Geology Reference
In-Depth Information
a copper coin (hardness circa 3.5) and a steel penknife blade or geological
hammer (hardness circa 5.5) may prove diagnostic. For example, mica or
calcite will scratch easily with a coin or penknife; feldspars may scratch a
penknife blade with difficulty, but quartz will always scratch it easily.
Table 3.4 contains sufficient information for good examples of the 10 most
common minerals that occur in coarse-grained igneous rocks to be identi-
fied. Also included are eight of the more abundant accessory minerals whose
One direction - basal
example - mica
Two directions - prismatic
example - calcite
example - galena
Three directions - cubic
Figure 3.8
Examples of different types of cleavage.
