Sheet mineral (4)

I believe the answer is:

mica

'sheet mineral' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for mica that I've seen before include "Semitransparent insulating mineral" , "Muscovite with perfect cleavage" , "Transparent mineral" , "Flexible mineral found in igneous and metamorphic rocks" , "Isinglass" .)

I've seen this clue in the New York Times.

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