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Delaware State Symbols: State Mineral Sillimanite

06.12.2016

Delaware's state mineral is sillimanite.  Sillimanite is one of three alumino-silicate polymorphs; the other two are kyanite and andalusite.  Sillimanite is named after Benjamin Silliman (an American chemist  and the first professor to teach minerology at Yale University) by George Thomas Bowen who first described this rock.  It was first found in Connecticut in 1824.  In Delaware, sillimanite is found in the Brandywine Springs area in Newcastle County and near the Hoopes Reservoir.  The material found in Deleware is massive and fibrous.  It was named the state mineral because of the huge boulders of it found in the region.  This picture is a piece of sillimanite from Delaware. Photo from mindat.org

Usually colorless, gray or white, but can, on rare occasions, be brown, pale yellow, yellow-green, blue-green, or blue.  Generally transparent to transluscent, sillimanite can be splintery, acicular (like needles) or fibrous (as is the case in Delaware).  Sillimanite is common in metamorphosed sedimentary rock and can be found in biotite schist with quartz segragations.  This pic is of a piece of sillimanite schist thin section under a microscope from geosec.biz.  Gotta get me one of those scopes!

While it is 6.5 to 7.5 on the Mohs scale, sillimanite has perfect cleavage making it very difficult to cut and facet. Native American Indians used the fibrous schist for making tools. Facet grade sillimanite has been found in Burma (Myanmar) and in Sri Lanka.  This photo is of a facet grade rough crystal from rruff.com

It makes a pretty faceted stone though, so a good cutter can come up with some nice stones for setting.  This photo is from skywalker.cochise.edu

  Finally, due to the fibrous growth, sillimanite can be cut into cabochons that show a nice cat's eye effect.  I've even seen a few that had star effects.  This photo is of a cat's eye sillimanite from gemselect.com

 

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