What mineral is this. nearly rectangular in shape and approximately 5 cm by 7 cm in size. blackish in color with a shiny but nonmetallic luster. peels off in very thin, flexible sheets and crumbles rather easily.

The mineral we are talking about here is Biotite Mica.

Explanation:

Structure of this material is Rectangular in shape and color that we are suggested here is black which is true for biotite Mica And various between black to dark brown.

Because it A Shiny although the luster is nonmetallic in nature. It has a hardness that varies from 2.5 to 3. The cleavage being perfect as it can be Separated into different and thin sheets.

h let me think