Crystal-structure refinement of Fe3+-rich aerinite from synchrotron powder diffraction and Mössbauer data

Pale-blue fibres of an Fe³⁺-rich variety of aerinite from Tartareu (Catalunya, Spain) were studied by synchrotron powder diffraction. Crystal data and composition are: a = b = 16.9161(1), c = 5.2289(1) Å, V = 1296 ų, space group P3c1, D_calc = 2.46 g cm³; (Ca_4.31, Na_0.07) (Al_5.25, Si_0.43, Mg_0.33) (Fe _1.36,³⁺ Fe_0.64^2.5+) (Fe_1.2³⁺, Mg_0.5, Al_0.3) [Si₁₂O₃₆(OH)₁₂] [(CO₃)_0.75 (SO₃)_0.25(H₂O)_11.1], Z = 1. The oxidation states of Fe in these fibres were determined from Mössbauer spectroscopy. The model of the structure was refined with the Rietveld method to the residual value R_wp = 0.036 (χ² = 1.73). The unit cell of aerinite contains two similar basic building units (columns) formed by three pyroxene chains pointing inwards producing two different cation sites (M1a,b) at the centres of the resulting face-sharing octahedra. The composition of M1a is 68 % Fe³⁺ and 32 % mixed-valence iron (Fe^2.5+) and the composition of M1b is 60 % Fe³⁺, 25 % Mg²⁺ and 15 % Al³⁺. The presence of mixed-valence iron provides an explanation for the observed blue colour of aerinite. According to the electron-microprobe analysis, 0.43 Si⁴⁺ can not be hosted in the pyroxene chains and most probably occupy M2 positions. However, due to the small amount that it represents, i.e. 6 % of the M2 sites, these positions have been refined as Al³⁺ assuming an octahedral coordination (final M2–O average bond length of 1.92(6) Å). The large “channel” includes 0.75 CO₃²⁻ and 0.25 sulphur atoms, probably as sulphite groups, stacked along one threefold axis. Inspection of a polished thin section of the same Tartareu specimen from which the studied aerinite fibres were taken shows the presence of an additional deeper blue phase forming a stripe of approximately 0.2–0.3 mm thickness between the pale-blue aerinite and the laumontite substrate. The electron-microprobe analysis of this new phase indicates that it is related to aerinite but Si-richer and Fe- and Ca-poorer. Both phases also display a different optical behaviour under transmitted polarised light.