Formula: Fe2+3Si4O10(OH)2
Phyllosilicate (sheet silicate), pyrophyllite-talc group
Crystal System: Triclinic
Specific gravity: 3.01
Hardness: 1½ to 2
Streak: White
Colour: Greenish grey to olive green
Common impurities: Ti,Al,Mn,Ca,Na,K,H2O

Metamorphic environments
Hydrothermal environments

Minnesotaite occurs in low grade metamorphosed banded iron formations, and also as hydrothermal alteration around sulphide veins (Mindat). Associated minerals include quartz, stilpnomelane and siderite (Dana).


Howells River area, Labrador, Newfoundland and Labrador, Canada. The Formation at the western edge of the Labrador Trough is part of a relatively undisturbed sedimentary sequence. The finely banded silicate-carbonate members of this iron formation were studied, and in most of the assemblages early diagenetic features are overprinted by later ones which are the result of very low-grade metamorphic reactions. Of the three Fe-silicates, greenalite, stilpnomelane, and minnesotaite, greenalite appears to be of very early origin (sedimentary to diagenetic), stilpnomelane appears to be of diagenetic origin, and minnesotaite is generally concluded to be of very low-grade metamorphic origin. Minnesotaite, in well-crystallized rosettes and sprays, replaces earlier greenalite and stilpnomelane as well as much of the associated carbonates. Carbonate compositions are represented by members of the dolomite-ankerite series, siderite, and calcite. Quartz or chert occur throughout all of the assemblages. Minnesotaite is a reaction product of earlier silicate and carbonate-chert assemblages (CM 12.475-498).

There are three co-type localities, the Cuyuna North Range, Crow Wing county, the Mesabi Range, St. Louis county and Drill Core M4450, Mesabi Range, St. Louis county, all in Minnesota, USA.
Minnesotaite occurs in banded iron formations subjected to low-grade regional metamorphism. The most common associates are quartz, siderite, stilpnomelane, greenalite and magnetite. Any two, or three, or all five may be found together. Minnesotaite occurs only in microscopic plates or needles, the latter radiated or in sheaves. Sometimes it is so abundant that it may comprise 90% of the ore (AM 29.363-372).
Minnesotaite from Minnesota - Image

At the Overlook Mine, Cooke Mountain, Belcher Mining District, Ferry county, Washington, USA, a fayalite-bearing assemblage of iron silicates, magnetite and pyrrhotite, plus quartz and calcite, formed during alteration of a volcanogenic, carbonate-rich, massive magnetite-sulphide seafloor deposit. The temperature of the fayalite-forming environment was approximately 300oC. The fayalite is close to end-member composition, and the greenalite and minnesotaite are correspondingly Fe-rich. The paragenetic sequence fayalitegreenaliteminnesotaitequartz represents infiltration at approximately 300oC of an initially very reducing, Fe-rich hydrothermal fluid that became progressively more rich in SiO2 (CM 36.147-162).


greenalite and SiO2 to minnesotaite and H2O
Fe3Si2O5(OH)4 + 2SiO2 → Fe2+3Si4O10(OH)2 + H2O
CM 36.157).

greenalite and minnesotaite to grunerite and H2O
2Fe3Si2O5(OH)4 + 5Fe2+3Si4O10(OH)2 → 3Fe2+2Fe2+5Si8O22(OH)2 + 6H2O
CM 36.157).

minnesotaite to grunerite, SiO2 and H2O
7Fe2+3Si4O10(OH)2 → 3Fe2+2Fe2+5Si8O22(OH)2 + 4SiO2 + 4H2O
CM 36.157).

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