Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

B. K. Mohapatra ¹, B. R. Nayak ¹, R. K. Sahoo ¹

Affiliations
1 Minera10gy and Meta110graphy Division, Regional Research Laboratory, Bhubaneswar - 751 013, IN

Abstract

Characteristics of vernadite in manganese ores from Gangpur Group of rocks, Goriajhor Formation, Sundargarh district, Orissa, have been described. This terrestrial vernadite is distinguished from δ-MnO₂ in marine manganese nodules by its well ordered crystallinity and thinner fibrils of submicron size. It appears to be chemically homogenous with the proposed formula: (Ba, K, Fe) MnO₂.nH₂O. This vernadite probably originated through solution, precipitation and redeposition under supergene conditions and later transformed into cryptomelane or romanechite.

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B. R. Nayak ¹, B. K. Mohapatra ²

Affiliations
1 National Metallurgical Laboratory, Jamshedpur - 83 1 007, IN
2 Regional Research Laboratory, Bhuhaneswar - 751 013, IN

Abstract

The Precambrian manganese-Rich formation (Goriajhar Formation) Gangpur Group contains appreciable Fe, as reflected from the occurrence of hematite in different paragenesis. The first generation of hematite (Hm₁) exhibits dissimilar micromorphology, contains Mn in solid solution (MnO: 1 to <4%)and may be termed as Manganoan-Hematite. The different morphological types (Hm₁) recorded are: hexagonal plate, twinned idiomorph, suhhedral inclusion within quartz and braunite, blebs forming zone within spessartine garnet, crystal dusted with oriented intergrown ilmenite, haphazard platelets within Mn-aegirine, oriented grains forming alternate bands and fused grainns forming large porphyroblast. The hematite formed due to maartitisation is of second generation (Hm₂) and the one that occurs as vein fillings, often traversing the other two types, is of third generation (Hm₃). Both Hm₂ and Hm₃ are devoid of Mn content. The first generation hematite is of primary type. syngenetic with manganese minerals and developed at different stages of metamorphism. The hematite of other two generations is of hypogene origin, Hm₂ being formed due to low-temperature oxidation, while Hm₃ during solution and reprecipitation at meteoric condition.

Keywords

Hematite, Manganoan Hematite, Manganese Rich Formation, Manganese Ore, Gangpur Group, Orissa.

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B. K. Mohapatra ¹, B. R. Nayak ¹

Affiliations
1 Mineralogy and Metallography Division, Regional Research Laboratory, Bhubaneswar - 751 013, IN

Abstract

Manganite γ-MnOOH, in the manganese-rich rocks of Precambrian Gangpur Group, Sundargarh district, Orissa, shows dissimilar fabrics and belongs to three generations but has more or less similar chemical composition. It occurs as patchy inclusions, accicular crystallites, egg-shaped micro-nodules, medium to coarse-grained mosaics, vein and fissure fillings and as wedge-shaped linings. The manganite is either formed froin Mn-oxide (jacobsite and vredenburgite) phases through retrogressive metamorphic transformation (Ma1) or generated through pseudomorphic replacement of Mn-sillicate (rhodonite and tcphroite) or Mn-carbonate (rhodochrosite) by chemical weathering under supergene conditions (Ma2). The vein filling type is the latest in its paragenesis (Ma3). Despite different mineral lineages, manganites are chemically similar though crystallites located in veins and fissures are relatively pure.

Keywords

Economic Geology, Manganite, Manganese, Gangpur Group, Orissa.

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