Chemistry Reference
In-Depth Information
3.2.3.4 Occurrences of amphiboles
By the wide range of their chemical compositions, amphiboles have extremely
varied deposits, reflecting both the chemical composition of the rock and its
conditions formation. In a group of amphiboles belonging to a complex
of related rocks (magmatic series, metamorphic suite), changes in chemical
composition and the laws of substitution between them, reflect very pre-
cisely the geological evolution of these rocks. It is worthwhile to study them
even if the experimental data available are still insufficient to interpret these
geochemical results in terms of precise physical. (emperature, pressures, etc.)
estimations.
Metamorphic rocks
The metamorphic facies (recalled in the introduction ยง1.3.1 and Figure 1.9)
are largely defined by the presence, or absence, of amphiboles and their
nature.
In
low to medium pressure metamorphism
, amphiboles are generally
calcic amphiboles and appear in rocks derived from basic to intermediate
igneous rocks and in carbonate rocks.
In
rocks deriving from igneous rocks
, in greenschist facies, amphibole
is a non-aluminous calcic amphibole of the tremolite-actinolite series (with
chlorite, epidote, albite, etc.). In the epidote-amphibolite facies, amphibole
is a hornblende associated with epidote and sodic plagioclase (An
20).
Amphibolite facies itself is defined by the critical association hornblende
and calcic plagioclase. With increasing metamorphism the composition
of amphibole varies: amphibole becomes progressively richer in sodium,
changing from terms richer in tschermakitic component to terms richer in
edenitic component. Amphiboles of low pressure are less rich in pargasitic
component than the ones under pressure. These compositional changes
induce variations of the optical properties: green to bluish green at low
temperature, amphibole becomes brownish green and then brown green at
higher temperatures.
Cummingtonite appears in the upper amphibolite facies where it results
from destabilization of the iron-rich component of hornblende into cum-
mingtonite
<
calcic plagioclase. Cummingtonite coexists with a hornblende
that is then more magnesian and less aluminous (edenitic type). At high tem-
peratures amphiboles (hornblende and cummingtonite) disappear and are
replaced by the association orthopyroxene
+
+
calcic plagioclase characteristic
of granulite facies.
In
metamorphic carbonate rocks
, the chemical compositions of amphi-
boles also vary depending on the intensity of metamorphism. The varia-
tions are larger than in the igneous rocks but they are less clearly correlated
to the intensity of metamorphism due to the initial variability of the rocks
(calcitic or dolomitic marble, proportions of clay, silica, etc.). Amphiboles
