Page 176 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES OXIDES
It is often found as idiomorphic octahedral sections, but also as HABIT Small, acicular prisms common.
skeletal grains or granular aggregates. Lamellae of hematite are often in CLEAVAGE { 110} and { 100} prismatic cleavages good.
a triangular pattern. Lamellae and blebs of ilmenite in a fine 'frosty' *RELIEF Exceptionally high.
texture of ulvospinel in magnetite represents slowly cooled titaniferous *BIREFRINGENCE Extremely high, interference colours usually masked by mineral colour.
magnetite. Also, exsolved lamellae and blebs of dark grey spinels may TWINNING Common in a number of different planes.
be present. VHN = 530-599. Polished Rutile is light grey with a slight bluish tint. R 0 = 20% and R . = 23%.
Occurrence Magnetite is found usually with other Fe-Ti-0 minerals in igneous and section The bireflectance is weak but usually distinct. Rutile has about the same
metamorphic rocks and skarns. It also occurs as a heavy mineral in brightness as magnetite. It is strongly anisotropic in greys but anisotropy
sediments and sedimentary rocks and in high temperature hydrothermal is often masked by abundant bright colourless yellow to brown internal
veins with sulphides. lt represents reducing conditions relative to reflections. In iron-rich varieties, internal reflections are less abundant
hematite. and reddish.
Distinguishing Compared with magnetite, ilmenite is similar but often pinker and Rutile occurs as prismatic to acicular isolated crystals or as aggregates
features anisotropic; sphalerite is softer, usually has internal reflections and of crystals and in spongy porphyroblasts. It usually occurs as small
occurs in a different association; and chromite is very similar in isolation grains. Multiple and simple twins are common. VHN = 1074-1210.
but is darker and may show internal reflections. A magnetised needle Occurrence Rutile is associated with other Fe-Ti-0 phases in pegmatites, igneous
may be used to confirm the magnetism of magnetite grains in polished and metamorphic rocks. It is a heavy mineral in sediments. It is often
section. produced from ilmenite on wall rock alteration by hydrothermal sol-
utions, e.g. greisenisation. Rutile occurs within quartz crystals as long
Rume TiOz thread-like crystals.
Rutile may contain some Fe or Nb. The polymorphs anatase and brook- Distinguishing Compared with rutile, hematite is whiter and brighter and rarely shows
ite are almost identical to rutile in polished section. features internal reflections; ilmenite is slightly pinkish and does not show inter-
Crystals Rutile is tetragonal, a: c = 1: 0.6442. Crystals are commonly prismatic, nal reflections; and cassiterite tends to be more equant and is darker.
often slender to acicular (Fig. 3.13). Twinning on { 011} is common and Notes The low temperature Ti0 2 polymorphs anatase (uniaxial -ve) and
is often repeated or geniculated. There is a distinct cleavage on { 110} as brookite (orthorhombic, small 2V +ve) have a similar occurrence to
well as a cleavage on { 100}. rutile. In polished section anatase lacks twinning and is only very weakly
Thinsection no 2.605-2.613 anisotropic. Anatase occurs associated with clay minerals in sedimen-
n. 2.899-2.901 tary rocks.
a 0.286-0.296
Uniaxial +ve (crystals are length slow)
D = 4.23-5.5 H = 6-61f2 Spinel MgAlzO•
The two common aluminous spinels are spinel MgAl,O. and hercynite
2
•coLouR Reddish brown or yellowish (sometimes opaque). FeAl 2 0 4 • The general formula of aluminous spinels is M · M~· o., with
PLEOCHROISM Common, although sometimes weak with o pale yellow, and e pale M2+ = Mg, Fe, Mn, Zn and M' • = AI. There is extensive solid solution
brown dark brown, red. including AI' • :=:;Fe'+, Cr'•.
Crystals Spinel is cubic and usually of octahedral habit (Fig. 3.14). Twinning on
{ 111} may be repeated. There is a poor { 111} cleavage. D = 3.55
(D = 4.40 hercynite).
rutile
crystal

rutile: twinning rutile: twin
Figure 3.13 Figure3.14
on (Oil) on (031)
Rutile crystal Spinel crystal spinel twin on
and twinning. and twinning. spinel crystal ( Ill )
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