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Titanite (Sphene) Outline Crystal Structure of Titanite

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Titanite (Sphene) Outline Crystal Structure of Titanite 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. Outline 1 Chemical composition 2 Physical properties 3 Optical properties 4 Crystal structure 5 Titanite stability 6 Occurrences 7 Uses Crystal structure of titanite Ca polyhedron Ti octahedron Si tetrahedron O atoms 1 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- >> Ti4+ + (OH-, F -) Notice that the above charges balance. This particular substitution may occur in up to 30% of cations in titanite. 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 o CaTiO(SiO4) and SnTiO(SiO4) at 700 C and 7kbar. SnTiO(SiO4) is the chemical composition of malayaite. Malayaite and the high temperature form of titanite are they are isostructural. Physical properties 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 Specific gravity: 3.4 - 3.55 2 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. • aa: 1.900 • bb: 1.907 • gg: 2.032 Optical data • dd: 0.14 • 2V = 27o • Optically positive Crystal structure of Titanite Ca polyhedron Ti octahedron Si tetrahedron O atoms 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 3 Thermodynamic stability of Titanite Theoretical stability at 1200 K and 25 kbar Si – O bond Ti – O bond Ca – O bond length length length At 25 kbar Decrease by Decrease by Decrease by 0.003 Å 0.008 Å 0.015 Å At 1200K Increase by Increase by Increase by 0.004 Å 0.017 Å 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). o A space group transition occurs at ~220 C: P2 1/a A2/a 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 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. 4 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... 5.
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