If the mineral forms under favorable conditions, the ordered atomic arrangement may be expressed in the external crystal form. Rock crystal quartz exhibits external crystal form. Massive, coarsely crystallized quartz possesses an ordered internal, but not external, arrangement. Physical properties include hardness, specific gravity, color, streak, cleavage, luster, crystal form. Because minerals are chemical compounds, they have a definite composition expressed by a formula.
Some formulas are simple.
E.g., the formula for quartz is SiO 2 . Other formulas are complex. Notice that 'definite' is not the same as 'fixed' with regard to chemical composition. Many minerals vary in composition, but the variation is within definite limits.