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Afmite, Al (sub 3) (OH) (sub 4) (H (sub 2) O) (sub 3) (PO (sub 4) )(PO (sub 3) OH).H (sub 2) O, a new mineral from Fumade, Tarn, France; description and crystal structure

Anthony R. Kampf, Stuart J. Mills, George R. Rossman, Ian M. Steele, Joseph J. Pluth and Georges Favreau
Afmite, Al (sub 3) (OH) (sub 4) (H (sub 2) O) (sub 3) (PO (sub 4) )(PO (sub 3) OH).H (sub 2) O, a new mineral from Fumade, Tarn, France; description and crystal structure
European Journal of Mineralogy (March 2011) 23 (2): 269-277

Abstract

The new mineral afmite, Al (sub 3) (OH) (sub 4) (H (sub 2) O) (sub 3) (PO (sub 4) )(PO (sub 3) OH).H (sub 2) O, is triclinic with space group P1 and cell parameters a=7.386(3), b=7.716(3), c=11.345(4) Aa, alpha =99.773(5), beta =91.141(6), gamma =115.58(5) degrees , V=571.6(3) Aa (super 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 1 1/2. The mineral is flexible, but not elastic, has an irregular fracture and three cleavage directions: {001} perfect, {010} and {110} good. The measured density is 2.39(3) g/cm (super 3) and the calculated density is 2.391 g/cm (super 3) based upon the empirical formula. Optical properties (white light): biaxial (+), alpha =1.554(1), beta =1.558(1), gamma =1.566(1), 2V (sub meas.) =70(5) degrees and 2V (sub calc) =71 degrees . Electron microprobe analyses provided Al (sub 2) O (sub 3) 40.20 and P (sub 2) O (sub 5) 38.84 wt% and CHN analyses provided H (sub 2) O 25.64 wt%, total 103.68 wt%. Normalized EMP analyses and water based on the structure yield Al (sub 2) O (sub 3) 36.41, P (sub 2) O (sub 5) 35.17 and H (sub 2) O 28.42, total 100.00 wt%. Infrared and Raman spectra were consistent with the PO (sub 3) OH, OH and H (sub 2) O as indicated by the crystal-structure determination. The strongest powder X-ray diffraction lines are [d (sub obs) (Aa),I (sub 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 (sub 1) =10.4% for 1262 F (sub o) >4sigma F reflections) consists of chains of AlO (sub 6) octahedra parallel to [110] in which edge-sharing octahedral dimers share corners with individual octahedra. Both PO (sub 4) and PO (sub 3) OH tetrahedra link the chains into sheets parallel to {001} and the PO (sub 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 (sub 2) O 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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 23
Serial Issue: 2
Title: Afmite, Al (sub 3) (OH) (sub 4) (H (sub 2) O) (sub 3) (PO (sub 4) )(PO (sub 3) OH).H (sub 2) O, a new mineral from Fumade, Tarn, France; description and crystal structure
Affiliation: Natural History Museum of Los Angeles County, Mineral Sciences Department, Los Angeles, CA, United States
Pages: 269-277
Published: 201103
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 14
Accession Number: 2011-044464
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N43°39'30" - N43°39'30", E02°29'58" - E02°29'58"
Secondary Affiliation: University of British Columbia, CAN, CanadaCalifornia Institute of Technology, USA, United StatesUniversity of Chicago, USA, United States
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201125
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