Laboratory Manual for Introductory Geology 4e

TABLE 4.2 Occurrence of common rock-forming minerals
Mineral
Igneous
Sedimentary
Metamorphic
Plagioclase feldspar
Potassium feldspar
Quartz
Hornblende
Actinolite
Augite
Muscovite
Biotite
Chlorite
Olivine
Garnet
Andalusite
Kyanite
Sillimanite
Halite
Calcite
Dolomite
Gypsum
Talc
Serpentine
Graphite
Forms only in
these rocks
Forms commonly in
these rocks
Occurs as clasts in
these rocks
it is usually easy to determine how many minerals are in a coarse-grained rock because
color and luster are as easy to determine in rocks as in minerals. But it is difficult to
determine the hardness and cleavage of small grains without interference from their
neighbors. And when a rock contains many mineral grains cemented together, it may
be difficult to distinguish between the hardness of individual minerals and the strength
of the cement holding them together.
Extra care must be taken with rocks to be sure you are measuring the mineral
properties you think you are measuring. In very fine-grained rocks, geologists have
to use microscopes and even more sophisticated methods, such as X-rays, to identify
the minerals. Keep the following tips in mind as you determine the properties of
minerals in rocks in Exercises 4.5 and 4.6.
■ Color: Whenever possible, look at a rock’s weathered outer surface and a freshly
broken surface. Weathering often produces a surface color different from the color of
unaltered minerals, and this difference can be helpful for identification. For example,
regardless of whether plagioclase feldspar is dark gray, light gray, or colorless, weathering
typically produces a very fine white coating of clay minerals. Weathering of a
fine-grained rock can help distinguish dark gray plagioclase (white weathering) from
dark gray pyroxene (brown weathering), which might otherwise be hard to tell apart.
A rock’s color depends on its grain size as well as the color of its minerals. All
other things being equal, a fine-grained rock appears darker than a coarse-grained
rock made of the same minerals.
■ Luster: Using a hand lens, rotate the rock in the light to determine how many
different kinds of luster (and therefore how many different minerals) are present.
4.6 IDENTIFYING MINERALS IN ROCKS
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