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A mineralogical and crystal-chemical study on the fluorine-rich minerals, found in the metasomatized benmoreitic lavas from Biancavilla (Mount Etna, Sicily, Italy), is in progress. Besides the fluoro-edenite and fluorophlogopite minerals, recently accepted by the IMA as new fluorine mineral species, fluorapatite crystals, characterized by high As contents (until 15% wt of As2O5), were also recovered in the same products. Arsenicum is only observed in the rim of the analyzed crystals, where an increase of REE, minor and trace elements (La, Ce, Nd, Sr, Y, etc.) is also highlighted. In addition, the bulk rock analyses show significant enrichments in As content (until to 1033 ppm).
The fluorapatite crystals, with prismatic exagonal habit and micrometric dimensions (200-400 m in length), show a particular resinous luster and orange-reddish color. By SEM-BSE images a light rim is evident (As-rich) respect to a dark core (As-free). Interestingly, many thin lines (<1 microns in width), elongated parallel to C crystal axis, are visible in the inner part of the crystals. The lines, black in polarized light and white in SEM-BSE images, show high Fe contents by EDS spectra and are probably constituted by hematite lamellae. In the same rocks, inclusions of thin hematite lamellae were already individualized in the fluoro-edenite millimetric crystals. Moreover, a recent paper reports that hematite lamellae have been also found in the fluorapatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CSSD) project. Laser Ablation analysis (LA-ICP-MS) allowed to carry out REE, minor and trace element contents. EMPA micro-analysis, obtained on a rim-core-rim traverse perpendicular to the fluorapatite crystal elongation (about 200 m length and 0.5 m width), also allowed to observe that As, Si, and Na content increase in the rim, while the fluorine content decreases. The powder diffraction pattern obtained grounding many crystals of the As-bearing fluorapatite allowed to calculate a medium value of the unit cell parameters: a = 9.403(5) Å, c = 6.893(4) Å, and V = 527.78(46) Å3, which correspond to values comprised between As-free and As-rich fluorapatite.
Present mineralogical results, added to previous petrological data on the same volcanic materials, confirm that both the host rock-type and the fuorine-dominant apatite were interested from a metasomatic process, in which hot fluids with an enrichment of fuorine, REE, and other incompatible elements, allowed the crystallization of fluorapatite with As-rich rims.
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