Chemistry Reference
In-Depth Information
of the tremolite - actinolite series appear from the biotite isograd. Tremo-
lite is stable at temperatures higher than those of the greenschist facies
in dolomitic marbles. In the amphibolite facies, amphiboles of the other
carbonate rocks are hornblende often rich in pargasitic constituent and/or
richterites.
Some
rocks of particular chemical composition
contain uncommon
amphiboles. Grunerite is restricted to metamorphic iron ores or sediments
rich in iron (Collobrieres Var; Pierrefitte Hautes Pyrenees, France). Rie-
beckite also appears in sediments rich in iron (Saint-Veran, Hautes Alpes,
France). Anthophyllite appears in highly magnesian rocks (ultrabasic or
ultramafic rocks initially more or less rich in olivine and orthopyroxene).
It has a very limited range of stability: at temperatures lower than about
700°C it is replaced by chlorite and serpentine; at temperatures higher than
800°C, it is replaced by orthopyroxene. Anthophyllite is also described in
rocks of sedimentary origin, probably having undergone metasomatic phe-
nomena: cordierite - anthophyllite gneiss (famous examples in Finland),
contact metamorphism of the Pyrenean lherzolites.
In high-pressure metamorphism, blueschist facies is defined by the pres-
ence of amphibole of the glaucophane - riebeckite series (crossite). Crossites
have a compositions similar to the one of the association chlorite
albite.
Thus, these minerals appear in rocks of more varied composition in low to
medium pressure metamorphism: meta-basites, but also meta-greywackes
and meta-pelites.
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Igneous rocks
Besides tschermackite which is peraluminous, amphiboles are meta-alumi-
nous (hornblendes) or alkaline (arfvedsonite-riebeckite series, katophorite)
minerals.
Common hornblende is an important and frequent constituent of acidic
to intermediate
volcanic rocks
, saturated in silica or not. It is less abundant
in basic rocks. Crystallization of amphibole instead of pyroxene depends on
water pressure. Crystallization of one or other of these minerals influences
the subsequent evolution of the magma. Hornblendes of volcanic rocks usu-
ally belong to the pargasite-ferrohastingsite series.
Basaltic hornblende is common in mafic to intermediate rocks, in satu-
rated silica or not. It is generally accepted that these minerals derive from a
primary common hornblende by oxidation during or after the eruption.
Cummingtonite is a rare constituent of some dacites.
In alkaline series, katophorite and barkevicite appear in mafic to inter-
mediate rocks, kaersutite (Ti) in intermediate to acidic rocks, while the dif-
ferentiated rocks show an amphibole of riebeckite (Fe
3+
) type. Ferrorichterite
occur in sodium-rich alkaline rocks (syenites and granites). Richterite also
occurs in carbonatites and meteorites.
