Page 91 - Laboratory Manual in Physical Geology

C. Scanning tunneling microscope (STM) image of galena showing
B. When struck with a hammer, galena breaks along flat the orderly arrangement of its lead and sulfur atoms. Each sulfur
cleavage surfaces (planes of weak chemical bonding within atom is bonded to four lead atoms in the image, plus a lead atom
the crystal) that have a silvery color, like metal, and intersect beneath it. Similarly, each lead atom is bonded to four sulfur atoms
at 90° angles to form shapes made of cubes. in the image, plus a sulfur atom beneath it.

x1 Blue = S (sulfur) atoms, Orange = Pb (lead) atoms
nm = nanometer = 1 millionth of a millimeter

FIGURE 3.2 Crystal shape, cleavage, and atomic structure. Galena is lead
A. Galena mineral crystals form
cubic shapes that tarnish to a sulfide—PbS. It is an ore mineral from which lead (Pb) and sulfur (S) are extracted.
dull gray color. (STM image by C.M. Eggleston, University of Wyoming)

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■ Halide minerals contain a halogen ion (F , Cl , Br , Opal is a residue of hydrated silicon dioxide that forms
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or I ) combined with a metal. Examples are halite: light-colored translucent masses like this. Notice its lack
NaCl and fluorite: CaF . of crystals and cleavage. This “precious” opal has been
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■ Phosphate minerals contain phosphate ions (PO ) polished to enhance its internal flashes of color.
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combined with other elements. An example is apatite:
Ca F(PO ) (OH, F, Cl).
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■ Native elements are elements in pure form, not com-
bined with different elements. Examples include graph-
ite: C, copper: Cu, sulfur: S , gold: Au, and silver: Ag.
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How Are Minerals Related to Rocks?
Most rocks are aggregates of one or more mineral crys-
tals. For example, mineral crystals comprise all of the

rocks in FIGURE 3.1 . Notice that you can easily detect
the mineral crystals in FIGURES 3.1A and 3.1B by their
flat faces , which are an external feature of the internal
geometric framework of their atoms. However, the crys-
tals in many rocks have grown together in such a crowded
way that few faces are visible ( FIGURES 3.1C ). Some rocks Limonite forms dull powdery yellow-brown to dense
are also cryptocrystalline made of crystals that are only dark brown masses like this. Notice its lack of crystals
,
visible under a microscope ( FIGURE 3.1D ). and cleavage. It is a residue of hydrated iron oxide and/or
hydrated iron oxyhydroxide that you know as rust.
Earth is sometimes called the “third rock” (rocky
planet) from the Sun, because it is mostly made of rocks.
But rocks are generally made of one or more minerals, FIGURE 3.3 Mineraloids. Opal and limonite are naturally-
occurring inorganic materials, but they are amorphous (non-
which are the natural materials from which every inorganic
crystalline; they never form crystals). This makes them mineraloids
item in our industrialized society has been manufactured.
(amorphous mineral-like materials), rather true minerals, but they
Therefore, minerals are the physical foundation of both are described, identified, and listed as minerals.
our rocky planet and our human societies.
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