Gorceixite from catalão, goiás, Brazil: Rietveld crystal structure refinement

Abstract

Gorceixite is an abundant aluminum phosphate in Brazilian laterite phosphate ores and is one of the most important gangue minerals. Catalão I mine, Goiás, Brazil, one of the biggest phosphate mines in Brazil, contains great amounts of gorceixite, whose structural formula, as suggested by microprobe analysis, is (Ba0.76Ca0.19Sr 0.10Ce0.01)Σ1.06(Al2.74Fe 3+0.25)Σ2.99 (P0.96Si 0.03)Σ0.99O4(PO3OH)(OH) 6.02. Its structure, rich in Ca, Sr, and Fe, was refined by the Rietveld method applied to X-ray powder diffraction data for trigonal and monoclinic structural models. The statistical parameters for the two models are so similar that they do not allow clear distinction of which is the better structural model for gorceixite. The cell parameters obtained for the trigonal model, space group R3m, are a = 7.0791(3) Å, c = 17.089(1) Å, V= 741.68(7) Å3, Z= 3, and for the monoclinic model, space group Cm, a =12.195(8) Å, b = 7.040(5) Å, c = 7.055(5) Å, β= 125.19(5)°, V = 495 Å3, Z= 2. A particular feature of the Catalão gorceixite is the replacement of some Al in the X-site by Fe 3+, with minor distortions in crystallochemical sites. These features may alter physical-chemical mineral characteristics, such as density and heavy metal retention capacity.