Geologia USP. Série Científica (Oct 2007)
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 importantgangue minerals. Catalão I mine, Goiás, Brazil, one of the biggest phosphate mines in Brazil, contains great amounts ofgorceixite, whose structural formula, as suggested by microprobe analysis, is (Ba0.76Ca0.19Sr0.10Ce0.01)Σ1.06(Al2.74Fe3+0.25)Σ2.99(P0.96Si0.03)Σ0.99O4(PO3OH)(OH)6.02. Its crystal structure, rich in Ca, Sr, and Fe, was refined by the Rietveld method appliedto X-ray powder diffraction data for trigonal and monoclinic structural models. The statistical parameters for the two modelsare so similar that they do not allow clear distinction of which is the better structural model for gorceixite. The cell parametersobtained 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 themonoclinic 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 Fe3+, with minor distortions incrystallochemical sites. These features may alter physical-chemical mineral characteristics, such as density and heavy metalretention capacity.
