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.] Amorphous Minerals An amorphous mineral, unlike a crystalline mineral, does not have a definite, orderly arrangement of its atoms. Because of this lack of internal structure, the mineral occurs in masses that have no regular geometric shapes, and it has no crystal form of its own. Only a few minerals are amorphous. SOME DISTINGUISHING PROPERTIES OF MINERALS