Titanite

Titanite (Sphene)
Titanite from Capelinha, Brazil
General
Category	nesosilicate
Chemical formula	CaTiSiO5
Identification
Colour	Reddish brown, gray, yellow, green, or red
Crystal habit	Flattened wedge-shaped crystals, also massive
Crystal system	Monoclinic - prismatic
Twinning	Contact and penetration on [100], lamellar on [112]
Cleavage	Distinct on [110] imperfect on [100] and [112]
Fracture	Sub-conchoidal
Mohs scale hardness	5 to 5.5
Lustre	Subadamantine tending to slightly resinous
Streak	Reddish white
Diaphaneity	Translucent to transparent
Specific gravity	3.48 to 3.60
Optical properties	Biaxial (+)
Refractive index	nα = 1.843 - 1.950 nβ = 1.870 - 2.034 nγ = 1.943 - 2.110
Birefringence	δ = 0.100 - 0.160
Pleochroism	strong
Other characteristics	Radioactive - may be metamict
References	[1][2][3][4]

Titanite

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.^[5]

Nomenclature

The International Mineralogical Association Commission on New Minerals and Mineral Names (CNMMN) adopted the name titanite and 'discredited' the name sphene^[6] as of 1982^[7], although commonly papers and books initially identify the mineral using both names^[8][9]. Sphene was the most commonly used name until the IMA decision,^{[citation needed]} although both were well known^[4]. Some authorities^[who?] think it is less confusing as the word is used to describe any chemical or crystal with oxidized titanium such as the rare earth titanate pyrochlores series^[10] and many of the minerals with the perovskite structure^[11]. The name sphene continues to be publishable in peer-reviewed scientific literature, e.g. a paper by Watson et al. was published in early 2008 in the journal Contributions to Mineralogy and Petrology.^{[citation needed]} Sphene persists as the informal name for titanite gemstones.

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. Associated with this, under the petrological microscope one often sees pleochroic or metamict halos in minerals around 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, Mane, Gouverneur, Diana, Rossie, Fine, Pitcairn, Brewster, New York ^[4] and California in the USA .

Uses

Titanite is a source of titanium dioxide, TiO₂, used in pigments.

As a gemstone, titanite is usually some shade of chartreuse and is prized for its exceptional dispersive power (0.051, B to G interval) which exceeds that of diamond.^{[citation needed]}

References

Wikimedia Commons has media related to: Titanite