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Tuesday, February 28, 2012

The Emerald Green Garnet Andradite

Demantoid an emerald green variety of andradite garnet on a matrix of serpentine,
Photo by Rob Lavinsky

Andradite is another species longing for the garnet family that is classified as a nesonsilicate with a formula approximating Ca3Fe2Si3O12­.  This family of garnets includes three different varieties they come in various colors. There is one variety where titanium ions substitute in the formula producing a black stone that in the gem trade is called melanite, or is sometimes referred to as titanian andradite. The second stone in this group is referred to as the demantoid garnet that is found ophiolite deposits where it is associated with chromium. The stones are often the vivid green in color that occur in small crystals rarely over one carat in size. Because of their rarity they are often worth more than $1000 per carat in the world. The third type of stone in this family is called Topazolite that is yellowish green in color and is sometimes found in a high enough quality so they can be cut into faceted gems.

Melanite garnet crystals
Photo by Rob Lavinsky

This garnet occurs in skarns in impure limestone that has been intruded by igneous rock. Sometimes it is also found in calcium rich igneous rocks. Probably the most common place to find andradite is in a rock called serpentine that also produces asbestos. Andradite garnets have been found in the asbestos mines around Thetford Mines, Quebec in small crystals. Serpentine is thought by geologists to be metamorphosed oceanic crust that was caught up in a mountain building event where some of the ocean crust was incorporated into a continental landmass.

Topazolite crystals
Photo by Rob Lavinsky

There are several other minerals that are commonly associated with andradite including this vesuvianite, chlorite, epidote, spinel, calcite, dolomite and magnetite. One of the varieties of andradite is often found near iron mines where the over is magnetite. It is found as braunite on the rock dumps at the old Tillie Foster iron mine in Brewster, New York.

A faceting grade crystal of demantoid.  Sometimes faceted gems cut from this stone can sell for well over $1,000 per carat.  Most of these stones are small rare;y affording faceted gems weighing much more then a carat.
Photo by Rob Lavinsky

Andradite is found in Italy around the area of Mt. Vesuvius and other volcanic areas that are underlain by limestone.  It is also found in Russia in the Urals.  In the United States this stone is found in Arizona and California.  On the East Coast several occurrences are in the Danbury, Connecticut area where andradite occurs in limestone deposits along Rt. 7 where the limestone is in close proximity to intrusions of granite where it is associated with danburite and idocrase.  This is a mineral that should be found in serpentine or contact altered limestone anywhere they are found.

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