Shimmery mineral (4)

I believe the answer is:

mica

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

I've seen this clue in The New York Times and the USA Today.

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