ROMANÈCHITE
Romanèchite – (Ba,H2O)2(Mn4+,Mn3+)5O10 – is a relatively frequent and important oxide that is part of Mn ores. It is the main component of those materials called “psilomelane”. Its identification is only possible through X-ray diffractometry and/or chemical analysis.
Chemically, it is similar to hollandite and macroscopically it can be confused with minerals of the Cryptomelane Group.
Despite its importance as a Mn ore, the information available on the behavior of romanechite under the microscope is very poor, and several classic literatures on ore minerals do not even address it.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Grayish black, black.
Habit: It generally occurs with a reniform, botryoidal, stalagtitic, colloform, or banded habit. Idiomorphic crystals are very rare, can be flattened up to 200 microns in size, acicular to fibrous, in divergent aggregates.
Cleavage: No.
Tenacity: No information available.
Twinning: Twins formed by prismatic to acicular crystals resembling sagenite in their arrangement may occur.
Fracture: No information available.
Mohs Hardness: 5 – 6
Parting: No.
Streak: Black to brownish black.
Lustre: Sub-metallic, dull when massive.
Diaphaneity: Opaque.
Density (g/cm³): 4.70 – 5.
2. Geology and Deposits
Romanèchite is a weathering product of Mn minerals, including oxides, carbonates and silicates.
It occurs in sedimentary deposits and in replacement deposits in limestones and dolomites.
It is the main component of “psilomelane” and “desert varnish,” which are Mn oxide-rich coatings formed on the surface of rocks in arid regions.
It can occur in plumose agates.
3. Mineral Associations
It occurs associated with some common minerals such as quartz, hematite, barite, fluorite, and goethite.
It occurs with other Mn minerals such as todorokite, chalcophanite, hausmannite, birnessite, braunite, rhodochrosite, and pyrolusite.
It occurs with some rare minerals such as sulfates (jarosite, natrojarosite) and arseniosiderite.
4. Transmitted Light Microscopy
This does not apply because romanèchite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: No information available.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white, medium bluish-gray to brownish.
Pleochroism: Distinct, somewhere between bluish-gray and brownish-gray.
Reflectivity: 22.22 – 28.99%
Bireflectance: Strong.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, ranging from cream to dark brown.
Internal reflections: No.
May be confused with: No information available.
