BIRNESSITE
Birnessite – (Na,Ca)0.5(Mn4+,Mn3+)2O4·1.5H2O – is a common oxide, yet it is rarely conclusively identified. It occurs in Mn deposits primarily as an alteration product of other Mn minerals.
It is classified within the Birnessite Group and may contain various impurities, such as Cl, Cu, Co, Fe, K, Ni, Mg, S, and Si. Due to the extremely small grain size, identification under a microscope is impossible.
1. Characteristics
Crystal system: Monoclinic prismatic, pseudohexagonal.
Color: Black.
Habit: Generally massive, earthy, or fine-grained; dull. Spherulitic. Rare acicular crystals in divergent aggregates up to 50 microns in size.
Cleavage: No.
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 1.5
Parting: No.
Streak: Dark brown to brown.
Lustre: Sub-vitreous, resinous, dull, earthy, etc.
Diaphaneity: Transparent.
Density (g/cm³): 3.0
2. Geology and Deposits
Birnessite is a common and important Mn mineral in many soils; it also precipitates in lakes, oceans, and groundwater.
In deposits containing Mn minerals, it is a common alteration product of other Mn minerals, including Mn silicates (rhodonite).
It is a component of bacterially precipitated Mn oxides.
It can form Mn dendrites and the crusts known as “desert varnish.” At the type locality (Birness, Scotland), it forms black crusts and cement between pebbles within a concretion found in an unconsolidated glacial sedimentary deposit.
It occurs as a regular constituent of marine Mn nodules.
It can mineralize fungal hyphae, as observed in a siderite block and in soil.
3. Mineral Associations
It occurs associated with many other Mn minerals, such as cryptomelane, pyrolusite, ramsdellite, todorokite, romanechite, alleghanyite, ranciéite, tephroite, nsutite, serandite, chalcophanite, and kingsmountite.
It also occurs with calcite, aegirine, microcline, cummingtonite, garnet (spessartine), eudialyte, leucophanite, and Fe oxides.
It forms as a weathering product of ore containing franklinite and willemite, thus occurring alongside franklinite.
4. Transmitted Light Microscopy
Refraction indices: nω: 1.730 nε: 1.690
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Dark brown, without pleochroism.
Relief: High.
Cleavage: Due to the extremely small grain size (maximum of 50 microns), cleavage is not visible.
Habits: Massive, colloform, laminated, banded, radial.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.040, corresponding to intense and varied colors up to the end of the second order.
Extinction: Paralell.
Elongation sign: No information available.
Twins: No information available.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(-), pseudo-uniaxial.
2V angle: No.
Alterations: No information available.
May be confused with: Many other minerals. It is not possible to identify birnessite under a transmitted-light microscope.
5. Reflected Light Microscopy
Sample preparation: The polishing hardness is greater than the hardness of rancieite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray.
Pleochroism: Very weak.
Reflectivity: ~10%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak to distinct anisotropy in shades of gray.
Undulose extinction.
Internal reflections: No. Occasionally, it may show brown internal reflections.
May be confused with: Many other minerals.
It is not possible to identify birnessite using reflected light; indeed, traditional literature on ore minerals often does not even mention the mineral. Only very fragmentary information is available.
General Characteristics:
Grain shape: occurs as reniform to botryoidal masses. It can form laminated to radially crystallized masses.