• journal_title：European Journal of Mineralogy
• Contributor：Anthony R. Kampf ; Stuart J. Mills ; George R. Rossman ; Ian M. Steele ; Joseph J. Pluth ; Georges Favreau
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2011/0023-2093
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：23
• issue：2
• firstpage：269
• section：Articles

The new mineral afmite, Al₃(OH)₄(H₂O)₃(PO₄)(PO₃OH)·H₂O, is triclinic with space group P1̄ 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) ų 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 {11̄ 0} 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 {11̄ 0} 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), 2V_meas. = 70(5)° and 2V_calc = 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 [d_obs(Å),I_obs,(hkl)]: 11.089,100,(001), 3.540,81,(01̄3,1̄1̄2), 5.484,79,(002,101), 2.918,60(1̄22), 3.089,33(1̄13,201), 4.022,30,(102,1̄12), 6.826,23,(010). The crystal structure, solved from twinned data, (R₁ = 10.4 % for 1262 F_o > 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.