Most minerals are crystalline. In crystalline minerals, combinations of
atoms are arranged in ordered patterns, which are repeated over and
over. This orderly internal structure of atoms is a characteristic of
each crystalline mineral, as mineralogists are able to determine by
using X-rays and special microscopes.
_Crystals._—
When a mineral occurs as a well-formed individual crystal, it has a
definite, precise shape. The kind of crystal shape it has depends on its
own type of crystalline internal structure. A well-formed crystal has
smooth, flat, outer surfaces called _crystal faces_, which are arranged
together to form prisms, cubes, pyramids, and many other geometric
shapes. For example, quartz, a common Texas mineral, is commonly found
as a six-sided, prism-shaped crystal that is topped by pyramid-like
forms. Pyrite, another common mineral, occurs as cube-shaped crystals.
We can identify some minerals more readily by learning to recognize
their crystal shapes.
[Illustration: A scalenohedron, one of the many crystal forms of
calcite.]
_Imperfect crystals._—
A crystalline mineral commonly forms under conditions that do not permit
it to become a well-shaped crystal. Although the mineral may show a few
crystal faces, it does not have a complete crystal shape and so is
described as _massive_, or is said to occur in _masses_. Some of the
minerals that make up rocks occur as crystalline masses. For example,
_calcite_ is a crystalline mineral that occurs in the metamorphic rock
_marble_ without its normal crystal shape.
Many crystalline minerals occur as incomplete and imperfect crystals
that are grouped together in various arrangements. If these incomplete
crystals are arranged around a common center like the spokes of a wheel,
they are said to be _radial_ or _radiated_. If the groups of incomplete
crystals look like bundles of strings or fibers, they are described as
_fibrous_. If they are in rounded masses that resemble bunches of
grapes, they are called _botryoidal_. If they look like fish scales,
they are described as _scaly_. Some crystalline minerals are made up of
tiny grains that are grouped together like the grains in a lump of
sugar. A mineral occurring in this way is described as _granular_. More
descriptions of crystalline minerals are found in the section on Texas
rocks and minerals (pp. 43-98).
[Illustration: Barite specimen showing radial form.]