Page 90 - Laboratory Manual in Physical Geology

Crystal Crystal
faces faces

6 1
A.
5 2 A rock made of two large, visible, quartz mineral crystals. Crystal
faces (flat outside surfaces) merge into three dimensional crystal
3 forms (geometric shapes). Crystal growth was unobstructed, except
4 where the two crystals touched and grew together (x1).
Top view: Crystal growth was unobstructed so
crystal faces are developed (x1).

x2
Side view:
Deformed
crystal faces
among
crowded C.
intergrown Crystal growth of the
crystals (x1). calcite mineral crystals
in this rock (marble)
was obstructed in
every direction. The
crystals grew together
B. as a dense mass of
Rock made of many quartz mineral crystals. Note odd-shaped crystals Intergrown
how crystal growth was obstructed as the sides of instead of perfect crystals outlined
many crystals grew together (side view), but tips of crystal forms. in black
the crystals (top view) grew unobstructed into
six-sided pyramids. Iron impurity gives the purple
amethyst variety of quartz its color.

Thin section (x30). The layers of
agate are made of long intergrown
quartz mineral crystals.

D.
Slice of rock (agate) cut with a diamond saw and polished. The layers are made of quartz mineral crystals that are
cryptocrystalline (not visible in hand sample). They can only be seen in a thin section (thin transparent slice of the
rock mounted on a glass slide) magnified with a microscope to 30 times larger than their actual size (x30).

FIGURE 3.1 Minerals and rocks. Most rocks are made of one or more mineral crystals.
Mineral Properties, Identification, and Uses ■ 75