Al-B substitution in the system albite (NaAlSi (sub 3) O (sub 8) )-reedmergnerite (NaBSi (sub 3) O (sub 8) )

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 degrees C and 750 degrees C in the pressure range 0.2-3.0 GPa. As starting material, glasses close to NaBSi (sub 3) O (sub 8) (R100), NaB (sub 0.75) Al (sub 0.25) Si (sub 3) O (sub 8) (R75), and NaB (sub 0.5) Al (sub 0.5) Si (sub 3) O (sub 8) (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 degrees 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 degrees 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 degrees 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 T (sub 1o) , T (sub 1m) , T (sub 2o) , T (sub 2m) . For reedmergnerite a pressure-induced high/low transition appears to occur within the pressure range 1.0-1.5 GPa at 750 degrees 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.