Humite

Humite
Humite specimen, Italy
General
Category	Nesosilicates
Formula (repeating unit)	(Mg,Fe)7(SiO4)3(F,OH)2.
Strunz classification	9.AF.50
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H–M Symbol: (2/m 2/m 2/m)
Space group	Pbnm
Unit cell	a = 10.24, b = 20.72 c = 4.73 [Å]; Z = 4
Identification
Color	White, yellow, dark orange, brown
Cleavage	Poor
Fracture	Uneven to subconchoidal
Tenacity	Brittle
Mohs scale hardness	6-6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	3.20 – 3.32
Optical properties	Biaxial (+)
Refractive index	nα =1.607-1.643, nβ=1.619-1.653, nγ=1.639-1.675
Birefringence	0.0320
2V angle	Calculated = 70-78°, measured = 68-81°
Dispersion	Weak, r > v
References

Humite is a mineral found in the volcanically ejected masses of Vesuvius. It was first described in 1813 and named for Abraham Hume (1749–1838).