Chemistry Reference
In-Depth Information
Ferro-magnesian amphiboles
- anthophyllite Mg 7 Si 8 - gedrite Mg 5 Al 2 Si 6 series: orthorhombic amphib-
oles (all the other amphiboles are monoclinic).
- cummingtonite (Mg, Fe) 7 Si 8 - grunerite (Fe, Mg) 7 Si 8 series.
They are quite rare minerals which occur primarily or exclusively
(anthophyllite-gedrite series) in high grade regional metamorphism of mafic
and ultramafic rocks, and for grunerite in metamorphosed iron deposits.
Calcic amphiboles
Calcic amphiboles are divided into three groups:
￿
non-aluminous calcic amphiboles form a continuous series tremolite
(Mg) - actinolite Ca 2 (Fe, Mg) 5 Si 8 - ferroactinolite (Fe);
￿
aluminous calcic amphiboles or hornblendes. These are the most com-
mon amphibole, both common in igneous and metamorphic rocks.
They are generally described as a proportion of the end members: eden-
ite - ferroedenite, tschermakite - ferrotschermakite, pargasite - hasting-
site - ferrohastingsite. There is a gap of solubility at low temperatures
between the tremolite-ferroactinolite series and hornblende; beyond
720°C the solubility is complete;
￿
brown hornblendes are distinguished by their high iron content (par-
ticularly ferric iron) (basaltic hornblende, barkevicite) and/or titanium
(kaersutite) and their high sodium contain. Under the microscope they
have a brown, reddish brown, yellowish brown color. These are miner-
als of volcanic rocks (and igneous rocks kaersutite, barkevicite) of basic
to intermediate composition. Kaersutite and barkevicite are character-
istic of alkaline rocks.
Sodi-calcic amphiboles
Sodic-calcic amphiboles have NaCa position X. They are classified into
two groups:
￿
the barroisite (- ferrobarroisites) are aluminous amphiboles, whose
A site is empty. These minerals are mainly found in high-pressure
metamorphism.
￿
presence of sodium in A site places the amphibole of the richterite-
ferrorichterite-katophorite group in a position intermediate to sodic
amphiboles. Ferrorichterite-katophorite contain ferric iron whose pres-
ence induces yellow brown (ferrorichterite) to reddish brown colors
(katophorite) in thin section. Katophorite is a mineral of basic alkaline
rocks. Richterite is found in metamorphic marbles and skarns.
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