MAS-NMR studies of glasses and glass-ceramics based on a clinopyroxene–fluorapatite system

Abstract

Glasses and glass-ceramics based on the ternary system CaMgSi2O6–NaAlSi2O6–Ca5(PO4)3F (diopside (Di)–jadeite (Jd)–fluorapatite (FAp)) have been studied using magic angle spinning (MAS) NMR, powder X-ray diffraction (XRD) and thermal calorimetry. In this silicate system Na2O and Al2O3 have been progressively substituted for CaO and MgO: (CaMgSi2O6)1−x–(NaAlSi2O6)x–(Ca5(PO4)3F)y where 0 ≤ x ≤ 30 mol% and y = 7 mol%. 29Si MAS-NMR of the glasses showed a peak corresponding to a mixed Q2/Q3 silicate species which slightly decreased in chemical shift on increasing Na2O and Al2O3 substitution. After heat treatment of the glasses two major crystalline phases: a silicate solid solution (clinopyroxene) and FAp have been identified by XRD, with the silicate phase crystallising out first. A glassy phase coexisted with these crystalline phases and became more evident with increasing Na2O and Al2O3 content. As expected, no crystalline Jd has been found from XRD or MAS-NMR. 27Al MAS-NMR spectra of the glasses shows only Al(IV) to be present on addition of Al2O3. Upon heat treatment, a new, sharp Al(VI) peak at −4 ppm assigned to the solid solution was observed, which corresponds to Al3+ in a six coordinate state, replacing Mg2+ along the crystalline silicate chain structure with the replacement of either an additional Mg2+ or Ca2+ by Na+ to maintain the charge balance. The amount of Al(VI) formed on heat treatment grows linearly with addition of Al2O3, but does not exceed ca. 20% from the total aluminium concentration present in the system. The signal from the Al(IV) in the heat treated material belongs to the residual aluminosilicate glassy phase. 31P MAS-NMR shows the presence of mixed calcium/magnesium/sodium amorphous orthophosphate species in the glass, which upon heat treatment, changes to crystalline Ca5(PO4)3F. 23Na MAS-NMR of the heat treated glass-ceramics shows a Jd-like environment at the lowest Na2O and Al2O3 content. This site gets increasingly disordered with increasing Na2O and Al2O3 content, which is reflected in the changing 23Na MAS-NMR shift.