Geology Reference
In-Depth Information
olivine
dunite
90
90
peridotities
lherzolite
40
40
pyroxenite or
olivine websterite
pyroxenites
10
10
websterite
10
10
orthopyroxene
clinopyroxene
Figure 3.14 Triangular plot showing proportions of olivine, orthopyroxene and
clinopyroxene for classification of ultramafic rocks (rocks with 90% or more of
mafic minerals). The commonest ultramafic rocks are lherzolite and harzburgite.
of feldspathoid minerals may allow a distinction to be made between alkali
syenite, syenite and an abundant group of plagioclase-rich rocks - the diorites,
gabbros and a less common group discussed later, the anorthosites (compare
Figures 3.11 and 3.13). Strictly, the distinction between diorite and gabbro
lies in the composition of the plagioclase feldspar which must contain more
than 50% of the albite (NaAlSi 3 O 8 ) molecule in diorite and more than 50%
of the anorthite (CaAl 2 Si 2 O 8 ) molecule in gabbro. As the compositions of pla-
gioclase feldspars cannot be determined in the field, the use of a secondary
characteristic is - the nature of the common mafic mineral (Figure 3.10) - to
make a preliminary distinction between diorite (with biotite and amphibole)
and gabbro (usually with pyroxene). It is emphasised once more that, wher-
ever possible, these provisional field names should be re-examined in the light
of subsequent microscope studies using a more detailed version of the QAPF
diagram (Figure 3.11 and appendix Figure A.1).
Among the rock-types introduced so far are two major acid-intermediate-
basic geochemical series, or associations, the occurrence of which tends to be
mutually exclusive.
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