Talc

Talc: mineral with the chemical formula Mg3[(OH)2Si4O10]. It ranges from transparent to opaque, with a luster from pearly to greasy, and colors from colorless to white, greenish, yellowish, and reddish. It crystallizes in the Monoclinic System and does not occur as large, well-developed crystals but rather as reniform, foliated, and scaly aggregates. It is found in fractures within Schistose, Dolomitic, and Limestone rocks.

(Photo: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0/Wikimedia Commons, License CC BY-SA 3.0)
(Photo: Didier Descouens/Wikimedia Commons, License CC BY-SA 4.0)

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