Chemistry Reference
In-Depth Information
3.3.2.2 Igneous rocks
Peraluminous igneous rocks:
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either belong to peraluminous suites (for example, the Laouzas granite,
Montagne Noire, France)
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or are the differenciated members of mainly metaaluminous suite (like
the ordinary calcalkaline series) such as evolved granites, aplites and
pegmatites.
The peraluminous character is expressed by the presence of minerals
such as muscovite, garnet, cordierite, aluminous biotite, sometimes topaz
and/or beryl, seldom andalusite and, exceptionally, kyanite.
The garnets from differentiated granitoids, aplites and pegmatites belong
to the almandine-spessatine series.
The habit of cordierite may be either euhedral crystals, clearly of mag-
matic origin, or nodules (“chestnuts”) with inclusions of sillimanite and
biotite, probably metasomatic origin. Both habits sometimes may occur
simultaneously in a same sample:
Cordierite and topaz are also found in some rare rhyolites.
Kimberlites, peridotites, biotite peridotites contain a pyrope-rich garnet
containing a greater or lesser proportion of chromium.
3.3.2.3 Metasomatic rocks
Greisens are rocks made of quartz and muscovite formed by leaching of
alkalis from quartzo-feldspathic rocks. Greisens frequently contain topaz
and beryl, accompanying mineralization in tin and/or tungsten.
Further leaching of the alkalis leads to “secondary hydrothermal quartz-
ites”, rocks formed mostly of quartz with aluminous minerals: andalusite,
corundum, pyrophyllite, diaspore, etc. These rocks develop at the expense of
granitoids, volcanic rocks and, more rarely, sedimentary rocks in an environ-
ment of volcanic or subvolcanic rocks of acid to intermediate composition.
The quartz
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pink andalusite cores of pegmatites come from similar
leaching.
This is probably the same phenomenon that produces quartz
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pink anda-
lusite lenses or quartz
blue kyanite lenses (with some staurolite), that are
frequent in schists and quartz
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sillimanite nodules and lenses in gneisses.
Corundum occurs in pegmatites associated with granitoids or nepheline
syenites. The presence of corundum implies leaching of alkalis and above all
the absence of quartz. Nepheline syenite pegmatites contain no quartz. It is
probable that the granitic pegmatites have undergone desilication in contact
with limestone.
Emerald occurs in biotitites derived ultramafic rocks by potassic meta-
somatism due to fluids, probably of perigranitic (/pneumatolitic) origin.
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