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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
Environments
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).
Localities
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 fayalite →
greenalite → minnesotaite →
quartz 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).
Alteration
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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