Cronstedtite

(repeating unit)Fe²⁺
₂Fe³⁺
(Si,Fe³⁺
O
₅)(OH)
₄IMA symbolCro^[1]Strunz classification9.ED.15Crystal systemTrigonalCrystal classDitrigonal pyramidal (3 m)
(same H-M symbol)Space groupP31m (no. 157)Unit cella = 5.486 Å,
c = 7.095 Å; Z = 1^[2]IdentificationColorBlack, dark brown-black, green-blackCleavagePerfect on {001}TenacityElasticLusterSub-MetallicStreakDark olive greenDiaphaneityTranslucentSpecific gravity3.34 – 3.35Optical propertiesBiaxial (−)Refractive indexn_α = 1.720 n_β = 1.800 n_γ = 1.800Birefringenceδ = 0.080PleochroismVisibleDispersionr < v moderateReferences^[2][3][4]

Cronstedtite is a complex iron silicate mineral belonging to the serpentine group of minerals. Its chemical formula is Fe²⁺
₂Fe³⁺
(Si,Fe³⁺
O
₅)(OH)
₄.

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.^[5]

See also

• List of minerals
• List of minerals named after people

References

MicasTalcsPyrophyllite seriesKaolinitesSerpentinesCorrensitesSmectites and vermiculite familyChloritesAllophanesSepiolitesPyrosmalitesStilpnomelanes