Chemistry Reference
In-Depth Information
gradually restrict the field of composition of the chlorite: appearance of
biotite, then of aluminous magnesian and/or ferriferrous silicates (cordier-
ite, garnet, staurolite) and finally alumina silicates. Usually chlorite dis-
appears in the andalusite or kyanite zone (see the section on aluminous
minerals, 3.3).
Chlorite is a common mineral in the blueschist facies in association
with albite, glaucophane, phengite, etc. In white schist facies, the associa-
tion talc - kyanite replaces magnesian chlorite.
In the basic rocks of igneous origin, the association chlorite - albite -
epidote - actinolite is characteristic of the greenschist facies. This associa-
tion is replaced by the paragenesis hornblende - epidote - albite/oligoclase
in the epidote - amphibolite facies and by hornblende - calcic plagioclase in
the amphibolite facies.
Steatite (soapstone) is a rock made of talc, chlorite, actinolite, etc., and
of low to medium grade metamorphism of ultramafic rocks (for example,
dunite, harzburgite).
Chlorite is also present in the intermediate and acidic rocks where it
replaces the primary biotite and amphibole.
Secondary mineral
Chlorite is a common alteration mineral of ferromagnesian minerals (biotite,
amphibole, pyroxene, olivine, garnet, cordierite, etc.) It may be supergene
alteration, but most often an alteration associated with hydrothermal
fluids circulation or deuteric alteration the end of crystallization of igne-
ous rocks, or even a retrograde mineral in the lowering of temperature in
metamorphism.
Biotites are altered frequently into a green chlorite, with Prussian blue
interference colors. Titanium contained in the biotite recrystallizes in rutile
in grains, often showing the habit of sagenite (60° twinned needles).
Cordierite is typically altered into a yellowish isotropic chlorite.
Metasomatic rocks
The rocks, once called “spilites” (a term now obsolete), show occurrences
and textures of basaltic lava, but they are formed of albite and chlorite
(
calcite-actinolite-epidote). Comparison to similar lavas shows a leaching
of calcium and sodium enrichment. They are commonly attributed to
oceanic hydrothermal metamorphism, but they may also occur in a conti-
nental environment.
Propylitization (propylitic alteration) is an alteration developed in mafic
to intermediate (igneous) rocks around hydrothermal sulfide deposits: it
transforms the primary rock into an association epidote-albite-chlorite-
pyrite (sometimes with hematite and magnetite).
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