RANCIÉITE
Ranciéite – (Ca,Mn2+)0.2(Mn4+,Mn3+)O2·0.6H2O – is a rarer oxide, a minor component of Mn ores, with 75.34% MnO2.
It is classified in the Birnessite Group, in the Ranciéite-Takanelite Series, the two being isostructural.
Macroscopically it can be confused with todorokite, but X-ray diffractograms are diagnostic, somewhat variable depending on the degree of hydration.
The literature presents very little data on the behavior of ranciéite under the microscope.
1. Characteristics
Crystal system: Trigonal rhombohedral
Color: Black, brownish, violet, silvery gray.
Habit: Lamellar in radial aggregates. Polycrystalline aggregates. Stalactitic, incrustations.
Cleavage: No information available.
Tenacity: No information available.
Twinning: No information available.
Fracture: No information available.
Mohs Hardness: 2.5 – 3
Parting: No information available.
Streak: Dark brown.
Lustre: Metallic
Diaphaneity: Opaque
Density (g/cm³): 3.2
2. Geology and Deposits
Ranciéite is typically a product of weathering or alteration of Mn deposits, generally forming in limestone or in limestone caves. It also occurs in cavities in limonite.
3. Mineral Associations
It is associated with some common minerals such as quartz, microcline, opal, calcite, goethite, and other Fe hydroxides.
It also occurs with other Mn oxides such as birnessite, todorokite, and others.
Sometimes it occurs associated with rarer minerals such as aegirine, eudialyte, and tundrite-(Ce).
4. Transmitted Light Microscopy
The literature states that ranciéite exhibits a brown color under transmitted light and is uniaxial.
5. Reflected Light Microscopy
Sample preparation: Its hardness upon polishing is less than that of birnessite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White-gray in coarse aggregates. Fine-grained masses show much lower reflectivity.
Compared to the color of todorokitite, the color of rancieite is somewhat darker.
Pleochroism: Distinct, ranging from off-white to creamy yellowish-gray.
Reflectivity: ~15%
Bireflectance: No information available.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy ranging from bluish-gray to bluish-black. This anisotropy is stronger than that of hausmannite.
Undulating extinction.
Internal reflections: No.
May be confused with: todorokite, but todorokite presents internal reflections.
General Characteristics:
Grain shape: occurs in aggregates of coarse and fine granulation with a fibrous texture. Forms crusts around todorokitite.
