Minerals—The Building Blocks of Rocks - The Changing Earth: Exploring Geology and Evolution - page 70

that the four oxygen atoms surround a silicon atom, which
occupies the space between the oxygen atoms, thus forming
a four-faced pyramidal structure (Figure 3.12b). The silicon
atom has a positive charge of 4, and each of the four oxygen
atoms has a negative charge of 2, resulting in a radical with a
total negative charge of 4 (SiO 4 ) -4 .
Because the silica tetrahedron has a negative charge, it
does not exist in nature as an isolated ion group; rather, it com-
bines with positively charged ions or shares its oxygen atoms
with other silica tetrahedra. In the simplest silicate minerals,
the silica tetrahedra exist as single units bonded to positively
charged ions. In minerals that contain isolated tetrahedra,
the silicon-to-oxygen ratio is 1:4, and the negative charge
of the silica ion is balanced by positive ions (Figure 3.12c).
Olivine [(Mg,Fe) 2 SiO 4 ], for example, has either two mag-
nesium (Mg +2 ) ions, two iron (Fe +2 ) ions, or one of each to
offset the -4 charge of the silica ion.
Silica tetrahedra may also join together to form chains
of indefi nite length (Figure 3.12d). Single chains, as in the
pyroxene minerals, form when each tetrahedron shares two
of its oxygens with an adjacent tetrahedron, resulting in a
silicon-to-oxygen ratio of 1:3. Enstatite, a pyroxene-group
mineral, refl ects this ratio in its chemical formula MgSiO 3 .
Individual chains, however, possess a net -2 electrical charge,
so they are balanced by positive ions, such as Mg +2 , that link
parallel chains together (Figure 3.12d).
The amphibole group of minerals is characterized by a
double-chain structure in which alternate tetrahedra in two
parallel rows are cross-linked (Figure 3.12d). The formation
of double chains results in a silicon-to-oxygen ratio of 4:11,
so each double chain possesses a -6 electrical charge. Mg +2 ,
Fe +2 , and Al +2 are usually involved in linking the double
chains together.
In sheet-structure silicates, three oxygens of each tet-
rahedron are shared by adjacent tetrahedra (Figure 3.12e).
Such structures result in continuous sheets of silica tetrahe-
dra with silicon-to-oxygen ratios of 2:5. Continuous sheets
also possess a negative electrical charge satisfi ed by positive
ions located between the sheets. This particular structure
accounts for the characteristic sheet structure of the micas ,
such as biotite and muscovite, and the clay minerals.
Three-dimensional networks of silica tetrahedra form
when all four oxygens of the silica tetrahedra are shared by
adjacent tetrahedra (Figure 3.12f). Such sharing of oxy-
gen atoms results in a silicon-to-oxygen ratio of 1:2, which
is electrically neutral. Quartz is a common framework
silicate.
Geologists recognize two subgroups of silicates:
ferromagnesian and nonferromagnesian silicates. The
ferromagnesian silicates are those that contain iron (Fe),
magnesium (Mg), or both. These minerals are commonly
dark and more dense than nonferromagnesian silicates.
Some of the common ferromagnesian silicate minerals
are olivine, the pyroxenes, the amphiboles, and biotite
(

ferromagnesian silicates (Figure 3.13b). The most common
minerals in Earth's crust are nonferromagnesian silicates
known as feldspars. Feldspar is a general name, however, and
two distinct groups are recognized, each of which includes
several species. The potassium feldspars are represented by
microcline and orthoclase (KAlSi 3 O 8 ). The second group of
feldspars, the plagioclase feldspars , range from calcium-rich
(CaAl 2 Si 2 O 8 ) to sodium-rich (NaAlSi 3 O 8 ) varieties.
Quartz (SiO 2 ) is another common nonferromagnesian
silicate. It is a framework silicate that can usually be recog-
nized by its glassy appearance and hardness. Another fairly
common nonferromagnesian silicate is muscovite, which is a
mica (Figure 3.13b and see Geo-Focus on page 76 and 77).

Carbonate Minerals
Carbonate minerals , those that contain the negatively
charged carbonate radical (CO 3 ) -2 , include calcium carbonate
(CaCO 3 ) as the minerals aragonite or calcite (

Figure 3.14a).
Aragonite is unstable and commonly changes to calcite, the
main constituent of the sedimentary rock limestone. A num-
ber of other carbonate minerals are known, but only one of
these need concern us: Dolomite [CaMg(CO 3 ) 2 ] forms by
the chemical alteration of calcite by the addition of mag-
nesium. Sedimentary rock composed of the mineral dolo-
mite is dolostone (see Chapter 7).

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Other Mineral Groups
In addition to silicates and carbonates, geologists recognize
several other mineral groups (Table 3.1). Even though min-
erals from these groups are less common than silicates and
carbonates, many are found in rocks in small quantities and
others are important resources. In the oxides, an element
combines with oxygen as in hematite (Fe 2 O 3 ) and magnetite
(Fe 3 O 4 ). Rocks with high concentrations of these minerals
in the Lake Superior region of Canada and the United States
are sources of iron ores for the manufacture of steel. The re-
lated hydroxides form mostly by the chemical alteration of
other minerals.
We have noted that the native elements are minerals
composed of a single element, such as diamond and graph-
ite (C) and the precious metals gold (Au), silver (Ag), and
platinum (Pt) (see Geo-inSight on pages 72 and 73). Some
elements, such as silver and copper, are found both as native
elements and as compounds and are thus also included in
other mineral groups; argentite (Ag 2 S), a silver sulfi de, is an
example.
Several minerals and rocks that contain the phos-
phate radical (PO 4 ) -3 are important sources of phosphorus
for fertilizers. The sulfi des, such as galena (PbS), the ore of
lead, have a positively charged ion combined with sulfur
(S -2 ) (Figure 3.14b), whereas the sulfates have an element
combined with the complex radical (SO 4 ) -2 , as in gypsum
(CaSO 4 · 2H 2 O) (Figure 3.14c). The halides contain the hal-
ogen elements, fl uorine (F -1 ) and chlorine (Cl -1 ); examples
are halite (NaCl) (Figure 3.14d) and fl uorite (CaF 2 ).

Figure 3.13a).
The nonferromagnesian silicates lack iron and mag-
nesium, are generally light colored, and are less dense than

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