Substitution du manganèse bivalent par du calcium dans les minéraux du groupe : braunite, neltnérite, braunite II

Baudracco-Gritti, Célestine

doi:10.3406/bulmi.1985.7840

Cited by 3 publications

(9 citation statements)

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“…The CaO concentrations in braunite and Capoor silica-depleted braunite are lower than in braunite II but considerably larger than those in partridgeite and bixbyite ( Fig. 10C), suggesting substitution of Ca for Mn 2 § in braunite group minerals (Baudracco-Gritti, 1985;Abs-Wurmbach et al, 1983). A1203 concentrations (Fig.…”

Section: + 3+mentioning

confidence: 91%

Mineralogy and mineral chemistry of oxide-facies manganese ores of the Postmasburg manganese field, South Africa

Gutzmer¹,

Beukes²

1997

Mineral. mag.

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The diagenetic to very low-grade metamorphic manganese ores of the Postmasburg manganese field provide a unique example of oxide-facies manganese ores in a Palaeoproterozoic palaeokarst setting. The ores are composed mainly of braunite group minerals, including braunite, partridgeite and bixbyite, with rare braunite II and Ca-poor, silica-depleted braunite. Iron-poor partridgeite is distinguished from Fe-rich bixbyite and the occurrence of Ca-poor, silica-depleted braunite is reported for the first time. Braunite and partridgeite formed during early diagenesis but remained stable under greenschist facies metamorphic conditions. In contrast, bixbyite is apparently a product of metasomatic remobilisation under peak metamorphic conditions. It is suggested that local variations of the metamorphic mineral association reflect variations of the host rock composition and that they are not related to changing P-T conditions of metamorphic alteration, a model promoted by previous authors. The phase chemistry of braunite, braunite II and bixbyite is explained by the existing polysomatic stacking model for the braunite group. However, the chemical composition of partridgeite and Ca-poor, silica-depleted braunite can only be explained by introducing a distinct module layer, with partridgeite composition, to the existing polysomatic stacking model.

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Section: + 3+mentioning

confidence: 91%

Mineralogy and mineral chemistry of oxide-facies manganese ores of the Postmasburg manganese field, South Africa

Gutzmer¹,

Beukes²

1997

Mineral. mag.

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“…Braunite has close to the ideal composition of braunite I, (Mn+2)6(SiMn+3)024 (Baudracco-Gritti, 1985). The composition ( fig.…”

Section: Braunitementioning

confidence: 63%

Microbanded manganese formations; protoliths in the Franciscan Complex, California

Huebner¹,

Flohr²

1990

Professional Paper

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The Buckeye manganese deposit, 93 km southeast of San Francisco in the California Coast Ranges, preserves a geologic history that provides clues to the origin of numerous lenses of manganese carbonate, oxides, and silicates that occur with interbedded radiolarian chert and metashale of the Franciscan Complex. Compositionally and mineralogically laminated Mn-rich protoliths were deformed and dismembered, in a manner that mimics in smaller scale the deformation of the host complex, and then were incipiently metamorphosed at blueschistfacies conditions. Eight phases occur as almost monomineralic protoliths and mixtures: rhodochrosite, caryopilite, chlorite, gageite, taneyamalite, braunite, hausmannite, and laminated chert (quartz). Braunite, gageite, and some chlorite and caryopilite layers were deposited as gel-like materials; rhodochrosite, most caryopilite, and at least some hausmannite layers as lutites; and the chert as turbidites of radiolarian sand. Some gel-like materials are now preserved as transparent, sensibly isotropic relics of materials that fractured or shattered when deformed, creating curved surfaces. In contrast, the micrites flowed between the fragments of gel-like materials. The orebody and most of its constituent minerals have unusually Mn-rich compositions that are described by the system MnO-Si02-02-C02-H20. High values of Mn/Fe and U/Th, and low concentrations of Co, Cu, and Ni, distinguish the Buckeye deposit from many high-temperature hydrothermal deposits and hydrogenous or diagenetic manganese and ferromanganese nodules and pavements. This chemical signature suggests that ore deposition was related to fluids from the sediment column and seawater. Tungsten is associated exclusively with gageite, in concentrations as high as 80 parts per million. The source of the manganese is unknown; because basalts do not occur near the deposit, it was probably manganese leached from the sediment column by reducing solutions. Low concentrations of calcium (CaO approximately 0.6 weight percent) suggest that the host sediments formed beneath the carbonate-compensation depth. The most probable cause of the microbanding is changing proportions of chemical fluxes supplied to the sediment-seawater interface. The principal fluxes were biogenic silica from the water column, carbon dioxide from organic matter in the sediment column, 02 and other seawater constituents, and Mn+2-bearing fluid. The presence of A1203 and Ti02 (supplied by a detrital flux) in the metashale but not the ore lens suggests rapid ore deposition. Material supply-rate changes were probably due to a complex combination of episodic variations in the hydrothermal flux and periodic flows of radiolarian sand (silica and C02 fluxes) that may be related to climate variations.

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“…Braunite, braunite II and neltnerite are isostructural; they have the same tetragonal space group I41/acd, and their structural arrangement is composed of one distorted cubic [Mn 2+ O8] site, three non-equivalent distorted [Mn 3+ O6] octahedra, and one [Si 4+ O4] tetrahedron [23,27]. In natural braunites, the sites occupied by Mn 3+ can contain minor quantities of Fe 3+ and Al, whereas Mn 2+ can be replaced by Ca and small amounts of Mg, Cu, Ti, Ba, Na and K [26,[29][30][31]. Various arrangements of layers (modules) parallel to (001), stacked in various proportions and with different stacking vectors, influence c-axis dimension in the above three members, which have the following cell parameters: (a: 9.42, 9.46, 9.43 Å and c: 18.70, 18.85, 37.77 Å in braunite [23], neltnerite [25], and braunite II [24], respectively).…”

Section: Resultsmentioning

confidence: 99%

“…Braunite ) is a member of a polysomatic series containing Mn 3+ , whose structural and chemical features were previously described in Geller [21], de Villiers and Herbstein [22], de Villiers [23,24], Baudracco-Gritti et al [25], Baudracco-Gritti [26], de Villiers and Buseck [27] and de Villiers et al [28]. The composition of these minerals can be described by the general formula M 8 O 12 , through substitutions of Si 4+ , Mn 2+ and Ca 2+ for Mn 3+ , by having, as a consequence, a considerable range of compositions.…”

Section: Resultsmentioning

confidence: 99%

“…exchange in braunite up to (Ca0.8Mn 2+ 0.2)Mn 3+ 6SiO12 at P = 15 kb, T = 800 °C, and fO2 of the Mn2O3/MnO2 buffer. Indeed, it would seem quite feasible to have a continuous variation in the Ca/Mn 2+ ratio on the cubic distorted site, and the extent of distortion should not create a barrier for the continuous exchange of Mn 2+ or Ca atoms on this site, in accordance with the following formula [26]:…”

Section: Resultsmentioning

confidence: 99%

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Manganese-Containing Inclusions in Late-Antique Glass Mosaic Tesserae: A New Technological Marker?

2020

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The present study focuses on manganese-containing inclusions identified in late-Antique glass tesserae, light brown/amber and purple in colour, from Padova (Italy), in order to clarify the nature of these inclusions, never identified in glass mosaics until now, and provide new insights on the production technologies of such kinds of tesserae. Multi-methodological investigations on manganese-containing inclusions were carried out in this work by means of optical microscopy (OM), scanning electron microscopy (SEM), micro-X-ray diffraction (micro-XRD), electron backscattered diffraction (EBSD), electron microprobe (EMPA), and micro-Raman spectroscopy. The combination of analytical results shows that inclusions are crystalline, new-formed phases, mainly composed of manganese, silica and calcium, and are mineralogically ascribed as a member of the braunite-neltnerite series, with unit-cell parameters closer to those of neltnerite. However, the low Ca content makes such crystalline compounds more similar to braunite, in more detail, they could be described as Ca-rich braunite. The occurrence of such crystalline phase allows us to constrain melting temperatures between 1000 and 1150 °C, and to hypothesize pyrolusite, MnO2, as the source of manganese. In addition, it is worth underlining that the same phase is identified in tesserae characterised by different colours (light brown/amber vs purple due to different manganese/iron ratios), glassy matrices (soda-lime-lead vs soda-lime) and opacifiers (cassiterite vs no opacifier). This suggests that its occurrence is not influenced by the “chemical environment”, revealing these manganese-containing inclusions as a new potential technological marker.

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Substitution du manganèse bivalent par du calcium dans les minéraux du groupe : braunite, neltnérite, braunite II

Cited by 3 publications

References 0 publications

Mineralogy and mineral chemistry of oxide-facies manganese ores of the Postmasburg manganese field, South Africa

Mineralogy and mineral chemistry of oxide-facies manganese ores of the Postmasburg manganese field, South Africa

Microbanded manganese formations; protoliths in the Franciscan Complex, California

Manganese-Containing Inclusions in Late-Antique Glass Mosaic Tesserae: A New Technological Marker?

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