Sol-Gel Synthesis of Protoenstatite
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ABSTRACT Protoenstatite, a high-temperature polymorph of enstatite (MgSiO 3 ), is generally not stable at room temperature, and is difficult to synthesize. Using a recently developed, hydrogen peroxideassisted, sol-gel synthesis, protoenstatite was synthesized in a form that was stable at room temperature. Its crystallization was strongly dependent on processing conditions, particularly on the manner in which the xerogel was formed and fired. Xerogels prepared by evaporation, spraydrying and freeze-drying were compared by XRD, HTXRD, BET, TG/DTA, and 29 Si NMR methods. When samples were prepared by evaporation or spray-drying, the result was a mixture of polymorphs. Only the freeze-dried precursor yielded protoenstatite at a lower temperature and within a shorter time than any previously reported.
INTRODUCTION Protoenstatite is a high temperature, metastable polymorph of enstatite that may convert to the low temperature polymorphs orthoenstatite and clinoenstatite by quenching or shearing. Some of the known synthesis procedures use quenching to trap the protoenstatite. Polymorphism of enstatite has been studied since the beginning of the century; see Atlas[l] and Smith[2] for early reviews of the literature and Smyth[3] and Buseck et al.[4] for more recent reviews. Protoenstatite has been used as (1) a component in ceramic bodies with excellent mechanical strength and low thermal expansion[5], and (2) more recently as an enzyme immobilizer[6]. Several synthesis procedures for protoenstatite have been described. Atlas used fluxes such as 2% LiF to grow protoenstatite from a glass, and concluded that when starting from natural enstatite or the metal oxides, the tendency to convert to clinoenstatite increased[l]. Smith prepared protoenstatite for crystal structure determination from a MgSiO 3 glass under hydrothermal conditions (four days at 1080 'C and 500 bar)[7]. Yamaguchi et al. prepared an enstatite precursor by pouring a mixture of Mg and Si alkoxides into ammonium hydroxide, but could not obtain pure protoenstatite even after heating to 1400 'C[8]. Usov et al. produced a mixture of periclase, quartz, and forsterite by heating a mixture of Si0 2 and MgCO 3 below 1200 °C but, at 1200 °C or above, formed protoenstatite that was stable when cooled to room temperature if the grain size was less than 5 mm[9, 10]. Lee et al. prepared MgSiO 3 glass from a melt then allowed it to crystallize up to 1400 'C, and produced pure protoenstatite with a very small amount of clinoenstatite after a water quench; long times at lower temperatures produced a disordered mixture of orthoenstatite and clinoenstatite[ 11 ]. Recently, a novel, hydrogen-peroxide assisted route for the synthesis of enstatite sols was developed[12]. Xerogels from these sols did not yield fully crystalline materials, even after firing to 1250 'C. A similarly prepared forsterite xerogel yielded pure crystalline forsterite as low as 750 'C[13]. Similarly prepared enstatite produced protoenstatite when the xerogel was prepared by freeze-drying. Three different
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