The GemHunter

Professor Hausel's Guide to Finding Gemstones, Diamonds, GoldRocks & Minerals 

Garnet - a Gem of a Mineral

"Just a little quartz for the foundation, some magnesium here, a little chromium there, maybe a pinch of aluminum, a bit of iron - ah heck, just throw in the bucket. Sit on it for a million years or cook it in a pressure cooker, and walla, we have garnet gemstones that ants can't resist" 

-  W. Dan Hausel

 

Ants, ants, ants - these little buggers bite with emphasis in Arizona and leave their mark for days. But in Wyoming, ants work for a living - yes, they are employed as miners to dig for gemstones just to decorate their mounds so you and I can pluck them off their hills. But don't worry, there be plenty more for ants to find and decorate their mounds. 


Left- excellent pyrope garnet I picked up from anthill in Butcherknife Draw in the Green River Basin of Wyoming. It was  faceted in Sri Lanka.


But where do these gems come from - how do the ants find them. Well, a friend of mine, Dr. Tom McCandless, worked on a research project and placed a group of beer bottle chips on grid patterns around some anthills to see what the little buggers would do. Sure enough, when he later returned, the ants had sampled the beer bottle fragments on their way to a AAA (Alcoholic Ants Anomalous) meeting, and placed them on their anthill. So, the Green River Basin ants gather chips and pebbles from the surrounding soil at the surface, and drag them to their hills. 


Those tiny gems can be cut, and a few are quite large. If I remember correctly, one of the larger stones I collected in the Leucite Hills was a 12 to 14 mm peridot. That must have been one heck of a strong ant! Actually, ants work in teams, and a large gemstone like that is moved by a team. After all, what else do they have to do?


There are many varieties of garnet. Some produce extraordinary and beautiful gems yet garnet is a common accessory mineral in micaceous (almandine-amphibolite-grade) metamorphic rocks known as schist and gneiss. And some are found in kimberlite, granite and in granitic pegmatite. In metamorphic or pegmatitic rock, garnet can range from millimeter-size to single crystals of 5 to 6 inches in diameter. Many are purplish-red, yellow-orange or reddish-brown in color.


Six pure end-member garnet subspecies are known to mineralogists. These vary in color, specific gravity, chemistry, and index of refraction and include: (1) pyrope [Mg3Al­2(SiO4)3], (2) almandine [Fe3Al­2(SiO4)3], (3) spessartine [Mn3Al­2(SiO4)3], (4) grossular [Ca3Al­2(SiO4)3], (5) andradite [Ca3Fe­2(SiO4)3] and (6) uvarovite [Ca3Cr­2(SiO4)3]. Natural garnets form solid solutions or solid chemical mixtures of these end members and the pure end member compositions are uncommon. Thus, garnets are described as solid-solutions (hybrids). Names have been given to some garnets with intermediate compositions, such as rose-red to purple rhodolite garnet, which has a chemical composition of 2:1 mixture of pyrope to almandine. Another intermediate variety with a composition between pyrope and almandine (1:1 mixture) is referred to as pyrope-almandine (also known as Mozambique garnet) which exhibits a striking dark orange-red to red color.


Garnets have relatively high specific gravity (3.5 to 4.3) and hardness (6.5 to 7.5). The specific gravity results in the mineral showing up in heavy black sand concentrates when panning for goldGarnets crystallize in the isometric crystal system with no cleavage. They are transparent to translucent and often exhibit well-formed dodecahedral or trapezohedral crystal habit. Garnets are used for abrasives; although, excellent museum quality specimens are found, and many transparent to translucent garnets with good color are used as semiprecious gemstones.


Occurrence. Garnet is a common mineral in many metamorphic terrains, particularly where rocks have been metamorphosed to amphibolite-grade (moderate pressure and temperature) facies and are aluminum-rich. Such rocks typically contain abundant black mica and may also have amphibole with porphyroblasts of garnet. Such rocks are typically found in many mountain uplifts in North America. They can also be found in what is known as skarn - limestone (and other carbonate-rich rocks) that has been heated and altered by the intrusion of hot magma. 


Localities. A number of garnet localities are known in Wyoming and many thousands remain to be discovered worldwide. In Wyoming, these include translucent to opaque almandine garnet with good dodecahedral habit located in the Teton Mountains and chlorite pseudomorphs after garnet from the Sierra Madre near Encampment. These latter pseudomorphs exhibit excellent dodecahedral habit, are opaque, and completely to nearly completely replaced by chlorite mica, even though many retain the former dodecahedral crystal habit of the garnet inherited by the chlorite mica. Excellent, 3 to 4-inch diameter pseudomorph garnets occur in chlorite schist at the Oldman prospect (NE section 14, T14N, R84W). The deposit located south of Encampment along the Copper Creek road forms a narrow schist (<10 feet wide) on both sides of the road about 1/2 mile south of the Oldman Ranch. The garnet-chlorite-schist crops out over a distance of approximately 2,000 feet, and has large, dodecahedral, chlorite pseudomorphs after garnet. Several 0.5 to 3-inch diameter pseudomorphs have been collected. The interior of many of the pseudomorphs contain primary, reddish-brown, almandine garnet (Hausel, 2014).


Extraordinary pyrope-almandine garnet megacrysts occur in kimberlite in the State Line district (T12N-10N, R72W). Some of these rounded megacrysts are as large as 5 inches across. Due to assimilation in the kimberlite magma during emplacement, these garnets never exhibit any crystal faces, are always rounded with smooth surfaces, and show signs of shearing (rehealed fractures) from great depth.


In the same area (section 11, T12N, R72W), some pegmatite was quarried for feldspar during the 1940s along Highway 287 east of the State Line diamond district. These have uncommon euhedral garnet (Osterwald and others, 1966). At one of quarry, about 500 feet east of Highway 287, fractured, fist-size, opaque, euhedral garnets were found.


Hundreds of rounded pyrope and almandine garnets were found in breccia pipes in the Greater Green River Basin near Cedar Mountain & in anthills near Butcherknife Draw. These are small, transparent, pyrope, pyrope-almandine, and spessartine garnets (typically <8 mm in diameter) found in anthills in association with emerald-green chromian diopside & chromian enstatite. Some collectors faceted some of these producing attractive emerald-green, yellow-orange, and reddish-purple gems. A few diamonds have also been found in anthills and lamprophyre dikes. Similar garnets are reported in anthills in the Bighorn basin north of Thermopolis.


Even though there are several great garnet localities in Wyoming, one of my favorite localities is in Alaska. And of course, there are garnets in Arizona, CaliforniaColorado, IdahoMontana, Nevada, TexasUtah, and even in the Appalachian Mountains. There are many places to hunt garnet and other gemstones in the US. For more information on garnets - please search my blogs - Gem Garnet and Wyoming Garnets

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