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Category Silicate mineral
(repeating unit)
Strunz classification 9.EC.05
Crystal symmetry Triclinic pinacoidal
H-M symbol: (1)
Space group: C1
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
Crystal system Triclinic
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]

It occurs associated with quartz, siderite, stilpnomelane, greenalite and magnetite.[1][4] 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]