Geology Reference
In-Depth Information
manner, which provides compelling evidence for an internal
orderly structure. Another way we can determine that min-
erals with no obvious crystals are actually crystalline is by
their
cleavage
, the property of breaking or splitting repeatedly
along smooth, closely spaced planes. Not all minerals have
cleavage planes, but many do, and such regularity certainly
indicates that splitting is controlled by internal structure.
As early as 1669, the Danish scientist Nicholas Steno
determined that the angles of intersection of equivalent
crystal faces on different specimens of quartz are identical.
Since then, this
constancy of interfacial angles
has been dem-
onstrated for many other minerals, regardless of their size,
shape, age, or geographic occurrence (Figure 3.8c). Steno
postulated that mineral crystals are made up of very small,
identical building blocks, and that the arrangement of these
building blocks determines the external form of mineral
crystals, a proposal that has since been verifi ed.
The defi nition of a mineral contains the phrase
a nar-
rowly defined chemical composition
because some minerals
actually have a range of compositions. For many minerals,
the chemical composition does not vary. Quartz is composed
of only silicon and oxygen (SiO
2
), and halite contains only
sodium and chlorine (NaCl). Other minerals have a range of
compositions because one element can substitute for another
if the atoms of two or more elements are nearly the same size
and the same charge. Notice in
◗
Figure 3.9 that iron and
magnesium atoms are about the same size; therefore, they
can substitute for each other. The chemical formula for the
mineral olivine is (Mg,Fe)
2
SiO
4
, meaning that, in addition
to silicon and oxygen, it may contain only magnesium, only
iron, or a combination of both. A number of other miner-
als also have ranges of compositions, so these are actually
mineral groups with several members.
The last criterion in our defi nition of a mineral,
characteris-
tic physical properties
, refers to such properties as hardness,
color, and crystal form. These properties are controlled by
composition and structure. We will have more to say about
the physical properties of minerals later in this chapter.
Mineral composition is shown by a chemical formula, which
is a shorthand way of indicating the numbers of atoms of
different elements that make up a mineral. The mineral
quartz consists of one silicon (Si) atom for every two oxy-
gen (O) atoms and thus has the formula SiO
2
; the subscript
number indicates the number of atoms. Orthoclase is com-
posed of one potassium, one aluminum, three silicon, and
eight oxygen atoms, so its formula is KAlSi
3
O
8
. Some miner-
als known as
native elements
consist of a single element and
include silver (Ag), platinum (Pt), gold (Au), and graphite
and diamond, both of which are composed of carbon (C).
BY GEOLOGISTS
Geologists have identified and described more than 3500
minerals, but only a few—perhaps two dozen—are common.
One might think that an extremely large number of minerals
Negatively
charged ions
Positively
charged ions
2
-
1
-
1
+
2
+
3
+
4
+
0.39
0.26
1.40
1.36
0.99
1.00
Sodium
Calcium
Aluminum
Silicon
Oxygen
Fluorine
0.15
1.37
0.63
0.49
1.84
1.81
Iron
2
+
Iron
3
+
Carbon
Potassium
Sulfur
Chlorine
1 Ångstrom = 10
- 8
cm
0.72
Magnesium
◗
Figure 3.9
Sizes and Charges of Ions Electrical charges and relative sizes of ions common in minerals. The
numbers within the ions are the radii shown in Angstrom units.
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