Paragénesis de zeolitas alcalinas en un afloramiento de basaltos olivínicos amigdaloides de Junín de Los Andes, Neuquén, Patagonia, Argentina

the optical characteristics, x-ray diffraction, unit cell, chemistry and crystallization conditions of an alkaline zeolite assemblage -were studied. the zeolites have been found infilling amygdales and in the matrix of tertiary olivine basalts from junin de los andes, patagonia. the secondary minerals (albite, pumpellyite, interestratified chlorite/smectite (c/s), smectite, iddingsite, magnetite and hematite) are replacing phenocrysts ofplagioclase, olivine and the matrix of the rocks. zeolites are sodic, calcosodic and potassic in composition. na-chabazite, analcime, ca-k phillipsite and paulingite are the first zeolites that crystallized. they were followed by acicular-fibrous zeolites as natrolite/nagonnardite, thomsonite, mesolite and scolecite. the formation of the first zeolites is consistent with low temperature (<100 °c), low pressure (<0.1 gpa) and alkaline (ph 9-10) hydrothermal/geothermal meteoric fluids, which were favored by tensional fractures. the activity of si decreased during the alteration process and the temperature increased toward 200 °c allowing the analcime formation followed by the equilibrium assemblage natrolite/nagonnardite-thomsonite-scolecite and mesolite.