Abstract
Sr-substitution in the crystal structure of bredigite has been studied in detail. Samples of a hypothetical solid-solution series with nominal composition Ca7-xSrxMg[SiO4]4 (x = 0, 2, …,7) have been prepared from sinter reactions in the temperature range between 1275 and 1325 °C and characterized using powder and single-crystal X-ray diffraction. Synthesis runs between x = 1 and x = 4 resulted in compounds with increasing Sr contents, for which single-crystal diffraction studies revealed the following Sr/(Sr + Ca) atomic ratios: 0.133,0.268, 0.409 and 0.559. They are isostructural to the pure calcium end-member (x = 0) and adopt the orthorhombic space group Pnnm. Evolution of the unit-cell parameters and cell volumes of the solid-solution series are defined by linear or nearly linear trends when plotted against the Sr/(Sr + Ca) atomic ratio. Replacement of calcium with strontium atoms on the different sites shows clear preferences for specific positions. For the experiment with x = 5, formation of bredigite-related single-crystals with Sr/(Sr + Ca) = 0.675 was observed. These samples, however, exhibited a halved c lattice parameter when compared with the corresponding value in the Pnnm structure, pointing to a compositionally induced phase transition somewhere in region between Sr/(Sr + Ca) = 0.559 and 0.675. The crystal structure of this new phase with composition Ca2.32Sr4.82Mg0.86[SiO4]4 was successfully determined in space group Pbam. Basic crystallographic data are as follows: a = 18.869(2) Å, b = 6.9445(8) Å, c = 5.5426(6) Å, V = 726.28(14) Å3, Z = 2. Structure determination was accomplished using charge flipping. Subsequent least-squares refinements resulted in a residual of R(|F|) = 2.70% for 822 independent reflections and 87 parameters. The Pbam- and the Pnnm-structures are in a group-subgroup relationship and topologically related. Both are based on so-called pinwheel-like MSi6O24 clusters consisting of a central magnesium-dominated [MO6]-octahedron as well as six attached [SiO4]-tetrahedra. The clusters are linked into chain-like elements running along [001]. Linkage between the chains is provided by mixed Sr/Ca positions with 6 to 10 oxygen ligands. Differences between the two phases result from changes in Sr-Ca site occupancies in combination with displacements of the atoms and tilts of the tetrahedra. The distortion pattern has been studied using group-theoretical methods including mode analysis. Notably, for the samples with x = 6 and x = 7—the latter corresponding to the hypothetical pure strontium end-member composition—no bredigite-type phases could be identified, indicating that there is an upper limit for the Sr-uptake.
Highlights
Bredigite is an exotic phase, presently known from only a handful of localities listed in the mindat.org database
Bredigite occurs in quite a number of different petrological settings all of which are related to pyrometamorphism (Tilley and Vincent 1948; Bridge 1966; Grapes 2011; Žáček et al 2015; Kahlenberg et al 2019)
Diffraction experiments point to a klassengleiche phase transition of index 2 somewhere in the compositional range between Sr/(Sr + Ca) = 0.559 and 0.675
Summary
Bredigite (ideally C a7Mg[SiO4]4) is an exotic phase, presently known from only a handful of localities listed in the mindat.org database When compared with the data for the pure calcium end-member, samples with nominal compositions C a6.5Sr0.5 Mg[SiO4]4, Ca6SrMg[SiO4]4 and Ca6.5Ba0.5 Mg[SiO4]4 showed a significant increase in the values of the unit-cell volumes pointing to a successful substitution among the larger divalent cations. Due to the limitations of structural studies based on X-ray powder diffraction data in the era before the Rietveld method became a standard tool for this purpose, Moseley and Glasser (1982) did not perform any more detailed crystallographic investigations on their polycrystalline samples. For the samples Bred-1 to Bred-5, crystals showing bredigite-related unit cells could be found From each of these sets, the crystal with the best overall diffraction quality was selected for full data collection (see Table 1). For the crystals from samples Bred-1 to Bred-4 the values of the lattice parameters indicated the presence of bredigite-type phases isostructural to the pure calcium endmember. The same program was employed for the calculation of the effective coordination numbers (ECoN) as well as volumes and distortion indices of the M Ox-polyhedra which can be found in Tables 4 and 5
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