Richterite

Richterite is a sodium calcium magnesium silicate mineral belonging to the amphibole group. If iron replaces the magnesium within the structure of the mineral, it is called ferrorichterite; if fluorine replaces the hydroxyl, it is called fluororichterite. Richterite crystals are long and prismatic, or prismatic to fibrous aggregate, or rock-bound crystals. Colors of richterite range from brown, grayish-brown, yellow, brownish- to rose-red, or pale to dark green. Richterite occurs in thermally metamorphosed limestones in contact metamorphic zones. It also occurs as a hydrothermal product in mafic igneous rocks, and in manganese-rich ore deposits. Localities include Mont-Saint-Hilaire, Quebec, and Wilberforce and Tory Hill, Ontario, Canada; Långban and Pajsberg, Sweden; West Kimberley, Western Australia; Sanka, Myanmar; and, in the US, at Iron Hill, Colorado; Leucite Hills, Wyoming; and Libby, Montana. The mineral was named in 1865 for the German mineralogist Hieronymous Theodor Richter (1824–1898).

Richterite
Richterite. Wilberforce, Monmouth Township, Haliburton County, Ontario, Canada.
General
Category	Inosilicates
Formula (repeating unit)	Na(NaCa)Mg5Si8O22(OH)2
Strunz classification	9.DE.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Brown, yellow, red, or green
Crystal habit	Prismatic; acicular or asbestiform
Twinning	Simple or multiple parallel to {100}
Cleavage	Perfect
Fracture	Uneven, brittle
Mohs scale hardness	5-6
Luster	Vitreous
Streak	Pale yellow
Diaphaneity	Transparent to translucent
Specific gravity	3.0-3.5
Optical properties	Biaxial (-)
Refractive index	nα = 1.615 nβ = 1.629 nγ = 1.636
Birefringence	δ = 0.021
Pleochroism	Strong: pale yellow, orange, and red
2V angle	68° measured
References	[1][2][3][4]