Where is cleavage?
Cleavage is a mineral’s tendency to break along flat, parallel surfaces (cleavage planes). Excellent cleavage direction reflects light in one direction, it is considered basal, or book cleavage. Biotite and muscovite have excellent cleavage. Good cleavage reflects light in one direction from a set of many small flat surfaces, augite and gypsum are examples. Poor cleavage directions reflect light form surfaces that are hard to detect, and this is true for sulfur.
