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FIGURE 17.5 Here are some large quartz crystals with some
well-developed shapes.

silicates (Table 17.1). Note, however, that the silicates can contain
FIGURE 17.4 The structural unit for a crystal of table salt, some other elements in addition to silicon and oxygen. The sili-
sodium chloride, is cubic, as you can see in the individual grains.
cate minerals are by far the most abundant, making up about
+4
92 percent of Earth’s crust. When an atom of silicon (Si ) com-
–2
bines with four oxygen atoms (O ), a tetra hedral ionic structure
Myths, Mistakes, & Misunderstandings of (SiO 4 ) forms (see Figure 17.6). All silicates have a basic silicon-
–4
oxygen tetrahedral unit either isolated or joined together in
Healing Crystals? the crystal structure. The structure has four unattached electrons
Practitioners of crystal healing have long held that certain on the oxygen atoms that can combine with metallic ions such as
crystals aid physical and mental healing. Quartz crystals, for iron or magnesium. They can also combine with the silicon atoms
example, are believed to “draw out” pain and “align” healing of other tetrahedral units. Some silicate minerals are thus made
energy. Other varieties of quartz are believed to have other up of single tetrahedral units combined with metallic ions. Other
healing abilities. Those who wear amethyst, for example, are silicate minerals are combinations of tetrahedral units combined
“helped” as the amethyst relaxes the mind and nervous system. in single chains, double chains, or sheets (Figure 17.7).
Citrine is believed to be a self-esteem crystal that helps increase The silicate minerals can be conveniently subdivided into
knowledge of self and overcomes fear, worry, or depression.
No scientific studies have found that crystals are beneficial. two groups based on the presence of iron and magnesium. The
Healing crystals seem to belong in the same category of folk basic tetrahedral structure joins with ions of iron, magnesium,
medicine as wearing copper bracelets to treat arthritis or am- calcium, and other elements in the ferromagnesian silicates.
ber beads to protect against colds. Such folk medicine remedies Examples of ferromagnesian silicates are olivine, augite, horn-
appear to serve as placebos, nothing more. blende, and biotite (Figure 17.8). They have a greater density and
a darker color than the other silicates because of the presence of
the metal ions. Augite, hornblende, and biotite are very dark in
color, practically black, and olivine is light green.
SILICATES AND NONSILICATES The nonferromagnesian silicates have a light color and a low
Silicon and oxygen are the most abundant elements in Earth’s density compared to the ferromagnesians. This group includes
crust, and, as you would expect, the most common minerals con- the minerals muscovite (white mica), the feldspars, and quartz
tain these two elements. All minerals are classified on the basis (Figure 17.9).
of whether the mineral structure contains these two elements or The silicate minerals can also be classified according to the
not. The two main groups are thus called the silicates and the non- structural differences, or arrangements of the basic tetrahedral

436 CHAPTER 17 Rocks and Minerals 17-4