Original paper

Synthesis and chemical variability of Mg-staurolite in the system MgO-Al2O3-SiO2-H2O as a function of water pressure

Fockenberg, Thomas

European Journal of Mineralogy Volume 7 Number 6 (1995), p. 1373 - 1380

19 references

published: Dec 27, 1995
manuscript accepted: Jul 17, 1995
manuscript received: May 15, 1995

DOI: 10.1127/ejm/7/6/1373

BibTeX file

ArtNo. ESP147050706010, Price: 29.00 €

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Abstract

Abstract Mg-staurolites were synthesized from a gel with the oxide ratio 4 MgO: 9 Al2O3: 8 SiO2 at water pressures from 20 to 50 kbar and the constant temperature of 800°C. They form euhedral grains up to 10 μm in length. The chemical composition is strongly correlated with water pressure and, compared to the ideal formula of Mg3-4Ali8Si8O46(OH)2-4 the magnesium content increases only slightly between pressures of 20 and 30 kbar, whereas a strong decrease in the aluminum content is accompanied by an increase in the amount of hydrogen. This suggests the following substitutions: 3 H+ ↔ Al3+ and Mg2+ + H+ ↔ Al3+. At higher pressures, a further increase in hydrogen is accompanied by a decrease in magnesium content related to the substitution 2H+↔ Mg 2+. The change in substitution mechanisms may reflect an increase in the amount of sixfold coordinated magnesium at high pressure. Surprisingly, the lattice parameters of Mg-staurolites are not measurably influenced by their chemical variability

Keywords

MASH-systemMg-staurolitesynthesischemical composition