Solid solution and unmixing in the (Ba, K, Na)-feldspars; 2, An X-ray powder diffraction study of the ternary feldspars

Gels of different compositions in the BaAl (sub 2) Si (sub 2) O (sub 8) - KAlSi (sub 3) O (sub 8) - NaAlSi (sub 3) O (sub 8) system were crystallized at 650-900 degrees C at P of 1-3 kbar. A region containing two feldspars was found near the albite corner of the ternary diagram, whereas the Ba-rich and K-rich feldspars were found to be homogeneous. Preliminary results are presented in two ternary diagrams for the variations in cell volume and beta lattice angle with compositions. Where two feldspars were observed, one of them was always richer in the10albite component than the bulk compositions of the starting gel, suggesting that pure albite is unable to accommodate the larger K and Ba ions at lower T whereas considerable Na is accommodated in (Ba,K)-feldspar at similar T.