• journal_title：The Canadian Mineralogist
• Contributor：Fernando Cámara ; Elena Sokolova ; Yassir Abdu ; Frank C. Hawthorne
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.1.1
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：1
• firstpage：1
• section：Articles

The crystal structures of niobophyllite, K₂ Na Fe²⁺₇ (NbTi) (Si₄O₁₂)₂ O₂ (OH)₄ O, kupletskite-(Cs), Cs₂ Na Mn₇ Ti₂ (Si₄O₁₂)₂ O₂ (OH)₄ F, and Sn-rich astrophyllite, K₂ Na Fe²⁺₇ Ti₂ (Si₄O₁₂)₂ O₂ (OH)₄ F, all members of the astrophyllite group, have been refined in space group P1̅ to R₁ values 5.96, 3.33 and 3.53% for 3576, 3490 and 3791 observed [F_o > 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, (K_1.79 Cs_0.03)_∑1.82 Na_1.01 (Fe²⁺_4.02 Mn_1.98 Fe³⁺_0.55)_∑6.55 (Nb_1.46Ti_0.55Ta_0.01)_∑2.02 [(Si_7.77 Al_0.20)_∑7.97O₂₄] O_2.73 [(OH)_4.13F_0.13]_∑4.26; kupletskite-(Cs), (Cs_1.42K_0.36Ca_0.08Pb_0.06Na_0.05Sr_0.02)_∑1.99 (Na_0.73Ca_0.27)_∑1 (Mn_3.74 Fe²⁺_1.82Li_0.68Zn_0.50Fe³⁺_0.13Mg_0.11)_∑6.99 (Ti_1.54Nb_0.45Zr_0.02)_∑2.01 [(Si_7.90Al_0.05)_∑7.95O₂₄] O_2.03 [(OH)_4.26 F_0.71]_∑4.97; Sn-rich astrophyllite, (K_1.70Ca_0.04Cs_0.03Pb_0.01)_∑1.78 (Na_0.96Ca_0.04)_∑1 (Fe²⁺_6.18Mn_0.45Fe³⁺_0.26Ca_0.09Zn_0.02)_∑7 (Ti_0.74Sn_0.62 Nb_0.44Zr_0.16Ta_0.02Mg_0.02)_∑2.00 [(Si_7.75 Al_0.16)_∑7.91O₂₄] O₂ [(OH)_4.356F_0.65]_∑5.00; Z = 1. In Sn-rich astrophyllite, Sn⁴⁺ 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⁺ and producing a potential new mineral of the astrophyllite group. The general formula of the astrophyllite-group minerals should be written as A₂ B C₇ D₂ (T₄O₁₂)₂ O₂ (OH)₄ X_0–1 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.