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Penberthycroftite, [Al (sub 6) (AsO (sub 4) ) (sub 3) (OH) (sub 9) (H (sub 2) O) (sub 5) ].8H (sub 2) O; a second new hydrated aluminium arsenate mineral from the Penberthy Croft Mine, St. Hilary, Cornwall, UK

Ian E. Grey, John Betterton, Anthony R. Kampf, C. M. Macrae, F. L. Shanks and J. R. Price
Penberthycroftite, [Al (sub 6) (AsO (sub 4) ) (sub 3) (OH) (sub 9) (H (sub 2) O) (sub 5) ].8H (sub 2) O; a second new hydrated aluminium arsenate mineral from the Penberthy Croft Mine, St. Hilary, Cornwall, UK
Mineralogical Magazine (December 2016) 80 (7): 1149-1160

Abstract

Penberthycroftite, ideally [Al (sub 6) (AsO (sub 4) ) (sub 3) (OH) (sub 9) (H (sub 2) O) (sub 5) ].8H (sub 2) O, is a new secondary aluminium arsenate mineral from the Penberthy Croft mine, St. Hilary, Cornwall, England, UK. It occurs as tufts of white, ultrathin (sub-micrometre) rectangular laths, with lateral dimensions generally <20 mu m. The laths are flattened on {010} and elongated on [100]. The mineral is associated with arsenopyrite, bettertonite, bulachite, cassiterite, chalcopyrite, chamosite, goethite, liskeardite, pharmacoalumite-pharmacosiderite and quartz. Penberthycroftite is translucent with a white streak and a vitreous to pearly lustre. The calculated density is 2.18 g/cm (super 3) Optically, only the lower and upper refractive indices could be measured, 1.520(1) and 1.532(1) respectively. No pleochroism was observed. Electron microprobe analyses (average of 14) with H (sub 2) O obtained from thermogravimetric analysis and analyses normalized to 100% gave Al (sub 2) O (sub 3) =31.3, Fe (sub 2) O (sub 3) =0.35, As (sub 2) O (sub 5) =34.1, SO (sub 3) =2.15 and H (sub 2) O=32.1. The empirical formula, based on nine metal atoms and 26 framework anions is [Al (sub 5.96) Fe (sub 0.04) (As (sub 0.97) Al (sub 0.03) O (sub 4) ) (sub 3) (SO (sub 4) ) (sub 0.26) (OH) (sub 8.30) (H (sub 2) O) (sub 5.44) ] (H (sub 2) O) (sub 7.8) , corresponding to the ideal formula [Al (sub 6) (AsO (sub 4) ) (sub 3) (OH) (sub 9) (H (sub 2) O) (sub 5) ].8H (sub 2) O. Penberthycroftite is monoclinic, space group P2 (sub 1) /c with unit-cell dimensions (100 K): a=7.753(2) Aa, b=24.679(5) Aa, c=15.679(3) Aa and beta =94.19(3) degrees . The strongest lines in the powder X-ray diffraction pattern are [d (sub obs) in Aa(I) (hkl)] 13.264(46)(011); 12.402(16)(020); 9.732(100)(021); 7.420(28)(110); 5.670(8)(130); 5.423(6)(131). The structure of penberthycroftite was solved using synchrotron single-crystal diffraction data and refined to wR (sub obs) =0.059 for 1639 observed (I>3sigma (I)) reflections. Penberthycroftite has a heteropolyhedral layer structure, with the layers parallel to {010}. The layers are strongly undulating and their stacking produces large channels along [100] that are filled with water molecules. The layers are identical to those in bettertonite, but they are displaced relative to one another along [001] and [010] such that the interlayer volume is decreased markedly (by approximately 10%) relative to that in bettertonite, with a corresponding reduction in the interlayer water content from 11 H (sub 2) O per formula unit (pfu) in bettertonite to 8 H (sub 2) O pfu in penberthycroftite.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 80
Serial Issue: 7
Title: Penberthycroftite, [Al (sub 6) (AsO (sub 4) ) (sub 3) (OH) (sub 9) (H (sub 2) O) (sub 5) ].8H (sub 2) O; a second new hydrated aluminium arsenate mineral from the Penberthy Croft Mine, St. Hilary, Cornwall, UK
Affiliation: CSIRO Mineral Resources, Clayton, Victoria, Australia
Pages: 1149-1160
Published: 201612
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 23
Accession Number: 2017-005914
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N50°00'00" - N50°30'00", W05°30'00" - W05°10'00"
Secondary Affiliation: Haslemere Educational Museum, GBR, United KingdomNatural History Museum of Los Angeles County, USA, United StatesMonash University, AUS, AustraliaAustralian Synchrotron, AUS, Australia
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: 201705
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