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Category Silicate mineral
(repeating unit)
Strunz classification 9.EC.05
Crystal system Triclinic
Crystal class Pinacoidal (1)
(same H-M symbol)
Space group P1
Unit cell a = 5.623(2) Å,
b = 9.419(2) Å,
c = 9.624(3) Å;
α = 85.21(3)°, β = 95.64(3)°,
γ = 90.00°; Z = 2
Color Greenish gray to olive-green
Crystal habit Occurs as microscopic needles or platelets, the needles occur in radiating clusters or in sheaves; also fibrous
Twinning Inferred based on X-ray patterns
Cleavage Perfect on {001}, micaceous
Fracture Uneven and irregular
Tenacity Brittle
Mohs scale hardness 1.5 - 2
Luster Greasy to waxy, dull
Diaphaneity Translucent
Specific gravity 3.01
Optical properties Biaxial (-)
Refractive index nα = 1.578 - 1.583 nβ = 1.578 - 1.622 nγ = 1.615 - 1.623
Birefringence δ = 0.037 - 0.040
Pleochroism X= pale green, Z= colorless to pale greenish yellow
2V angle Measured: 4°
Dispersion r < v moderate
References [1][2][3]

Minnesotaite is an iron silicate mineral with formula: (Fe2+,Mg)3Si4O10(OH)2. It crystallizes in the triclinic crystal system and occurs as fine needles and platelets with other silicates.[1] It is isostructural with the pyrophyllite-talc mineral group.[2]


Minnesotaite was first described in 1944 for occurrences in the banded iron formations of northern Minnesota for which it was named. Co-type localities are in the Cuyuna North Range, Crow Wing County and the Mesabi Range in St. Louis County.[2][4]

It occurs associated with quartz, siderite, stilpnomelane, greenalite and magnetite.[1][5] In addition to the low grade metamorphic banded iron formations it has also been reported as an alteration mineral associated with sulfide bearing veins.[2]


  1. ^ a b c Handbook of Mineralogy
  2. ^ a b c d Minnesotaite on Mindat.org
  3. ^ Minnesotaite on Webmineral
  4. ^ Gruner, John (1946). The Mineralogy and Geology of the Taconites and Iron Ores of the Mesabi Range, Minnesota. Office of the Commissioner of the Iron Range Resources and Rehabilitation. p. 8,38. 
  5. ^ Gruner, John W., The Composition and Structure of Minnesotaite, a Common Iron Silicate in Iron Formations, American Mineralogist, 1944, 29, 363-372