Geology Reference
In-Depth Information
Table 3.2 General rules.
Geochemical
term
Definition wt%
Approximate
range of colour
index 1
Possible field
descriptions
Acid
> 65
5 - 25
Felsic/leucocratic
Intermediate
52 - 65
25 - 55
Intermediate/mesocratic
Basic
45 - 52
55 - 85
Mafic/melanocratic
Ultrabasic
<
45
85 - 100
-
1 Note that there is a general correlation, in which felsic/salic rocks are leucocratic, with
low colour index and of acid chemical composition, whereas mafic/femic rocks are melano-
cratic, with high colour index and of basic/ultrabasic chemical composition. However, there
are exceptions to this generalisation. For example, rocks composed of Ca-rich plagioclase,
termed anorthosites , are felsic/salic leucocratic rocks with low colour index but of basic or
intermediate chemical composition. Similarly, rocks composed of Na-, Ca-, Mg- carbonates,
termed carbonatites , are leucocratic rocks with low colour index and of ultrabasic chemical
composition.
fresh surfaces, consider next the other features of field specimens - their texture
and mineral content.
3.3 Texture, Grain-Size/Shape and Fabric
Collectively, the relationships between the constituent crystals and glass (where
present) in an igneous rock are known as texture . When describing the texture
of a field sample, examine:
1. The grain-size of the rock which can reflect the rate at which it crystallised.
2. The fabric , or geometrical characteristics and arrangement of the crystals
including, where possible, observations of the number of minerals present
and the characteristic shapes, or habits , of their crystals.
3. The overall homogeneity of the specimen (that is, whether it is uniform
and equigranular, or contains mineral segregations, banding and irregular
inclusions).
These features all provide clues to the physical conditions under which the
magma crystallised; thus, whereas the colour of a specimen is generally related to
its chemical composition (for example, granites are usually lighter in colour than
gabbros), the texture reflects its history and mode of occurrence. For example,
rapid chilling leads to the formation of a glass, whereas the nucleation of crystals
at many sites in a magma erupted at the surface and still subject to fast cooling,
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