Minnesotaite

Minnesotaite
Minnesotaite
General
Category	Silicate mineral
Formula (repeating unit)	(Fe^2+,Mg)3Si4O10(OH)2
IMA symbol	Mns[1]
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
Identification
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	[2][3][4]

Minnesotaite is an iron silicate mineral with formula: (Fe^2+,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. Handbook of Mineralogy
3. 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