Titanite (Sphene)

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Titanite (Sphene)
CaTiO(SiO4)
By Dominic Papineau and Tiffany Yesavage
Titanite refers to titanium which was named after
the Titans, the mythical first sons of the Earth.
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Outline
1 Chemical composition
2 Physical properties
3 Optical properties
4 Crystal structure
5 Titanite stability
6 Occurrences
7 Uses
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Crystal structure of titanite
Ca polyhedron
Ti octahedron
Si tetrahedron
O atoms
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Chemical substitutions
In Titanium octahedra
Most commonly Al3 or Fe3 replaces Ti4.
Occasionally, Fe2 and REE such as Nb5 and Ta5
may also substitute for Ti4.
For Oxygen
O2- is commonly replaced by either F- or OH-.
The most common substitution that occurs in
titanite involves this coupled substitution
(Al,Fe)3 O2- gtgt Ti4 (OH-,
F-) Notice that the above charges balance. This
particular substitution may occur in up to 30 of
cations in titanite.
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Chemical substitutions (continued)
For Calcium
Sr, Ba, Na, Mn and REE such as Ce and Nd may
substitute for Ca2.
U, Th and radiogenic Pb may also substitute for
Calcium. Like zircon and apatite, titanite may be
used in order to determine the ages of rocks.
In the Silica tetrahedra
The only element observed to substitute for Si4
is Al3.
A complete solid solution has also been shown to
exist between CaTiO(SiO4) and SnTiO(SiO4) at
700oC and 7kbar. SnTiO(SiO4) is the chemical
composition of malayaite. Malayaite and the high
temperature form of titanite are they are
isostructural.
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Physical properties
Color gray, brown, green, yellow, black,
transparent to translucent
Luster resinous to adamantine
Cleavage 110 distinct
Hardness 5 - 5.5 Because of titanites lack of
hardness, it weathers quickly in most rocks
Specific gravity 3.4 - 3.55
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Optical properties
Titanite under thin section can be determined
from its sphenoidal shape as seen in a thin
section.
Because titanite has a very high birefringence,
it will appear as a high order white under
crossed nicols. Titanite can also be recognized
by its very high relief.
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Crystal structure of Titanite
Ca polyhedron
Ti octahedron
Si tetrahedron
O atoms
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Crystallographic data of Titanite
Crystal class Monoclinic
Point group 2/m
Space group C2/c
Unit cell parameters
a 7.039 7.088 Å b 8.643 8.740 Å c
6.527 6.584 Å ß 113.74o - 114.15o V
369.6 Å3 Z 4
Calculated density 3.52 g/cm3
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Thermodynamic stability of Titanite
Theoretical stability at 1200 K and 25 kbar
Si O bond length Ti O bond length Ca O bond length
At 25 kbar Decrease by 0.003 Å Decrease by 0.008 Å Decrease by 0.015 Å
At 1200K Increase by 0.004 Å Increase by 0.017 Å Increase by 0.027 Å
Source Dempsey and Strens (1976)
Experimental studies showed that the bonding
stabilities in titanite at 1200 K are 0.002 Å
less than the calculated values (Taylor and
Brown, 1976).
A space group transition occurs at 220oC
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Metamictization of Titanite
Full symbols represent heated specimen
Open symbols represent unheated specimens
The regression curves were calculated only with
the data from the heated specimens
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Occurrences of titanite
In igneous rocks
Common accessory mineral in granite,
granodiorites, diorites, syenites.
In metamorphic rocks
Crystals can be large in gneisses, chlorite
schists, and marble.
It also occurs with iron ores, pyroxene,
amphibole, scapolite, zircon, apatite, feldspar
and quartz.
Locations on Earth
Russia, Switzerland, Italy, Norway, United States
(NY, Pa, Mt, and Ma), Canada (Ont, Que), Brazil,
and Pakistan.
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Uses of titanite
As a source of titanium oxide for use in paint
pigments.
As a minor gemstone.
In brief, titanite is not very useful...