29Si MAS NMR study of diopside–Ca-Tschermak clinopyroxenes: Detecting both tetrahedral and octahedral Al substitution

Roberta L. Flemming; Robert W. Luth

doi:10.2138/am-2002-0104

Abstract

We have studied short-range cation ordering across the diopside (CaMgSi₂O₆)-Ca-Tschermak pyroxene (CaAI₂SiO₆) (Di-CaTs) solid solution in samples synthesized at 1400 °C and 2 GPa, for 24 hours. Peak positions in ²⁹Si MAS NMR spectra are sensitive to A1 substitution, both in the cornersharing NN tetrahedral sites on the single chain and in one of the three NN octahedral Ml sites. The substitution of A1 for Mg on Ml causes the ²⁹Si chemical shift to be shielded by about the same magnitude as the deshielding caused by substitution of A1 for Si in NN tetrahedra, causing severe peak overlap among central peaks. Two pairs of the unique local environments have very similar chemical shifts, leaving only four peaks resolved in the spectrum, for which six site assignments have been made.

²⁹Si MAS NMR peak intensities for end-member CaTs are consistent with 70% of the Si in the tetrahedral chain being locally ordered into alternating ^[4]Al-O-Si-O-^[4]Al linkages, although space group C2/c would suggest that Si and A1 are long-range disordered over the tetrahedral chain. CaTs clearly violates Lowenstein’s (1954) Al-O-Al avoidance principle, with 0.175 Al-O-Al linkages per formula unit (based on six O atoms per formula unit), as calculated from observed ²⁹Si MAS NMR peak intensities. For intermediate members of the solid solution, site populations have been modeled using a random distribution model (RD), an Al-O-Al avoidance model (AA) and a modified AA model which couples the cation distribution between the tetrahedral and octahedral sites (AAOC). The RD model fits the ²⁹Si MAS NMR peak intensities poorly. The AA and AAOC models fit the ²⁹Si NMR data well to at least 75% CaTs, suggesting that diopside-rich Di-CaTs clinopyroxenes obey Lowenstein’s Al-O-Al avoidance principle. For compositions >75% CaTs, observed cation distributions deviate considerably from that predicted by Al-O-Al avoidance, indicating formation of Al-O-Al linkages. The AAOC model yields an improved fit to the observed ²⁹Si MAS NMR intensities, particularly near Di₅₀CaTs₅₀, implying the presence of ^[4]Si-O-^[6]Mg and ^[4]Al-O-^[4]Al couples. This suggests the presence of short-range ordering on both tetrahedral and octahedral sites in Di-CaTs pyroxenes.

The high sensitivity to octahedral A1 substitution observed in these chain silicates is not observed by ²⁹Si MAS NMR of more-polymerized silicates, such as framework and sheet structures. Thus, ²⁹Si MAS NMR of pyroxenes has potential to reveal tetrahedral-octahedral cation coupling and octahedral cation ordering information, which is potentially important for geothermobarometry of aluminous pyroxenes.

²⁹Si MAS NMR study of diopside–Ca-Tschermak clinopyroxenes: Detecting both tetrahedral and octahedral Al substitution

Abstract

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