SYNTHESIS AND CHEMISTRY OF UNUSUAL EXCESS-SI ALUMINOUS ENSTATITE IN THE SYSTEM MGO-AL2O3-SIO2 (MAS)

Synthetic enstatites were prepared at 800 degrees C/25 kbar from six g els along the binary joins Mg-2[Si2O6] (= En) - Mg2Al[AlSiO6] (= MgTsc h) and En - square(0.5)Mg(0.5)Al[Si2O6] (= Mg-Es = ''Mg-Eskola pyroxen e'') as well as in the ternary range between these joins. Microprobe a nalyses of the nm products exhibit aluminum contents in pyroxene betwe en 0.132 and 0.333 Al p.f.u. (based on 6 oxygens), which is much more than indicated by the classical equilibrium isopleth (0.08 Al p.f.u.). In all these enstatites the ratios of Si/Mg were larger than 1 so tha t they clearly plot off the En - MgTsch join towards SiO2. As the calc ulated structural formulae show cation deficiences, and as no OH-bands were observed by infrared spectroscopy, these ternary enstatite compo sitions are explained by a combination of the two substitutions Al-[6] + Al-[4] for Mg + Si and 2 Al-[6] + square for 3 Mg-[6], essentially in the ratio 2:1 thus plotting along the join MgSiO3-Al2SiO5. Stacking faults in the plane (010) in synthetic enstatites with clinoenstatite lamellae, but no amphibole bands were detected by TEM. With rising de grees of substitution the cell parameters a, c, and V increase, while for the b direction a contraction was observed.