Synthesis and structure of a stuffed derivative of α-quartz, Mg_0.5AlSiO₄

Abstract

A structural derivative of quartz with the composition Mg 0.5 AlSiO 4 has been grown from glass and characterized using synchrotron X-ray diffraction (XRD), transmission electron microscopy (TEM), and 29 Si nuclear magnetic resonance (NMR) spectroscopy. Rietveld analysis of the XRD data indicates that the framework of Mg 0.5 AlSiO 4 is isostructural with a-quartz, rather than b-quartz, as is consistent with previous theoretical modeling (Sternitzke and Müller 1991). Al and Si exhibit long-range disorder over the framework tetrahedral sites, indicated by the absence of the superlattice reflections corresponding to the doubling of c relative to that of quartz. Nevertheless, 29 Si NMR measurements show that Al and Si exhibit partial short-range order with an ordering degree of 56%. Electron diffraction reveals superlattice reflections indicative of doubled periodicities along the a-axes. Fourier electron density maps show that Mg occupies channel sites that each are bonded to six O atoms, in contrast to the tetrahedral coordination of Li in the b-quartz-type framework for b-eucryptite, LiAlSiO 4 . Furthermore, the concentrations of Mg in adjacent channels are different, resulting in framework distortions that generate the superstructures along a.

Mg‐bearing quartz solid solutions as structural intermediates between low and high quartz

“…Si-O-Si angles are strongly affected at low unit cell volumes (ie high SiO 2 content) ( Figure 4A) and soon stabilize around ~148°, in accordance with the recent results of Xu et al 24 The longer-range Si-Si-Si angles within the Qss helical chains parallel to c show a continuous variation over the studied compositional range ( Figure 4B), manifesting a gradual tetrahedral tilt that leads to an increasing "pseudo-hexagonality" of the c-channels (see the progression in Figure 5F,D,B,C). Such relations are in agreement with the description provided by Xu et al, 24,25 who referred to a tetrahedral rotation angle δ and remarked the intermediate value obtained for Mg-Qss at 299 K (~9.4°), between those of room-temperature low quartz (15.8°) and high quartz (0°). In turn, the distorted octahedral oxygen coordination of Mg appears to achieve progressively higher regularity with increasing unit cell volume (ie decreasing SiO 2 content, Figure 4C).…”

“…5 Within this work, the Qss stoichiometry had to be estimated through the sof of Mg obtained during Rietveld refinements, since all specimens were obtained from the same parent glass. The computed values ( Figure 3B and Table 2) matched satisfactorily the scattered trend of the available literature sources, 5,24,26 which were indicated based on the nominal composition of the parent glass annealed to form Qss. However, the determination using the sof proved less successful at comparatively high SiO 2 content (see the four rightmost data points in Figure 3B).…”

“…The structure of Qss was then refined, enabling the sof of Mg and all symmetrically unconstrained atomic coordinates; refinement of the atomic displacement parameters proved instead unstable and was therefore omitted, fixing their values to those previously determined by other authors. 24 The uncertainty of the values was obtained multiplying by 3 the estimated standard deviations provided by the program. The obtained structures were sketched using F I G U R E 3 A, Plot of the room-temperature lattice parameters obtained for Mg-bearing Qss within this work and from some literature references, 5,14,24,26 as well as for pure quartz 31 (* labels the value pertaining to high quartz at 600°C).…”

“…24 The uncertainty of the values was obtained multiplying by 3 the estimated standard deviations provided by the program. The obtained structures were sketched using F I G U R E 3 A, Plot of the room-temperature lattice parameters obtained for Mg-bearing Qss within this work and from some literature references, 5,14,24,26 as well as for pure quartz 31 (* labels the value pertaining to high quartz at 600°C). The plot contains also the values obtained from literature sources for Li-bearing Qss, 10,18,20 the software VESTA.…”

Inversion of quartz solid solutions at cryogenic temperatures

“…Note that Qss phases can be also obtained if Mg and Zn are employed as stuffing cations instead of Li 10,11,20‐23 . Particularly in the case of Mg doping, however, the univocal assignment of their structure to a high or low quartz symmetry is still debated 24‐26 …”