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Afmite, Al_3(OH)_4(H_2O)_3(PO_4)(PO_3OH)•H_2O, a new mineral from Fumade, Tarn, France: description and crystal structure

Kampf, Anthony R. and Mills, Stuart J. and Rossman, George R. and Steele, Ian M. and Pluth, Joseph J. and Favreau, Georges (2011) Afmite, Al_3(OH)_4(H_2O)_3(PO_4)(PO_3OH)•H_2O, a new mineral from Fumade, Tarn, France: description and crystal structure. European Journal of Mineralogy, 23 (2). pp. 269-277. ISSN 0935-1221.

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The new mineral afmite, Al_3(OH)_4(H_2O)_3(PO_4)(PO_3OH)·H_2O, is triclinic with space group Pl and cell parameters a = 7.386(3), b = 7.716(3), c = 11.345(4) Å, a = 99.773(5), β = 91.141(6), y = 115.58(5)°, V = 571.6(3) Å^3 and Z = 2. It occurs, sometimes in association with matulaite and variscite, in fractures and solution cavities in shale/siltstone at Fumade, Tarn, France. The formation is probably largely the result of remobilisation and crystallisation during low-temperature hydrothermal activity and/or weathering and ground water activity. Afmite forms in cockscomb aggregates of diamond-shaped tablets on {001}, ubiquitously contact-twinned on {001} and also commonly twinned by rotation on [010] with {010} and {110} composition planes, forming star-like sixlings. The streak of the mineral is white, the luster is pearly, and the Mohs hardness is about 11/2. The mineral is flexible, but not elastic, has an irregular fracture and three cleavage directions: {001} perfect, {010} and {1Formula 0} good. The measured density is 2.39(3) g/cm^3 and the calculated density is 2.391 g/cm^3 based upon the empirical formula. Optical properties (white light): biaxial (+), α = 1.554(1), β = 1.558(1), y = 1.566(1), 2V_(meas). = 70(5)° and 2V_(calc) = 71°. Electron microprobe analyses provided Al_2O_3 40.20 and P_2O_5 38.84 wt% and CHN analyses provided H_2O 25.64 wt%, total 103.68 wt%. Normalized EMP analyses and water based on the structure yield Al_2O_3 36.41, P_2O_5 35.17 and H_2O 28.42, total 100.00 wt%. Infrared and Raman spectra were consistent with the PO_3OH, OH and H_2O as indicated by the crystal-structure determination. The strongest powder X-ray diffraction lines are [d_(obs)(Å),I_(obs),(hkl)]: 11.089,100,(001), 3.540,81,(013,112), 5.484,79,(002,101), 2.918,60(122), 3.089,33(113,201), 4.022,30,(102,112), 6.826,23,(010). The crystal structure, solved from twinned data, (R_1 = 10.4 % for 1262 F_o > 4σF reflections) consists of chains of AlO_6 octahedra parallel to [110] in which edge-sharing octahedral dimers share corners with individual octahedra. Both PO_4 and PO_3OH tetrahedra link the chains into sheets parallel to {001} and the PO_4 tetrahedra further serve to link two sheets together into a thick slab in which tetrahedral (T) and octahedral (O) layers alternate, forming a T-O-T-O-T sandwich. The linkage between these sandwiches in the c direction is via hydrogen bonding with isolated H_2O groups in the interlayer region. Afmite is closely related structurally to the turquoise-group minerals and specifically to planerite. The recently described mineral kobokoboite probably has a closely related sheet structure.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 2011 E. Schweizerbart’sche Verlagsbuchhandlung. Received 13 October 2010. Modified version received 11 January 2011. Accepted 13 January 2011. Published online February 2011. Chief Editor Roland Oberhänsli and reviewers Peter Elliot and especially Andrew M. McDonald are thanked for helpful comments that significantly improved the paper. Jochen Schlueter is acknowledged for his assistance in arranging the CHN analysis at the University of Hamburg. This study was funded, in part, by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County and the White Rose Foundation at Caltech. The first set of structure data was collected at ChemMatCARS, Sector 15, Advanced Photon Source at Argonne National Laboratory. ChemMatCARS Sector 15 is principally supported by the National Science Foundation/Department of Energy under grant number CHE-0535644. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357.
Funding AgencyGrant Number
John Jago Trelawney EndowmentUNSPECIFIED
White Rose FoundationUNSPECIFIED
NSF/Department of Energy (DOE)CHE-0535644
Department of Energy (DOE) Office of ScienceDE-AC02-06CH11357
Subject Keywords:afmite, new mineral, crystal structure, acid phosphate, Al-phosphate, planerite, kobokoboite, Fumade, France.
Issue or Number:2
Record Number:CaltechAUTHORS:20110623-141938942
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Official Citation:Anthony R. Kampf, Stuart J. Mills, George R. Rossman, Ian M. Steele, Joseph J. Pluth, and Georges Favreau Afmite, Al3(OH)4(H2O)3(PO4)(PO3OH)·H2O, a new mineral from Fumade, Tarn, France: description and crystal structure Eur J Mineral 2011 23: 269-277.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24187
Deposited By: Ruth Sustaita
Deposited On:24 Jun 2011 17:15
Last Modified:03 Oct 2019 02:54

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