Environmental Engineering Reference
In-Depth Information
Fracture
The appearance of the surface obtained by breaking the mineral in a direction other than
that of the cleavage, or by breaking a mineral that has no cleavage, provides fracture char-
acteristics. Fracture can be fibrous, hackly (rough and uneven), or conchoidal. The last
form is common in fine-grained and homogeneous minerals such as quartz and volcanic
glass.
Specific Gravity (SG or G
s
)
The ratio between the mass of a mineral and the mass of an equal volume of water defines
its specific gravity, expressed as
G
s
W
a
/(
W
a
W
w
)
(5.1)
where
W
a
is the weight of the test specimen and
W
w
is the weight of the specimen sub-
merged in water. Quartz and calcite, for example, have
G
s
2.65, and any variation from
that amount is caused by impurities.
Hardness
Hardness refers to the ability of a mineral to resist scratching relative to another mineral.
The hardness scale of minerals assigned by Friedrich Mohs is given in Table 5.4. It signi-
fies that each mineral, if used in the form of a sharply pointed fragment, will scratch
smooth surfaces of all minerals preceding it on the table.
Some useful hand tests are:
Window glass has a hardness of about 5.5.
●
Pocket knife blade has a hardness of about 5.
●
Brass pinpoint has a hardness a little over 3 (can scratch calcite).
●
Fingernail is a little over 2 (can scratch gypsum).
●
Summary
The identification characteristics of some common minerals including streak, luster, color,
hardness, specific gravity, cleavage, and fracture are given in
Table 5.5.
Laboratory Methods
Chemical Tests
A simple test is a sample's reaction to hydrochloric acid. Calcite, a constituent of lime-
stone, is differentiated from most other minerals by its vigorous effervescence when
treated with cold hydrochloric acid. Dolomite will react to hydrochloric acid only if the
specimen is powdered.
TABLE 5.4
The Mohs Scale of Mineral Hardness
1. Talc
6. Feldspar
2. Gypsum
7. Quartz
3. Calcite
8. Topaz
4. Fluorite
9. Corundum
5. Apatite
10. Diamond
