LIPSCOMBITE FROM CERRO BLANCO, CÓRDOBA, ARGENTINA: DESCRIPTION AND GENESIS.

F. COLOMBO; E.V. PANNUNZIO MINER; H.D. GAY; R. LIRA; M. DORAIS

Rocks & Minerals

Heldref Publications

Año: 2007 vol. 82 p. 234 - 234

0035-7529

Lipcombite, ideally Fe2+Fe3+2(PO4)2(OH)2, occurs at the El Criollo pegmatite (64º 39´ 9´´W,  31º 21´ 28´´ S), Cerro Blanco pegmatite group, 8 km west of Tanti, Córdoba Province, Argentina. In one of the intermediate zones of the pegmatite, near the core-margin, there is a nodule of triplite about 2 meters in diameter, discontinuously enveloped by the lipscombite-bearing assemblage. The primary paragenesis consists of massive quartz with euhedral greenish-white fluorapatite crystals to 20 x 10 mm long and abundant interstitial sulfides (mostly pyrite, with minor chalcopyrite). Most apatite crystals have a spongy texture, with phosphosiderite and hentschelite microcrystals in the voids. Rare U- and Hf-bearing zircon crystals to 2 mm long occur in the fluorapatite. Sulfides are strongly replaced by a compact mass of black color and almost matte luster. In polished section under reflected light it can be seen that the aggregate is formed mainly by anhedral interlocking grains of lipscombite (very weakly anisotropic) with many inclusions of nonreflecting minerals and bladed hematite with a sieved texture. Full-profile refinement of the X-ray diffraction pattern using the Rietveld method led to the identification of lipscombite, hematite, fluorite, phosphosiderite, goethite, and quartz. Another phase also present is very likely barbosalite (P21/c), dimorphous with lipscombite. Hentschelite was not detected, but is has been found in the same sector in similar samples. Cell parameters of lipscombite (refined in space group P43212) are a 7.389(2), c 12.961(5) (in Å), V 707.6 Å3. The iron phosphates (lipscombite/barbosalite) are compositionally quite homogeneous. An average of four electron microprobe analyses (wavelength dispersive mode) gives (in weight percent) P2O5 35.72, SiO2 0.07, FeO53.93, MnO 0.40, Al2O3 0.22, Na2O 0.19, CaO 0.12, MgO 0.01, F 0.27, Cl not detected, total  90.93. No attempt was made to calculate an empirical formula using EMPA data because of the variable Fe2+/Fe3+ ratio reported in natural lipscombite. This assemblage was formed under oxidizing and acid conditions (as evidenced by the presence of phosphosiderite), due to the breakdown and replacement of sulfides (mostly pyrite) by the action of phosphate-bearing hydrothermal solutions derived from the dissolution of fluorapatite. Calcium and fluorine precipitated as fluorite (confirmed by EDS), whereas sulfur from the sulfides must have left the system.  In spite of the presence of large amounts of triplite [(Mn,Fe,Mg,Ca)2(PO4)(F,OH)] nearby, the lipscombite/barbosalite has a very low Mn content. The phosphates-oxides-fluoride association closely reflects the chemistry of nearby primary phases and is one more evidence of the importance of the local environments at a very small scale in determining the secondary species assemblages.