Structure determination of the blue mineral pigment aerinite from synchrotron powder diffraction data : The solution of an old riddle

The structure of aerinite, a blue fibrous silicate mineral associated with the alteration of ophitic rocks in the southern Pyrenees, has been determined by applying the direct methods modulus sum function to synchrotron powder diffraction data. This mineral was the blue pigment commonly used in most Catalan romanic paintings between the XI-XV centuries. The studied specimen comes from the Camporrells-Estopanyà area (Huesca, Spain). The unit cell dimensions are a = b = 16.8820(9), c = 5.2251(3) Å, the space group is P3c1 and the structural formula is (Ca_5.1Na_0.5)(Fe³⁺AlFe²⁺_1.7Mg_0.3)(Al_5.1Mg_0.7)[Si₁₂O₃₆(OH)₁₂H]·[(CO₃)_1.2(H₂O)₁₂] with Z = 1 and D_c = 2.52 g/cm³ (Fe²⁺/Fe³⁺ ratio from Mössbauer spectroscopy; carbonate content confirmed by infrared spectroscopy). The model of the structure obtained by direct methods was refined with the Rietveld method to the residual value R_wp = 0.0937 (χ² = 1.05). A bond valence analysis shows the plausibility of the refined model.

The crystal structure of aerinite can be best understood by introducing cylindrical basic building units consisting on three pyroxene chains pointing inwards to accommodate tri- and divalent metal cations at the centres of the resulting face-sharing octahedra. The average composition of these cationic sites is Fe³⁺_0.25,Al³⁺_0.25,Fe²⁺_0.43,Mg²⁺_0.07, the mean cation-oxygen bond length is 2.054(9) Å and the intercationic distance is 2.61 Å. Out of the three symmetry-independent three-fold rotation axes in the unit cell, two are occupied by such cylindrical units and the third by CO₃ groups. Consequently, each unit is surrounded by three similar ones which are, however, shifted by 0.93 Å along c. Between two such units, i.e., tangential to both cylindrical envelopes, a four-row wide slab of a brucite-like layer is found. The two inner octahedra are predominantly filled with Al and Mg atoms, the two outer with Ca, Na and some vacancies [average values: d(Al-O) = 1.936(53) Å (6x), ∠(O-Al-O) = 90.1(6.5)° (12x) and 172.7(4.1)° (3x); d(Ca-O) = 2.42(6) Å (6x), ∠(O-Ca-O) = 90(23)° (12x) and 158(14)° (3x)]. The internal O atoms of the brucite-like layer are hydroxyl groups, the intermediate are unshared basal O atoms of the neighbouring pyroxene chains, while the external ones are water molecules forming relatively strong H-bridges with the partially disordered CO₃ groups. Presumably, the hydroxyl groups in the brucite-like layer also form H-bridges with the apical O atoms of the neighbouring pyroxene chains to compensate for the defect of charge caused by the presence of divalent cations in the face-sharing octahedra.