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Cronstedtite
Cronstedtite – Salsigne Mine – France
General
Category	Silicate minerals
Formula (repeating unit)	Fe²⁺ ₂Fe³⁺ (Si,Fe³⁺ O ₅)(OH) ₄
IMA symbol	Cro^[1]
Strunz classification	9.ED.15
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3 m) (same H-M symbol)
Space group	P31m (no. 157)
Unit cell	a = 5.486 Å, c = 7.095 Å; Z = 1^[2]
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	n_α = 1.720 n_β = 1.800 n_γ = 1.800
Birefringence	δ = 0.080
Pleochroism	Visible
Dispersion	r < v moderate
References	^[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]

References

Wikimedia Commons has media related to Cronstedtite.

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ ^a ^b Webmineral.com
^ Mindat.org
^ American Mineralogical Society
^ Browning et al. (1996) Geochimica et Cosmochimica Acta

v t e Phyllosilicates
Micas	Aluminoceladonite Anandite Annite Aspidolite Balestraite Bityite Biotite Boromuscovite Bramallite Celadonite Chernykhite Chromphyllite Clintonite Eastonite Ephesite Ferroaluminoceladonite Ferroceladonite Fluorannite Fluorokinoshitalite Fluorophlogopite Fluorotetraferriphlogopite Garmite Glauconite Gorbunovite Hendricksite Illite Kinoshitalite Lepidolite Luanshiweiite Manganiceladonite Margarite Masutomilite Montdorite Muscovite Nanpingite Norrishite Orlovite Oxyphlogopite Paragonite Phengite Phlogopite Polylithionite Preiswerkite Roscoelite Siderophyllite Suhailite Tainiolite Tetraferriannite Trilithionite Voloshinite Wonesite Yangzhumingite
Talcs	Minnesotaite Talc Willemseite
Pyrophyllite series	Ferripyrophyllite Pyrophyllite
Kaolinites	Bismutoferrite Dickite Halloysite Hisingerite Kaolinite Nacrite
Serpentines	Amesite Antigorite Berthierine Brindleyite Caryopilite Chrysotile Cronstedtite Fraipontite Greenalite Guidottiite Kellyite Lizardite Manandonite Népouite Odinite Pecoraite
Corrensites	Aliettite Brinrobertsite Corrensite Dozyite Hydrobiotite Karpinskite Kulkeite Lunijianlaite Rectorite Saliotite Tosudite
Smectites and vermiculite family	Beidellite Ferrosaponite Montmorillonite Nontronite Saponite Sauconite Swinefordite Vermiculite Volkonskoite Yakhontovite
Chlorites	Baileychlore Borocookeite Chamosite Clinochlore Cookeite Donbassite Gonyerite Nimite Pennantite Sudoite
Allophanes	Allophane Chrysocolla Imogolite Neotocite
Sepiolites	Falcondoite Ferrisepiolite Sepiolite
Pyrosmalites	Friedelite Mcgillite Nelenite Pyrosmalite Schallerite
Stilpnomelanes	Bannisterite Franklinphilite Lennilenapeite Parsettensite Stilpnomelane
Structural groups mainly; based on rruff.info/ima, modified Minerals portal

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References