garnet group

Crystals that are shaped like garnets but chemically, they may or may not bear any resemblance to garnets.

Pyrope Garnet

This is a special type of garnet called pyrope garnet. The name comes from the Greek pyro, meaning fire. Pyrope and other members of the aluminum part of the garnet group have a higher specific gravity and hardness, and are usually red. Calcium garnets like the previously mentioned andradite and uvarovite are the ones that are usually green and have a lower hardness and specific gravity.

Pyrope garnet is difficult to distinguish from almandine, but pyrope usually has fewer flaws and inclusions. However, garnet jewelry is usually almandine garnet because almandine is much more common and inexpensive.

If you would like some pyrope garnet it can be found nearby in Kansas, all around the Nemaha Uplift (or Nemaha Ridge), which is in the area between Salina and Manhattan, and extending south into Oklahoma. Basically, garnets are found anywhere near previous volcanic activity. The one pictured is from Apache County, Arizona. They are also found in Africa and other places. For lots and lots of information about this particular specimen, see its page on the RRUFF here.

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Andradite Garnet

andradite garnet

Credit: Aaron Palke/Gemological Institute of America

Since it’s January, it’s a good time to read about this garnet originally posted by Chemistry in Pictures.

“This gemstone isn’t pure andradite garnet [Ca₃Fe₂(SiO₄)₃], but its flaws produce its mesmerizing colors. Some gemologists think that this rainbow explosion arises because the garnet’s different elements aren’t regularly spaced from the core of the gemstone to the outside. For example, in some regions, aluminum atoms might have worked their way into the structure and replaced the iron atoms. These irregularities create mismatched sheets of atoms that then bend and stretch. This makes the stone birefringent, meaning that light travels through it at two different speeds. Under cross-polarized lighting conditions, rays of light that enter get misaligned by the time they exit, so they then interfere with each other and highlight some colors in certain spots, producing the spectrum seen here. The black flecks are tiny pieces of magnetite that were enveloped by the crystal as it grew.”

Spessartine

You may remember our earlier post about uvarovite, the green garnet.  Spessartine is another garnet group mineral.  Notice how similar the crystals look, except for spessartine being reddish orange (and a bit of yellow in there, too) and uvarovite being green.  It is named after where it was first found: the German city Spessart, in the state of Bavaria.  The specimen pictured was found in the Broken Hill district in New South Wales, Australia.  It can also be found in Madagascar, China, Myanmar, India, Afghanistan, Israel, and parts of the United States such as Maine and Colorado.  Look for it in granite pegmatite and also sometimes in metamorphic phyllites.

If you liked these posts, you may wish to research the other garnet group minerals: almandine (purplish-red to orangish-red), andradite (can be black, green, or yellow-green), grossularite (brown to yellow), pyrope (very dark red to black), and tsavorite (light to dark green).

Uvarovite

Uvarovite is a garnet group mineral, meaning that it’s structurally and compositionally similar to garnets, but unlike garnets, it is green. Uvarovite is named after Count Sergei Semenovitch Uvarov, a Russian statesman who was also quite the rockhound. It’s usually found in Russia, Spain, and Canada.  The photo above is an extreme close-up, and may be misleading.  Uvarovite crystals are usually about 2 mm long, so in “real life” it looks more like a plate of drusy quartz.  Here’s how it looks from a bit farther away:

A plate of tiny green sparkly crystals of uvarovite.

Photo by Rob Lavinsky, retrieved from Mindat http://www.mindat.org/photo-118217.html

This specimen is from the Saranovskii Mine in Russia, and I think it’s quite pretty, don’t you?