Page 175 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES OXIDES
Ilmenite
Martite
grains of ilmenite with
'heavy mineral' grain of
magnetite replaced by hematite exsolvcd hematite

X POLS
500 ~-'"' PPL

Occurrence Hematite is found with other Fe-Ti-0 minerals in igneous and centrates. Magnesium-rich ilmenites occur in kimberlites but also in
metamorphic rocks as well as sedimentary rocks, especially banded iron contact metamorphosed rocks.
formation. Hematite in veins can be primary but it frequently forms by Distinguishing Compared with ilmenite, magnetite is slightly brighter and usually
oxidation of other primary iron-bearing minerals, for example in features bluish grey in direct comparison, isotropic and strongly magnetic, and
gossans. rutile shows abundant internal reflections.
Distinguishing Compared with hematite, stibnite has a distinct bireflectance is softer
features and has a good cleavage; ilmenite is pinkish and darker; and,cinnabar Magnetite Fe,O.
Magnetite often contains Ti, Cr or Mn. Titaniferous magnetite often
has abundant internal reflections and is softer.
contains ulvospinel Fe,TiO. in solid solution.
Note Martite is magnetite pseudomorphed by an intergrowth of hematite.
Crystals Magnetite is an inverse spinel. It is cubic, commonly occurring as
octahedra and combinations of the octahedron and rhombic dodeca-
Ilmenite FeTiO,
hedron. Twinning is common on { 111}, the usual spinel twin. D = 5.2.
Ilmenite may contain Mn or Mg, the magnesian end member being
geikielite and the manganiferous end member being pyrophanite. It may Polished section Magnetite is grey, sometimes with a brownish or pinkish tint indicative
of titanium. (Uivospinel is brownish grey). R = 21 %, making magnetite
also contain Fe'+, which represents a solid solution towards hematite
Fe,O,. much darker than pyrite and hematite. Magnetite is isotropic with good
Crystals Ilmenite is trigonal, a :c = 1: 1.3846, and occurs as tabular { 0001} extinction.
crystals. Twinning occurs on { 0001} and multiple twinning on { 1011}. Magnetite
There is no cleavage but there is a parting parallel to { 1 Oi 1}. D = 4. 7.
a grain of magnetite with
Thin section In very thin flakes ilmenite is red, uniaxial - ve. exsolved ilmenite
Polished Ilmenite is slightly pinkish or brownish light grey with a weak pleochro-
section ism. R 0 = 21%, which is similar to magnetite, and R . = 18% Ani-
sotropy is moderate but only distinct in some orientations; tints are
greenish, bluish and brownish greys.
Ilmenite is sometimes idiomorphic but is usually intergrown with
other Fe-Ti-0 minerals. It often contains lamellar inclusions of hematite
or other Fe-Ti-0 minerals. Occasionally lamellar twins may be present.
VHN = 519-703, varying with chemical composition.
Occurrence Ilmenite is found with other Fe-Ti-0 minerals in igneous rocks
200 fln1 PPL
(especially of mafic composition) and metamorphic rocks, and also (but
rarely) in veins and pegmatites. Detrital ilmenite is usually altered to
leucoxene which is enriched in TiO,. It occurs in heavy mineral con-
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