Chemistry:Norbergite

Norbergite
Norbergite crystals on calcite from Oakssaung Hill, Mogok, Mandalay, Burma (size: 4.2 x 3.3 x 2.3 cm)
General
Category	Nesosilicates
Formula (repeating unit)	Mg3(SiO4)(F,OH)2
Strunz classification	9.AF.40
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 8.747(6), b = 4.710(4) c = 10.271(8) [Å]; Z = 4
Identification
Formula mass	202.00 g/mol
Color	Tan, yellow, yellow-orange, orange-brown, pink with purplish tint, white
Crystal habit	Granular, tabular crystals rare
Fracture	Uneven to subconchoidal
Tenacity	Brittle
Mohs scale hardness	6–6.5
|re|er}}	Vitreous to resinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.177
Optical properties	Biaxial (+)
Refractive index	nα = 1.563 nβ = 1.567 nγ = 1.590
Birefringence	δ = 0.027
Pleochroism	Pleochroism: X = pale yellow; Y = very pale yellow; Z = colorless
2V angle	44 – 50°
Other characteristics	May fluoresce canary-yellow under SW UV
References

Norbergite is a nesosilicate mineral with formula Mg₃(SiO₄)(F,OH)₂. It is a member of the humite group.

It was first described in 1926 for an occurrence in the Östanmoss iron mine in Norberg, Västmanland, Sweden, for which it is named. It occurs in contact metamorphic zones in carbonate rocks intruded by plutonic rocks or pegmatites supplying the fluorine. Associated minerals include dolomite, calcite, tremolite, grossular, wollastonite, forsterite, monticellite, cuspidine, fluoborite, ludwigite, fluorite and phlogopite.