GEM GARNET

 Some garnets produced extraordinary and beautiful gems. Garnet is a common accessory mineral in many micaceous (almandine-amphibolite-grade) metamorphic rocks (schists and gneisses) and in many kimberlites. When in metamorphic or pegmatitic rocks, garnets typically 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 with some of the largest garnets found in kimberlite and in pegmatites.

Gem pyrope from Butcherknife Draw, Green River Basin, Wyoming.

Six pure end-member garnet subspecies are recognized. These vary in color, specific gravity, chemistry, and index of refraction and include pyrope [Mg3Al­2(SiO4)3], almandine [Fe3Al­2(SiO4)3], spessartine [Mn3Al­2(SiO4)3], grossularite [Ca3Al­2(SiO4)3], andradite [Ca3Fe­2(SiO4)3] and uvarovite [Ca3Cr­2(SiO4)3]. Garnets in nature form solid solutions or mixtures of the end members: pure end member compositions are uncommon. Thus, garnets are often described as solid-solutions. Names have been given to some garnet species of intermediate composition such as the rose-red to purple rhodolite garnet, which has a chemical composition of 2:1 mixture of pyrope to almandine. Another intermediate variety of garnet with a composition between pyrope and almandine (1:1 mixture) is referred to as pyrope-almandine (also known as Mozambique garnet) that exhibits a striking dark orange-red to red color.  O4]3

Garnets have relatively high specific gravity (3.5 to 4.3) and hardness (6.5 to 7.5).  The specific gravity results in garnets showing up in heavy black sand concentrates in placer deposits.

 Garnets crystallize in the isometric crystal system & have no cleavage. They are typically transparent to translucent and often exhibit well-formed dodecahedral or trapezohedral habit. Garnets are used for abrasives; although, excellent museum quality garnets are often found. Some transparent to translucent garnets are used as semiprecious gemstones.

Two faceted pyrope garnets from Butcherknife Draw, Green River Basin. The garnet on the left has a distinct spessartine component giving it an orange color. The garnet on the right has the distinct purplish color of pyrope.

 Pyrope garnet is purple-red, but may also be yellow orange when in contains some spessartine in solid solution. It has a specific gravity of 3.5 to 3.8 and exhibits distinct rounded habit with no visible crystal faces. Pyrope has relatively high magnesium and chromium content and is associated with ultramafic igneous and metamorphic rocks. Essentially all pyrope garnet in Wyoming, Colorado and Montana have been xenocrysts in kimberlite in the State Line and Iron Mountain districts and Missouri Breaks, or have occurred as porphyroblasts in garnet peridotite xenoliths (nodules) hosted by kimberlite or have been found as detrital grains in stream sediment samples and anthills.  Pyropes have also been found in mafic breccia pipes (lamprophyres) along Cedar Mountain and in nearby anthills in the Butcherknife Draw area of the southern Green River Basin.  The largest pyrope-almandine garnet found in Wyoming was about 5 inches across and found in kimberlite in the State Line district south of Laramie.

 Almandine. Almandine garnets are red to reddish brown and have specific gravities of 3.85 to 4.32.  Almandine often exhibits good dodecahedral habit.

 Chlorite pseudomorph after almandine from Oldman property, Wyoming. Note the distinct dodecahedral crystal habit.

Spessartine garnet is orange red, orange to yellow orange with a specific gravity of about 4.2.  Garnets of this type have been described in granite pegmatite in the Eagle Rock-Happy Jack area of the southern Laramie Range and in pegmatites at Copper Mountain in the Owl Creek Mountains.

 

Occurrence. Garnet is a common mineral in many metamorphic environments, particularly where the rocks have been metamorphosed to yomingites grade facies and are aluminum-rich.  Such rocks typically contain abundant black mica and may also have amphibole with porphyroblasts of garnet.

 

Localities. A number of garnet localities have been reported. These include translucent to opaque almandine garnet with good dodecahedral habit from 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 they retain dodecahedral crystal habit. Some excellent 3 to 4-inch diameter crystals have been collected from 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 garnet pseudomorphs have been collected from this area. The interior of many of the pseudomorphs contain primary, reddish-brown almandine garnet.

Large pyrope-almandine megacryst from Schaeffer kimberlite complex, Wyoming.

Some extraordinary pyrope-almandine garnet megacrysts have been found in kimberlite in the State Line district (T12N-10N, R72W).  Some rounded megacrysts as large as 5 inches across, have been found in this region. Due to assimilation in the kimberlite magma during emplacement, these garnets never exhibit any crystal faces and are always rounded with smooth surfaces.

In the same area, near Tie Siding (section 11, T12, R72W), some pegmatite was quarried for feldspar during the 1940s along Highway 287 east of the State Line diamond district. These contain uncommon euhedral garnet (Osterwald and others, 1966).  At one of quarry, about 500 feet east of Highway 287, some fractured, fist-size, opaque, euhedral reddish-brown almandine garnets were recovered (personal field notes, 1979).

Parcel of pyrope and chromian diopsides from diamondiferous lamprophyre at Cedar Mountain, Wyoming. Essentially all of these stones are gem-quality and facetable.

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 and pyrope-almandine garnets (typically <8 mm in diameter) found in anthills in association with emerald-green chromian diopside & chromian enstatite. Some collectors have faceted some of these to produce attractive emerald-green, yellow-orange, and reddish-purple gems. A few diamonds have been found in the area in anthills and lamprophyres. Similar garnets are found in anthills in the Bighorn basin north of Thermopolis.

 

 

 

 

 

 

 

 

 

Above - gem pyropes & spessartine cut from rough collected in anthills from Butcherknife Draw, Green River Basin, Wyoming. I was first told about this garnets in anthills years ago by Tom McCandless.