Delaware State Mineral
Delaware State Mineral, Sillimanite
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Sillimanite
Al2SiO5
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Orthorhombic
Crystals
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Hardness:
6-7 |
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Specific
Gravity: 3.23
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Vitreous
Luster |
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Perfect
(010) Cleavage
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Sillimanite,
a white to tan to green aluminum silicate, (Al2SiO5)
occurs in high temperature, aluminum-rich metamorphic rocks. In Delaware,
it is found in the Hoopes Reservoir and Brandywine Springs areas.
In 1977,
the Delaware General Assembly, acting on a proposal by the Delaware Mineralogical
Society, established sillimanite as the Delaware State Mineral. This act
recognizes the geological and mineralogical significance of the large
masses of this mineral found as boulders at Brandywine Springs, an occurrence
that was recognized as important in the 6th (1892) edition of Dana's System
of Mineralogy. The Brandywine Springs boulders are remarkable for their
size and purity. The sillimanite has a fiberous texture reminiscent of
wood and could potentially be cut into cabochon gems showing a chatoyant
("cat's eye") effect. Sillimanite has no industrial value and is not mined
as an ore or raw material.
Sillimanite
forms at temperatures greater than 550oC, and its coarse grain
size at Brandywine Springs indicates a prolonged period of high-temperature
metamorphism of the rocks. These conditions are confirmed by the absence
of muscovite and the occurrence of the pair sillimanite + K-feldspar (second
sillimanite zone) in the schists/gneisses and by the presence of micropegmatites
in the fold noses of the schists/gneisses, which are interpreted as partial
melts of the rock under high-temperature conditions.