Reference:
▪ Kampf, A.R., Nash, B.P., Dini, M., Molina Donoso, A.A. (2017): Juansilvaite, Na5Al3[AsO3(OH)]4[AsO2(OH)2]2(SO4)2·4H2O, a new arsenate-sulfate from the Torrecillas mine, Iquique Province, Chile. Mineralogical Magazine, 81, 619–628.
Abstract:
The new mineral juansilvaite (IMA2015-080), Na5Al3[AsO3(OH)]4[AsO2(OH)2]2(SO4)2·4H2O, was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with anhydrite, canutite, halite, sulfur and a mahnertite-like phase. Juansilvaite occurs as bright pink blades to about 0.5 mm in length grouped in tightly intergrown radial aggregates and also as opaque dull pale pink rounded aggregates. Blades are flattened on {001}, elongated on [100] and exhibit the forms {001}, {111} and {20-1}. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is ca. 2½, tenacity is brittle and fracture is irregular. Cleavage is very good on {001}. The measured density is 3.01(2) g·cm–3 and the calculated density is 3.005 g·cm–3. Optically, juansilvaite is biaxial (+) with α = 1.575(1), β = 1.597(1), γ= 1.623(1) and 2V = 86(1)° (measured in white light). Dispersion is r < v, slight, and the orientation is X = b; Z ^ c = 27° in the obtuse angle β. The pleochroism is X > Y ≈ Z in shades of pale pink. The mineral is slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is Na4.95Al2.28Fe3+0.50Mn3+ 0.21Cu0.04As5.92S1.83O36H17.37. Juansilvaite is monoclinic, C2/c, a =18.1775(13), b = 8.6285(5), c = 18.5138(13) Å, β = 90.389(6)°, V = 2903.7(3) Å3 and Z = 4. The eight strongest X-ray powder diffraction lines are [dobs Å(I)(hkl)]: 9.25(100)(002), 7.20(34)(-111), 4.545(34)(400), 3.988(39)(-114), 3.363(42)(314), 3.145(66)(-512,420), 2.960(68)(-422,422) and 2,804(33)(131-423). The structure of juansilvaite (R1 = 3.82% for 2040 Fo > 4σF reflections) contains layers made up of alternating corner-linked Al-O octahedra and acid-arsenate tetrahedra. Na cations occur both peripheral to the layers and within cavities in the layers. An SO4 tetrahedron and an H2O group also are in the interlayer region.
▪ Kampf, A.R., Nash, B.P., Dini, M., Molina Donoso, A.A. (2017): Juansilvaite, Na5Al3[AsO3(OH)]4[AsO2(OH)2]2(SO4)2·4H2O, a new arsenate-sulfate from the Torrecillas mine, Iquique Province, Chile. Mineralogical Magazine, 81, 619–628.
Abstract:
The new mineral juansilvaite (IMA2015-080), Na5Al3[AsO3(OH)]4[AsO2(OH)2]2(SO4)2·4H2O, was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with anhydrite, canutite, halite, sulfur and a mahnertite-like phase. Juansilvaite occurs as bright pink blades to about 0.5 mm in length grouped in tightly intergrown radial aggregates and also as opaque dull pale pink rounded aggregates. Blades are flattened on {001}, elongated on [100] and exhibit the forms {001}, {111} and {20-1}. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is ca. 2½, tenacity is brittle and fracture is irregular. Cleavage is very good on {001}. The measured density is 3.01(2) g·cm–3 and the calculated density is 3.005 g·cm–3. Optically, juansilvaite is biaxial (+) with α = 1.575(1), β = 1.597(1), γ= 1.623(1) and 2V = 86(1)° (measured in white light). Dispersion is r < v, slight, and the orientation is X = b; Z ^ c = 27° in the obtuse angle β. The pleochroism is X > Y ≈ Z in shades of pale pink. The mineral is slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is Na4.95Al2.28Fe3+0.50Mn3+ 0.21Cu0.04As5.92S1.83O36H17.37. Juansilvaite is monoclinic, C2/c, a =18.1775(13), b = 8.6285(5), c = 18.5138(13) Å, β = 90.389(6)°, V = 2903.7(3) Å3 and Z = 4. The eight strongest X-ray powder diffraction lines are [dobs Å(I)(hkl)]: 9.25(100)(002), 7.20(34)(-111), 4.545(34)(400), 3.988(39)(-114), 3.363(42)(314), 3.145(66)(-512,420), 2.960(68)(-422,422) and 2,804(33)(131-423). The structure of juansilvaite (R1 = 3.82% for 2040 Fo > 4σF reflections) contains layers made up of alternating corner-linked Al-O octahedra and acid-arsenate tetrahedra. Na cations occur both peripheral to the layers and within cavities in the layers. An SO4 tetrahedron and an H2O group also are in the interlayer region.