Chemistry Reference
In-Depth Information
for example, was then being traded in western Asia and Egypt (von Rosen
1990). Emeralds seem to have been favored during the Hellenistic period,
while amethyst, sapphire, and diamond became relatively popular during
Roman times. Many of these gemstones, set in jewelry, have survived to the
present day (Hackens and Moucharte 1987).
Most gemstones are minerals; only a few are other inorganic or organic
materials. Out of an estimated 3000 minerals known on the crust of the earth,
however, only about 100 are valued as precious and semiprecious gem-
stones. When extracted from the crust of the earth, most gemstones have a
rough surface and irregular shape, and to reveal and enhance their beauty
they have to be cut and polished. Minerals of gem quality are formed and
found in the most varied environments. Diamonds, for example, occur
mainly among igneous rocks but also in sand or gravel deposits. Sapphires
and rubies are usually found within metamorphic rocks.
Jade
is, in itself, a
type of metamorphic rock. Turquoise and opal are formed in sedimentary
rocks as a result of groundwater seeping through and dissolving rocks; when
the water evaporates, the dissolved solids finally precipitate to form the
stones.
TEXTBOX
21
THE STRUCTURE OF SOLIDS: AMORPHOUS AND
CRYSTALLINE MATERIALS
Solid materials can be classified, on the basis of how their constituent par-
ticles (atoms, ions, or molecules) are arranged in space, into two groups:
amorphous
and
crystalline solids
.
Amorphous solids
, literally “solids
without shape,” such as obsidian and glass (see Chapters 2 and 3), are
devoid of any regular internal arrangement. Their component particles are
distributed at random within the bulk of the solid and lack short- or long-
range geometric order. In
crystalline solids
such as table salt and quartz,
the component particles occupy definite, fixed positions; they are arranged
in a characteristic, symmetric, and repeating pattern that extends from
one edge of the solid to the other. A crystalline solid is therefore a solid
characterized by an ordered, regular, and symmetric three-dimensional
arrangement of its component particles (see Fig. 17).
Crystalline Solids
Crystalline solids have a regular geometric shape bound by plane surfaces
that intersect at characteristic angles. Their shape results from the arrange-
ment of the particles (atoms, ions, or molecules) within the crystals, in an
