Chemistry Reference
In-Depth Information
crystals of the same substance may be long and narrow. The angle between
any single pair of crystal faces, whether the crystals are short and wide or
long and thin, will be the same in all the crystals of any substance, regard-
less of their dimensions.
All minerals and many solid substances of inorganic as well as organic
origin exhibit characteristic crystal structures. The crystal structure of the
primary minerals
, for example, is formed when hot magma cools down; as
the temperature of the magma decreases, the component atoms of min-
erals become spontaneously ordered in unique, symmetric arrangements.
When the mineral finally solidifies, the solid preserves this symmetric
arrangement in its characteristic crystalline structure. In
secondary miner-
als
the crystals are formed as a result of the integration, at a specific tem-
perature and pressure, of small crystallites that came out of solution and
precipitated (Goldman 1991).
The Study of Crystals
The structure of crystalline materials is studied using
crystallographic tech-
niques
. Based mainly on the use of penetrating radiation, such as X- and
gamma rays but also on other physical techniques,
crystallographic studies
reveal the exact position of atoms within solids. Such studies thus clarify
the relationship between the structure and the properties of crystalline
solids. Graphite and diamond provide illuminating examples of how the
extremely different properties of these two substances are determined
solely by the spatial arrangement of the atoms of a single element, carbon
(see Textbox 19).
Gemstones can usually be identified and characterized by a number of
physical properties; if they are minerals, mainly by their crystal structure,
but also by their density, hardness, color, and other physical properties. All
gemstones of any one type of mineral have the same crystal structure (see
Textbox 21), but the crystal structure varies from one type of gemstone to
another.
Mineral gemstones that have the same basic chemical composition, that
is, are composed of the same major elements and differ only in color, are con-
sidered as variations of the same mineral species. As gemstones, however,
minerals that have the same composition and crystalline structure but
exhibit different colors are classified as different gemstones.
Beryl
, for
example, a mineral (composed of beryllium aluminum silicate), includes a
pink variety, known by the gemstone name of
morganite
, and also a well-
known green variety,
emerald
. Table 18 lists and classifies, by composition
and color, gemstones that have been appreciated since antiquity.
