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