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mineral will depend on two factors: (1) the kinds of atoms of 17.3 MINERAL-FORMING PROCESSES
which it is composed and (2) the way the atoms are packed in
the crystal lattice. A diamond, for example, has carbon atoms Mineral crystals usually form in a liquid environment, but they
arranged in a close-packed structure and has a specific gravity can also form from gases or in solids under the right condi-
of 3.5. Graphite, however, has carbon atoms in a loose-packed tions. Two liquid environments where minerals usually form are
structure and has a specific gravity of 2.2. To obtain an exact (1) water solutions and (2) a solution of a hot, molten mass of
specific gravity, a mineral sample must be pure and without melted rock materials known as magma. Magma may cool and
cracks, bubbles, or substitutions of chemically similar elements. crystallize to solid minerals either below or on the surface of Earth.
These conditions are difficult to meet, and a range of specific Magma that is forced out to the surface is also called lava. Lava is
gravities is sometimes specified for certain minerals. the familiar molten material associated with an erupting volcano.
There are a few other properties and tests that can be used for High kinetic energy prevents the forming of bonds between at-
a few minerals such as taste, feel, melting point, reaction to a mag- oms when water solutions and magma are very hot. As cooling
net, and so forth. Some of these special properties, such as the occurs, the kinetic energy is reduced enough that bonds will begin
double image seen through a calcite crystal, might identify an un- to form. Atoms then combine into small groups called nucleation
known mineral in an instant. Otherwise, an analysis of the other, centers that grow into crystals. Crystals can also form from cooler
more general properties will be needed. In general, the properties water solution, but dissolved ions must be highly concentrated
of minerals are used to find out what an unknown mineral is not, for this to happen. In a concentrated solution, the ions are close
that is, what possible minerals can be ruled out by certain tests. enough together that their charges can pull them together. When
this happens, crystals form and are precipitated from the solution.
Mineral-forming processes are influenced by temperature,
EXAMPLE 17.1
pressure, time, and the availability and concentration of ions in
Specific gravity is a measure of the density of a mineral. It is the ratio solution. Different minerals are stable under different conditions
of the density of the mineral to the density of water and is calculated
of temperature and pressure. Clearly, a given mineral can form
by dividing the density of a mineral by the density of water. A 4.08 g
only if the ions of which it is made are present in the environ-
crystal of the mineral augite measures 1.43 cm long by 0.98 cm wide by
ment. Ion concentrations in solution must be high enough to per-
0.89 cm high, and its cleavage causes it to break into a rectangular solid.
mit the formation of the embryonic nucleation centers required
What is its specific gravity?
for crystal growth. The amount of time available for crystal for-
mation and the amount of water present determine the size of the
SOLUTION crystals. Large crystals result from long, slow cooling of magma
The volume of the mineral can be calculated using the formula for the or from high water content in the magma, both of which favor
volume of a rectangular solid. Determine the density of the mineral the free migration of ions. Rapid cooling and low water content,
specimen by dividing its mass by its volume, calculated from the di- on the other hand, suppress ion migration and result in small or
mensions of the specimen. Then divide this by the density of water to even microscopic crystals. Sudden chilling can even prevent crys-
determine specific gravity. tal growth altogether, resulting in glass, a solid that cooled too
ρ
mineral
_ m _

L = 1.43 cm SG = ρ water and ρ mineral = quickly for its atoms to move into ordered crystal structures.

V
W = 0.98 cm and V = L × W × H Early in the twentieth century, N. L. Bowen conducted a
H = 0.89 cm __ series of experiments concerning the sequence that minerals
m

(L × W × H )
m = 4.08 g __ crystallized in a cooling magma. The sequence became known
∴ SG = ρ water as Bowen’s reaction series. The series, as shown in Figure 17.11,
g
_

ρ water = 1 is arranged with minerals at the top that crystallize at higher
4.08 g
c m 3 ___

(1.43 cm)(0.98 cm)(0.89 cm)
___ temperatures and minerals at the bottom that crystallize at

SG = ? = lower temperatures. Bowen’s idea was that different crystals
g
_
1

3
c m separated from a homogeneous mixture, and the separation
caused a change in the remaining materials available to form
g
__ __
4.08

(1.43)(0.98)(0.89) (cm)(cm)(cm)
__ __ crystals. In greater detail, consider that the minerals at the top

=

g
1 _ of the series are ferromagnesian silicates, minerals that are rich

c m 3 in iron and magnesium. Minerals at the bottom of the series
_ are nonferromagnesian silicates, minerals that are rich in silicon
4.08
=
1.25 and generally lack iron and magnesium. Since the ferromagne-
= 3.26 sian silicates crystallize at a higher temperature, these minerals
are the first to form in a cooling magma. If magma crystallizes
directly, it will contain the ferromagnesian minerals listed at the
EXAMPLE 17.2 top of the series. On the other hand, if it cools slowly, perhaps
A 6.39 g crystal of the mineral potassium feldspar measures 1.82 cm far below the surface, the minerals containing the iron and mag-
long by 1.18 cm wide by 1.16 cm high, and its cleavage causes it to break nesium will form crystals that sink toward the bottom of the
into a rectangular solid. What is its specific gravity? (Answer: 2.56) liquid magma. Thus, the remaining magma, and the minerals
that crystallize later, will become progressively richer in silicon
440 CHAPTER 17 Rocks and Minerals 17-8