Chemistry Reference
In-Depth Information
The
banded iron formations
that have undergone high grade meta-
morphism, contain iron-rich pyroxenes: orthopyroxene close to ferrosilite,
hedenbergite, ferriaugite.
Clinopyroxenes of the diopside-hedenbergite series occur in
carbon-
ate rocks
, impure marbles and calc-silicate-gneisses in a wide temperature
range: the diopside isograd in these rocks corresponds roughly with that of
aluminous silicates (garnet, cordierite, etc.) in metapelites.
Jadeite
quartz association defines a high-pressure sub-facies in
blue-
schist facies
(pressure over 7-8 kb). But in the absence of quartz, jadeite
can be stable at lower pressures. Some rocks deficient in silica and alumina,
in particular iron-rich formations in blueschist facies, may contain aegir-
ine associated with an amphibole of the glaucophane-riebeckite series (for
instance, Saint-Veran, Hautes Alpes, France).
Pyroxenes of the
eclogites
are omphacite. But the composition of those
omphacites depends on the type of eclogite: the relatively low temperature
eclogites in the areas of Alpine metamorphism are rich in jadeite component.
Increase of temperature promotes the introduction of the Tschermak mol-
ecule. Eclogites in enclaves in alkali basalts contain aluminous diopside.
+
Metasomatic rocks
Pyroxenes are major components of the skarns. In skarns of high tempera-
ture (often at the magmatic stage) pyroxenes are commonly aluminous
diopside. In most common hydrothermal skarns, pyroxenes belong to the
diopside-hedenbergite-johannsenite series.
Aegirine appears in fenites, metasomatic halos around alkaline com-
plexes, particularly carbonatites.
3.2.5 Olivine group
3.2.5.1 Structure and chemical composition
Minerals of the olivine group are nesosilicates which crystallize in the
orthorhombic system. Isolated (SiO
4
)
4-
tetrahedra are linked by divalent
cations in octahedral position, occupying two non-equivalent sites. Hence
the general formula:
X
2
SiO
4
X
=
Mg, Fe
2+
, Mn, Ca, Ni, Cr, Ti
Aluminium does not enter the tetrahedral.
The most important minerals in this group are
olivines
, which form a
continuous series between the magnesian end member, Mg
2
SiO
4
,
forster-
ite
and the iron end member, Fe
2
SiO
4
,
fayalite
. The composition of olivine
is noted by its proportion of forsterite, Fo%. Forsterite can contain up to
