Garnet


Garnet is a dense and hard silicate mineral which occurs in many rock types but it is especially common in some metamorphic rocks like schist and amphibolite. Garnet is also a common rock-forming mineral in some igneous rocks. It is hard and resistant to weathering which makes it a very frequent component of sandy sediments. Garnet is almost nowhere a dominant mineral (it is one of the principal minerals in eclogite) but it is present in variable amounts in a wide variety of rock types and sediments. It is easily noticeable because of intense and contrasting coloration (mostly red) and because it often stands out from the rock’s surface.

Garnet is actually a mineral group, not a single mineral. These minerals share similar crystal structure but they have a variable chemical composition. The general chemical formula of garnets are X3Y2(SiO4)3, where X cations are mostly Fe2+, Mn2+, Mg, and Ca and the Y cations are Al, Fe3+, and Cr3+.

Garnets are divided into two groups.Those with Al in Y structural site are the pyralspites and those with Ca in the X site are ugrandites. These strange names are derived from the first letters of the single minerals in these groups. Pyrope, almandine, and spessartine make up the pyralspite and uvarovite, grossular, and andradite are the members of the ugrandite group.

Here are the common members of the garnet group and their chemical composition:

Mineral Composition Group
PYROPE Mg3Al2(SiO4)3 Pyralspite
ALMANDINE Fe3Al2(SiO4)3 Pyralspite
Spessartine Mn3Al2(SiO4)3 Pyralspite
Grossular Ca3Al2(SiO4)3 Ugrandite
ANDRADITE Ca3Fe2(SiO4)3 Ugrandite
Uvarovite Ca3Cr2(SiO4)3 Ugrandite

Pure endmembers, however, are very rare. There is an extensive solid solution within pyralspite and ugrandite groups but only limited amount of substitutions are possible between these groups. Hence the need to separate them into two groups. Specific name of a garnet group mineral depends on the dominant cation. Garnet is an almandine if Fe2+ is the main cation in the X site. Almandine is the most widespread mineral of the garnet group. Uvarovite is commonly described as a common garnet group mineral although it is rare in nature and occurs only in specific chromium-rich rocks. Possible replacements in the lattice are not restricted to those mentioned above but these are the most important ones.

Garnet (almandine) is a common mineral in metamorphic rocks that formed when buried in crust under the load of at least 10 km of rocks and sediments1. This rock sample is a schist (metamorphosed clay-rich sediments) that contains many common Al-bearing porphyroblasts like almandine (red, equant), staurolite (dark, elongated), and kyanite (light blue, elongated) in a light-colored groundmass of muscovite (mica). Width of sample is 7 cm.


Garnet group minerals crystallize in the cubic system. Hence, they all show roughly equant dimensions (no elongation). Garnet in rocks may demonstrate beautifully developed crystal faces. Garnets are outstandingly dense minerals for a silicate mineral with such a composition. Their specific gravities range from 3.58 (pyrope) to 4.32 (almandine). This is a result of close packing of the crystal structure which allows garnets to be common minerals deep in the crust and mantle. Garnet is also physically hard, some garnets are even harder than quartz. This property and a lack of cleavage makes it a potentially good abrasive material and garnet is indeed frequently used for that purpose.

Rocks that host garnets are relatively good guides that help to identify the specific garnet species. Red equant grains in mica schists belong to iron-rich garnet almandine. Pyrope is a Mg-rich garnet that occurs in (originally) deep-seated rocks like peridotite, kimberlite, eclogite, or serpentinite. Spessartine, the manganese-rich variety of the pyralspite group, is common in magmatic rocks, especially pegmatite. Ugrandites are typical garnets in metamorphosed calcareous rocks like skarns. This is simplified approach, of course. For example: almandine also occurs in igneous rocks, not only metamorphic rocks. And spessartine also occurs in skarns.

The color of garnet is primarily controlled by its composition. Pyralspites are either red, orange, purple, or even almost black. Grossular and andradite are yellowish brown to black or green. Uvarovite is bright green. Garnet crystals are beautiful because they are intensely colored and often have nicely developed crystal faces. Hence, garnet is also used as a semiprecious gemstone.

Almandine is dark red in rocks but small almandine grains in sand are light red or pink. These grains are picked from a beach sand. Redondo Beach, California, USA. Width of view is 10 mm.

Almandine grains from Emerald Creek, Idaho, USA. Width of view is 15 mm.

Andradite (demantoid) crystals. Andradite is not usually green but demantoid is a green variety of andradite that owes its color to chromium that is partly in place of iron in the crystal lattice. Width of view is 30 mm.

Beach sand from Sri Lanka that contains lots of heavy minerals like almandine (pink) and spinel (dark red). Width of view 20 mm.

Garnet concentrated from a beach sand in Australia. Garnet is used as an abrasive material. Width of view 20 mm.

Garnet-rich fraction of heavy minerals sorted out by running water near the coastline at Pfeiffer Beach, California.

A closer look to the Pfeiffer Beach sand. Pink mineral is garnet. Width of view 8 mm.

Almandine grains are often present in granitic igneous rocks (S-type granites that have a sedimentary protolith). Width of sample from Estonia is 8 cm.

Sometimes garnet crystals are very concentrated in beach sand. This sand sample is from Nome in Alaska which also contains gold. Width of view is 10 mm.

This sand sample obviously comes from a weathered metamorphic terrane. It is composed of schistose lithic fragments, mica, garnet, and plagioclase feldspar, among others. Width of view is 20 mm.

References

1. Wood, B. J. (2007). Garnet. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 7. 686-687.
2. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.


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