Geology Reference
In-Depth Information
(see below); for example freshly-broken feldspar crystals may show slightly
jagged edges which, under a hand lens, are seen to be a composite of many
flat cleavage surfaces intersecting at or near 90
◦
(Figure 3.8). The perfect
single cleavage of mica, which forms the brightly reflecting surfaces, often
allows small flakes to be dislodged with a penknife or finger nail. In contrast,
cleavage in pyroxenes and amphiboles is seen only occasionally as fine stri-
ations within the grains, whereas the characteristic cross-sections of euhedral
crystals (eight-sided and six-sided lozenge shapes, respectively, for pyroxenes
and amphiboles, Table 3.4) are much more diagnostic.
3.
Lustre
describes the reflective properties of a mineral and is assessed by
turning the specimen until the mineral surfaces, particularly any cleavage
surfaces, are caught by the light. Lustre varies from
dull
(non-reflecting),
through
resinous
and
silky
to
bright
, which may be metallic, glassy or vitre-
ous (which literally also means glassy but here is defined slightly differently)
depending on the mineral concerned. A brightly coloured silicate mineral
which is opaque to the transmission of light may have a
vitreous
lustre, like
glazed pottery, such as clean crystals of amphibole and pyroxene. On the
other hand, a similar translucent (weakly transparent) or transparent mineral
may appear
glassy
(for example, quartz and fresh olivine).
4.
Habit
refers to the characteristic morphology of euhedral crystals. Of course,
most crystals in igneous rocks are not euhedral and, given the random mixture
of cross-sections through three-dimensional mineral grains represented in a
hand-specimen, many grains may need to be examined before a charac-
teristic shape is found (for example, pyroxene and amphibole habits are
distinctive only when viewed close to one particular planar cross-section).
Some minerals have quite well-defined characteristic shapes in most euhe-
dral cross-sections, for example lath-shaped feldspars which are long and
thin, but rectangular, dodecahedral or trapezohedral garnets and acicular
(that is, needle-like) tourmalines. Others, such as quartz and olivine, are
less commonly euhedral.
Twinning
results from crystal growth in different
orientations on either side of a common atomic structural plane. It can be
visible in large crystals of feldspar, where it provides the only practical
way of distinguishing alkali feldspar (simple twins only) from plagioclase
(simple and multiple twins) in hand specimen (see Table 3.4). Twinning is
best observed by rotating cleavage or crystal faces in sunlight; individual
twins are seen to have slightly different angles for optimum reflection and,
at the hand specimen scale, are commonly seen only in alkali feldspars.
5.
Hardness
, a property related to the strength and uniformity of atomic bonding
within a crystal, is described using a simple but non-linear scale of increasing
hardness from 1 to 10 known as
Mohs' scale
(Figure 3.9). If the mineral
grains are large and fresh enough (weathered grains are often softer than the
original), then a series of scratching tests using a finger nail (hardness circa 2.5),
