Page 95 - Laboratory Manual in Physical Geology

the brassy mineral in FIGURE 3.7 has a dark gray streak,
but the reddish silver mineral has a red-brown streak. A
C mineral’s streak is usually similar even among all of that
mineral’s varieties.
If you encounter a mineral that is harder than the
streak plate, it will scratch the streak plate and make
a white streak of powder from the streak plate. The streak
of such hard minerals can be determined by crushing a
tiny piece of them with a hammer (if available). Otherwise,
record the streak as unknown.

Hardness (H). A mineral’s hardness is a measure of its
B resistance to scratching. A harder substance will scratch
A
a softer one ( FIGURE 3.8 ). German mineralogist Friedrich
Mohs (1773–1839) developed a quantitative scale of rela-
FIGURE 3.6 Native elements. The native elements are tive mineral hardness on which the softest mineral (talc)
minerals composed of just one element, like gold nuggets. has an arbitrary hardness of 1 and the hardest mineral

A. When freshly formed or broken, native copper (Cu, naturally- (diamond) has an arbitrary hardness of 10. Higher-
occurring pure copper) has a reflective metallic luster like this numbered minerals will scratch lower-numbered minerals
freshly-minted copper coin. However, these dendritic clusters of (e.g., diamond will scratch talc, but talc cannot scratch
native copper crystals have tarnished to nonmetallic dull brown
( A ) and/or green ( B ) colors. diamond). Mohs Scale of Hardness ( FIGURE 3.9 ) is widely
used by geologists and engineers. When identifying a
mineral, you should mainly be able to distinguish miner-
als that are relatively hard (6.0 or higher on Mohs Scale)
surfaces when possible. If you think that a mineral’s from minerals that are relatively soft (less than or equal to
luster is submetallic , between metallic and nonmetallic, 5.5 on Mohs Scale). You can use common objects such as

then it should be treated as metallic for identification a glass plate ( FIGURE 3.9 ), pocket knife, or steel masonry
purposes. nail to make this distinction as follows.

Streak. Streak is the color of a mineral or other sub- ■ Hard minerals: Will scratch glass; cannot be scratched
stance after it has been ground to a fine powder (so fine with a knife blade or masonry nail.
that you cannot see the grains of powder). The easiest ■ Soft minerals: Will not scratch glass; can be scratched

way to do this is simply by scratching the mineral back with a knife blade or masonry nail.
and forth across a hard surface such as concrete, or a
square of unglazed porcelain (called a streak plate ). The You can determine a mineral’s hardness number on
color of the mineral’s fine powder is its streak. Note that Mohs Scale by comparing the mineral to common objects

A: MINERAL B: FRAGMENT
CRYSTAL OF MINERAL
(CUBE) CRYSTAL

Streak Streak
plate plate

Color: brassy Color: reddish silver
Streak: dark gray Streak: red-brown
Luster: metallic (M) Luster: metallic (M) to nonmetallic (NM)

FIGURE 3.7 Streak tests. Determine a mineral’s streak (color in powdered form) by scratching it across a streak plate with significant force,

then blowing away larger pieces of the mineral to reveal the color of the powder making the streak. If you do not have a streak plate, then
determine the streak color by crushing or scratching part of the sample to see the color of its powdered form.
80 ■ L ABOR ATORY 3