Smith College - Geology 222b - Petrology

Petrographic Data File

Titanite
A.K.A. Sphene
Property
Value
Comments
Formula CaTiOSiO4  
Crystal System Monoclinic (2/m) Beta = 119.9°
Crystal Habit Wedge-, and elongated rhomb-shaped crystals are common, but rounded and irregular grains are also common. Grains typically are not apparent in hand specimens.

Diamond-shaped cross-sections can be seen in thin sections. Titanite has very high positive relief.

H=5-51/2
Cleavage Distinct prismatic ceavage {110} may control grain orientation. Uneven fracture. Brittle. Partings parallel to twins may be more prominent than cleavage.
Twinning Simple twins may occur on {100}.  
Color/Pleochroism

Tan-brown or yellowish color. Nonpleochroic to weakly pleochroic:

x almost colorless
y pale yellow
z brownish

 Color appears the same in plane- and crossed polarized light.
Optic Sign Biaxial (+)
2V 20-40°  
Optic Orientation

X /\ a=-6° to -21°
Y=b
Z /\ c=36-51°

  
Refractive Indices
alpha = 
beta = 
gamma = 
1.843-1.950
1.970-2.034
1.943-2.110
Max Birefringence 0.100-0.192 Birefringence is high order white or beige (like calcite) with little variation when the stage is rotated.
Elongation cannot be easily determined  
Extinction Because of high dispersion, some grains do not extinguish completely.  
Dispersion  r > v Optic axis dispersion is very strong, inclined dispersion in weak.
Distinguishing Features Very High Relief. Extreme birefringence; same brownish color in plane and crossed polars. Diamond/wedge-shaped crystal habit
Occurrence Titanite is a common accessory in many igneous and metamorphic rocks, of which it can be the dominent Ti-bearing contituent. Titanite also occrs as detrital grains in the heavy mineral fraction of clastic sediments.
Editors Sarah Pistone ('06), Janelle McAtamney ('07), Molly Peek ('18)
Sources

Philpotts, Anthony R., 1989. Petrogrophy of Igneous and         Metamorphic Rocks. Waveland Press, Inc.: Prospect Heights,         Illinois. 61 p.
Nesse, William D., 2000. Introduction to Mineralogy. Oxford         University Press: New York, NY. 322 p.
MacKenzie W.S., and Guilford C., 1980. Atlas of rock-forming         minerals in thin section. Halsted Press: New York, NY. 97p.


 
Titanite (Sphene) grain under plane polarized light.
Photograph of a Titanite grain taken in plane polarized light (click for rollover image).

These photos are meant to show the extremely high interference colors displayed by titanite, and the consistent color under plain polarized light and crossed polars.

Characteristic wedge shape of the crystals.
WWW Images

Union College
Brock University

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