Articles

by
Peter Rosewarne

This month we are checking out the garnet group of minerals, which most people will have come across in the form of jewellery, as mineral specimens and even in mundane articles such as sandpaper. The previous fluorite article started with some “C” words that apply to the mineral and in the same vein, the following apply to garnets, with some qualifiers; cubic, contain calcium (some), colourful (some), cleavage-free, conchoidal fracture, costly (some), contain chrome (some), and common (mostly).

The group consists of 15 members of which only seven are widely known, with 10 varieties that I could discover. They belong to the silicate class of minerals and occur in virtually all colours, including a fairly recently discovered blue variety from Madagascar. They are common in metamorphic rocks and some ultramafic igneous rocks and, being relatively hard, are common as detrital grains in sediments. This article concentrates on garnets as mineral specimens with passing reference to their significance as gems. As always before we get on to discussing individual occurrences, here is some brief technical information, or you can skip the next section and go straight to the photographs. Talking of which, all specimens except two are from the Rosey Collection and all photographs by the author.

The name garnet is derived from the resemblance of crystals to pomegranate seeds. Garnet became a fashionable Christian name in the late Victorian era. One of the most important people that I have come across with this name is Sir Garnet Wolseley, a prominent British army official who featured in the history of South Africa back in the early 1900s.

Background Information  Garnets are nesosilicates, meaning that the silicate tetrahedra are isolated in the crystal lattice. They crystallise in the isometric system with the rhombic dodecahedron (d) and trapezohedron (n) being the most common forms, sometime modified by each other, as shown in Figure 1. They come in red, pink, orange, brown, green, yellow, and hues in between, black and colourless and even blue. Their hardness ranges between 6.5 and 7.5, depending on chemistry. Figure 1.

The seven common-to-rare garnets that are familiar to mineral collectors and as gems are:

·        Almandine (Fe,Al)
·         Spessartine (Mn,Al)
·         Pyrope (Mg,Al)
o   Collectively known as the pyralspite series, and
·         Andradite (Ca,Fe)
·         Grossular (Ca,Al)
·         Uvarovite (Ca,Cr)
o   Collectively known as the ugrandite series, and
·         Hydrogrossular (“Transvaal jade”)

In nature, pure end-members do not occur and garnets are classified according to the dominant molecule present. Colour is not a reliable characteristic for identifying garnets and optical features and chemical analysis are the surest ways to accurate identification.

Garnets are widely used as gems and in industrial applications such as abrasives, and several of the types listed above probably feature in most mineral collections. Australia is the largest producer of garnet for abrasive applications, followed by India, both from beach-sand deposits. Historically, pyrope for gem use mainly came from Bohemia in the Czech Republic. In ancient times almandine and pyrope were known as carbuncle. Garnet crystals were used as bullets in Asia and the southwest of the USA.

Descriptions and Photographs

Almandine is the commonest garnet and is characteristic of metamorphic rocks such as schists, gneisses and granulites. It is named after the town of Alabanda in Asia Minor and is the state gemstone for Connecticut, USA. It is commonly a reddish-brown to dark brown colour to almost black and often shows a tinge of violet. A classic location for almandine is the Zillertal area in the Swiss Alps, where large well-formed crystals occur in mica schist. An example is shown in Figure 2. The crystal is 5 cm and forms a perfect rhombic dodecahedron, typical of almandine. Giant almandine crystals up to 15 cm are found at the Sedalia Copper Mine in Colorado, USA and an example of a 9.5 cm crystal is shown in Figure 3.

The red-pink gem variety of almandine-pyrope is known as rhodolite. Colour-change pinkish rhodolite garnet occurs in the Lindi District of Tanzania; when exposed to cool LED light they change to purple, similar to the colour of amethyst.

Figure 2. Almandine from the Zillertal area


Figure 3. Almandine from Sedalia Copper Mine, USA

Pyrope is a characteristic mineral of ultrabasic igneous rocks such as garnet peridotite, kimberlite and eclogite. It is usually a deep red colour and derives its name from the Greek for fire, pyropos. It is popular as a gemstone and extensive weathered volcanic deposits in Bohemia have been mined since the middle of the fifth century. It is an important geochemical tool used in the search for diamondiferous kimberlite pipes as the so-called G10 ‘facies,’ characterised by low Ca and high Cr, is an indicator of suitable conditions having been present for the formation and preservation of diamonds. It is generally not found in euhedral crystals but rather as rounded grains of a few millimetres to a centimetre or so surrounded by pyroxene or olivine, as shown in Figures 4 and 5.  

Figure 4. Pyrope garnets in eclogite, Norway


Figure 5. Pyrope garnets in garnet peridotite, Norway

Spessartine is named after an occurrence in the Spessart (= forest) Mountains of Germany. It is characteristically found in metamorphic rocks such as schists but also in pegmatites. Marienflϋss in the Kaokoveld area of Namibia is the main source of gem-grade spessartine and the bright orange crystals are called “Mandarin” garnets. They occur embedded in mica schist. Another important source of bright orange gem spessartite is Iseyin in Nigeria. Similar orange crystals occur in the Merelani Hills area of Tanzania, but these often have inclusions. An example in mica schist is shown in Figure 6, the largest garnet being 1.5 cm. Very attractive mineral specimens have originated from the Tongbei area of China with deep red to dark brown trapezohedral crystals associated with smoky quartz and white to creamy feldspar, as shown in Figures 7 and 8, and coatings of small, bright orange crystals, as shown in Figure 9.

Figure 6. Spessartite, Tanzania


Figure 7. Spessartite and smoky quartz, China


Figure 8. Gemmy spessartite on smoky quartz, Tongbei, China


Figure 9. Spessartite, Tongbei, China


Figure 10. Spessartite on quartz, Shigar Valley, Pakistan

Very attractive specimens also come from Pakistan and Afghanistan, with large reddish-brown crystals associated with white/creamy feldspar. The best of these are arguably the “best of” mineral specimens of the garnet group. Check out the  “Contemporary Masterpiece” from Ikons in Figure 10 to see what I mean. The specimen is 11 cm tall and is possibly the world’s best garnet matrix specimen.

A light pink and orange to red pyrope-almandine-spessartine garnet found in the Umba area in northeast Tanzania is known in the trade as Malaya garnet.

The extremely rare blue garnets from Madagascar are pyrope-spessartine and derive their colour from a combination of Mn and traces of vanadium (V).

An unusual rock called coticule is found in the Ardennes area of Belgium consisting of minute spessartine crystals, chlorite and quartz. It is a straw-yellow to slightly green rock which occurs in layers mostly <5 cm thick in phyllite. It is thought to have originated as a clay, rich in volcanic ash before being metamorphosed. It is the world’s main source of whetstone used to sharpen steel knives and tools and has been used as such since ancient times.

Grossular was first discovered in Russian Siberia and because the crystals were green it was called grossular after the Latin for gooseberry. It occurs in a number of other colours including brown, yellow and pink. It is the state gemstone for Vermont, USA. So-called “Raspberry” grossular occurs in rhyolitic rocks in the Sierra de Cruces in Mexico. The pink colour is caused by Mn atoms occupying sites in the outer crystal lattice. They characteristically have black cores with high titanium. An example is shown in Figure 11 and a large crystal (5.2 cm) in Figure 12. Garnets from this locality that lack Mn look like Figure 13 (largest crystal is 4 cm).

Figure 11. Raspberry grossular


Figure 12. Grossular garnet


Figure 13. Grossular garnet
All 3 above from Sierra de Cruces Range, Mexico

Tsavorite is the green gem variety from the Merelani Hills area of Tanzania and into Kenya. The green colour is caused by Cr and V. An example from the internet is shown in Figure 14.

Figure 14. Tsavorite

Hessonite is a honey-coloured variety with beautiful examples having come from the Jeffrey Mine in Canada (see Figures 15 and 16). These crystals show characteristic striations caused by repetition interference of the dodecahedron with the trapezohedron crystal form. Dodecahedral crystals without the striations are shown in Figure 17. The ancient name for hessonite, lyncurium, came from a belief that it was formed from lynx urine and earth.

Figure 15. Grossular from Jeffrey Mine, Canada 


Figure 16. Grossular var. Hessonite, Jeffrey Mine


Figure 17. Grossular var. Hessonite, Jeffrey Mine

Other classic localities occur in Italy, such as the Piedmont and Bellecombe valleys where reddish-brown crystals occur on clinochlore and diopside (Figures 18, 18a). Well-formed brown dodecahedral crystals from Peru are shown in Figure 19.

Figure 18. Hessonite on clinochlore, Piedmont Valley, Italy


Figure 18a. Hessonite on clinochlore, Piedmont Valley, Italy


Figure 19. Grossular from Peru

Andradite is a characteristic garnet of skarns formed by the metamorphism of impure limestones and is usually a somewhat drab brown colour. It is named after a Brazilian statesman and geologist. Figure 20 shows a typical-looking andradite garnet plate from Dal’negorsk, Russia. From about 1994 attractive andradite garnets from the Kayes Region of Mali, in a range of colours from brown to green to yellow, hit the mineral market. Analysis of these garnets shows that many are grossular-andradite in chemistry. Some of the brown type are shown in Figures 21 and 22 (crystal is 3.5 cm).  

Figure 20. Andradite garnets, Russia


Figure 21. Andradite cluster, Kayes Region, Mali


Figure 22. Andradite, Kayes Region, Mali

An attractive reddish-brown to red variety well known to local and international collectors is characteristic of the Wessels and N’Chwaning II mines in the Kalahari Manganese Field of South Africa (see Figure 23). In association with hematite they make unique and sought-after collector specimens, as shown in Figure 24 (12 cm across).

Figure 23. Andradite, KMF, SA


Figure 24. Andradite on hematite, Wessels mine, SA

Traces of Cr give rise to the green variety demantoid (diamond-like dispersion) which would be as valuable as diamond if it were harder. Examples from Italy, Madagascar and Namibia are shown in Figures 25, 26 and 27. Topazolite (topaz-like) is a honey-brown variety with nice examples coming from Madagascar, with an example in Figure 28. Melanite (Greek for black) is an attractive opaque black variety, the ‘colour’ being caused by titanium. Collector examples come from Mexico and an example is shown in Figure 29.

A type of iridescent andradite called rainbow garnet comes from Sonora in Mexico. 

Figure 25. Demantoid garnet, Italy


Figure 26. Demantoid, Madagascar


Figure 27. Demantoid Tubussis, Namibia


Figure 28. Topazolite, Madagascar


Figure 29. Melanite, Mexico

Uvarovite is the rarest of the main garnets and is named after a Russian statesman. It is characteristic of metamorphosed chromite deposits and the bright green colour is caused by the presence of Cr in the crystal lattice. The type area is Saranovskii in the Urals, Russia, where small lustrous dark green rhombohedral crystals occur within chromite layers. These crystals are usually small, up to a few millimetres and very bright and gemmy. An example from the Saranovskii Mine is shown in Figure 30 at left. The second most important locality is the polymetallic deposit at Outokumpu in Finland where larger crystals up to 2 cm occur associated with chromite and other minerals but these larger crystals are always cloudy and/or have cracks.

Figure 30. From Russia


Figure 30b. Close up of uvarovite crystals

Hydrogrossular (water-bearing) is better known in South Africa and the trade as “Transvaal jade.” Outcrops of pink, green and grey hydrogrossular occur in the Rustenburg area and its visual resemblance to jade and physical properties led it to being called Transvaal Jade. It was formed by the alteration of plagioclase feldspar in anorthosites of the Bushveld Complex, possibly by dyke intrusions, often leaving behind bands and stringers of black chromite. An example of the green variety with chromite bands is shown in Figure 31. The green colouration is due to the presence of Cr and the pink colouration to the presence of Mn.

Figure 31.

Concluding Remarks

This is not a scientific article but has rather concentrated on some interesting background information, localities and numerous photographs. As with others in this series of articles, I've enjoyed the ride and learnt more about this attractive group of minerals and I hope you have too. If you get a chance, check out Garnets: Great Balls of Fire and Bruce Cairncross’s book for some beautiful photographs and more interesting background information. The last photograph is of a “drawer-full of garnets” (Figure 32 below). And with that I am taking a break for a while…

References

Cairncross, B. (2019). Minerals and Gemstones of East Africa. Struik.

Thompson, W.A. (2007). Ikons: Classic and Contemporary Masterpieces. Supplement to The Mineral Record Vol. 38, No. 1 January-February 2007. Tucson.

Gilg, HA. et al. Eds. (2008). Garnets: Great Balls of Fire. Lithographie. Connecticut.

Thomas, A. (2008). Gemstones: Properties, identification and use. New Holland Publishers. London.

Treasures of the Earth. (1995). Volume 1: The Minerals and Gemstones Collection. Orbis.