Chemistry Reference
In-Depth Information
Forms of low-temperature anatase and brookite are very often confused
with rutile under the microscope. These minerals occur in igneous and meta-
morphic rocks. They are mostly minerals of alteration of other titaniferous
minerals, titanite and ilmenite.
3.5.5.2 Ilmenite
Ilmenite FeTiO 3 may contain limited amounts of magnesium and manganese
in substitution to iron. There is a complete solid solution between ilmenite
FeTiO 3 and hematite Fe 2 O 3 at temperatures above 1050°C. At lower tem-
peratures, there is a solvus and ilmenite commonly contain up to 5% Fe 2 O 3 .
The common paragenesis ilmenite (containing hematite in solid solution)
- titaniferous magnetite allows a determination of the temperature and oxy-
gen fugacity. Ilmenite may also contain traces of V, Cr, Al.
The mineral is opaque, and cannot be precisely determined with the
petrographic microscope.
Ilmenite is a ubiquitous mineral in igneous rocks. Ilmenites from kimber-
lites and peridotite xenoliths may contain up to 12-13 wt% MgO · So Mg-
ilmenite is a guide for diamond exploration. In anorthosite, ilmenite associated
with rutile commonly form cumulative bodies likely to be of economic inter-
est (for example, Quebec, New York state, Mexico). In the alkaline series of
Velay, ilmenite only appears in differentiated basalts/basanites, hawaiites and
mugearites. Its molar contents of hematite are growing from 6 to 8% from the
basalts to the mugearites. It is absent in more evolved terms. Manganese-bearing
ilmenites are found in differentiated igneous rocks and carbonatites.
Ilmenite is a mineral easily reworked in the sediments. With rutile it belongs
to the family of heavy minerals concentrated in beach sands, which may some-
times have an economic interest (Florida, India, Senegal, Madagascar).
Ilmenite is a common mineral in metamorphic rocks as well as of ortho-
and para-derivation. At high pressures (eclogite facies and granulite facies of
high pressure), ilmenite is replaced by rutile according to reactions such as:
FeTiO 3
+
SiO 2
FeSiO 3
+
TiO 2
ilmenite
+
quartz
=
orthopyroxene
+
rutile (Hayob et al. 1993)
3FeTiO 3
+
Al 2 SiO 5
+
2SiO 2
Fe 3 Al 2 Si 3 O 12
+
3TiO 2
ilmenite
+
sillimanite
+
quartz
almandin
+
rutile (Feenstra and Engi,
1998)
CaFeSi 2 O 6
+
2CaAl 2 Si 2 O 8
+
2FeTiO 3
Ca 3 Al 2 Si 3 O 12
+
Fe 3 Al 2 Si 3 O 12
+
2TiO 2
clinopyroxene
+
plagioclase
+
ilmenite
Ca-Fe garnet
+
rutile
(Mukhopadhyay et al ., 1992)
3CaFeSi 2 O 6
+
3CaAl 2 Si 2 O 8 +
3FeTiO 3
Ca 3 Al 2 Si 3 O 12
+
2Fe 3 Al 2 Si 3 O 12
3CaTiSiO 5
clinopyroxene
+
+
plagioclase
+
ilmenite
Ca-Fe garnet
+
titanite
(Mukhopadhyay et al ., 1992)
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