Minnesotaite

Minnesotaite
Minnesotaite
General
CategorySilicate mineral
Formula
(repeating unit)		(Fe2+,Mg)3Si4O10(OH)2
IMA symbolMns[1]
Strunz classification9.EC.05
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit 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 = 2
Identification
ColorGreenish gray to olive-green
Crystal habitOccurs as microscopic needles or platelets, the needles occur in radiating clusters or in sheaves; also fibrous
TwinningInferred based on X-ray patterns
CleavagePerfect on {001}, micaceous
FractureUneven and irregular
TenacityBrittle
Mohs scale hardness1.5 - 2
LusterGreasy to waxy, dull
DiaphaneityTranslucent
Specific gravity3.01
Optical propertiesBiaxial (-)
Refractive indexnα = 1.578 - 1.583 nβ = 1.578 - 1.622 nγ = 1.615 - 1.623
Birefringenceδ = 0.037 - 0.040
PleochroismX= pale green, Z= colorless to pale greenish yellow
2V angleMeasured: 4°
Dispersionr < v moderate
References[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

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c Handbook of Mineralogy
  3. ^ a b c d Minnesotaite on Mindat.org
  4. ^ Minnesotaite on Webmineral
  5. ^ 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.
  6. ^ Gruner, John W., The Composition and Structure of Minnesotaite, a Common Iron Silicate in Iron Formations, American Mineralogist, 1944, 29, 363-372