CRYSTAL
HABIT/FORM
Crystals have smooth flat faces and regular geometric shapes (e.g.,
potassium feldspar).
Cleavage planes may be seen intersecting
crystal faces.
The arrangement of atoms in a mineral determines
the shape of its crystals. Some minerals commonly occur as well developed
crystals, and their crystal forms are diagnostic.
Quartz commonly occurs
as hexagonal prisms, with pyramid-like shapes at the top.
Pyrite occurs as cubes
or pyritohedrons (forms with 12 pentagonal faces).
Calcite occurs as
rhombohedrons or more complex, 12-sided forms called scalenohedrons.
Distinguishing crystal faces from cleavage
planes:
1) Crystal faces are normally
smooth, whereas cleavage planes, though also smooth, commonly are broken
in a step-like fashion;
2) Some crystal faces have fine
grooves or ridges on their surfaces, whereas cleavage planes do not. Striations
also can be seen on crystal faces (e.g., plagioclase),
but these persist through the mineral.
3) The mineral will not break
parallel to crystal faces.