Rutile

Rutile
Wine-red rutile crystals from Binn Valley in Switzerland (Size: 2.0 x 1.6 x 0.8 cm)
General
Category	Oxide minerals
Formula (repeating unit)	TiO2
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, c = 2.9587 [Å]; Z = 2
Identification
Color	Reddish brown, red, pale yellow, pale blue, violet, rarely grass-green; 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 submetallic
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 (589nm)
Birefringence	0.296 (589nm)
Pleochroism	Weak to distinct brownish red-green-yellow
Dispersion	strong
Fusibility	Fusible in alkali carbonates
Solubility	Insoluble in acids
Common impurities	Fe, Nb, Ta
References

Acicular to Prismatic crystals, elongated and

Rutile is a mineral composed primarily of titanium dioxide, TiO₂.

Rutile is the most common natural form of TiO₂. Three rarer polymorphs of TiO₂ are known:

Rutile has among 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 4.5μm.

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 is a common accessory mineral in high-temperature and high-pressure metamorphic rocks and in igneous rocks.

Thermodynamically, rutile is the most stable polymorph of TiO₂ at all temperatures, exhibiting lower total free energy than metastable phases of anatase or brookite. Consequently, the transformation of the metastable TiO₂ polymorphs to rutile is irreversible. As it has the lowest molecular volume of the three main polymorphs; it is generally the primary titanium bearing phase in most high-pressure metamorphic rocks, chiefly eclogites.