Zeolites in alkaline rocks of the Kaiserstuhl volcanic complex, SW Germany - New microprobe investigation and the relationship of zeolite mineralogy to the host rock

Low-grade zeolite facies mineralisation in the Kaiserstuhl volcanic complex results from alteration of alkaline volcanic rocks. Nine different zeolite species are known from the volcanic complex: analcime, chabazite-Ca, faujasite-Na, faujasite-Mg, natrolite, offretite, phillipsite-K, phillipsite-Ca and thomsonite. Their occurrence as well as their chemical composition depends on the chemical composition of the host rock, local hydrological features and porosity of the rock. New chemical analyses are presented, in order to give chemical compositions of Kaiserstuhl zeolite species, which were never analysed before, e.g. thomsonite from the Strümpfekopf-Totenkopf area, as well as a re-evaluation of zeolite species from classic localities. New microprobe analyses of barrel-shaped offretite from the Limberg area clearly show that the postulated epitaxial intergrowth of offretite and erionite does not occur. Textural and chemical observations require that the elements for zeolite formation derived from the hydration and palagonitisation of basaltic glass, and alteration of primary phases (e.g. sodalite, leucite). The zeolite species depends on the primary mineralogy and chemical composition of the host rock: Analcime occurs as pseudomorphs after leucite, which is a rock forming mineral in most lavas and tuffs of tephritic composition, and natrolite is derived from hydrothermal alteration of sodalite-group minerals in phonolitic intrusions.