Cronstedtite

Cronstedtite is a complex iron silicate mineral belonging to the serpentine group of minerals. It has a formula of Fe²⁺₂Fe³⁺(Si,Fe³⁺O₅)(OH)₄.

Cronstedtite
Cronstedtite - Salsigne Mine - France
General
Category	Silicate minerals
Formula (repeating unit)	Fe2+2Fe3+(Si,Fe3+O5)(OH)4
Strunz classification	9.ED.15
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) (same H-M symbol)
Space group	P31m
Unit cell	a = 5.486 Å, c = 7.095 Å; Z = 1[1]
Identification
Color	Black, dark brown-black, green-black
Cleavage	Perfect on {001}
Tenacity	Elastic
Luster	Sub-Metallic
Streak	Dark olive green
Diaphaneity	Translucent
Specific gravity	3.34 - 3.35
Optical properties	Biaxial (-)
Refractive index	nα = 1.720 nβ = 1.800 nγ = 1.800
Birefringence	δ = 0.080
Pleochroism	Visible
Dispersion	r < v moderate
References	[1][2][3]

It was discovered in 1821 and named in honor of Swedish mineralogist Axel Fredrik Cronstedt (1722–1765). It has been found in Bohemia in the Czech Republic and in Cornwall, England.

Cronstedtite is a major constituent of CM chondrites, a carbonaceous chondrite group exhibiting varying degrees of aqueous alteration. Cronstedtite abundance decreases with increasing alteration.[4]

Cronstedtite- Cornwall, England