Publication Date:
2013-10-02
Description:
Rossiantonite, ideally Al 3 (PO 4 )(SO 4 ) 2 (OH) 2 (H 2 O) 10 ·4H 2 O, triclinic (space group P ), a = 10.3410(5), b = 10.9600(5), c = 11.1446(5) Å, α = 86.985(2), β = 65.727(2), = 75.064(2)°, V = 1110.5(1) Å 3 , 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/cm 3 . Electron microprobe analyses, with H 2 O measured by thermogravimetric analysis, provided the following empirical formula based on 28 O apfu: Al 2.96 Fe 0.03 P 1.01 S 2 H 30.02 O 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 PO 4 and Al O 6 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 AlO 6 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.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
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