A MULTINUCLEAR NMR-STUDY OF SYNTHETIC PARGASITE

Atomic order in synthetic end-member pargasite, NaCa2[6](Mg4Al)[4](Si6Al2)O22(OH)2, has been examined by high-resolution H-1, Al-27, and Si-29 magic-angle-spinning (MAS) NMR spectroscopy, with and without cross-polarization, and by infrared spectroscopy. The spectra indicate that Al occurs only on T1, M2, and M3 sites and are consistent with 1.5Mg + 0.5Al on M2 and 0.5Mg + 0.5Al on M3, [4]Al being ordered onto TI sites without Al avoidance (Al-O-Al linkages are allowed). Assuming random mixing on sites in these ratios, we calculate a maximum configurational entropy for subordered synthetic pargasite of 38 J/(K.mol). This is much lower than the value of 56 J/(K.mol) indirectly estimated by Westrich and Holloway (1981) from their phase equilibrium study of the low-pressure dehydration of synthetic pargasite. However, we show that it is possible to reconcile their experimental results with our estimate of the configurational entropy. Our results suggest that synthetic pargasite is likely to be a good experimental analogue for natural pargasite.