Garnets

For my first Trivial Pursuits of the year I thought it would be good to talk about a birthstone, and garnet -- which happens to be the birthstone for January -- is an interesting and varied topic. (Amethyst is the birthstone for February, but it doesn’t have the complexity of the garnets.) Actually, I will talk about the garnet family because there are lots of different garnets.

Garnets are cubic minerals with the general formula A3B2Si3O12 where A can be Mg, Ca, Mn2+, and Fe2+ and B can be Al, V3+, Cr3+, Mn3+, and Fe3+. Even though garnets are cubic, you will probably never see a natural crystal of garnet in the shape of a cube. In fact they don’t form octahedra either - or tetrahexahedra or trisoctahedra. They only form as dodecahedra and trapezohedra, crystals with some of both, and some with hexoctahedral modifications.  

In addition to the common elements, there are a few weird elements that can be found in some natural garnets but, for a change, I don’t want to get involved with the weird stuff too much. At this point I should also mention that man has created several materials of unusual composition with the garnet structure that are not likely to ever be found in nature, such as YAG (yttrium aluminum garnet) and GGG (gadolinium gallium garnet), but I will not dwell on these either.

Simple multiplication of the 4 A elements and the 5 B elements indicates that there are at least 20 possible garnet end-members. Not all of these have been found yet and some may never be but there are still a lot of garnets around. And don’t forget, most garnets will have a little bit of everything in them; natural garnets that are even close to an end-member in composition are quite rare.

All of the Al varieties are common: they are Mg-Al-pyrope, Ca-Al-grossular, Mn-Al-spessartine, and Fe-Al- almandine (not an -ite ending in the group). The Fe3+ garnets are represented by Mg-Fe-majorite, Ca-Fe-andradite, and Mn-Fe-calderite. An Fe-Fe garnet isn’t known but should be possible - keep looking. The Cr garnets are Mg-Cr-knorringite and Ca-Cr-uvarovite, while the only V garnet is goldmanite which was originally described from the Grants, N.M. area. There are no Mn3+ garnet end-members known but these may turn up someday.

In addition to these end-member garnet names, we can find a number of names given to certain garnets to help sell them as gems. These include demantoid, a beautiful green, Cr bearing andradite; tsavorite, a very beautiful, emerald green, KV bearing grossular: topazolite and verdolite, yellow and green varieties of andradite; rhodolite, a pinkish-purple almandine; and Malaya (ma-lye-ah) a pinkish-orange pyrope- spessartine.

The various garnets are found in a wide variety of rock types. The Ca-garnets are almost invariable found where igneous intrusions have invaded into limestones. The calcite from the limestone combines with silica and the other metals which emanate from the intrusion. Well, all except Goldmanite which forms by combining Ca from calcite, V from tyuyamunite, and silica from a basalt sill. Mg garnets (pyrope, Cr-pyrope, and knorringite) tend to come from ultramafic igneous rocks. Spessartine garnets tend to come from acid igneous rocks like rhyolites and pegmatites. Almandines are the common garnet of regionally metamorphosed rocks like schists and gneisses.

Well, which one is the birthstone and what is the significance of this birthstone?

All of the red garnets - almandine, pyrope, rhodolite - qualify as the January birthstone. There is no real significance to the birthstone. It is essentially a sales gimmick to help sell more stones to people. (Wait a minute, I sell jewelry myself.) Hey Lady! Wanna buy a nice necklace with your birthstone? What about a ring or a pair of earrings? You just gotta have something with your own, personal birthstone.

Republished by permission of Albuquerque Gem and Mineral Club