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 (repeating unit) | (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]