²⁹Si magic-angle NMR spectroscopy of low-temperature ordered plagioclase feldspars

Abstract

Plagioclase feldspars are among the most abundant silicate minerals in the Earth's crust, and yet their subsolidus phase relations remain, perhaps, the most enigmatic¹. Their complex behaviour, which involves ordered (commensurate and incommensurate) structures, displacive transitions and three miscibility gaps, is generally attributed to variations in the ordering of aluminium and silicon between tetrahedral sites of the feldspar framework structure as a function of temperature and composition^1,2. The similarity in X-ray scattering factors of Al and Si atoms, however, has hindered conventional attempts to elucidate the exact structural relations. In the context of feldspar mineralogy, the recent advent of magic-angle sample spinning nuclear magnetic resonance spectroscopy (MAS NMR) has many implications, because it provides a powerful and direct method for determining the local distributions of Al and Si– in solids^3–8. The ²⁹Si-MAS NMR spectra of natural and synthetic plagioclases are equally complex, however, in containing many overlapping peaks and varying according to the chemistry and thermal history of each sample examined^8–11. We present here new spectra, obtained for a suite of carefully characterized ordered samples from meta-morphic and slowly cooled igneous rocks, which shed light on: (1) the sensitivity of the feldspar framework to small amounts of Al/Si disorder; (2) the substitution mechanism of Si for Al in the solid solution from pure anorthite (CaAl₂Si₂O₈) towards albite (NaAlSi₃O₈); and (3) structural variations in the incommensurate ordered structure of intermediate compositions. In the wider context of silicate chemistry, NMR studies on feldspars provide insights into the subtle influences of continuous solid solution on crystal structures with changing composition. This is also the first attempt to examine the local structure of an incommensurate phase by NMR spectroscopy.