Afmite, Al3(OH)4(H2O)3(PO4)(PO3OH)·H2O, a new mineral from Fumade,
Tarn, France: description and crystal structure

Kampf AR
Mineral Sciences Department
Natural History Museum of Los Angeles County, Los Angeles, CA 90007, USA

Mills SJ
Department of Earth and Ocean Sciences
University of British Columbia, Vancouver, BC, Canada V6T 1Z4

Rossman GR
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125, USA

Steele IM, Pluth JJ
Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA

Favreau G
421 Avenue Jean Monnet, 13090 Aix-en-Provence, France


The new mineral afmite, Al3(OH)3(H2O)3(PO4)(PO3OH)·H2O, is triclinic with space group P1 and cell parameters a = 7.386(3), b = 7.716(3), c = 11.345(4) Å, α = 99.773(5), β = 91.141(6), γ = 115.58(5) V = 571.6(3) Å3 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 {100} composition planes, forming star-like sixlings. The streak of the mineral is white, the luster is pearly, and the Mohs hardness is about 1½. The mineral is flexible, but not elastic, has an irregular fracture and three cleavage directions: {001} perfect, {010} and {1-10} good. The measured density is 2.39(3) g/cm3 and the calculated density is 2.391 g/cm3 based upon the empirical formula. Optical properties (white light): biaxial (+), α = 1.554(1), β = 1.558(1), γ = 1.566(1), 2Vmeas. = 70(5) and 2Vcalc = 71. Electron microprobe analyses provided Al2O3 40.20 and P2O5 38.84 wt% and CHN analyses provided H2O 25.64 wt%, total 103.68 wt%. Normalized EMP analyses and water based on the structure yield Al2O3 36.41, P2O5 35.17 and H2O 28.42, total 100.00 wt%. Infrared and Raman spectra were consistent with the PO3OH, OH and H2O as indicated by the crystal structure determination. The strongest powder X-ray diffraction lines are [dobs(Å),Iobs,(hkl)]: 11.089,(100,001), 3.540,81,(0-13,-1-12), 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, (R1 = 10.4 % for 1262 Fo > 4σF reflections) consists of chains of AlO6 octahedra parallel to [110] in which edge-sharing octahedral dimers share corners with individual octahedra. Both PO4 and PO3OH tetrahedra link the chains into sheets parallel to {001} and the PO4 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 H2O 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.


0.4 mm wide aggregates of afmite crystals