Page 162 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES
SULPHIDES
Pyrite Pyrrhotine is an alternative name recommended by the International
cataclased pyrite cube (white) Mineralogical Association.
veined and replaced by Crystals Both monoclinic and hexagonal, a: c = 1: 1.6502, varieties of pyrrho-
chalcopyrite (grey)
tite occur, and these are commonly intergrown. Crystals are commonly
tabular to platy with twinning on { 10l2}. There is no cleavage. D = 4.6.
Polished Pyrrhotite is brownish or pinkish white with a weak but usually visible
section pleochroism. R = 40 % with R 0 being darker andRe being lighter in the
case of hexagonal pyrrhotite. Anisotropy is strong with yellowish, green-
ish or bluish grey tints.
Pyrrhotite is usually xenomorphic, often occurring as polycrystalline
aggregates or as inclusions in pyrite. Multiple twinning, often spindle-
shaped due to deformation, is common. Ex solved lamellae (or flames) of
1000 .,_m PPL
white pentlandite are common. VHN = 230-318.
Occurrence The presence of pyrrhotite indicates a relatively low S availability. It is
common in igneous rocks, metamorphic rocks and stratiform massive
Bravoite Cu + Fe + S deposits. It forms in the reaction
layers of bravoite (grey) in
zoned pyrite (white) on
calcite (black)
in contact metamorphic aureoles. In veins, it is usually taken to indicate
precipitation from relatively high temperature, acid, reducing solutions.
Distinguishing Hexagonal and monoclinic pyrrhotite are not easily distinguished in
features polished section. A magnetic colloid may be used to stain monoclinic
pyrrhotite (Craig and Vaughan 1981). Compared with pyrrhotite,
ilmenite is darker and harder; bornite is browner (soon tarnishing to
purple) and essentially isotropic; and chalcopyrrhotite (rare) is isotropic
and browner than pyrrhotite.
500 .,_m I PPL Notes Pyrrhotite alters readily along irregular fractures to a mixture of iron
minerals including marcasite, pyrite, magnetite and limonite. Although
Occurrence Pyrite is a common sulphide occurring in most rocks and ores. Organic rare in sedimentary rocks and common in metamorphosed equivalents,
material, carbonates and quartz are all readily replaced by pyrite. especially near synsedimentary stratiform sulphide deposits, pyrrhotite
Distinguishing Compared with pyrite, marcasite is whiter and strongly anisotropic;
is not thought to be necessarily a metamorphic mineral formed by
features chalcopyrite is distinctly yellow and much softer; arsenopyrite is whiter breakdown of pyrite. It may be of hydrothermal exhalative origin and
and tends to form rhomb shapes; and pentlandite is whiter, softer and could persist in sea-floor sediments provided the sulphur availability was
often shows alteration along octahedral partings. low.
Notes Melnikovite is poorly crystallised colloform iron sulphide which appears
brownish and porous. It probably consists of FeS 2 and hydrous FeS. It Sphalerite ZnS
tarnishes rapidly. Sphalerite usually contains Fe and sometimes Cd, Mn or Hg in solid
Bravoite is nickeliferous pyrite (Fe,Ni)S 2 , often with some Co. It is solution.
similar to pyrite but brownish, slightly darker and anisotropic. It usually Crystals Sphalerite is cubic. It has the diamond structure (see Fig. 3.3) but is
occurs as idiomorphic centres or as layers in zoned pyrite. more complex than one might suspect; there are many polytypes. Cry-
stals are commonly tetrahedral and dodecahedral (Fig. 3. 7). Twinning
Pyrrhotite Fe 1-xS
about the [ 111 J axis leads to simple and complex twins. There is a
Pyrrhotite may contain some Ni, Co or Mn. It is cation deficient relative perfect {011} cleavage. D = 3.9.
to the stoichiometric mineral troilite FeS. Nickeliferous pyrrhotite Thin section Pure ZnS is transparent and colourless but sphalerite is opaque when
probably contains pentlandite. iron rich. It has very high relief and is usually yellow to brownish in
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