Environmental Engineering Reference
In-Depth Information
Streak
Streak refers to the color produced by scratching a sharp point of the mineral across a plate
of unglazed porcelain. It is most significant for dark-colored minerals, since streak does
not always agree with the apparent color of the mineral. Some feldspars, for example,
appear black but yield a white streak.
Luster
Luster refers to the appearance of light reflected from a mineral, which ranges from metal-
lic to nonmetallic to no luster. Pyrite and galena have a metallic luster on unweathered
surfaces. Nonmetallic luster is described as vitreous (quartz), pearly (feldspar), silky (gyp-
sum), and greasy (graphite). Minerals with no luster are described as earthy or dull
(limonite and kaolinite).
Cleavage
Cleavage is used to describe both minerals and rock masses. In minerals it refers to a par-
ticular plane or planes along which the mineral will split when subjected to the force of a
hammer striking a knife blade. Cleavage represents a weakness plane in the mineral,
whereas crystal faces represent the geometry of the mineral structure, although appear-
ances may be similar. For example, quartz exhibits strong crystal faces but has no cleav-
age. Types of mineral cleavage are given in Figure 5.1; rock-mass cleavage is described in
Section 6.3.2.
Cleavage in one direction
Examples:
Muscovite mica
Biotite mica
Topaz
Chlorite
Cleavage in two directions
Cleavage at right angles gives
four smooth surfaces.
Example: Feldspar
Cleavage in two directions
Cleavage not at right angles gives four smooth
surfaces.
Example: Hornblende
Cleavage in three directions
Cleavage at right angles gives six smooth
surfaces (cubic cleavage).
Example: Galena, Halite, Sylvite
Cleavage in three directions
Cleavage not at right angles gives six smooth
surfaces (rhombic cleavage).
Example: Calcite
Cleavage in four directions
Cleavage gives eight smooth surfaces
(octahedral cleavage).
Example: Fluorite, Diamond
Cleavage in six directions
Cleavage gives 12 smooth surfaces
(dodecahedral cleavage).
Example: Sphalerite
FIGURE 5.1
Types of mineral cleavage. (From Simpson, B.,
Minerals & Rocks
, Octopus Books, London, 1974. With
permission.)
