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Natropharmacoalumite, NaAl4[(OH)4(AsO4)3].4H2O, a new mineral of the pharmacosiderite supergroup and the renaming of aluminopharmacosiderite to pharmacoalumite

Published online by Cambridge University Press:  05 July 2018

M. S. Rumsey ,
S. J. Mills  and
J. Spratt
M. S. Rumsey*
Affiliation:
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
S. J. Mills
Affiliation:
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver BC, Canada V6T 1Z4
J. Spratt
Affiliation:
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
*
*E-mail: michr@nhm.ac.uk
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Abstract

Natropharmacoalumite, ideally NaAl4[(OH)4(AsO4)3]·4H2O, is a new mineral from the Maria Josefa Gold mine, Rodalquilar, Andalusia region, Spain. It occurs as colourless, intergrown cubic crystals with chenevixite, kaolinite, jarosite and indeterminable mixtures of Fe and Sb oxyhydroxides. Individual crystals are up to 0.5 mm on edge, although crystals are more commonly ˜0.25 mm across and occur in patchy aggregates several millimetres across. The mineral is transparent with a vitreous to adamantine lustre. It is brittle with an imperfect cleavage, irregular fracture and a white streak. The Mohs hardness is ˜2.5 with a calculated densityof 2.56 g cm–3 for the empirical formula. Electron microprobe analyses yielded Na2O 2.52%, K2O 1.49%, Al2O3 29.50%, As2O5 48.84% and H2O was calculated in line with the structural analysis as 16.28% totalling 98.63%. The empirical formula, based upon 20.21 oxygen atoms, is [Na0.57K0.22(H3O)0.21]Σ1.00Al4.05(As2.97O12)(OH)4·4H2O. The five strongest lines in the X-ray powder diffraction pattern are [dobs(Å), Iobs,(hkl)]: 7.759,100,(100); 4.473,40,(111); 3.870,50,(200); 2.446,9,(301); 2.331,12,(311). Natropharmacoalumite is cubic, space group with a = 7.7280(3) Å, V = 461.53(3) Å3 and Z = 1. The crystal structure was solved by direct methods and refined to R1 = 0.063 for 295 reflections with F>4σ(F). The structure conforms broadly to that of the general pharmacosiderite structure type, with Na as the dominant cation in cavities of strongly distorted Al octahedra and As tetrahedra. A new group nomenclature system for minerals with the pharmacosiderite structure has been established, including the renaming of aluminopharmacosiderite to pharmacoalumite.

Keywords

natropharmacoalumitepharmacosiderite groupnew mineral speciessodium aluminium arsenateMaria Josefa mineRodalquilarSpainmineral nomenclatureredefinitionaluminopharmacosideritepharmacoalumite
Type
Research Article
Information
Mineralogical Magazine , Volume 74 , Issue 5 , October 2010 , pp. 929 - 936
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2010

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