Page 160 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES
SULPHIDES
Distinguishing Compared with molybdenite, tungstenite WS is very similar; graphite is Thin section Both minerals are deep red, uniaxial - ve.
2
features morphologically similar but much darker; and tetradymite Bi Te S is Polished Both minerals are light grey, often slightly bluish. R = 28- 31 % (pyrar-
2 2
brighter. section gyrite) and R = 25-28 % (proustite), which makes them similar in
Notes Molybdenite polishes poorly because of smearing. brightness to tetrahedrite. Bireflectance is distinct and anisotropy strong
in greys. Red internal reflections are common and more abundant in
Pentlandite (Fe,Ni) 9 S 8 proustite.
Pentlandite usually contains about equal amounts of Fe and Ni. It often Both minerals occur as isolated crystals but are common as inclusions in
contains Co and sometimes Cu or Ag in solid solution. galena. Simple and multiple twinning may be present. VHN: pyragyrite
Crystals Pentlandite is cubic but rarely occurs as well shaped crystals. There is no 50-97 1 cleavage, 98-126 II cleavage; proustite VHN 109-135.
cleavage but a parting on { 111}. D = 5.0. Occurrence Pyrargyrite is more common than proustite. They are associated with
Polished Pentlandite is very slightly yellowish white (cream) with R = 52 % other sulphosalts, especially tetrahedrite-tennantite, in low tempera-
section which is similar to pyrite. It is isotropic. ture Pb + Zn mineralisation and Ag + Ni + Co veins. The ruby silvers
It occurs commonly as 'flame' lamellae in pyrrhotite and as veinlets or and similar Ag minerals may be significant silver carriers in base metal
xenomorphic grains associated with pyrrhotite. The octahedral parting mineralisation.
{ 111} is often well developed, resulting in triangular cleavage pits. Distinguishing There are some rare complex sulphides which resemble the ruby silvers.
Alteration also takes place along this parting. VHN = 202-230. features Cinnabar is quite similar but the anisotropy tints are greenish grey.
Pentlandite
Pyrite FeSz
exsolved pentlandite (Pn)
'flames'in pyrrhotite (Po): Pyrite may contain some Ni or Co. Auriferous pyrite probably contains
also coarse pentlandite inclusions of native gold and cupriferous pyrite probably contains inclu-
showing pits due to
octahedral parting sions of chalcoyprite.
Crystals Pyrite is cubic, crystals most commonly being modifications of cubes
(Fig. 3.6). The { 011} twin plane and [ 001 J twin axis produce penetra-
tion twins. There is a poor { 001} cleavage. D = 5.01.
Thin section Pyrite is opaque, often occurring as euhedral crystals or aggregates of
small rounded grains. Alteration to limonite results in brownish or
reddish coloured rims or brown staining.
Polished Pyrite is white, often with a slight yellowish tint especially in small
2()() ~m PPL section grains. R = 54 %, resulting in pyrite usually appearing very bright. It is
only ideally isotropic in ( 111) sections, and the weak anisotropy in very
dark green and brown can usually be seen in well polished grains.
Occurrence Pentlandite, usually associated with pyrrhotite and other
Pyrite is usually idiomorphic but is occasionally intergrown with other
Cu + Ni + Fe + S phases, is common in mafic igneous rocks, e.g.
sulphides, e.g. sphalerite. Grains are often cataclased. Framboidal
norites, and some massive sulphide deposits.
pyrite is common in sedimentary rocks. Growth zoning in pyrite is
Distinguishing Compared with pentlandite, pyrite is yellowish, often weakly aniso-
features tropic and harder, and pyrrhotite is darker, brownish, anisotropic and enhanced by etching. Zonation of inclusions is common. Inclusions of
slightly harder. other sulphides are common, e.g. chalcopyrite, pyrrhotite. Fractures in
pyrite often contain introduced sulphides, e.g. chalcopyrite, galena.
Pyrargyrite Ag,SbS, VHN = 1027- 1240.
Pyrargyrite and proustite Ag,AsS, are known as the 'ruby silvers'
because they are translucent with a deep red colour. Extensive solid
solution occurs between the two minerals.
Crystals Pyrargyrite is triagonal, a: c = 1:0.7892 and proustite is trigonal,
a :c = 1:0.8039. Both minerals are commonly prismatic [ 0001 J with
Figure3.6
twinning, sometimes complex, on {1014}. There is a distinct {lOll}
Typical pyrite
cleavage. D = 5.85 (D = 5.57 proustite). crystals. pyritohedron
148
149