Chemistry Reference
In-Depth Information
Under the microscope, spinel is colorless. Spinels containing iron or tita-
nium are colored in green, gray green, brown, green, brown, reddish brown
to opaque. Magnetite and chromite are opaque and cannot be determined
with the petrographic microscope.
3.5.1.2 Occurrences
Metamorphic rocks
Minerals of the spinel-hercynite group are minerals of high grade metamor-
phism. Spinel
s.s.
appears in calcareous-dolomitic rocks. The spinels of the
spinel-pleonaste-hercynite group occur in pelitic rocks. Spinel is incompat-
ible with quartz, spinel
quartz association being replaced by sapphirine or
cordierite. Hercynite is compatible with quartz.
Magnetite characterizes a large family of skarns characterized by an
oxygen fugacity intermediate between that of “reduced” skarns with heden-
bergite (ferrous iron) and “oxidized” skarns with andradite (ferric iron).
Rather pure, titatium-poor magnetite are mostly developed in exoskarns.
Magnetites of endoskarn are characterized by high titanium content.
Hematite and limonite of the sediments and iron formations are reduced
to magnetite during metamorphism. Magnetite is an essential constituent of
banded iron formations in particular.
+
Igneous rocks
During fractional crystallization, chromites crystallize in basic and ultra-
basic terms, magnetites appear in more evolved terms. In both cases these
minerals evolve from magnesium-richer to iron-richer members.
Spinelles of kimberlites and lherzolites in enclaves in basalts belong to a
magnesiochromite (Mg, Cr)-pleonastes (Fe, Cr)-picotite (Mg-Fe, Al) series.
Alpine peridotites are one of the two major types of chromium deposits
(for example: New Caledonia, Cuba, Turkey, Albania) (Bouladon, 1986).
Chromite form stratiform orebodies in peridotitic and/or gabbroic cumu-
lates. The ore is either massive or made of crystals or nodules of chromite
dispersed in a matrix of silicates (“leopard” or “antileopard” structures). It
is an aluminous and magnesian chromite (refractory chromite) which may
contain inclusions of heavy PGMs (Pt, Ir, Os). “Podiform” orebodies form
irregular masses in harzburgites. Such bodies are often of tectonic origin
(with perhaps a hydrothermal reworking). They are made of metallurgical
chromite rich in iron, low in aluminum. Cu, Ni, Co are recovered as byprod-
ucts from the processing of these chromites (these elements are presumably
contained in associated sulfides).
Mafic-ultramafic complexes of the Bushveld type are the other type of
chromium deposit (Willemse, 1969, Von Gruenenwaldt et al., 1985). In the
