As K sources, the feldspathoids nepheline and balliranoite and biotite mica could be considered, with high weathering rates of the latter according to the Goldich dissolution series (Goldich, 1938).
The feldspathoids nepheline and balliranoite, zeolite analcime and feldspar sanidine, which include Na in their chemical composition, may be the minerals that release Na, making it available in solution.
A linear K release over the period of 1 year was observed for the feldspathoid nepheline at pH 5, with reduced K release at pH 7.
Back-scattered electron and secondary electron images of altered feldspathoids confirmed zones with different ratios of intimately intergrown thomsonite and gonnardite resembling primary feldspathoid zoning and shaping.
Both the studied rock samples are porphyritic, with a small amount of phenocrysts formed by ferrifassaite passing to sodic ferrifassaite with kaersutite rims, oligoclase-andesine, and minor zeolitized feldspathoid. Andesine to oligoclase ([An.sub.27-30]) predominate over sodic sanidine and scarce clinopyroxene and titanomagnetite in the matrix of fresh trachyandesite facies.
A small amount of pseudomorphs after feldspathoid (sodalite?) is also present.
The microphenocrysts of totally zeolitized feldspathoid are clouded, reminding primary sodalite crystals only in their sub-rounded morphology (see Plate I, 3).