Minnesotaite

(repeating unit)(Fe²⁺,Mg)₃Si₄O₁₀(OH)₂IMA symbolMns^[1]Strunz classification9.EC.05Crystal systemTriclinicCrystal classPinacoidal (1)
(same H-M symbol)Space groupP1Unit cella = 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 = 2IdentificationColorGreenish gray to olive-greenCrystal habitOccurs as microscopic needles or platelets, the needles occur in radiating clusters or in sheaves; also fibrousTwinningInferred based on X-ray patternsCleavagePerfect on {001}, micaceousFractureUneven and irregularTenacityBrittleMohs scale hardness1.5 - 2LusterGreasy to waxy, dullDiaphaneityTranslucentSpecific gravity3.01Optical propertiesBiaxial (-)Refractive indexn_α = 1.578 - 1.583 n_β = 1.578 - 1.622 n_γ = 1.615 - 1.623Birefringenceδ = 0.037 - 0.040PleochroismX= pale green, Z= colorless to pale greenish yellow2V angleMeasured: 4°Dispersionr < v moderateReferences^[2][3][4]

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.^[2] It is isostructural with the pyrophyllite-talc mineral group.^[3]

Occurrence

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.^[3][5]

It occurs associated with quartz, siderite, stilpnomelane, greenalite and magnetite.^[2][6] In addition to the low grade metamorphic banded iron formations it has also been reported as an alteration mineral associated with sulfide bearing veins.^[3]

References