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Juansilvaite, Na (sub 5) Al (sub 3) [AsO (sub 3) (OH)] (sub 4) [AsO (sub 2) (OH) (sub 2) ] (sub 2) (SO (sub 4) ) (sub 2) .4H (sub 2) O; a new arsenate-sulfate from the Torrecillas Mine, Iquique Province, Chile

Anthony R. Kampf, Barbara P. Nash, Maurizio Dini and Arturo A. Molina Donoso
Juansilvaite, Na (sub 5) Al (sub 3) [AsO (sub 3) (OH)] (sub 4) [AsO (sub 2) (OH) (sub 2) ] (sub 2) (SO (sub 4) ) (sub 2) .4H (sub 2) O; a new arsenate-sulfate from the Torrecillas Mine, Iquique Province, Chile
Mineralogical Magazine (June 2017) 81 (3): 619-628

Abstract

The new mineral juansilvaite (IMA2015-080), Na (sub 5) Al (sub 3) [AsO (sub 3) (OH)] (sub 4) [AsO (sub 2) (OH) (sub 2) ] (sub 2) (SO (sub 4) ) (sub 2) .4H (sub 2) O, 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 up to approximately 0.5 mm long 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 {201}. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is approximately 21/2, tenacity is brittle and fracture is irregular. Cleavage is very good on {001}. The measured density is 3.01(2) g cm (super -3) and the calculated density is 3.005 g cm (super -3) Optically, juansilvaite is biaxial (+) with alpha = 1.575(1), beta = 1.597(1), gamma = 1.623(1) and 2V = 86(1) degrees (measured in white light). Dispersion is r < v, slight, and the orientation is X = b; Z^c = 27 degrees in the obtuse angle beta . The pleochroism is X > Y approximately 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 Na (sub 4.95) Al (sub 2.28) Fe (super 3+) (sub 0.50) Mn (super 3+) (sub 0.21) Cu (sub 0.04) As (sub 5.92) S (sub 1.83) O (sub 36) H (sub 17.37) . Juansilvaite is monoclinic, C2/c, a = 18.1775(13), b = 8.6285(5), c = 18.5138(13)A, beta = 90.389(6) degrees , V = 2903.7(3)A (super 3) and Z = 4. The eight strongest powder X-ray diffraction lines are [d (sub obs) A(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 (R (sub 1) = 3.82% for 2040 F (sub o) > 4sigma F reflections) contains layers made up of alternating corner-linked Al-O octahedra and acid-arsenate tetrahedra. Sodium cations occur both peripheral to the layers and within cavities in the layers. An SO (sub 4) tetrahedron and an H (sub 2) O group also are in the interlayer region.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 81
Serial Issue: 3
Title: Juansilvaite, Na (sub 5) Al (sub 3) [AsO (sub 3) (OH)] (sub 4) [AsO (sub 2) (OH) (sub 2) ] (sub 2) (SO (sub 4) ) (sub 2) .4H (sub 2) O; a new arsenate-sulfate from the Torrecillas Mine, Iquique Province, Chile
Affiliation: Natural History Museum of Los Angeles County, Department of Mineral Sciences, Los Angeles, CA, United States
Pages: 619-628
Published: 201706
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 21
Accession Number: 2017-077400
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
S20°58'13" - S20°58'13", W70°08'17" - W70°08'17"
Secondary Affiliation: University of Utah, USA, United States
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201741
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