Page 165 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES SULPHIDES
Figure 3.7 Distinguishing Compared with sphalerite, magnetite is often pinkish, harder and never
Typical features has internal reflections; limonite is bluish grey, usually has reddish
sphalerite internal reflections and is usually replacing iron-bearing minerals ; and
crystals. tetrahedrite is brighter, greenish or bluish grey and only very rarely
shows internal reflections.
Notes Wurtzite (hexagonal ZnS) is very similar to sphalerite in polished sec-
spinel-type twin
tion, but it is rare.
colour with dark brown bands due to Fe zonation. Oxidation of iron-
bearing varieties leads to brown staining, especially in fractures. Sphal- Stibnite Sb2S,
erite is isotropic but is sometimes anomalously anisotropic, revealing Crystals Stibnite is orthorhombic with a: b :c = 0. 9926: 1: 0.3393. Crystals are
fine lamellar twinning probably due to stacking polytypes. At usually prismatic [ 001 J, often slender to acicular. Twinning on { 130} is
>.. = 589 nm,n = 2.369 (pure ZnS), 2.40 (5.46% Fe), 2.43 (10.8% Fe) rare. There is a perfect { 010} cleavage and imperfect { 100} and { 110}
and 2.47 (17.06% Fe). cleavages. D = 4.63.
Polished Sphalerite is grey with R = 17 %. It is darker than most ore minerals but Thin section Stibnite is opaque. However, it is transparent using infra-red transmit-
section brighter than the gangue minerals. It is isotropic. Pure ZnS has abundant ted light.
internal reflections but, with increasing Fe content, opacity increases Polished Stibnite has a pronounced bireflectance with R = 30 to 40%. It is light
and internal reflections become fewer and brownish or reddish. section grey lla, brownish light grey lib and white lie. The anisotropy is very
Sphalerite is rarely idiomorphic. It usually occurs as rounded grains in strong, with tints ranging from light bluish grey to brown. Extinction is
aggregates. It also is found as zoned colloform masses. Irregular frac- straight.
tures are common and the cleavage often results in severe pitting. Stibnite often occurs as acicular or bladed crystals or granular aggre-
Multiple twinning is often visible. Zonation of iron, seen as brown bands gates. The usually well developed cleavage traces are deformed. Defor-
in transmitted light or by internal reflection, does not visibly change mation twinning is common. VHN = 42-109.
brightness. Sphalerite usually contains inclusions, especially of chalco-
pyrite, as blebs or lamellae. VHN = 186-209. Stibnite
bladed stibnite grains showing
Sphalerite
distinct bireflectance (white to
chalcopyrite (white) blebs and grey) and cleavage traces:
interstitial vemlets in in quartz (black)
sphalerite (grey)

200 iJ.m PPL
500 iJ.m PPL

Occurrence Sphalerite is common in stratabound, vein and massive sulphide
Occurrence Stibnite is found in low temperature hydrothermal veins, usually with
deposits. Sphalerite, typically very low in Fe content, also occurs with quartz. It is associated with complex Sb-bearing and As-bearing sul-
galena, pyrite and chalcopyrite in calcareous nodules or veinlets prob- phides, pyrite, gold and mercury.
ably of diagenetic origin. Fe-rich sphalerite often occurs with pyrrhotite, Distingt1ishing Compared with stibnite, hematite has a smaller bireflectance, weaker
as it is the activity of FeS rather than the abundance of Fe that controls features anisotropy, is harder and lacks cleavage. Some lead-antimony sulphides
the iron content of sphalerite. Sphalerite is often associated with galena.
are very similar to stibnite.
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