Fluor-uvite

Fluor-uvite is a tourmaline mineral with the chemical formula CaMg₃(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₃F. It is a rare mineral that is found in calcium rich contact metamorphic rocks with increased amounts of boron. Uvite is trigonal hexagonal, which means that it has three equal length axes at 120 degrees, all perpendicular to its fourth axis which has a different length. Uvite is part of the space group 3m. Uvite's hardness has been measured to be 7.5 on the Mohs hardness scale. The color of uvite widely varies, depending on the sample, but is mostly deep green or brown. In regard to uvite's optical properties, it is uniaxial (-) and anisotropic, meaning that the velocity of light in the mineral depends on the path that it takes. In plane polarized light, uvite is colorless to pale yellow and shows weak pleochroism.

Fluor-uvite
Uvite on quartz
General
Category	Cyclosilicate tourmaline group
Formula (repeating unit)	CaMg3(Al5Mg)(Si6O18)(BO3)3(OH)3F
IMA symbol	Fluvt
Strunz classification	9.CK.05
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	R3m
Identification
Color	Black, greenish-black, brownish-black, brown, green, colorless
Cleavage	None Observed
Fracture	Irregular/Uneven, Conchoidal
Tenacity	Brittle
Mohs scale hardness	7.5
Luster	Vitreous, Resinous
Streak	Light-brown, light-green, or white
Diaphaneity	Transparent, Translucent
Density	2.97 - 3.14 g/cm3
Optical properties	Uniaxial (-)
Birefringence	δ = 0.018 - 0.029
Pleochroism	Weak
References

Uvite was first found in 1929 in Uva Province, Sri Lanka, hence the name.

Uvite has no use, but is commonly found in mineral specimen collections. The mineral is sought after by collectors because of its pronounced colors, crystal structure, and often large crystal size.