Abstract
As a first step towards accurate quantification of the polysomatic states of biopyriboles, we have studied the polysomatic transformation between amphibole and hydrous triple-chain silicate (TCS) in the synthetic system Na2O-MgO-SiO2-H2O (NMSH). The reaction is: 4Na2Mg4Si6O16(OH)2 TCS ⇔ 3Na2.67Mg5.33Si8O21.33(OH)2.67. Amphibole We have characterised a polysomatic intergrowth of amphibole and TCS (synthesized at 2 kbar/(653° C) by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), infrared spectroscopy and 29Si magic-angle-spinning (MAS) NMR spectroscopy. The sample is a fine-scale lamellar intergrowth of double- and triple-chain structures; lamellae are 27 Å to hundreds of Ångströms wide. The 29Si MAS NMR spectrum of the intergrowth is explicitly a superposition of the individual amphibole and TCS spectra. By ensuring that the recycle delay time used considers the longest spin-lattice relaxation time (ca. 900 s), the relative amounts of double- and triple-chain structures can be quantified by simple deconvolution of the spectrum. The relative amounts of double- and triple-chain structures are 42 ± 5 and 58 ± 5 mol%, respectively. With regard to quantifying populations of chain multiplicities in biopyriboles, we believe that 29Si NMR is more accurate than the conventional HRTEM fringe-counting method (Maresch and Czank 1983, 1988), and is far superior to XRD and infrared spectroscopy, which suffer from high sensitivity to particle size and calibration problems. 29Si MAS NMR can provide an accurate means of monitoring the progress of polysomatic reactions in biopyriboles. It is likely to be most effective for samples containing only a few different chain multiplicities (e.g. m = 1, 2, 3 and perhaps 4), such as occur in natural pyroxenes and amphiboles.
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Welch, M.D., Rocha, J. & Klinowski, J. Characterization of polysomatism in biopyriboles: Double-/triple-chain lamellar intergrowths. Phys Chem Minerals 18, 460–468 (1992). https://doi.org/10.1007/BF00200969
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DOI: https://doi.org/10.1007/BF00200969