Titanite [CaTi(SiO₄)O]

Structure Monoclinic
Space Group : P121/a1 (No. 14)
a=7.069Å, b=8.722Å, c=6.566Å
a=90.00, b=113.86, g=90.00
Z=4

Atomic Positional Parameters
Ca1 4e 0.2424 0.9184 0.7512
Ti1 4e 0.5134 0.7542 0.2495
Si1 4e 0.7486 0.9330 0.7490
O1  4e 0.7499 0.8202 0.2502
O2  4e 0.9108 0.8162 0.9347

O3  4e 0.3827 0.9608 0.1459
O4  4e 0.9122 0.3165 0.4368
O5  4e 0.3813 0.4601 0.6468

Reference
JA Speer and GV Gibbs (1976) American Mineralogist, 61, 238.

Mineral Chemistry
Titanite, or sphene, is a common accessory mineral of granites, granodiorites, diorites, and (nepheline) syenites. It may also be found in metamorphic gneisses, chlorite schists, and crystalline limestone. In titanite, the SiO₄ groups are independent, and are linked by titanium atoms octahedrally coordinated by six oxygens (TiO₆), and by calcium atoms coordinated by seven oxygens (CaO₇). This mineral can accommodate many elements other than calcium, titanium, silicon and oxygen. Calcium may be replaced by strontium, barium, by rare earth elements or thorium; titanium may be substituted for by aluminium or iron, and oxygen by hydroxide, fluoride or chloride.

Malayaite (CaSnSiO₅) and vanadomalayaite (CaVOSiO₄) are tin and vanadium analogs of titanite.

The CHIME figure shows calcium ions as large blue spheres to make clearer the seven-fold coordination of these ions with oxygen. The monoclinic unit cell is outlined.