A _mineral_ can be compared to a word of our language. We combine
letters to form a word, and nature combines certain chemical elements to
form each particular mineral. For example, calcite, a mineral that is
abundant in Texas, is always made up of the same proportions of the same
three elements: calcium, carbon, and oxygen.
[Illustration: A mineral is made up of chemical elements. The mineral
_calcite_, for example, always consists of the same proportions of
calcium, carbon, and oxygen.]
Each mineral has its own characteristic internal structure and other
properties. At ordinary temperatures, nearly all the minerals are solids
rather than gases or liquids. (Water and mercury are the principal
exceptions.) In addition, minerals are inorganic rather than being
composed of plant or animal matter.
When a single chemical element is found alone in nature as a _solid_, it
is considered to be a mineral, too. Gold, silver, copper, lead, and
sulfur are some of the chemical elements that can occur alone as solid
minerals. When they occur this way, we refer to them as _native_ silver,
_native_ copper, or _native_ sulfur. Although the element mercury is a
liquid rather than a solid at ordinary temperatures, it too is a mineral
when it occurs alone in nature. It is then called _native_ mercury.