The crystal structure of an Fe³⁺-rich variety of aerinite has been refined from synchrotron powder diffraction data. The specimen comes from Tartareu (Catalunya, Spain) and is formed by pale-blue fibres. Unit cell dimensions and composition: a = b = 16.9161, c = 5.2289 Å, V= 1296 ų with space group P3c1 and D_calc= 2.46 g/cm³; [(Ca_4.3,Na_.1) (Al_5.3,Si_.4,Mg_.3) (OH)₁₂ (H₂O)_11.1] [(Fe³⁺_1.36,Fe^2.5+_0.64) (Fe³⁺_1.2,Mg_0.5,Al_0.3) Si_11.9P_0.1O₃₆] (CO₃)_.75 (SO₃)_.25 Z=1. Oxidation states of Fe 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 contains two similar basic building units (columns) formed by three pyroxene chains pointing inwards with different cation sites at the centres of the resulting face-sharing octahedra. These sites are occupied by different cation species compositions. One is formed by 68% Fe³⁺ and 32% mixed valence iron (Fe^2.5+), while the other has 60% Fe³⁺, 25% Mg²⁺ and 15% Al³⁺. According  to the electron microprobe data and to the crystal chemical study approximately 12% of the more internal metal positions of the brucite-like layer that connects each two columns, show substitution of Al³⁺ with Si⁴⁺ (6.6%) and Mg²⁺ (5%). The unit cell contains in the middle of the channel 0.75 CO₃^2- and 0.25 sulphur atoms, presumable as SO₃^2-, stacked along one of the three ternary axis and mutually separated by 5.23 Å.

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1. Advances of the constrained S-FFT direct phasing method to powder diffraction data