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The Collaborative International Dictionary
Gieseckite

Gieseckite \Gie"seck*ite\, n. [Named after Karl Giesecke.] (Min.) A mineral occurring in greenish gray six-sided prisms, having a greasy luster. It is probably a pseudomorph after el[ae]olite.

Wiktionary
gieseckite

n. (context mineral English) A greenish-grey variety of muscovite.