Short-Range Structure of Invert Glasses along the Pseudo-Binary Join MgSiO₃−Mg₂SiO₄: Results from ²⁹Si and ²⁵Mg MAS NMR Spectroscopy

Abstract

The short-range structure of "invert" glasses along the pseudobinary join MgSiO(3)-Mg(2)SiO(4) has been studied using (29)Si and (25)Mg MAS NMR spectroscopy. The results indicate a progressive compositional evolution in Q speciation that approximately follows a statistical distribution. The Mg(2)SiO(4) glass shows an abrupt deviation from this trend with the presence of nearly 40% of the Si atoms as (Si(2)O(7))(6-) dimers, i.e., Q(1) species. Mg(2+) ions are present in predominantly octahedral coordination in all glasses. When taken together, these results indicate that glasses with MgO contents between 50 and 60 mol % are characterized by a structure consisting primarily of at least three types of Q species and MgO(6) octahedra. On the other hand, the structure of glasses with >60 mol % MgO appears to consist of Q(0) and Q(1) species with structural connectivity being primarily provided by the MgO(6) octahedra. The possible consequences of such compositional evolution of structure on the ability of glass formation in this system are discussed.

Relationship between topological order and glass forming ability in densely packed enstatite and forsterite composition glasses

“…In the case of MgSiO 3 glass, the RMC model suggests that MgO 4 is the major unit, which is consistent with the coordination number derived from a combination of X-ray and neutron diffraction data (16,17). However, the result of two recent NMR studies (14,15) suggests that MgO 6 octahedra are the primary units in both MgSiO 3 glass and Mg 2 SiO 4 glass (although 5-fold sites could not be ruled out). Attempts to include large fractions of MgO 6 polyhedra in our RMC simulations failed, even if the coordination number was calculated out to large distances i.e., 3.0 Å.…”

“…The X-ray total structure factors SðQÞ together with the experimental data (40) The DFT calculations explain the discrepancy between the NMR and diffraction results, because NMR probes chemical shifts which are very sensitive to the electronic environment of the nuclei, while diffraction is a direct probe of the average coordination number through known neutron scattering lengths or the number of electrons surrounding an atom (provided the partial functions are known). Previous studies (14,15) concluded that for MgSiO 3 and Mg 2 SiO 4 glasses, the NMR shifts are in line with the octahedral crystalline environment, although in this (38). The structural glass model presented here at ambient pressures does however explain the low viscosity of highly mobile ultramafic lavas.…”

“…The Mg 2 SiO 4 composition glass has been formerly synthesized using a rapid quench method and studied using vibrational spectroscopy (11,12), however, the advantage of the levitation technique is that it allows us to synthesize a high-purity bulk sample. The structure of these glasses was recently studied using theoretical simulation (13), spectroscopic measurements (7,(13)(14)(15), diffraction measurements (16)(17)(18)(19), and atomistic modeling on the basis of diffraction data (17)(18)(19) In this paper we model and analyze the structure of silica glass (SiO 2 ), enstatite glass (MgSiO 3 ), and forsterite (Mg 2 SiO 4 ) glass employing the RMC modeling technique and the density function theory (DFT) of electronic structure using measured and published synchrotron X-ray and neutron diffraction data (16)(17)(18). Special attention is paid to ring statistics, cavity volumes, and the connectivity of short-range structural units in the glasses.…”

Low temperature MIR to submillimeter mass absorption coefficient of interstellar dust analogues

“…Sen et al (2009) have determined the relative proportion of Q i species in 9 glasses from a series of (MgO) y (SiO 2 ) 100−y glasses with y in the range 50-66.7 (y=50, 60, 66.7 correspond to x=0.5, 0.40, 0.33 in the notation adopted here). The samples X35, X40 and X50A were synthesized by the same team using the same technique as the study of Sen et al (2009) species in respective proportions of about 50-60 and 50-40%. As expected, the X35 sample close to the forsterite composition, is less polymerized than the the X50A sample, of enstatite composition, the X40 being intermediate.…”

Recent Advances in Solid-State 25Mg NMR Spectroscopy

“…Besides the use of large volume samples, there are also other ''brute force'' approaches frequently used since the earlier reports of solid-state 25 Mg NMR, including isotopic enrichment and the use of magnetic fields as strong as possible. 6,37,39,42,43,[45][46][47][48][49][50][51][52] There is no doubt that the increase in the number of reports involving solid-state 25 Mg NMR has been driven by the wider availability of high-field magnets (e.g. !14.1T).…”