Grossular Garnet

Not all garnet is red, although that's the way to bet. There are six garnet minerals belonging in two groups: an aluminum-silicate group called the pyralspite series (from pyrope, almandine and spessartine) and a calcium-silicate group called the ugrandite series (from uvarovite, grossular and andradite). Within each group the compositions blend smoothly between members, so classifying a garnet specimen is often an arbitrary exercise best left to experts. The best way to tell is to note the color and the rocks in which the garnet is found.

Grossular is sometimes called grossularite. This grossular crystal is from Michoacan, Mexico. I didn't collect it myself, but good specimens usually come from metamorphosed limestones where calcium is abundant and iron absent. Its composition is Ca₃Al₂(SiO₄)₃, the SiO₄ or silica groups being isolated, not linked as they are in many other silicates. The other ugrandite garnets, andradite and uvarovite, have iron and chromium in place of the aluminum (Al). The garnets usually form good crystals with this form—round with 12 sides (dodecahedral)—or with 24 sides (trapezohedral).

Grossular, like all garnets, has a hardness of 6 to 7.5 and no internal cleavage. When crushed, garnets break into tough, jagged grains (as shown here). This once made garnet the choice for top-quality sandpaper.

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