A few minerals are composed of only one element, such as diamond
(carbon) and native copper, but most minerals are chemical compounds
that contain several elements.
Most minerals grow into distinctive shapes if they are free to grow. A
familiar example is the formation of salt crystals that grow on a saucer
of evaporating salt water. The distinctive shapes of crystals are called
their habits, and the flat surfaces that develop are called crystal
faces, the angles of which may be used to identify the mineral.
The individual atoms of a crystal always arrange themselves in the same
way, so that each mineral breaks characteristically. Some minerals break
more easily in particular directions and present a flat, smooth surface.
This characteristic is called cleavage and the cleavage surfaces,
although sometimes confused with crystal faces, may be useful for
identifying a particular mineral. The manner in which a mineral breaks
when the broken surface does not include cleavage surfaces is called its
fracture, and this too may give a clue to the identification of the
mineral.
There are other physical features useful in identifying minerals. Some
of them, such as color, are quite obvious. The color of the powder left
when a mineral is scraped on a rough, white surface, such as unglazed
porcelain or tile, is called streak. Luster refers to the brightness of
light reflected from the mineral’s surface. Transparency and
translucency refer to the mineral’s ability to transmit light, and
tenacity is a measure of its toughness.
Two special physical characteristics of minerals are important to their
identification—specific gravity and hardness. Specific gravity simply
means the ratio of the weight of the mineral to an equivalent volume of
water. For example, if a mineral has a specific gravity of 4, then a
cubic inch of the mineral weighs as much as 4 cubic inches of water.
Hardness is measured by the ability of one mineral to scratch another,
and a set of ten standard minerals has been selected for determining
this characteristic. The listing below, from soft to hard, is known as
Mohs scale.
1—Talc
2—Gypsum
3—Calcite
4—Fluorite
5—Apatite
6—Orthoclase
7—Quartz
8—Topaz
9—Corundum
10—Diamond
A rough measure of hardness can be made by using handy objects. Your
fingernail has a hardness ranging from 2 to 3, a penny is a little
harder than 3, window glass ranges from less than 5 to approximately 6
in hardness, and a knife blade is generally in the range of 5 to 6.