sillimanite

Delaware State Symbols: State Mineral Sillimanite

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

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