Page 189 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES NATIVE ELEMENTS
{100} and {023}. There is a perfect {010} cleavage and a parting on Crystals Niccolite is hexagonal, a : c = 1 :1.3972. Crystals are rare. It is usually
massive, reniform with columnar structure. Repeated twinning occurs
{100} and {101}.
on {lOll}. There is no cleavage. D = 7.8. .
Thin section n. 2.150-2.269 Niccolite is pinkish or orange white with a pronounced pleochrOism,
Polished
n~ 2.195-2.328 section with R = 58 % (lighter, orange or yellowish) and R. = 52% (darker,
2.283-2.444 0
n y pinkish). The reflectance is similar to pyrite. Anisotropy is very strong,
a 0.133-0.175 the tints being bright bluish and greenish greys.
2Vy = 60°-70° +ve Niccolite usually occurs in xenomorphic or concentric, botryoidal
OAP is perpendicular to (010) masses with other Co + Ni + As + S minerals. Grains are often cata-
D = 7.18-7.61 H = 5-51/2. clased. Growth zonation is common, and botryoidal masses often con-
COLOU R Transparency decreases with increase in Fe content. Colour banding is tain radiating intergrown irregular lamellae. VHN = 328-455.
due to a variation in Fe : Mn ratio. Iron-rich wolframite is brownish red Niccolite
to dark green.
radiating intergrowth of
PLEOCHROISM Common, with a red, brown, yellow, f3 pale green to yellowish brown, niccolite in different
y red, green, dark brown. crystallographic
orientations
HABIT Elongate prismatic often occurring as thin flat crystals.
CLEAVAGE { 010} perfect.
RELIEF Extremely high.
*BIREFRINGENCE Extremely high, but colours hidden by mineral colour.
INTERFERENCE Seen on section perpendicular to (010), 2V large, but difficult to
FIGURE determine sign since high dispersion of mineral will make determination
colours difficult to see.
EXTINCTION Oblique extinction with y'cl = 17° to 27°. 500 ~m PPL
Polished Wolframite is slightly brownish grey. With R = 17% it is slightly brighter
section than cassiterite. Bireflectance is weak. Anisotropy is moderate and
distinct in bluish greys. Extinction is oblique. Reddish brown internal
reflections are common. Occurrence Niccolite occurs in Ni + Co + Ag + As + U deposits which are
Wolframite occurs as idiomorphic tabular or bladed crystals with simple probably low temperature hydrothermal veins and replacements. Such
twinning. Zoning is enhanced by weathering. Cleavage traces may be deposits are often associated with basic igneous rocks and organic-rich
observed. VHN = 357-394. sedimentary rocks.
Occurrence Wolframite is found in high temperature hydrothermal veins and peg- Distinguishing Compared with niccolite, marcasite is whiter, and arsenopyrite is whiter
matites usually associated with quartz and Sn, Au and Bi minerals. It is features and has a weaker anisotropy.
associated with granitic rocks and greisenisation. It is also found in Note Niccolite alters to green annabergite.
placers with cassiterite. Scheelite Ca W0 4 is a common associate and
may replace wolframite.
3.11 Native elements
Distinguishing Compared with wolframite, cassiterite is darker and has more abundant
features internal reflections, and sphalerite is isotropic and is often associated
with chalcopyrite. Copper Cu
Copper may contain As, Ag or Bi.
Crystals Copper is cubic. D = 8.95.
Polished Copper is bright metallic pink but tarnishes and darkens rapidly.
3.10 Arsenide
section R = 81%. It is isotropic, but with incomplete extinction and fine
scratches may cause false anisotropy.
Niccolite NiAs
Occurrence Copper occurs as small flakes, granular aggregates, porous masses or
Nickeline is the name recommended by the International Mineralogical dendrites. Zonal texture is not uncommon, and lamellar twinning may
Association. Niccolite may contain some Fe or Co. be revealed by etching. VHN = 120-143.
176 177