Rutile

Rutile
General
Category	Oxide minerals
Formula (repeating unit)	TiO2
IMA symbol	Rt[1]
Strunz classification	4.DB.05
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	P42/mnm
Unit cell	a = 4.5937 Å, c = 2.9587 Å; Z = 2
Identification
Color	Brown, reddish brown, blood red, red, brownish yellow, pale yellow, yellow, pale blue, violet, rarely grass-green, grayish black; black if high in Nb–Ta
Crystal habit	Acicular to Prismatic crystals, elongated and striated parallel to [001]
Twinning	Common on {011}, or {031}; as contact twins with two, six, or eight individuals, cyclic, polysynthetic
Cleavage	{110} good, {100} moderate, parting on {092} and {011}
Fracture	Uneven to sub-conchoidal
Mohs scale hardness	6.0–6.5
Luster	Adamantine to metallic
Streak	Bright red to dark red
Diaphaneity	Opaque, transparent in thin fragments
Specific gravity	4.23 increasing with Nb–Ta content
Optical properties	Uniaxial (+)
Refractive index	nω = 2.613, nε = 2.909 (589 nm)
Birefringence	0.296 (589 nm)
Pleochroism	Weak to distinct brownish red-green-yellow
Dispersion	Strong
Fusibility	Fusible in alkali carbonates
Solubility	Insoluble in acids
Common impurities	Fe, Nb, Ta
Other characteristics	Strongly anisotropic
References	[2][3][4][5]

Rutile is an oxide mineral composed of titanium dioxide (TiO₂), the most common natural form of TiO₂. Rarer polymorphs of TiO₂ are known, including anatase, akaogiite, and brookite.

Rutile has one of the highest refractive indices at visible wavelengths of any known crystal and also exhibits a particularly large birefringence and high dispersion. Owing to these properties, it is useful for the manufacture of certain optical elements, especially polarization optics, for longer visible and infrared wavelengths up to about 4.5 micrometres. Natural rutile may contain up to 10% iron and significant amounts of niobium and tantalum.

Rutile derives its name from the Latin rutilus ('red'), in reference to the deep red color observed in some specimens when viewed by transmitted light. Rutile was first described in 1803 by Abraham Gottlob Werner using specimens obtained in Horcajuelo de la Sierra, Madrid (Spain),^[6] which is consequently the type locality.