Original paper

A new synthetic hexagonal BeMg-cordierite, Mg2[Al2BeSi6018], and its relationship to Mg-cordierite

Hölscher, Angelika; Schreyer, Werner

European Journal of Mineralogy Volume 1 Number 1 (1989), p. 21 - 38

50 references

published: Mar 21, 1989

DOI: 10.1127/ejm/01/1/0021

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Abstract

Abstract The new end member of the cordierite group, Mg2[Al2BeSi6O18], which is related to Mg-cordierite, Mg4[Al2BeSi6O18], by the substitution Be + Si = Al + Al, was synthesized readily from gel at 1 bar and at low to intermediate pressures under both anhydrous and hydrothermal conditions. Contrary to other synthetic and natural cordierites, it is hexagonal at all temperatures investigated thus far (≥ 600 °C). It is isostructural with beryl, having the space group P6/mcc. Like other cordierites and beryl, the new phase can incorporate molecular H2O(type I only) in its channels. The mean refractive index of a hydrous BeMg-cordierite with 1.95 wt.-% H2O is 1.550 as compared to 1.535 for the anhydrous one. The crystals were found to be uniaxial negative. The lattice constants are a0 = 9.579(1) Å, c0 = 9.274(1) Å. MASS-NMR data indicate that all the Si-atoms are located within the six-membered tetrahedral rings (T2-site) thus producing the ring-silicate unit [Si6O18] as in beryl, while the ring-connecting tetrahedra T1 contain only Al and Be, although apparently not in complete disorder as required by hexagonal symmetry. While at high temperatures (1,350 °C-1,450 °C) and atmospheric pressure there is a complete series of solid solutions between Mg2(Al2Be)[Si6O18] and hexagonal high Mg-cordierite Mg2[Al4Si5O18], a large miscibility gap exists between the new phase and orthorhombic low Mg-cordierite at lower temperatures (850 °C), especially at elevated water pressure. At higher temperatures, the miscibility gap either closes as a two-phase loop or is intersected by the melting range. This new cordierite end member has thus far not been found in nature, probably because Be is usually incorporated into cordierite, together with sodium, by the substitution Na + Be = Al. However, the new phase should occur as a low-pressure, high-temperature product of the isochemical breakdown of the natural mineral surinamite, (Mg,Fe2+)3Al4BeSi3O16.

Keywords

cordieriteexperimental mineralogyberyllium substitutionNMRsolid solutionmiscibility gap