staurolite

stau·ro·lite  (stôr-lt)

n.

A brownish to black mineral, chiefly (FeMg)₂Al₉Si₄O₂₃(OH), often having crossed intergrown crystals and sometimes used as a gem.

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[Greek stauros, cross; see st- in Indo-European roots + -lite.]

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stauro·litic (-ltk) adj.

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staurolite  (stour-lt)

A brownish to black orthorhombic mineral, often having crossed intergrown crystals and found in mica schists and gneisses. Chemical formula: (FeMg)₂Al₉Si₄O₂₃(OH).