Al-B substitution in the system albite (NaAlSi3O8)?reedmergnerite (NaBSi3O8)
Wunder, Bernd; Stefanski, Johannes; Wirth, Richard; Gottschalk, Matthias
European Journal of Mineralogy Volume 25 Number 4 (2013), p. 499 - 508
published: Aug 1, 2013
ArtNo. ESP147052504002, Price: 29.00 €
The Al-B substitution in the system albite?reedmergnerite has been investigated experimentally by performing cold-seal hydrothermal and piston-cylinder synthesis experiments at temperatures of 450 °C and 750 °C in the pressure range 0.2?3.0 GPa. As starting material, glasses close to NaBSi3O8(R100), NaB0.75Al0.25Si3O8(R75), and NaB0.5Al0.5Si3O8(R50) composition were used. Run products were characterised by SEM, powder-XRD applying the Rietveld method, EMP- and TEM-analyses. In all but one experiment reedmergnerite or (Al, B)-bearing feldspar formed as the main phase beside quartz, coesite or jadeite in one run at 3.0 GPa, 750 °C. Amorphous phases were observed in the nominally dry experiments and were assumed to be due to incongruent melting under participation of absorbed water. Synthetic (Al, B)-feldspar from experimental runs show considerable Al-B solid solution. TEM-investigations show that the (Al, B)-feldspars are strongly twinned. No exsolution lamellae of varying compositions are visible, indicating that the (Al, B)-feldspars are chemically homogeneous. The reedmergnerite (Rd)-component in albite at constant temperature of 750 °C increases strongly with increasing pressure, with the highest Rd-content of about 50 mol% just below the albite ? jadeite + quartz equilibrium. The amount of Al substituting into reedmergnerite is small and the variation with pressure is not well constrained. At 3.0 GPa, 750 °C an albite content of 4 mol% is observed in reedmergnerite. Consequently, the wide miscibility gap of albite-reedmergnerite solid solution in albitic feldspars observed at low pressures strongly diminishes to higher pressures. The degree of ordering in the synthetic B-bearing feldspars was estimated from Rietveld-determined mean (T-O) distances of the four tetrahedral positions T1o, T1m, T2o, T2m. For reedmergnerite a pressure-induced high/low transition appears to occur within the pressure range 1.0?1.5 GPa at 750 °C. This experimental study indicates that albitic feldspars can reach maximum Rd-contents of up to about 20 mol% under late-stage Band Na-enriched pegmatitic conditions. At fluid-saturated conditions such feldspars can be transported during subduction to depths of about 40 km until their breakdown and formation of a B-bearing melt. Natural albitic feldspars, typically containing only traces of B, will decompose in subduction zones below about 50 km depths and produce a melt with low B-contents.