Silicate mineral (4)
I believe the answer is:
mica
'silicate mineral' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mica that I've seen before include "Isinglass" , "Transparent mineral" , "Flexible mineral found in igneous and metamorphic rocks" , "Shiny silicate mineral" , "Semitransparent insulating mineral" .)