HOW MINERALS OCCUR
Rocks are made up of minerals. In addition, minerals are associated with rocks in other ways. For example, minerals fill or coat cracks and cavities that have developed in some of the rocks. Minerals are either [crystalline] or [amorphous].
[Crystalline] Minerals
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.
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.