cronstedtite

{{Infobox mineral | name = Cronstedtite | boxwidth = | boxbgcolor = | image = Cronstedite France.jpg | imagesize = 260px | alt = | caption = Cronstedtite – Salsigne Mine – France | category = Phyllosilicate minerals | group = Kaolinite-Serpentine group, serpentine subgroup | formula = | IMAsymbol=Cro | molweight = | strunz = 9.ED.15 | dana = | system = Trigonal | class = Ditrigonal pyramidal (3&nbsp;m) (same H-M symbol) | symmetry = P31m (no. 157) | unit cell = a = 5.486&nbsp;Å, c = 7.095&nbsp;Å; Z&nbsp;=&nbsp;1 | color = Black, dark brown-black, green-black | colour = | habit = | twinning = | cleavage = Perfect on {001} | fracture = | tenacity = Elastic | mohs = | luster = Sub-Metallic | streak = Dark olive green | diaphaneity = Translucent | gravity = 3.34 – 3.35 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.720 nβ = 1.800 nγ = 1.800 | birefringence = δ = 0.080 | pleochroism = Visible | 2V = | dispersion = r < v moderate | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }}

Cronstedtite is a complex iron silicate mineral belonging to the serpentine group of minerals. Its chemical formula is .