Birthstone for January.
Garnet is a member of the nesosilicates which means that is has the orthosilicate ion attached to it [SiO4]4-. All species of garnets posses similar physical properties and crystal forms, but differ in chemical composition.
Master formula: X3Y2(SiO4)3
Pyralspite garnets has the Al in the Y spot of the formula as seen Almandine, Pyrope and Spessartine.
Ugrandite garnets have Ca in the X spot of the formula as seen in Andradite, Uvarovite and Grossular.
A – Grossular: Ca3Al2(SiO4)3 : Red, yellow, brown, black, green;
Tsavorite: Ca3Al2Si3O12 : Bright, vivid green; has traces of vanadium or chromium; found in Tanzania to Kenya.
Hessonite: Ca3Al2Si3O12 : Cinnamon stone, brown to yellow; lower hardness and density than other garnets.
B- Pyrope: Mg3Al2(SiO4)3 : Violet to red to black; indicator for high pressure rocks.
C- Almandine: Fe2+3Al2Si3O12 : Red to deep red; found metamorphic rocks and mica schists.
D- Spessartine: Mn3Al2(SiO4)3 : Orange yellow to violet red; granite pegmatite and certain low grade metamorphic phyllites.
E- Andradite: Ca3Fe3+2(SiO4)3 : Yellow, red, brown, green, black; found in deep seated igneous rocks, as well as serpentines, schists, and crystalline limestone.
F- Uvarovite: Ca3Cr2(SiO4)3 : Bright green in color; usually found as small crystals associated with chromite in peridotite, serpentine, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains of Russia and Outokumpu, Finland.
Crystal System: Rhombic Dodecohedron or cubic
Colors: Virtually all colors while blue is extremely rare.
Cleavage is indistinct
Mohs scale is 6.5 – 7.5
Garnets are not only used as gemstones but as abrasives as well.
Garnets have been known to man for thousands of years. Noah, it is said, used a garnet lantern to help him steer his ark through the dark night. Garnets are also found in jewelry from early Egyptian, Greek, and Roman times. Many an early explorer and traveler like to carry a garnet with him, for the garnet was popular as a talisman and protective stone, as it was believed to light up the night and protect its bearer from evil and disaster. Today, science has taught us that garnet’s proverbial luminosity comes from its high refractive index.
Garnet sand is harvested and used as abrasives. They sort it according to size and then glue it to sheets of paper and/or cloth for making sanding surfaces. They also use the sand as an abrasive which can be propelled by water or air so as to facilitate sand blasting. A common tool for this is in the automotive shops where they use garnet sand to sand blast spark plugs.
As a gemstone, red almandine is the red garnet most often found in jewelry because it is abundant and inexpensive. Pyrope and spessartine are reddish garnets that are commonly encountered in jewelry for the same reason. Green demantoid garnet has become popular in recent years. It has a dispersion of .057 that gives it a fire that exceeds that of diamond’s at .044. The green tsavorite has a bright, rich green color which is akin to that of the emerald. However, the greens cost a good bit more than the reds but the tsavorite makes a good alternative stone to emerald.
Garnet in Metamorphic rocks:
Most garnet forms at convergent boundaries where shale is being acted upon by regional metamorphism. The heat and pressure of metamorphism breaks chemical bonds and causes the mineral to recrystallize into structures that are more stable in the new environment. The aluminum garnet, almandine, generally forms in this environment. As these rocks are metamorphosed, the garnets start as tiny grains and slowly become enlarge over time. As they grow, they displace, replace, and include the surrounding materials.
The calcium garnets typically form when argillaceous, or clay bearing limestone is altered into marble by contact metamorphism along the edges of igneous intrusions. These are commonly androdite, grossular, and uvorvite, the slightly softer, typically green garnets with a lower specific gravity. Two calcium garnets are highly valued for their gem stone properties is the tsavorite, which is bright green grossular, and demantoid, a golden-green andradite.
Garnet in igneous rocks:
Garnet often occurs as an accessory mineral in igneous rocks such as granite. Many people are familiar with almandine garnet because of its dark red crystals in the igneous used in granite countertops. Spessartine is an orange garnet found as crystals in granite pegmatites. Pyrope is a red garnet that is brought to the Earth’s surface in pieces of peridotite that were torn from the mantle during deep source volcanic eruptions. Garnet is also found in basaltic lava flows.
Garnet in sedimentary rocks and sediments:
Garnets are relatively durable minerals. They are often found concentrated in the soils and sediments that form when granite bearing rocks are weathered and eroded. These alluvial garnets are often target of mining operations because they are easy to mine and remove from the sediment or soil by mechanical processing.