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Oxygen
Silicon

Tetrahedron
structure
A

B C D
FIGURE 17.7 (A) Isolated silicon-oxygen tetrahedrons do not share oxygens. This structure occurs in the mineral olivine. (B) Single
chains of tetrahedra are formed by each silicon ion having two oxygens all to itself and sharing two with other silicons at the same time.
This structure occurs in augite. (C) Double chains of tetrahedra are formed by silicon ions sharing either two or three oxygens. This structure
occurs in hornblende. (D) The sheet structure in which each silicon shares three oxygens occurs in the micas, resulting in layers that pull off
easily because of cleavage between the sheets.

FIGURE 17.8 Compare the dark colors of the ferromagnesian
silicates augite (right), hornblende (left), and biotite to the light-
colored nonferromagnesian silicates in Figure 17.9. FIGURE 17.9 Compare the light colors of the nonferro-
magnesian silicates mica (front center), white and pink orthoclase
(top and center), and quartz to the dark-colored ferromagnesian
silicates in Figure 17.8.
of small amounts of chemical impurities in the mineral that have
nothing to do with its basic composition. The mineral quartz, for
example, is colorless in its pure form, but other samples may ap- streak even though the colors of different samples may range from
pear milky white, rose pink, golden yellow, or purple. reddish brown to black, depending on variations in grain size.
A more consistent characteristic of a mineral is streak, the Hardness is the resistance of a mineral to being scratched
color of the mineral when it is finely powdered. Streak is tested (Figure 17.10). Classically, hardness is measured by using the
by rubbing a mineral across a piece of unglazed tile or porcelain, Mohs hardness scale, which is a list of 10 minerals in order of
which leaves a line of powdered mineral on the tile. Surprisingly, hardness (Table 17.2). The softest mineral is talc, which is
the streak of a mineral is more consistent than is the color of the assigned a hardness of 1. The hardest mineral is diamond, which
overall sample. The streak of the same mineral usually shows the is assigned a hardness of 10. A hardness test is made by trying to
same color even though different samples of the mineral may have scratch an unknown mineral or by using the unknown mineral
different colors. Hematite, for example, always leaves a red-brown to try to scratch one of the test minerals. If the unknown mineral

438 CHAPTER 17 Rocks and Minerals 17-6