Geology Reference
In-Depth Information
The minerals in rocks may have been broken down into small pieces. This is the
case in most sedimentary rock, which I explain in Chapter 7.
The rock minerals may have formed under conditions that prevented them from
growing large. An example of this situation are the minerals that create a rock
called
basalt,
which I also describe in Chapter 7.
Figure 6-1:
A large
quartz crystal.
Which minerals make up a rock depend on which elements are present, as
well as the conditions of temperature and pressure where the rock is forming. For
example, diamond and graphite are different minerals formed by the same ele-
ment: carbon. The crystal structure of graphite (commonly used as pencil lead) is
formed under conditions of low temperature and low pressure, similar to condi-
tions on earth's surface. Diamond crystals, while also composed of only carbon,
are formed under conditions of very high temperatures and pressures, deep with-
in the earth.
Identifying Minerals Using Physical Char-
acteristics
Because each mineral is composed of certain elements arranged in a specific crystal
structure, physical characteristics can be used to identify it. In this section I describe
the most common physical characteristics, or
properties
, used to identify minerals.
