Titanite, or sphene (from the Greek sphenos (σφηνώ), meaning wedge[4]), is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals including cerium and yttrium; calcium may be partly replaced by thorium.

Physical properties

Titanite, which is named for its titanium content, occurs as translucent to transparent, reddish brown, gray, yellow, green, or red monoclinic crystals. These crystals are typically sphenoid in habit and are often twinned. Possessing a subadamantine tending to slightly resinous lustre, titanite has a hardness of 5.5 and a weak cleavage. Its specific gravity varies between 3.52 and 3.54. Titanite's refractive index is 1.885-1.990 to 1.915-2.050 with a strong birefringence of 0.105 to 0.135 (biaxial positive) ; under the microscope this leads to a distinctive high relief which combined with the common yellow-brown colour and lozenge-shape cross-section makes the mineral easy to identify. Transparent specimens are noted for their strong trichroism, the three colours presented being dependent on body colour. Owing to the quenching effect of iron, sphene exhibits no fluorescence under ultraviolet light. Some titanite has been found to be metamict, in consequence of structural damage due to radioactive decomposition of the often significant thorium content. When viewed in thin section with a petrographic microscope, pleochroic halos can be observed in minerals surrounding a titanite crystal.

Occurrence

Titanite occurs as a common accessory mineral in intermediate and felsic igneous rocks and associated pegmatites. It also occurs in metamorphic rocks such as gneiss and schists and skarns.[1] Source localities include: Pakistan; Italy; Russia; China; Brazil; Tujetsch, St. Gothard, Switzerland;[4] Madagascar; Tyrol, Austria; Renfrew County, Ontario, Canada; Sanford, Maine, Gouverneur, Diana, Rossie, Fine, Pitcairn, Brewster, New York and California in the US.

SOURCE