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: <D-φ> = 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.

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.

The crystal structures of niobophyllite, kupletskite-(cs) and Sn-rich astrophyllite : revisions to the crystal chemistry of the astrophyllite-group minerals

F. Cámara
;
2010

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.
Astrophyllite group; Crystal structure; Electron-microprobe analysis; Kupletskite-(Cs); Mössbauer spectroscopy; Niobophyllite; Sn-rich astrophyllite
Settore GEO/06 - Mineralogia
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/478946
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