Flaky rock

I think it could be either:

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4 5

'flaky rock' 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" , "Shiny silicate mineral" , "Muscovite with perfect cleavage" , "Transparent mineral" , "Isinglass" .)

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

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