Titanite

Titanite (Sphene)
Titanite crystals on amphibole (image width 2 mm)
General
Category	nesosilicate
Formula (repeating unit)	CaTiSiO5
Strunz classification	9.AG.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Unit cell	a = 7.057 Å, b = 8.707 Å c = 6.555 Å; β = 113.81°; Z = 4
Identification
Colour	Reddish brown, gray, yellow, green, or red
Crystal habit	Flattened wedge-shaped crystals, also massive
Twinning	Contact and penetration on {100}, lamellar on {221}
Cleavage	Distinct on [110], parting on {221}
Fracture	Sub-conchoidal
Mohs scale hardness	5 to 5.5
Luster	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: X = nearly colorless; Y = yellow to green; Z = red to yellow-orange
2V angle	17 to 40° (measured)
Dispersion	r > v strong
Other characteristics	Radioactive - may be metamict
References

Titanite, 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.

The International Mineralogical Association Commission on New Minerals and Mineral Names (CNMMN) adopted the name titanite and 'discredited' the name sphene as of 1982, although commonly papers and books initially identify the mineral using both names. Sphene was the most commonly used name until the IMA decision, although both were well known. Some authorities 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 and many of the minerals with the perovskite structure. The name sphene continues to be publishable in peer-reviewed scientific literature, e.g. a paper by Hayden et al. was published in early 2008 in the journal Contributions to Mineralogy and Petrology. Sphene persists as the informal name for titanite gemstones.

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 luster, 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.