Titanite

Titanite (Sphene)
Titanite crystals on amphibole (image width 2 mm)
General
Category	Nesosilicate
Formula (repeating unit)	CaTiSiO5
IMA symbol	Ttn[1]
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, brown, gray, black, yellow, green, or red, colourless
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	5–5.5
Luster	Sub-adamantine tending to slightly resinous
Streak	Reddish white
Diaphaneity	Translucent to transparent
Specific gravity	3.48–3.60
Optical properties	Biaxial (+); very high relief
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	17–40° (measured)
Dispersion	r > v strong
Other characteristics	Radioactive – may be metamict
References	[2][3][4][5]

Titanite, or sphene (from Ancient Greek σφηνώ (sphēnṓ) 'wedge'),^[5] 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.^[6]