Mineralogy of rockbridgeite - frondelite series and its phosphate association from the Jocão pegmatite, Brazil

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Marco E. Ciriotti
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Mineralogy of rockbridgeite - frondelite series and its phosphate association from the Jocão pegmatite, Brazil

Messaggio da Marco E. Ciriotti » mer 15 ago, 2018 11:17

Referenza:
▪ Barre Ardizzi, J. & Atencio, D. (2018): Mineralogy of rockbridgeite - frondelite series and its phosphate association from the Jocão pegmatite, Galileia, Minas Gerais, Brazil. IMA2018 Abstract book. Poster presentations. Pegmatite mineralogy, geochemistry, classification and origins, 494.

Abstract:
The rockbridgeite group is composed by hydrated iron phosphates of general formula A2+2B3+3([PO3(O,OH)]3(OH)5, solved in space group Bbmm by Moore (1970) and represented by the isostructural members rockbridgeite, described and named by Frondel (1949) with ideal composition Fe2+Fe3+4(PO4)3(OH)5, and its Mn2+ equivalent frondelite, named and described by Lindberg (1949), with ideal composition Mn2+Fe3+
4(PO4)3(OH)5. Minerals of the rockbridgeite series crystalize as hydrothermal secondary phosphates in the lithium rich pegmatite of Galileia, and occurs as bottle green spherule aggregates with fibroradial habit when unaltered, to brownish-black botryoidal aggregates when oxidized. In thin section, they show a huge variation in pleocroism, mostly depending on the oxidation stage, associated minerals and crystal habit. Generally, it appears as the oxidation product of barbosalite (Baijot et al., 2014) and is commonly associated with a complex intergrowth of hydrothermal secondary phosphates (Fransolet, 2007) like hureaulite, phosphosiderite, leucophosphite, minerals of the jahnsite group, the mitridatite-robertsite series and a huge variation of Fe-Mn oxides. Phosphates nodules are found to crystalize in association or within fracture plane of silicates like albite, quartz and micas. diffraction echniques and MEV-EDS.
Analyses by MEV-EDS show a variation in their MnO and FeO content, from 1.70-3.37 wt.% MnOtot , and a FeOtot variation from 53-58 wt.%. The A2+ site is occupied predominantly by Fe2+, with minor Mn2+. Table 1 presents the chemical data obtain by Energy-Dispersive X-ray Spectroscopy (EDS) with solid detector Si(Li) Soft Inca Oxford and SEM microscopy of rockbridgeite samples and closely associated phosphates, with its petrological description in thin section.
Table 1: Rockbridgeite group minerals ideal formulae and calculated formulae of rockbridgeite and associated phosphates specimens from the Jocão pegmatite.

Ideal formulae
Rockbridgeite: Fe2+Fe3+4(PO4)3(OH)5
Phosphosiderite: Fe3+PO4.2H2O
Barbosalite: Fe2+ Fe3+2(PO4)2(OH)2

Calculated formulae
Rockbridgeite - Joc-alt_m2003 (Fe2+0.63Mn2+0.17Co0,03)Σ0.83Fe3+4(PO4)3.OH4.66
Phosphosiderite - J67-g4004 (Na0.02K0.02Ca0.02Mn0.02Co0.01Al0.04Fe3+0.94)Σ1.09[(P0.93Al0.06Si0.01)Σ1O4].2H2O
Barbosalite - Joc-alt_m3007 (Fe2+0.99Mn2+0.12)Σ1.11Fe3+2(PO4)2[O1.20(OH)0.80]Σ2.00
Marco E. Ciriotti

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