Understanding Garnet: Mineral Properties and Rock Occurrence

What is garnet

Garnet is a group of dense and hard silicate minerals which occurs in many rock types, but they are especially common in some metamorphic rocks like schist and amphibolite. Garnets are common rock-forming minerals in some igneous rocks.

Structure

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 is X₃Y₂(SiO₄)₃, where X cations are mostly Fe²⁺, Mn²⁺, Mg, and Ca and the Y cations are Al, Fe³⁺, and Cr³⁺.

Two groups

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:

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. It is an almandine if Fe²⁺ is the main cation in the X site. Almandine is the most widespread mineral of the 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.

Unusually dense mineral

Garnet group minerals crystallize in the cubic system – they all show roughly equant dimensions (no elongation). Garnet in rocks may demonstrate beautifully developed crystal faces. They 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. It 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.

Host rocks

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 variety almandine. Pyrope is a Mg-rich variety 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 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.

Garnet in sand

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 surface of the rocks.

Color

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.

Uses

Garnet is a well-known mineral not only because it is so widespread, but mostly because of its deep red color and beautiful crystal faces which make it a semi-precious gemstone. Industrially garnet is mostly used as an abrasive because of its hardness and irregular fracture. It is also used in water purification filters.

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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.