Original paper

Synthesis and chemistry of unusual excess-Si aluminous enstatite in the system MgO-Al2O3-SiO2 (MAS)

Fockenberg, Thomas; Schreyer, Werner

European Journal of Mineralogy Volume 9 Number 3 (1997), p. 509 - 518

20 references

published: Jun 2, 1997
manuscript accepted: Nov 25, 1996
manuscript received: Jun 2, 1995

DOI: 10.1127/ejm/9/3/0509

BibTeX file

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Abstract Synthetic enstatites were prepared at 800°C/25 kbar from six gels along the binary joins Mg2[Si2O6] (= En) - Mg2Al[AlSiO6] (= MgTsch) and En - □0.5Mg0.5Al[Si2O6] (= Mg-Es = "Mg-Eskola pyroxene") as well as in the ternary range between these joins. Microprobe analyses of the run products exhibit aluminum contents in pyroxene between 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 that they clearly plot off the En - MgTsch join towards SiO2. As the calculated structural formulae show cation deficiences, and as no OH-bands were observed by infrared spectroscopy, these ternary enstatite compositions are explained by a combination of the two substitutions Al[6] + Al[4] for Mg + Si and 2 Al[6] + □ 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 degrees of substitution the cell parameters a, c, and V increase, while for the b direction a contraction was observed


enstatiteMAS-systemhigh-pressure synthesis.