Sample in alkali granite
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A mica mineral. Clear, shiny, basal cleavage (one cleavage resulting in thin sheets). In this specimen we see right through the mica to the background cloth below.
The sheets of silica tetrahedra are ionically held together by potassium ions. Sheets are stacked in alternating layers, resulting in the distinctive basal cleavage. Observe that when the cyrstals grow freely they form hexagonal, six sided (edged) sheets, most easily seen is specimen 3.
Muscovite occurs occasionally in felsic igneous rocks. It is often recognized because light flashes sharply off its cleavages faces, almost like light off a mirror.