NSUTITE
Nsutite – (Mn4+,Mn2+)(O,OH)2 – is a very common oxide found in manganese ores. It occurs in most large Mn deposits; however, its identification is problematic – often leading to it going undetected – and, consequently, there are only sparse references to it in the literature.
It is classified within the Ramsdellite Group and was, for many decades, mistaken for intimately intergrown pyrolusite.
Nsutite can only be identified via X-ray diffraction (XRD). Due to the minute size of its grains, identification under a microscope is not possible.
1. Characteristics
Crystal system: Hexagonal.
Color: Gray-black.
Habit: Nsutite typically exhibits a massive or dense habit, though it may also be porous, fine-grained, or coarse-grained. Its crystals can be tabular or wedge-shaped, reaching up to 50 microns in length. It is rarely fibrous – with fibers reaching up to 2 mm – and can also appear in spherulitic, radiating, or colloform forms; when coarse-grained, it frequently displays desiccation (shrinkage) fractures.
Cleavage: No.
Tenacity: No information available.
Twinning: No information available.
Fracture: No information available.
Mohs Hardness: 6.5 – 8
Parting: No information available.
Streak: Gray-black.
Lustre: Metallic to earthy.
Diaphaneity: Opaque.
Density (g/cm³): 3.86 – 4.24
2. Geology and Deposits
Nsutite is a mineral of supergenic origin, formed under alkaline oxidizing conditions and deficiency in Ba and K.
It is generally formed by the oxidation of rhodochrosite and other Mn carbonates. It is much more common than ramsdellite, which is similar.
3. Mineral Associations
It occurs associated with quartz and goethite.
With phosphates such as strengite, torbernite, cacoxenite, beraunite, phosphosiderite, and paulkerrite.
It is naturally associated with other Mn-bearing minerals, such as rhodochrosite, pyrolusite, romanechite, chalcophanite, todorokite, cryptomelane, and akhtenskite.
4. Transmitted Light Microscopy
Not applicable, as nsutite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Nsutite takes a good polish slowly; it requires more time and patience. Its polishing hardness is greater than that of pyrolusite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White with a subtle cream or brownish tint.
Compared to the color of pyrolusite, the color of nsutite is slightly grayer and lighter.
Pleochroism: Grayish-white with a cream to bluish-gray-white tint.
Reflectivity: 25 – 35%. Nsutite-Mn is much darker than nsutite.
Bireflectance: Distinct.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Distinct to strong anisotropy between white-gray and dark gray. Cryptocrystalline aggregates are isotropic.
Internal reflections: No.
May be confused with: Conclusive identification of nsutite can only be obtained through X-ray diffraction.
General Characteristics:
Grain shape: occurs as cryptocrystalline aggregates or as coarse granular crystals, similar to pyrolusite grains. It may form radial aggregates of fine fibrous crystals.
Desiccation fractures can occur in fine-grained colloform aggregates. These result from the transition of nsutite-Mn to nsutite.
Alternating layers of nsutite with cryptomelane, pyrolusite, and birnessite may form.
