Rossiantonite, Al3(PO4)(SO4)2(OH)2(H2O)10•4H2O, a new hydrated aluminum phosphate-sulfate mineral from Chimanta massif, Venezuela: Description and crystal structure

Rossiantonite, ideally Al3(PO4)(SO4) 2(OH)2(H2O)10·4H 2O, triclinic (space group P1̄), a = 10.3410(5), b = 10.9600(5), c = 11.1446(5) Å, α = 86.985(2), β = 65.727(2), γ = 75.064(2)°, V = 1110.5(1) Å3, Z = 2, is a new mineral from the Akopan-Dal Cin cave system in the Chimanta massif (Guyana Shield, Venezuela). The mineral occurs as small (≤0.15 mm) and transparent crystals in a white to slightly pink fine-grained sand, filling spaces between boulders of weathered quartz sandstone. Associated phases are gypsum, sanjuanite, rare alunite, quartz and micro-spherules of amorphous silica. Rossiantonite is colorless with a white streak and vitreous luster. The mineral is brittle with irregular to sub-conchoidal fracture and it shows a poorly developed cleavage. Rossiantonite is biaxial and not pleochroic, with mean refractive index of 1.504. The calculated density is 1.958 g/cm3. Electron microprobe analyses, with H2O measured by thermogravimetric analysis, provided the following empirical formula based on 28 O apfu: Al2.96Fe 0.03P1.01S2H30.02O 28. The five strongest lines in the X-ray powder diffraction pattern, expressed as d (Å), I, (hkl) are: 4.647, 100, (210); 9.12, 56, (100); 4.006, 53, (220); 8.02, 40, (110); 7.12, 33, (011). The crystal structure, refined using 3550 unique reflections to R = 0.0292, is built of PO4 and Al O6 polyhedral rings, creating complex chains parallel b by sharing the OH-OH edge belonging to the Al(3) polyhedron. Three symmetrically independent Al sites can be identified, namely: Al(1), Al(2), and Al(3). Tetrahedral sites, occupied by P, share all their apexes with AlO6 octahedra. Unshared octahedral apexes are occupied by water molecules. Four additional water molecules are placed in between the previously identified chains. Two oxygen tetrahedra, occupied by S atoms, are connected along the chains by means of weak hydrogen bonding. The rossiantonite structure shows similarities with minerals belonging to the sanjuanite-destinezite group.