Geology Reference
In-Depth Information
Table 3.3
Grain-size descriptions in crystalline rocks.
Fine-grained
1
Few crystal boundaries distinguishable in the field or with
the aid of a handlens; mean grain size below 1mm. If the
rock is glassy, the term
hyaline
may be used
Medium-grained
2
Most crystal boundaries distinguishable with the aid of a
hand lens; mean grain size 1 - 5 mm
Coarse-grained
2
Virtually all crystal boundaries distinguishable with the
naked eye; mean grain size greater than 5 mm
1
Fine-grained and hyaline rocks may be termed
aphanitic
in texture.
2
Medium- and coarse-grained rocks may be termed
phaneritic
in texture.
such as in lava flow, tends to result in minute grains that usually cannot be
identified in the field. On the other hand, slow cooling, as in intrusions emplaced
below the surface, is accompanied by crystallisation at relatively few nucleation
sites and results in larger, easily observed crystals.
Grain size is usually the most obvious textural characteristic, and after exam-
ining the specimen with the aid of a hand lens, it should be assigned to one of
the categories in Table 3.3.
The terms in Table 3.3 are easily applied to rocks that are more or less
equigranular
(equal grain sizes), but many specimens are porphyritic (see
Figure 3.4), having a population of early-formed, large
phenocrysts
in a
finer grained
groundmass
or
matrix
. In such cases, the actual size of both
populations of crystals should be recorded. Note that another short-hand way of
describing porphyritic texture, once the phenocryst mineral has been identified,
is to use the hyphenated adjective '-phyric' along with the mineral name:
thus Figure 3.4d shows a
feldsparphyric
granite. But it may prove impossible
to continue the description of non-porphyritic (or
aphyric
) specimens in
this category without recourse to laboratory techniques. Field names such
as 'fine-grained felsic or mafic rock' are recommended on the basis that the
former are likely to contain mainly feldspars whereas the latter will contain
mainly mafic minerals.
The next stage in describing medium and coarse-grained rocks is to examine
their
fabric
, that is the
shape
of the crystals and their
relationship
to one another.
First compare and contrast the characteristics of different types of crystals in
the rock. Minerals left to crystallise 'freely' in a magma will form well-shaped
crystals which, in the two dimensions of a field sample surface, would show reg-
ular, straight-sided cross-sections, often of rectangular shape (but see Table 3.4
for common crystal habits). During the crystallisation of most igneous rocks
several minerals are competing for the available space and only the first-formed
crystals, such as any phenocrysts present, will have good crystal forms. Three
