SYSTEMS AND HABITS OF CRYSTALS
We are able to determine the system of any given crystal by examining its faces, their number and positioning; the crystal’s over-all proportions tell us its habit. The habit of a crystal may be acicular (needle-shaped), columnar, prismatic or tabular. Barytes, for example, can form either thick or thin tabular crystals, but both types of crystals must have the same arrangement of faces as they belong to the same system. On the other hand, two minerals can have the same habit but belong to different crystal systems. Two mineral species may, for instance, both form crystals with prismatic habit, yet one may belong to the hexagonal and the other to the tetragonal system. In some minerals, the system and habit of the crystal are a guide to the conditions under which the mineral was formed.