Minnesotaite

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]

Minnesotaite
Minnesotaite
General
Category	Silicate mineral
Formula	(Fe2+,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]

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]