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American Mineralogist; February 1994; v. 79; no. 1-2; p. 43-50
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Thermal reactions of chrysotile revisited; a 29 Si and 25 Mg MAS NMR study

K. J. D. MacKenzie, and R. H. Meinhold

New Zealand Institute for Industrial Research and Development, Lower Hutt, New Zealand

The thermal decomposition sequence of chrysotile samples from New Zealand and Canada has been found by 29 Si and 25 Mg MAS NMR to be more complex than previously indicated by X-ray diffraction and electron optical studies. The greater part of the mineral dehydroxylates, producing a Mg-rich X-ray amorphous phase containing essentially Mg in octahedral coordination and characterized by a broad 29 Si resonance centered at about -73 ppm with respect to tetramethylsilane. This transforms to forsterite, Mg 2 SiO 4 , at 670-700 degrees C, at which temperature the unreacted chrysotile (possibly the deepest layers of the fibrils) forms a different, Si-rich X-ray amorphous dehydroxylate. This phase, called dehydroxylate II, has a 29 Si chemical shift (-97 ppm) not unlike that of talc, but with Mg sites that may be distorted beyond NMR detection, because most of the 25 Mg spectral intensity is lost over this temperature range. Dehydroxylate II forms enstatite (MgSiO 3 ) and free silica at 770-800 degrees C, further enstatite being formed by reaction of the silica with forsterite at 1150 degrees C. The stoichiometry of these reactions is consistent with NMR measurements of the partitioning of Si between the various phases.

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