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1 CNR Istituto di Geoscienze e Georisorse (IGG), unità di Pavia, via Ferrata 1, I-27100 Pavia, Italy
2 Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, 24 rue Lhomond, F-75005 Paris, France
3 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
Correspondence: * E-mail: oberti{at}crystal.unipv.it
A re-examination of the chemistry and structure of nearly all the known occurrences of arrojadite and related minerals (dickinsonite and sigismundite) allowed understanding of the main substitution vectors and cation ordering schemes ruling the crystal-chemistry of these very complex phosphates. Electron microprobe analyses were done with a careful choice of the standards and of experimental conditions, and were coupled with LA-ICP-MS in situ analysis for Li, Be, and B. Structure refinement was done in a space group (Cc) with a lower symmetry than those used in previous studies (C2/c and its equivalents), which allowed a better understanding of the structure details and of cation ordering. The combined approach yielded a new formula for the arrojadite group, namely A2B2Ca1Na2+xM13Al (PO4)11(PO3OH1–x)W2, where A are either large divalent cations (Ba, Sr, Pb) plus vacancy, or monovalent (K, Na) cations; and B are either small divalent cations (Fe, Mn, Mg) plus vacancy, or monovalent (Na) cations. The number of hydroxyl groups in the arrojadite formula is generally 3 apfu, and can be lowered to 2 apfu in particular when the sum of non-(P,Al) cations is higher than 20 apfu.
We present in this paper the complete characterization of three samples (two of which are new members) that are crucial to fix the cornerstones of arrojadite crystal-chemistry. The sample from Rapid Creek (Yukon Territory) is the holotype for arrojadite-(KNa), and has unit formula K0.83Na5.01(Ca0.91Sr0.01)
= 0.92 (Fe9.342+Mg2.69Mn1.032+Zn0.01Li0.01) = 13.08 (Al1.04Ti0.02) = 1.06(OH1.97F0.03) =2.00[(P11.99Si0.01T1)O47(OH)1.00] [ideally, A1K A2Na B1Na B2Na Na1,2Na2 Na3
CaCa MFe13 Al (PO4)11 P1x (PO3OH) W(OH,F)2] and unit-cell dimensions: a = 16.5220(11), b = 10.0529(7), c = 24.6477 (16) Å, ß = 106.509(2)°, V = 3932.2(7) Å3 (Z = 4). The sample from Horrsjöberg (Värmland, Sweden) is the holotype material for arrojadite-(SrFe), and has unit formula Sr0.93Na3.20(Ca0.59Ba0.20Pb0.03K0.03) = 0.85 (Fe6.642+Mg3.61Mn3.332+Zn0.07Li0.01) = 13.66 (Sc0.04Al1.00) = 1.04 (OH1.10F0.90) = 2.00[(P11.95Si0.02) = 11.97O47(OH)1.00] [ideally, A1Sr A2B1Fe2+ B2Na1,2Na2 Na3CaCa MFe132+ Al (PO4)11 P1x(PO3OH) W(OH,F)2], and unit-cell dimensions a = 16.3992(7), b = 9.9400(4), c = 24.4434(11) Å, ß = 105.489(1)°, V = 3839.76(46) Å3. The sample from Branchville (Connecticut) is the holotype material for dickinsonite-(KMnNa), and has unit formula K0.50Na5.78(Ca0.51Sr0.05Ba0.01Pb0.01) = 0.58(Mn9.702+ Fe3.722+Li0.31Mg0.06Zn0.01)=13.80(Al0.91Fe0.093+Ti0.01)=1.00(OH1.97F0.03)=2.00[(P12.02Si0.01) = 12.03O47(OH)0.21] [ideally, A1K A2Na B1Mn B2Na1,2Na2 Na3Na CaCa MMn13 Al (PO4)11 P1x (PO4) W(OH, F)2] and unit-cell dimensions a = 16.6900 (9), b = 10.1013 (5), c = 24.8752 (13) Å, ß = 105.616(2)°, V = 4038.9(7) Å3.
Key Words: Crystal structure • arrojadite • analysis (chemical) • new minerals • arrojadite-(KNa) • arrojadite-(SrFe) • dickinsonite-(KMnNa) • optical properties • Raman spectroscopy • XRD data
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