| Minnesotaite | |
| Category: | Silicate mineral |
| Formula: | (Fe2+,Mg)3Si4O10(OH)2 |
| Imasymbol: | Mns[1] |
| Strunz: | 9.EC.05 |
| System: | Triclinic |
| Class: | Pinacoidal (same H-M symbol) |
| Symmetry: | P |
| 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 |
| Color: | Greenish gray to olive-green |
| 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, micaceous |
| Fracture: | Uneven and irregular |
| Tenacity: | Brittle |
| Mohs: | 1.5 - 2 |
| Luster: | Greasy to waxy, dull |
| Diaphaneity: | Translucent |
| Gravity: | 3.01 |
| Opticalprop: | Biaxial (-) |
| Refractive: | 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: | Measured: 4° |
| Dispersion: | r < 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]
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]