or sphene (from the Greek sphenos (σφηνώ), meaning wedge), 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.
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.
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. Source localities include:
Pakistan; Italy; Russia; China; Brazil; Tujetsch, St. Gothard,
Switzerland; Madagascar; Tyrol, Austria; Renfrew County, Ontario,
Canada; Sanford, Maine, Gouverneur, Diana, Rossie, Fine, Pitcairn,
Brewster, New York and California in the US.