The crystal structures of niobophyllite, kupletskite-(cs) and Sn-rich astrophyllite : revisions to the crystal chemistry of the astrophyllite-group minerals
Articolo
Data di Pubblicazione:
2010
Citazione:
The crystal structures of niobophyllite, kupletskite-(cs) and Sn-rich astrophyllite : revisions to the crystal chemistry of the astrophyllite-group minerals / F. Cámara, E. Sokolova, Y. Abdu, F.C. Hawthorne. - In: CANADIAN MINERALOGIST. - ISSN 0008-4476. - 48:1(2010 Feb), pp. 1-16.
Abstract:
The crystal structures of niobophyllite, K2Na Fe 72+ (NbTi) (Si4O12)2 O2 (OH)4 O, kupletskite-(Cs), Cs2Na Mn 7Ti2 (Si4O12)2 O 2 (OH)4 F, and Sn-rich astrophyllite, K2Na Fe72+ Ti2 (Si4O12) 2 O2 (OH)4 F, all members of the astrophyllite group, have been refined in space group P1̄ to R1 values 5.96, 3.33 and 3.53% for 3576, 3490 and 3791 observed [Fo > 4σF] unique reflections measured with MoKα radiation on a Bruker AXS diffractometer with a CCD SMART APEX detector. The crystal structures of all three minerals are topologically identical to that of triclinic astrophyllite. The crystals used in the collection of the X-ray intensity data were analyzed by electron microprobe, Fe3+ : (Fe2+ + Fe3+) ratios were measured by Mössbauer spectroscopy, and Li was determined by laser-ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS). The empirical formulae were calculated on an anion basis, (O + OH + F) = 31 apfu: niobophyllite, (K1.79Cs0.03)Σ1.82Na 1.01 (Fe4 022+ Mn1.98 Fe 0.553+)Σ6.55 (Nb1 46 Ti 0.55Ta0 01)Σ2.02 [(Si 7.77Al0.20)Σ7.97O24] O 2.73 [(OH)4.13F0.13]Σ4.26; kupletskite-(Cs), (Cs1.42K0.36Ca0 08Pb 0.06Na0.05Sr0.02)Σ1 99 (Na0.73Ca0.27)Σ1 (Mn3.74 Fe1.822+Li0.68Zn0.50Fe 0.133+Mg0.11)Σ6.99(Ti 1.54Nb0 45Zr0.02)Σ2.01. [(Si7.90Al0.05)Σ7.95O24] O2.03 [(OH)4 26 F0 71]Σ4 97; Sn-rich astrophyllite, (K1.70Ca0.04Cs 0.03Pb0.01)Σ1.78 (Na 0 96Ca0.04)Σ1 (Fe6.182+Mn0 45Fe0.263+.Ca 0.09Zn0.02)Σ7(Ti0.74Sn 0.62 Nb0.44Zr0.16Ta0.02Mg 0.02)Σ2.00 [(Si7.75Al0.16) Σ7.91O24] O2 [(OH)4.356F 0.65]Σ5 00; Z = 1. In Sn-rich astrophyllite, Sn 4+ replaces Ti4+ at the D site, and there is a corresponding expansion of the D octahedron: = 1.979 Å, where φ represents O, F, OH. The discovery of Sn-rich astrophyllite indicates that there is potential for a new species in the astrophyllite group with Sn4+as the dominant cation at the D site. We have identified three distinct A sites, A(1), A(2) and A(3) with coordinations [13], [5] and [13], respectively, that are separated by ≤1.2 Å such that locally adjacent sites cannot both be occupied. We suggest the possibility of Ca occupying the B site via the substitution BCa2++ A□ → BNa++AK +producing a potential new mineral of the astrophyllite group. The general formula of the astrophyllite-group minerals should be written as A 2 B C7 D2 (T4O12) 2 O2 (OH)4 X0-1 as this (i) preserves the correct formula of the silicate radical in the structure, and (ii) identifies the O2 anion component of the structure as not bonded to Si.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Astrophyllite group; Crystal structure; Electron-microprobe analysis; Kupletskite-(Cs); Mössbauer spectroscopy; Niobophyllite; Sn-rich astrophyllite
Elenco autori:
F. Cámara, E. Sokolova, Y. Abdu, F.C. Hawthorne
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