Chemistry Reference
In-Depth Information
TEXTBOX
7
MINERAL- AND ROCK-FORMING PROCESSES - CRYSTALLIZATION
AND PRECIPITATION
Most natural solids, including minerals, are formed by one of two very dif-
ferent processes:
crystallization
, which takes place when a hot melt cools
down slowly, or
precipitation
, which occurs mainly when a solid separates
from a water solution.
Crystallization
When a hot melt cools down, the particles that make up the melt (that is
to day, atoms, ions, or molecules) become rearranged, during the transi-
tion from the liquid to the solid state, into a regular, symmetric spatial
arrangement, known as a
crystal structure
, which is regularly repeated
throughout the bulk of the solid (see Textbox 21). Molten
magma
, for
example, is a very hot, fluid mixture of liquid and gas formed deep below
the crust of the earth. When magma emerges to the surface during vol-
canic eruptions (then known as
lava
), it cools down gradually. Depending
on the chemical composition of the lava and the prevailing environmen-
tal conditions, and providing it cools down slowly, the atoms that make up
lava become naturally grouped in orderly and symmetric patterns. As the
melt finally solidifies, it is said to
crystallize
, the solid preserves the orderly
and symmetric arrangement, known as the
crystal structure
, acquired while
solidifying. The
crystal structure
of each substance is unique and charac-
teristic, and provides a relatively easy way for characterizing minerals;
the internal structure of several crystalline substances is shown in
Figure 17.
When molten lava cools down rapidly, however, the orderly arrange-
ment of the component particles into symmetric structures is curbed and
the melt solidifies into an amorphous mass that lacks internal order. Such
a substance is known as a
mineraloid
. Although mineraloids have a mineral
appearance, they lack an internal orderly arrangement and are said to be
natural glasses
(see Textbox 27).
Obsidian
, widely used since prehistoric
times for making tools and decorative objects, is a type of mineraloid.
Another well-known mineraloid is
opal
, certain varieties of which have
long been appreciated as gemstones (Middlemost 1985).
Precipitation
Precipitation
, the other process by which minerals and rocks are formed,
takes place from a solution. When solid particles separate out from a solu-
tion as the water evaporates, or as a consequence of cooling or of the
