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Thursday, March 8, 2012

Beryl the Genstone of Many Colors

Beryl crystals from Pakistan var. aquamarine
Photo by Gia Cassa

As a mineral beryl is beryllium aluminum cyclosilicate possessing the chemical formula Be3Al2(SiO3)6­.   As a gem it comes in many colors although pure beryl is colorless of the variety goshenite named after the town of Goshen in the central Berkshires of Massachusetts its type locality.  It is tinted by different colors causing it to be colored blue, green, red, yellow and white.  This is a mineral that is usually found in pegmatites although it is often found in biotite schists and in intruded limestone.  A rare occurrence is found in Utah called bixbite that has been colored red by manganese as an impurity.  Other gems in this family include aquamarine, emerald, green beryl, heliodor and morganite

An emerald crystal from Colombia
Photo by Gery Parent

Aquamarine is the color of sea water a bluish green with green beryl as a sub-order of aquamarine.  Emerald of course is bright green that is usually found in heavily intruded limestone although some occurrences are in biotite schist.  Heliodor is the yellow variety of beryl that is sometimes galled “golden beryl.”  Morganite is sometimes found associated with both aquamarine and golden beryl, bit is differentiated by its pink color.  Red beryl is also called bixbite or red emerald because of its red coloration.  Of all the beryl’s bixbite is the rarest, but only occurs in small crystals allowing gems cut from it to be lass then 5 carets in weight,

Golden beryl or heliodor

Beryl aside from its use as a gem is also the ore of beryllium where it occurs in granitic pegmatites.  One such crystal was a giant that occurred in the Bumpus Quarry in Albany, Maine that was about 5.5 meters long by 1.2 meters in diameter that weighed around 18 metric tons.  The largest crystal of any kind of mineral was found in Madagascar is a beryl crystal that was 18 meters long by 3.5 meters in diameter.  That is the size as the boiler on a steam locomotive.

A crystal of red beryl var. bixbite from Utah
Photo by Rob Lavinsky

Beryl is found on all the continents wherever crystalline igneous or metamorphic rocks are found.  The gems are found in the core of pegmatites where at times terminated crystals are found growing into cavities or vugs in the rock.  Some beryl like the emeralds found in Columbia are found in a peculiar black limestone that has been intruded with granite that contain in addition to the beryl an abundant amount of pyrite that certainly predates the formation of the emerald crystals.. Beryl is also found associated with biotite schist.   Many of the emeralds found in the Ural Mountains of Russia are this type of deposit.  The emeralds found at the Crabtree Corners deposit in North Carolina are also an example of this kind of deposit.

A crystal of morganite on albite and quartz.
Photo by Rob Lavinsky

One of the most important deposits of aquamarine in the United States is found in the Maryall district of New Milford, Connecticut where they occur in a large pegmatite that that was originally mined for feldspar and mica.  This mine has produced some fine specimens of both aquamarine and heliodor beryl including a 44 carat heart on display in the American Museum of Natural History in New York City.  It is also known to produce many other minerals especially large garnets and uranium minerals.  

1 comment:

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