Geology Reference
In-Depth Information
red-brown colour whereas, with a few rare exceptions, clinopyroxenes are very
dark brown, green or black. Green orthopyroxene is distinguished from olivine
using crystal shape and cleavage (Table 3.4). However, a particularly distinc-
tive clinopyroxene that occurs in some ultramafic rocks in bright green
chrome
diopside
(CaMgSi
2
O
6
containing a few per cent of the NaCrSi
2
O
6
molecule).
The igneous rocks considered so far are all composed essentially of silicate
minerals. However, a rather rare group of igneous rocks, known as
carbonatites
,
contain more than 50% (by volume) of carbonate minerals and occur as intrusive
dykes and plugs and more rarely as extrusive lavas. These are subdivided further
using the major carbonate mineral present into calcite (CaCO
3
)-carbonatites,
known as
sovites;
dolomite (CaMg(CO
3
)
2
) carbonatites, known as
beforsites
,
which tend to be cream or pale-yellow; and
ferrocarbonatites
where yellow-
brown siderite (FeCO
3
) is the main mineral. Although these igneous rocks are
rare, they are associated with alkaline igneous rocks in many areas.
The terminology introduced so far in this section (cf. Figures 3.11 - 3.14)
applies exclusively to coarse-grained rocks. Finer grained examples in which
the requisite minerals can be identified in hand specimen should be given the
same names with the prior adjectives (Table 3.3, but see also Table 3.6) '
medium-
grained
'or'
fine-grained
' (for example, fine-grained granite). However, in the
special case of fine- or medium-grained mafic intrusive rocks, the terms
dolerite
(or
diabase
in the USA) are in common use and are equally as acceptable as
fine- or medium-grained gabbro.
Although most medium-grained rocks can be matched in mineral compo-
sition with the common rock-types described above, there is one group that
occurs in minor intrusions which has distinctive mineralogy and texture: the
lamprophyres
. These are medium-grained mesocratic or melanocratic (occasion-
ally ultramafic) porphyritic rocks with phenocrysts of biotite and/or amphibole
in a groundmass containing feldspars and/or feldspathoid minerals. They may
contain hydrothermal calcite and zeolites, and the primary igneous minerals are
frequently extensively altered to these minerals. They are difficult to classify
further due to the fine grain size and alteration of the groundmass. Biotite
bearing examples that are found in association with feldspar are commonly
termed
Minette;
Amphibole bearing examples with no feldspar
Monchiquite
.
Kimberlites
are also found in minor intrusions, pipe like bodies and erupted
units (see Chapter 6) and are usually characterised by significant inclusions
of mantle olivine. It is notoriously difficult to classify rocks like
Kimberlites
and
lamprophyres
in the field and often geochemical classification is required.
Readers are directed to the recent detailed classification paper by Tappe
et al
.
(2005) which outlines some of the specific issues associated with classifying
lamprophyres
and associated rock types.
Fine-grained rocks
have an individual set of names, although for each coarse-
grained rock type there is a fine-grained equivalent (Table 3.5). The distinctive
