Barbosalite and lipscombite in Cerro Blanco, Córdoba (Argentina): description and genesis of secondary phosphates in pegmatites with triplite and apatite

Abstract

The dimorphs lipscombite and barbosalite, ideally Fe2+Fe3+2(PO4)2(OH)2, occur in El Criollo pegmatite (64º39´9´´W, 31º21´28´´S), Cerro Blanco, Córdoba, Argentina. In one of the intermediate zones of this pegmatite there is a nodule of triplite discontinuously rimmed by the lipscombite-barbosalite-bearing assemblage. The primary paragenesis consists of quartz with euhedral fluorapatite crystals, zircon, and interstitial sulfides (pyrite>>chalcopyrite). Apatite is corroded and sulfides have been replaced by a greenish black mass of anhedral grains of lipscombite and barbosalite, with myriads of inclusions of fluorite, hematite, and scarce phosphosiderite. Cell parameters, obtained by Rietveld refinement, are (in Å): barbosalite (space group P21/n): a=7.326(2), b=7.446(2), c=7.401(2), ß=118.79(2)º; lipscombite (space group P43212): a=b=7.388(2), c=12.924(6). Chemical analyses of barbosalite and lipscombite (electron microprobe, n=4) gave (wt. %) P2O5: 35.72, SiO2: 0.07, FeO: 17.98, Fe2O3: 39.96, MnO: 0.40, Al2O3: 0.22, Na2O: 0.18, CaO: 0.12, MgO: 0.01, F: 0.27, H2O (calculated as 2-F): 4.45, Cl not detected, total corrected for F≡O (99.27). The amounts of FeO and Fe2O3 were calculated to give a Fe2+:Fe3+ ratio of 1:2 in the structural formula. In general, secondary phosphates in these pegmatites indicate an oxidizing and acidic environment and a strong control of the primary mineralogy, stressing the importance of the environment at a very local scale in determining the secondary species assemblage. Crystallizing minerals are very influenced by kynetic aspects of the dissolution and precipitation reactions, and not only by the thermodynamically stable structural configurations. Secondary phases containing H2O indicate that alteration continued to temperatures below 250ºC.