1. Titanite (Sphene) By Dominic Papineau and Tiffany Yesavage CaTiO(SiO 4 ) Titanite refers to titanium which was named after the Titans, the mythical first sons of the Earth.

2. Outline 4 Crystal structure 1 Chemical composition 6 Occurrences 3 Optical properties 2 Physical properties 7 Uses 5 Titanite stability

3. Crystal structure of titanite Ca polyhedron Ti octahedron Si tetrahedron O atoms

4. Chemical substitutions In Titanium octahedra : Most commonly Al 3+ or Fe 3+ replaces Ti 4+. Occasionally,Fe 2+ and REE such as Nb 5+ and Ta 5+ may also substitute for Ti 4+. Occasionally, Fe 2+ and REE such as Nb 5+ and Ta 5+ may also substitute for Ti 4+. For Oxygen: O 2- is commonly replaced by either F - or OH -. The most common substitution that occurs in titanite involves this coupled substitution: (Al,Fe) 3+ + O 2- >> Ti 4+ + (OH -, F - ) Notice that the above charges balance. This particular substitution may occur in up to 30% of cations in titanite.

5. For Calcium: Sr, Ba, Na, Mn and REE such as Ce and Nd may substitute for Ca 2+. 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 Si 4+ is Al 3+. A complete solid solution has also been shown to exist between CaTiO(SiO 4 ) and SnTiO(SiO 4 ) at 700 o C and 7kbar. SnTiO(SiO 4 ) is the chemical composition of malayaite. Malayaite and the high temperature form of titanite are they are isostructural. Chemical substitutions (continued)

6. Color: gray, brown, green, yellow, black, transparent to translucent Luster: resinous to adamantine Cleavage: {110} distinct Hardness: 5 - 5.5 Because of titanite’s lack of hardness, it weathers quickly in most rocks Physical properties Specific gravity: 3.4 - 3.55

7. Optical properties 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. under crossed nicols. Titanite can also be recognized by its very high relief. Titanite under thin section can be determined from its “sphenoidal shape” as seen in a thin section.  : 1.900  : 1.900  : 1.907  : 1.907  : 2.032  : 2.032  : 0.14  : 0.14 2V = 27 o Optically positive Optical data

8. Crystal structure of Titanite Ca polyhedron Ti octahedron Si tetrahedron O atoms

9. Crystallographic data of Titanite Point group: 2/m Crystal class: Monoclinic Unit cell parameters: a = 7.039 – 7.088 Å a = 7.039 – 7.088 Å b = 8.643 – 8.740 Å b = 8.643 – 8.740 Å c = 6.527 – 6.584 Å c = 6.527 – 6.584 Å β = 113.74 o - 114.15 o β = 113.74 o - 114.15 o V = 369.6 Å 3 V = 369.6 Å 3 Z = 4 Z = 4 Space group: C2/c Calculated density: 3.52 g/cm 3

10. 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 ~220 o C: P2 1 /a A2/a

11. 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

12. 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.

13. 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...