Zeolites in Pyroclastic Deposits in Southeastern Tenerife (canary Islands)

-The chemical and the mineralogical composition of a group of pumiceous tufts associated with recent salic volcanic episodes from Tenerife (Canary Islands) have been studied. The investigation focused on the two main types of pyroclastic deposits of the zone: ash-flows and ash-falls. The samples can be classified chemically as trachytic and phonolitic rocks with an intermediate silica content and a high percentage of alkali cations (Na + and K + ). The mineralogical composition, determined by X-ray diffraction, scanning electron microscopy, and optical microscopy, shows the occurrence ofzeolites (mainly phillipsite, with lesser chabazite and analcime), associated with the parent glass. K-feldspar (sanidine) and calcite are accessory minerals. Zeolites are significantly more abundant in the ash-flow deposits. Zeolite formation by hydrothermal weathering in closed-system conditions varies according to the nature and the origin of the pyroclastic deposits. Tenerife phillipsites differ from typical diagenetic, lacustrine, and deep-sea phillipsites, both in chemical and mineralogical features. Alkali cations exceed divalent cations in the unit-cell that, assuming a monoclinic symmetry, has the following parameters: a = 8.4610.55 ~, b = 14.21-14.40/~, c = 7.80--8.70 ~., and/3 = 105"--110 ~ Key Words--Ash-fall, Ash-flow, Chemical analysis, Geoautoclave, Phillipsite, X-ray diffraction, Zeolites, Zeolitization.