun, mirrored in the ocean.

Medieval Christians often used bloodstone to carve scenes of the crucifixion and martyrs, for which reason it was also dubbed the martyr's stone. According to the legend about the origin of bloodstone, it was first formed when drops of Christ's blood fell and stained some jasper at the foot of the cross. A beautiful example of carved bloodstone with the seal of the German Emperor Rudolf II can be seen at the Louvre in Paris.

Even today, finely pulverised bloodstone is used as a medicine and aphrodisiac in India. Perhaps that explains why it is now rather difficult to find fine specimens of bloodstone on the market. Bloodstone is mined in India, Australia, and the United States. (www.gemstone.org)

Part two will follow next month

Mineral of the Month - Ruby Corundum

Wed, 21 Nov 2012 14:51:41 +0100

Ruby from Afghanistan Ruby from Musina, S.A.

Specimen size 2,7 cm x 1,2 cm Specimen 2,5 cm, crystal 0,5 cm

Macro of Ruby corundum from Poona District, Maharashtra State, India
Specimen size 8,2 cm x 5,4 cm. Main crystal 1,6 cm x 0,7 cm

Crystal system:    Trigonal (hexagonal scalenohedral)        Hardness:    9 (defining mineral)

Density:    4                        Streak:        white

Cleavage:    None                    Composition:    Oxide mineral    Al2O3

Being the Christmas season I was requested that the MOM must be related or associated with the season – possibly a mineral of which the name resembles or relates to Christmas?   No luck there…. I could not find Christmasnite, Rudolphnite or St. Nickelsite in any book. Then I started looking at colours or maybe something red, white or green which are typical colours associated with Christmas. So I stumbled onto something red, something precious, something pretty, something that was for many centuries regarded as the birth stone for December, and something most of us would not mind finding in our stocking or under the tree on Christmas day …. a ruby.

A ruby is a pink to blood-red coloured gemstone which is a variety of the mineral corundum (aluminium oxide). The red colour is caused by the presence of minor quantities of the element chromium which replaces aluminium in the crystal structure. The name comes from “ruber”, Latin for red and as a matter of interest….or to add confusion, the name “corundum” is derived from the Tamil word “kuruntam” meaning “ruby”. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with sapphire, emerald, and diamond.

Corundum occurs mainly in metamorphic rocks and as large crystals in pegmatites. The largest documented single crystal of corundum measures 65 cm x 40 cm x 40 cm. Rubies normally occur as well-formed crystals showing good form. The crystals can sometimes fluoresce under long wave UV light (take note Jo).

International ruby localities include, Afghanistan, India, Burma, Vietnam, Russia, Tanzania (ruby embedded in green zoisite).

Considerable controversy seems to surround the question of the world’s largest ruby. One of the most famous is the Rajaratna Ruby which is 2 475 carats and was unveiled to the world at the end of 1986. Its owner, Mr G. Vidyaraj of Bangalore, India, inherited the stone from his ancestors, the kings of the Empire of Vijayanagar. Since then, the 125 West Ruby, which is certified by the Gemological Institute of America and weighs 18 696 carats, has claimed the prize, as has another stone from Myanmar reported to be 21 450 carats.

Prices of rubies are primarily determined by colour. The brightest and most valuable “red” is called pigeon blood red and commands a large premium over other rubies of similar quality. In traditional gemological terms, ruby has to be blood-red and of a clear, facetable quality to be considered to be a real ruby. In the wider usage, however, any corundum with a red or reddish colour has obtained the name “ruby”, and this name is usually applied in this way by mineral collectors.

Rubies have a hardness of 9 on the Mohs’s scale of mineral hardness. It can, therefore, scratch almost every other mineral. Among the natural gems only diamond is harder with a Mohs’s hardness of 10. Ruby corundum is, therefore, commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood.

The largest corundum crystal in the world, a grey 59 cm long stone weighing 151 kg, which is displayed in the Transvaal Museum in Pretoria, was found in the Limpopo Province. I am, however, not sure whether this refers to the same corundum specimen referred to in paragraph three above, since the dimensions differ slightly.

Corundum was mined from the early 1900s in a broad area from Polokwane to Musina to Leydsdorp. True gem quality ruby does not occur in South Africa, although some translucent ruby-red corundum occurs in grey-white host rock, south of Aggeneys, in the Northern Cape.

Corundum is not widespread in Namibia and some grey-brown to pale pink corundum is found in the Karasburg district. It is also found at Kyanite Kop, a prominent hill composed of corundum, kyanite and diaspore in the Windhoek district. Other districts where it is found include Omaruru, Damaraland, Karibib and Warmbad.

Zimbabwe was once, after Russia, the world’s largest producer of corundum. Some small faceting quality stones have also been found.

In 1837 Marc Antoine Gaudin made the first synthetic rubies by fusing alumina at a high temperature with small amounts of chromium as pigment. Later the “Verneuil process” (after Auguste Verneuil) was developed that allows the production of flawless single-crystal sapphires, rubies and other corundum gems of much larger sizes than normally found in nature. JDJ

All photos and specimens supplied by Johann de Jongh
References
Bruce Cairncross,Field Guide to Rocks & Minerals of Southern Africa (2004).
http://en.wikipedia.org
http://webmineral.com
www.mindat.org

Mineral of the Month - Quartz

Thu, 25 Oct 2012 04:55:31 +0100

We all think we know quartz, and can recognise it, but it has possibly greater variation than any other mineral. First, it is only one of several different silica minerals made of only silicon and oxygen, linked together in the proportions of two oxygen atoms per silicon atom, hence the chemical formula SiO2. The other minerals, with identical chemical composition are tridymite and cristobalite (both high temperature minerals) and coesite and stishovite (both high pressure minerals). There is a synthetic silica “mineral” called keatite, not found in nature, as well as two amorphous mineraloids, lechatelierite and the hydrated form of silica called opal. Some of these minerals themselves have high temperature variations or polymorphs, with high-quartz, high-tridymite, and high cristobalite. You are unlikely to see any of these varieties of silica except for low quartz and opal. But there is also great variation within these two species. Opal can be colourless (hyalite), plain white, green, blue, pink, brown, or filled with flashes of colour. Opal has no formal crystal structure, but consists of microscopic hydrated silica spherules, giving rise to flashes of colour when they are packed together in a regular way. Quartz itself is crystalline, with the atoms arranged in such a way that each silicon atom is linked to four oxygen neighbours, each in turn linked to another silicon atom, in a three dimensional network of SiO4 groups. These are arranged like a spiral staircase, around the vertical axis of the crystal (the one emerging from the points at each end of a doubly terminated crystal). Like a spiral staircase, the twist can go either left or right, giving rise to left-hand and right-hand crystals. You can sometime distinguish between them visually, if the trigonal trapezohedral faces are present. These are small faces that occur in the upper right hand corner of alternate prism (side) faces in right-hand quartz, and in the upper left corner of alternate prism faces in left-hand quartz. This is difficult to visualise but is clearly illustrated in many mineralogy textbooks, including Dana’s Manual of Mineralogy. Quartz can crystallize as clear quartz, called “Herkimer diamonds” if they are small, clear and doubly terminated, or as milky vein quartz filled with millions of microscopic bubbles of carbon dioxide. Citrine is yellow quartz, and purple quartz is amethyst, rose quartz is pink, while smoky quartz is brown to black, when it is known as ”cairngorm” after historical occurrences in Scotland.

Agate is a banded variety of quartz made up of sub-microscopic crystals, and can be any colour including the blue of blue-lace agate, or the banded red and yellow of jasper. Non-banded blue agate is called chalcedony, and the red variety is carnelian (or cornelian). Green agate is chrysoprase and the bright apple green chrysoprase from Mutorashanga in Zimbabwe, coloured by nickel, is appropriately called mtorolite. Aventurine can be green with inclusions of green mica, or brown with inclusions of haematite. Harlequin quartz crystals from Namibia have spangles of large haematite flakes; the rather misleadingly named aqualite has inclusions of blue apatite. This all makes for a lot of names for silica, but the real clincher is that “tiger’s eye” and “pietersite” are also varieties of quartz, both being pseudomorphs or replacements after fibrous asbestos. One could easily fill a cabinet with the different varieties of quartz, without duplications.
Natural citrine is coloured by a trace of ferric (Fe3+) iron, but most citrine we see is heat-treated amethyst. Amethyst is coloured by the natural irradiation of iron-bearing quartz. Heating amethyst coloured by trivalent iron (Fe3+) produces citrine. Heating amethyst coloured with divalent iron (Fe2+) produces greenish prasiolite (marketed here as “green amethyst”). Heating sometimes produces amethyst-citrine (ametrine). Smoky quartz is caused by the natural irradiation of an aluminium impurity. Heating can bleach the colour.

For anyone interested in quartz, and in particular twisted crystals or “gwindels”, must-reads are the article on gwindels in the March-April 2007 issue of the Mineralogical Record, and the brilliant and lucid letter on quartz crystal growth published in the November-December 2007 issue. The letter not only explains the “mysteries” of gwindel formation mentioned in the first article, but also has fascinating explanations of the growth rates of quartz crystals in different initial crystallographic orientations. DM

“Herkeimer Diamonds”

A discussion arose during Maurice’s quartz talk about “Herkimer diamonds” – a name given to small double-terminated quartz crystals, so named after a famous New York location where they are found. This is not a unique occurrence, and Herkimers have been given a variety of names depending on where they come from. An occurrence in Russia produces “Marmarosh diamonds”. Other names for this type of quartz, all ending with “diamonds” includes, Alaska, Alencon, Arkansas, Bohemia, Bristol, Cornish, Hot Springs, Irish, Isle of Wight, Lake George, Mexican and Vallum. With the exception of Herkimer diamonds (whether they are found at Herkimer or not), none should be used. TVJ

What are the Uses of Quartz?

Quartz is one of the most useful natural materials. Its usefulness can be linked to its physical and chemical properties. It has a hardness of seven on the Mohs Scale which makes it very durable. It is chemically inert in contact with most substances. It has electrical properties and heat resistance that make it valuable in electronic products. Its lustre, colour and diaphaneity make it useful as a gemstone and also in the making of glass.

Uses of Quartz in Glass Making. Geological processes have occasionally deposited sands that are composed of almost 100% quartz grains. These deposits have been identified and produced as sources of high purity silica sand. These sands are used in the glassmaking industry. Quartz sand is used in the production of container glass, flat plate glass, specialty glass and fiberglass.

Uses of Quartz as an Abrasive. The high hardness of quartz, seven on the Mohs Scale, makes it harder than most other natural substances. As such it is an excellent abrasive material. Quartz sands and finely ground silica sand are used for sand blasting, scouring cleansers, grinding media, and grit for sanding and sawing.

Uses of Quartz as Foundry Sand. Quartz is very resistant to both chemicals and heat. It is therefore often used as foundry sand. With a melting temperature higher than most metals it can be used for the molds and cores of common foundry work. Refractory bricks are often made of quartz sand because of its high heat resistance. Quartz sand is also used as a flux in the smelting of metals.

Uses in the Petroleum Industry. Quartz sand has a high resistance to being crushed. In the petroleum industry sand slurries are forced down oil and gas wells under very high pressures in a process known as hydraulic fracturing. This high pressure fractures the reservoir rocks and the sandy slurry injects into the fractures. The durable sand grains hold the fractures open after the pressure is released. These open fractures facilitate the flow of natural gas into the well bore.

Many Other Quartz Sand Uses. Quartz sand is used as a filler in the manufacture of rubber, paint and putty. Screened and washed, carefully sized quartz grains are used as filter media and roofing granules. Quartz sands are used for traction in the railroad and mining industries. These sands are also used in recreation on golf courses, volleyball courts, baseball fields, children's sand boxes and beaches.

Uses of Quartz Crystals. High quality quartz crystals are single-crystal silica with optical or electronic properties that make them useful for specialty purposes. USGS estimates that about ten billion quartz crystals are used every year. Electronics grade crystals can be used in filters, frequency controls, timers, electronic circuits that become important components in cell phones, watches, clocks, games, television receivers, computers, navigational instruments and other products. Optical-grade crystals can be used as lenses and windows in lasers and other specialized devices. Although some natural quartz crystals are used in these applications, most of these special crystals are now manufactured.

Quartz as a Gemstone. Quartz makes an excellent gemstone. It is hard, durable and usually accepts a brilliant polish. Popular varieties of quartz that are widely used as gems include: amethyst, citrine, rose quartz, and aventurine. Agate and jasper are also varieties of quartz with a microcrystalline structure.

Special Silica Stone Uses. "Silica stone" is an industrial term for materials such as quartzite, novaculite and other microcrystalline quartz rocks. These are used to produce abrasive tools, deburring media, grinding stones, hones, oilstones, stone files, tube-mill liners and whetstones.

Tripoli. Tripoli is crystalline silica of an extremely fine grain size (less than ten micrometers). Commercial tripoli is a nearly pure silica material that is used for a variety of mild abrasive purposes which include: soaps, toothpastes, metal polishing compounds, jewelry polishing compounds and buffing compounds. Tripoli is also used in brake friction products, fillers in enamel, caulking compounds, plastic, paint, rubber and refractories.

This information was obtained from the web page Geology.Com

Mineral of the Month - Braunite

Mon, 24 Sep 2012 14:31:13 +0100

    Braunite II, andradite garnet, ettringite                          Braunite II on andradite garnet
                       (Wessels mine)                             (Wessels mine
      Braunite II crystal (2,7 cm x 1,4 cm)                             specimen (4,1 cm x 3,5 cm)

Crystal system:        Tetragonal        Hardness:        6-6,5

Density:        4.8            Streak:            Black

Cleavage:        Perfect            Composition:        silicate    Mn2+Mn3+6(SiO4)O8

I think it is about time that our MOM features a mineral from a local locality. Many of our interesting discussions at the club regarding minerals from KMF have included the mineral braunite; and more specifically the difference between braunite and braunite II and the other minerals it is associated with and also sometimes mistaken for, such as hausmannite, hematite, bixbyite, manganite, kentrolite. Limited information is available on this mineral but I managed to compile the following.

Braunite was discovered in 1827 and was named after Alderman Wilhelm von Braun (1790-1872) being a minister in Gotha, Thuringia, Germany. He was a strong supporter of geology and mineralogy and supplied the original material for the description of braunite.

Braunite crystallizes in the tetragonal system as pyramidal striated crystals or as masses. It occurs in veins as a secondary mineral formed by weathering and associated with other manganese minerals.

Braunite is a silicate mineral and common impurities include iron, calcium, boron, barium, titanium, aluminum and magnesium. Braunite is found in Germany, Panama, Norway, Sweden, Italy, India, USA, Brazil, Australia, Namibia and South Africa.

Southern African localities:

Braunite

Braunite is one of the most important ore minerals of the manganese fields. It is dominant as a fine-grained form in the Mamatwan-type ore and is found in a coarse recrystallised form in the Wessels-type ore. Single crystals have not yet been found, other than in the Postmasburg manganese field, where braunite occurs with barite at Kapstewel, as small 2-3 mm octahedral crystals in massive braunite.

Other localities are in the Waterberg with the mineral psilomelane, at Weenen in KwaZulu-Natal and at Gendendal (Marico) associated with pyrolusite, psilomelane, dolomite, hausmannite, partridgeite, coronadite, heteraeolite and cryptomelane. I will not even try to pronounce these mineral names. I did not even know they existed, except maybe for the latter that vaguely resembles the name of a “crystal” from a famous super hero’s planet.

The Otjosondu manganese deposit, located 150 km northwest of Okahandja, is the largest manganese deposit in Namibia. The mine is virtually unknown for its collector-type minerals. Yet various minerals are found here which include braunite, with a high barium component. Probably the world’s largest euhedral crystal of braunite was collected at this location and forms part of the Desmond Sacco collection. The main crystal on this specimen measures 5,3 cm.

Braunite II

Braunite II is the name given to a silica-deficient analogue of braunite, in which the c-axis is twice the length of that of braunite. Braunite II is not regarded as a distinct mineral species, but rather as a compositional variety of braunite that was first discovered at Black Rock (De Villiers, 1945) and described as ferrian braunite. It was renamed braunite II by De Villliers and Herbstein in 1967 and its crystal structure was determined by De Villiers in the 1980s. It is a calcium iron bearing variant with the formula Ca (Mn3+, Fe3+)14SiO24. Black Rock mine is the type locality for this mineral.

Metallic–grey braunite II crystals have come from Wessels, Black Rock (up to 2 cm) and N’Chwanning II. The crystals are typically bi-pyramidal with complex terminations. In 1995, crystals up to 3 cm were found at Wessels mine. These were associated with red andradite garnet, hausmannite and occasionally yellow ettringite. This is the only locality in the world where macroscopic braunite II has been found. – JDJ



Two specimens with braunite II, hausmannite and andradite garnets probably also from Wessels.
Specimen on the left measures 13 cm x 11,5 cm and specimen on the right 20 cm x 11 cm.

References
Bruce Cairncross, Roger Dixon, Minerals of South Africa.
Bruce Cairncross, 2000 – The Desmond Sacco Collection – Focus on Southern Africa.
Bruce Cairncross, Nicolas Beukes, Jens Gutzmer, 1997 – The Manganese Adventure.
Michael O’Donoghue, 1885 – The Encyclopedia of Minerals & Gemstones.
http://en.wikipedia.org
http://webmineral.com
www.mindat.org

SPODUMENE

Fri, 20 Jul 2012 11:27:11 +0100

Crystal system: monoclinic Hardness: 6,5 - 7

Density:                       3,2            Streak:        White
Cleavage:        Perfect            Composition:    silicate    LiAlSi2O6

Spodumene is derived from the Greek word “spodoumenos”, which translates to “burnt to ash”, which refers to the ashy colour of early specimens. Spodumene is a relatively new mineral having been discovered in the last 300 years, and gem varieties have only been discovered in the last 130 years.

Crystals are prismatic, generally flattened and elongated and the termination is usually rounded. Crystal faces are often pitted and rough. The largest crystal ever mined was from the Etta Mine, Keystone, South Dakota, USA and measured 14,35 m X 0,8 m. Yes, that is metres! Weighing about 90 tons, it is a real heavy weight. Good luck with placing that in your display cabinet!

Spodumene is mined as a source of lithium although the latter only comprises 3,73 % of its molecular weight. Its refraction index is 1,66. Prism faces are deeply striated lengthwise and clear colourful varieties show strong pleochroic colour intensity variation when a crystal is viewed along the various axes. Other associated minerals include lepidolite, feldspars, quartz, tourmaline and topaz.

Spodumene is found in three gem varieties: kunzite, hiddenite and triphane.

Kunzite is the pink to light purple variety which was first found in the pegmatites of Pala, California, in 1902, and is named after the famous mineralogist George Frederick Kunz (1856 – 1932) who first identified it. It was not until the 1990s that this gemstone became a more mainstream gemstone, having been used only as a collector’s gemstone prior to that time.

Although kunzite is a very attractive pink gemstone, it does unfortunately have the nasty habit of colour fading after prolonged exposure to bright light and sunlight. Although the colour-fading effect is very slow, most people prefer to wear it in the evening to avoid sunlight exposure and it is, therefore, regarded as an “evening” stone for that reason.

Kunzite deposits are wide spread over the world with large deposits which makes it more affordable than most of the other traditional gemstones. Very large flawless crystals have been found, from which very large flawless faceted gemstones can be cut. The “Big Kahuna” for example measures over 30 cm in height.

Most kunzites in their natural form are very light in colour, and are commonly heat treated to intensify colour and remove brownish tones.

Kunzite is found in Afghanistan, Brazil, Canada, Finland, Madagascar, Mozambique, Nigeria, Pakistan, Sri Lanka and the USA.

Hiddenite is the less common pale-to-emerald green gem variety of spodumene. The first specimen of hiddenite was recovered in 1879 near the tiny settlement of White Plains, west of Stony Point, Alexander County, North Carolina. It is told that a young man named Lackey brought some specimens to J. Stephenson, a local trader who was also a keen mineral collector. He initially thought it was diopside. He in turn brought the discovery to the attention of exploration geologist William Earl Hidden, who had been commissioned by Thomas Edison……………that rings a bell, or rather sheds more light on the matter………..to search for any sources of platinum in North Carolina which was according to the reference source “stunningly unsuccessful”. Hidden sent some samples to J. Lawrence Smith, a well-known chemist and mineralogist. Smith correctly identified the specimens as being a variety of spodumene and named it “hiddenite” in honour of William Hidden. The settlement in which it was found was also later renamed to “Hiddenite”. The town was, therefore, named after the mineral and not the other way around, which is normally the case with the naming of new minerals. In the hey-days of the hiddenite mining during the 1880s to 1890s it was also referred to as “lithia emerald” and “Carolina emerald”.

One of the important mines in the above mentioned area is the Emerald Hollow Mine. This is the only emerald mine in the USA which is open to the public to search for emeralds. At the mine, more than 63 different types of gems and minerals can be found including emeralds, amethyst, sapphire, aquamarine, topaz, garnet, as well as hiddenite….So! Who’s game for a club outing?

In addition to the above mentioned type locality, hiddenite is also mined in Afghanistan, Brazil, China, Madagascar and Sri Lanka. A hiddenite crystal of 25 cm, weighing 7 500 carats, was found in Afghanistan.

People disagree about exactly what hiddenite is. Some are of the opinion that it must contain chromium, which is responsible for the brilliant deep green colour. Others say it must be a particular shade of green, and others claim that to be regarded as true hiddenite, it must come from the Hiddenite area, of North Carolina where it was discovered.

Hiddenite is generally not treated or enhanced, though deep emerald-green hiddenite can be formed through irradiation of other lighter forms of spodumene. Natural hiddenite from North Carolina is very rare and increasingly difficult to obtain.

Triphane.    Not much is mentioned about triphane, possibly because its colour, which varies between colourless and yellow, is less attractive than the pink and the greens of kunzite and hiddenite respectively.

Kunzite, hiddenite and triphane are however difficult to facet due to the perfect cleavage and splintery nature of the gemstone. It is very sensitive to knocks and will chip if hit or knocked too hard. Small gemstones are not commonly cut from the stone due to the perfect cleavage and strong pleochroism. For this reason, it is cut to show the deepest pink colour from the top of the gemstone. It is mostly used as a pendant stone and as a large decorating stone on ornamental objects. It is less commonly used in rings, necklaces, or other items where small stones are required.

On the local front only spodumene is found and unfortunately not the gem varieties. It is mined in some pegmatites in the Northern Cape where crystals can be several metres long. It occurs at Spodumene Kop, Norrabees (as referred to in Charlie’s article “Norrabees – Lepidolite and Spodumene - the lithium bearing ores” also included in this month’s newsletter), Noumas and Henkries in Namaqualand.

It is also found in the Piet Retief district, Mpumalanga, the Free State in the south-eastern section of the Vredefort dome and in KwaZulu-Natal north of Port Shepstone. Spodumene is also found in our neighbouring countries, i.e. Namibia, Zimbabwe and Botswana.    JDJ

Photos of various specimens from Afghanistan: From left to right: triphane (11 cm x 2,5 cm), “hiddenite” (9 cm x 3,5 cm), lilac kunzite (5,4 cm x 3 cm), pink kunzite (5,3 cm x 3 cm), pink kunzite (5,2 cm x 2,5 cm)

Photos of kunzite displaying strong pleochroism and terminations

All photos and specimens by Johann de Jongh

References
Cairncross, Bruce, 2004 – Field Guide to Rocks & Minerals of Southern Africa.
webmineral.com
www.galleries.com
www.giantcrystals.strahlen.org
www.minerals.net
www.mindat.org
www.wikipedia.org

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RECENT FACETINGS BY DUNCAN MILLER

Despite leading a full working life, and on Saturdays teaching club members theoretical mineralogy, gemmology and practical faceting, Duncan is busy at his own faceting machine in his spare time. He has produced the following marvellous stones. The kunzite was faceted some while ago but is topical to our lithium theme this month, and the morganite collection, cut all from one choice stone, is fresh off the lap.

Kunzite

1. The kunzite (spodumene) crystals are 5,5 cm and 3,5 cm long respectively. The left hand one shows the typical purplish-pink colour and the right hand one the characteristic striations on a naturally etched crystal. (It also happens to be twinned, but you can’t see that in the photograph because the twin plane is parallel to the plane of the photo.)

2. The faceted kunzite is a Barion cut, with a mass of 12,26 ct. It has tiny bubble inclusions, which in reality are hardly visible but annoyingly are picked up selectively by the camera.

Morganite

The 208,2 gram morganite rough before cutting

The morganite rough after sawing

3. BELOW The pear-shaped morganites (beryl) range in mass from 59,58 ct to 7,60 ct. The morganite briolettes are 51,28 ct and 51,21 ct, and are each 40 mm long. All the morganites were cut from a single 208,2 gram rough crystal belonging to Lorna Quinton.

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Norrabees - Lepidolite and Spodumene - the lithium bearing ores.

About 10 years ago, before I was a member of the Club, a few friends and myself proceeded to Steinkopf to look for “rocks” with the help of a GPS and information gathered from Geoscience data. The highlight of the trip was Norrabees 1. As we approached the koppie there was this soft purplish gash running from the crest to the base. Five excited, green-behind-the-ears, rock aficionados poured out the 4x4 and soon were scrambling around the lepidolite and spodumene slopes with oohs and ahs. When the first watermelon tourmaline turned up the hunt intensified. Then someone found some botryoidal spodumene amongst the lepidolite. Crystals!!! Finally, in a shallow cave that substituted as a mine, John Graham discovered some gemmy needles of hiddenite in situ.

Later in discussion with George Swanson, we asked why all this material was abandoned. He mumbled something about lithium and that the Chinese were not interested. Lepidolite is a micaceous mineral containing lithium. Spodumene is a lithium aluminium silicate. A single crystal of spodumene was found in a South Dakota mine that was 15 metres long and weighed ten tons. Even the green tourmaline has a smattering of lithium in it. Most of the lithium mined today comes from lake brines which means, that to obtain the concentrate, water is merely evaporated and the various salts cheaply separated. Chile is the main exporter. Economically Norrabees could not compete.

Watermelon tourmaline growing around spodumene at Norrabees

On working with the lepidolite from Norrabees in our workshop we found that it was too micaceous but the lapidarists loved the colour. We thus located some acceptable rough which produced some quite decent cabochons. There is a project now on to make eggs and then possibly spheres. The spodumene has badly weathered and as soon as you touch it with a machine it breaks into smaller and smaller chunks.

Lithium, being the lightest metal, produces very light alloys and it also strengthens the alloy. Thus it is used in armour-plating. In World War 1, Germany was blockaded and so couldn’t get tin. They made an alloy of lead, lithium and a few other minor ingredients called Bahnmetal as a substitute for tin which is still in use today.

The hydrogen in the hydrogen bomb is lithium hydride which is wrapped around an atomic bomb. To add insult to injury, the whole contraption is mounted in a casing with uranium imbedded. You thus have three explosions in one. All devastating and frightening, and that is why lithium production in nuclear countries is not published.

Of course we all know of our long-lasting lithium batteries and so to end on a cheery note, it was discovered in 1945 that lithium carbonate helps to cure manic depressives and was used extensively. With the world’s economy as it is, look after your lepidolite carefully.

Finally, lithium is an amazing element. In the universe it is not being produced anymore. For some obscure reason this element gets bypassed during a supernova and all that exists was manufactured during the Big Bang, or Small Bang, or other, whichever theory you subscribe to. So, again, hold onto your lepidolite. Lithium is getting more, and more, scarce with each passing day. Charlie Scharfetter.

Anorthoclase Crystal, Mount Erebus, Antarctica

Mon, 25 Jun 2012 08:47:43 +0100

Mount Erebus, the second highest volcano in Antarctica with a summit elevation of 3,794 metres, is located on Ross Island, which is also home to three inactive volcanoes. Mount Erebus is part of the Pacific Ring of Fire, which includes over 160 active volcanoes.

Geology and Volcanology
The mineral anorthoclase ((Na,K)AlSi3O8) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi3O8 and between 64 and 90 percent of NaAlSi3O8.

Anorthoclase crystal from Mt. Erebus

Occurrence
Anorthoclase occurs in high temperature sodium rich volcanic and hypabyssal (shallow intrusive) rocks. The mineral is typically found as a constituent of the fine grained matrix or as small phenocrysts which may occur as loose crystals in a weathered rock. Mount Erebus is currently the most active volcano in Antarctica and is the current eruptive zone of the Erebus hotspot. The summit contains a persistent convecting phonolitic lava lake, one of five long-lasting lava lakes on Earth. Characteristic eruptive activity consists of Strombolian eruptions from the lava lake or from one of several subsidiary vents, all lying within the volcano's inner crater.

Mount Erebus is classified as a polygenetic stratovolcano. The bottom half of the volcano is a shield and the top half is a stratocone (Mount Etna is like this as well). The composition of the current eruptive products of Erebus is anorthoclase-porphyric tephritic phonolite and phonolite, which constitute the bulk of exposed lava flow on the volcano. The oldest eruptive products consist of relatively undifferentiated and non-viscous basanite lavas that form the low, broad platform shield of Erebus.

Slightly younger basanite and phonotephrite lavas crop out on Fang Ridge — an eroded remnant of an early Erebus volcano — and at other isolated locations on the flanks of Erebus.

Lava flows of more viscous phonotephrite and trachyte erupted after the basanite. The upper slopes of Mount Erebus are dominated by steeply dipping (~30°) tephritic phonolite lava flows with large scale flow levees. A conspicuous break in slope at approximately 3 200 metres calls attention to a summit plateau representing a caldera less than 100 000 years old. The summit caldera itself is filled with small volume tephritic phonolite and phonolite lava flows. In the center of the summit caldera is a small, steep-sided cone composed primarily of decomposed lava bombs and a large deposit of anorthoclase crystals known as Erebus Crystals. It is within this summit cone that the active lava lake continuously degasses.

Web Reference: http://en.wikipedia.org/wiki/Mount_Erebus

Stromatolites - Living Representatives of the Most Ancient Organisms

Sat, 26 May 2012 12:47:52 +0100

The rocks in the hyper-saline waters of Lake Thetis, about 120 km north of Perth in Western Australia, are not quite what they seem. They are actually living things. Stromatolites are the oldest living life-forms on our planet.

They are formed through the activity of primitive unicellular organisms: cyanobacteria (blue-green algae) and other algae. These grow through sediment and sand, binding the sedimentary particles together, resulting in successive layers which, over a long period of time, harden to form rock. For at least three-quarters of the Earth's history stromatolites were the main reef building organisms, constructing large masses of calcium carbonate.

However their most important role in the history of the Earth has been that of contributing oxygen to the Earth's atmosphere. When stromatolites first appeared on Earth about 3,5 billion years ago there was little or no oxygen in the atmosphere. The organisms which construct stromatolites are photosynthetic. They take in carbon dioxide and water to produce carbohydrates, and in doing this they liberate oxygen into the atmosphere. It has been speculated that bacteria found in these organisms were responsible for increasing the level of oxygen in the atmosphere on Earth from much less than 1% to the present day level of 21%. lt was through the oxygen generating activity of stromatolites that other animal life on Earth was able to develop. These amazingly persistent living fossils form complex microbial communities. Conversely, it is believed that the decline in numbers of stromatolites is related to the evolution of animals that consumed cyanobacteria and algae. However, most living animals, which feed on the bacteria and algae of which stromatolites are composed, cannot tolerate the extremely saline conditions of places such as Lake Thetis, and as a result stromatolites can grow here successfully, undisturbed.

Western Australia has perhaps the best stromatolite fossils, giving a record through the eons of time. Fossils of the earliest known stromatolites, about 3,5 billion years old, are to be found near Marble Bar which lies on part of the ancient Pilbara Craton.

Stromatolites represent what is seen to be the biggest continuous biological lineage known in the world. The evolutionary events of the last 600 million years were in the time when most of the major groups of animals and plants on Earth evolved. If we look very closely they are primitive-celled organisms; these organisms have remained virtually unchanged during the comings and goings of all the animals and plants that have ever lived. Not only have they been found in some of the oldest rocks on Earth, but they have persisted with no other life forms for company. The existence of these ancient rocks extends three-quarters of the way back to the origins of the Solar System. These amazingly persistent living fossils form complex microbial communities. The long period of time over which these fossils have survived is amazing and these simple organisms have no peers.

The organisms’ existence is preserved in rocks by their fossilised remains, but also more commonly by the structures they created, domes or columns of sediment called stromatolites. They come in many shapes and sizes. It is extremely remarkable that the living stromatolites are only in a small number of places throughout the world. It is known that in the period from one billion to three billion years ago, stromatolites were prevalent on the shores of lakes and seas around the world. Such structures are still being formed today. Cervantes is one of several such sites in Western Australia where you can view them easily. Stromatolites grow as layers of sediment that have been trapped. These layers or mats slowly build on top of each other over many years with each stromatolite formation only growing at a rate of 5 cm in 100 years! They need light so are limited to shallow water where the sunlight can penetrate.

In South Africa the Malmani dolomite near Sudwala (and its equivalent Cambell Rand dolomite in the Northern Cape) are part of the Transvaal Supergroup that was deposited in a vast inland sea on the Kaapvaal Craton. Here too algal mats formed about 2,5 million years ago. On the upper part of the Sudwala Pass, there are small to large rounded or elongated dome-like structures visible in rocks alongside the road. The Barberton Greenstone Belt of eastern South Africa contains some of the most widely accepted fossil evidence for Archean life. These cell-sized prokaryote fossils are seen in rocks as old as 3,5 billion years. The Barberton Greenstone Belt is an excellent place to study the Archean Earth due to exposed sedimentary and metasedimentary rocks. The Barberton Greenstone Belt is located on the Kaapvaal craton, which covers much of the south eastern part of Africa, and was formed by the emplacement of granitoid batholiths. The Kaapvaal craton was once part of a supercontinent geologists term Vaalbara that also included the Pilbara craton of Western Australia. Though the exact timing is still debated, it is likely that Vaalbara existed from approximately 3,6 to 2,2 billion years ago, and then split into two different continents. Only in Australia though are modern-day algal mats or living stromatolites still to be seen. JW

References:
Lake Thetis Information pamphlet - DEC
Geological Journeys – Nick Norman & Gavin Whitfield
http://en.wikipedia.org/wiki/Archean_life_in_the_Barberton_Greenstone_Belt

All Photos are of Lake Thetis and were taken by Jo Wicht

Mineral of the Month - Opal

Wed, 18 Apr 2012 16:02:00 +0100

Common or potch opal, 6cm x 4cm Locality unknown
Opalised ammonite, 6,5 cm x 4,5 cm Madagascar

Crystal system:                Amorphous
Hardness:                        4,5 – 6,5
Density:                           1.9 – 2.3
Streak:                            White
Cleavage:                         None
Composition:                    Hydrated silica     SiO₂ nH₂

The word opal is adapted from the Roman term opalus, but the origin of this word is a matter of debate. Most modern references, however, suggest it is adapted from the Sanskrit (ancient Indo-Aryan language) word upala. Another claim is that the word is adapted from the Greek word, opillos. This word has two meanings, one is related to “seeing” and forms the basis of the English words like “opaque”, and the other is “other” as in “alias” and “alter”. It is claimed that apalus combines these uses, meaning to see change in colour. The argument for the Sanskrit origin is the strongest and the term first appeared in Roman references around 250 BC. At the time opal was valued above all other gems and was supplied by traders from the Bosphorus, who claimed the gems were being supplied from India. Okay! This concludes the history lesson for the day.

Opal is an amorphous form of silica related to quartz, and is therefore classified as a mineraloid and not a mineral. Between 3% and 21% of its total weight is made up of water, whereas quartz contains no water. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. Despite this almost all scientific references, including the well-known Dana’s System of Mineralogy, categorize opal together with true minerals. It only forms crystal shapes when it forms as pseudomorphs after another mineral.

During the Cretaceous period, some 135 to 65 million years ago, the central area of Australia was an inland sea. Fine marine sands rich in silica were deposited around the shoreline. The Great Artesian basin formed when the sea receded. Around 30 million years ago, deep weathering caused changes to the sediments. As the water filtered down, it picked up silica from sandstone, and carried the silica rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporated, it left behind a silica deposit. This cycle repeated over very long periods of time, and eventually opal was formed. Opalised shells, woods and reptilian bones of the Cretaceous period are also found, their remains dissolved by the solution and replaced with opal. The solution is believed to have a rate of deposition of approximately one cm in five million years at a depth of approximately 40 metres.

This diagram appeared in the Australian Geographic of July-September 1998.

Precious opal is one of the most precious gemstones and it shows a variable interplay of internal colours and even though it is a mineraloid, it has an internal structure. At micro scale precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close packed lattice. The coloured lights (called iridescence) that precious opals give off are caused by diffraction of white light from these tiny silica spheres inside the gemstone. A similar phenomenon occurs in iridescent bird wings. It is said that the larger the internal spheres, the larger the range of colours. In some instances one can make an opal more colourful simply by holding it and the heat in your hands will expand the spheres and increase the range of colours.

Depending on the conditions in which the opal formed, it can take on many colours. It ranges from clear, white, gray, red, orange, yellow, green, blue, magenta, rose, pink, olive, brown, black, etc. and if you think the range of colours is extensive; one source mentions over 70 varieties of opal, although not all are recognized as natural opal. This is pretty complex for a mineral that is not really a mineral. Black opal, crystal opal, boulder opal, white opal and fire opal, are just some of the more well-known varieties. Black opal is the most valuable and desired variety and can even fetch a higher price per carat than diamonds. White and precious fire opal can also be quite expensive.

Most precious opal is mined in Australia and is, therefore, also the national gemstone of Australia, which produces 97% of the world’s supply. Other worldwide deposits of precious opal occur in Mexico, USA, Ethiopia, Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Guatemala, Nicaragua, Brazil and Honduras. Very interesting to note is that in late 2008, NASA announced that they had discovered opal-bearing deposits on Mars …………. and no it’s not an April Fool’s joke.

The town of Coober Pedy in South Australia is a major source of precious opal. The world’s largest and most valuable gem opal “Olympic Australis” was found in 1956 at the “Eight Mile” opal field in Coober Pedy. It weighs 17 000 carats (3,45 kg) and is 28 cm long, with an height of 12 cm and width of 11 cm. It was valued at approximately AUD$ 2 500 000 in 2005 and consists of 99% gem opal and the remaining 1% being the soil still attached to it.

The largest opal matrix was also discovered in South Australia weighing 55 000 carats. It was discovered by Stuart Hughes and its dimensions are 30 cm x 20 cm x 4cm and is valued at US$ 1 000 000.

Just some additional useless information on Coober Pedy, with its population of approximately 3500 people, is that it is also a famous tourist destination since 80% of its population lives underground due to the summer temperatures that can exceed 50 degrees. These underground residents have all the modern facilities which you can expect from any normal house. The “Desert Cave Hotel” is also very prestigious accommodation for tourists with the world’s only underground bar, gaming room and art gallery. Although it is a small town, you can experience the incredible ethnic diversity with over 45 different nationalities.

On home turf precious opal has to date not been found in South Africa but common opal in a variety of colours and patterns occurs near Postmasburg and Pella in the Northern Cape, in the Pilansberg and near Soutpansberg.

In the Erongo region in Namibia, hyaline (also a variety of opal) is found mainly as colourless to white/light green globular masses on mineral specimens.

Erongo hyaline opal on quartz crystals in normal and shortwave UV light(photo and specimen JW)

Due to their unorganised structure, opals have no crystal shapes to hold them together and they may eventually dry out and crack. Opals are very fragile when exposed to the air as they lose molecules of water and develop tiny fractures. When heated, they can also lose their water content, decompose and may alter into chalcedony or quartz. As a word of caution, opal is apparently soluble in hot salt water. There is some good news, however, since it is insoluble in most acids.

Opals are also relatively soft and for this reason, they need to be set in jewellery in such a way that it affords them the maximum protection against scratching and knocks. Opals must also be cut slowly and with plenty of coolant due to their heat sensitivity. This might seem like a lot of trouble, but is all worth it when set in such a way that their beauty and colour play and flashes captivate the gem lover’s eye.

For gemstone use its natural colour is often enhanced by placing thin layers of opal on a darker underlying stone (doublets), such as basalt. Opal doublets are sometimes also coated with a thin layer or dome shaped clear quartz which not only protects the opal but also acts as a magnifier to emphasize the play of colour and are referred to as triplets.

Common opal is used in the industry as fine abrasives, filtering powders, in insulators and in ceramics. Common opals, also referred to as potch opals, are those stones without the play of colour and they occur worldwide.

The discovery of the ordered sphere structure of opals led to the fabrication of synthetic opal by Pierre Gilson in 1974. This material is distinguishable from natural opal by its regularity under magnification and it also does not fluoresce under UV light, unlike the real McCoy - JDJ

References:

Cairncross, Bruce, 2004 – Field Guide to Rocks & Minerals of Southern Africa.

Orbis Publishing Ltd, 1995, - Treasures of the Earth - The Minerals and Gemstones Collection.

Simon and Schuster, 1977 - Simon and Schuster’s Guide to Rocks and Minerals

www.minerals.net

www.mindat.org

www.traveltourimmigration.blogspot.com

www.wikipedia.org

All specimens and photographs by Johann de Jongh unless otherwise noted.