Characterization of synthetic pargasitic amphiboles (NaCa ₂ Mg ₄ M (super 3+) Si ₆ Al ₂ O ₂₂ (OH,F) (sub 2;) M (super 3+) = Al, Cr, Ga, Sc, In) by infrared spectroscopy, Rietveld structure refinement, and ²⁷ Al, ²⁹ Si, and ¹⁹ F MAS NMR spectroscopy

Natural and synthetic amphiboles can display long-range and short-range order-disorder that must be characterized for proper interpretation of synthesis experiments. In this study, octahedral M³⁺ (Al, Cr, Ga, Sc, In) analogues of pargasite and fluor-pargasite were synthesized and characterized by optical and scanning-electron microscopy; X-ray diffraction; infrared spectroscopy; ²⁷Al, ²⁹Si, and ¹⁹F magic-angle spinning (mas) nuclear magnetic resonance (nmr) spectroscopy; and Rietveld structure analysis. Although “end-member” pargasite was synthesized readily, yields were never quite 100%; cell dimensions were consistent with previous studies. Substitution of Cr, Ga, Sc, and In for octahedral Al reduced yields to less than 90%. Variation in cell volume with the cube of the average octahedral cation radius for this isostructural series was not linear, indicating incomplete substitution of these cations for Al. For the pargasites, the infrared spectra showed that the octahedrally coordinated trivalent cations were significantly disordered over the M(l,2,3) sites of the octahedral strip. In the fluor-pargasites, the trivalent cations tended to be more strongly ordered at M(2). The ²⁹Si mas nmr spectrum of scandium-fluor-pargasite was incompatible with complete ordering of tetrahedral Al at the T(l) site.