Weathering of leucite to clay minerals in tephrites of the Vico volcano,

ABSTRACT- The weathering of leucite has been studied in tephrites of the Vico volcano. The petrography was made on undisturbed samples by optic and scanning electron microscopy with microprobe analysis. We follow a weathering sequence of leucite crystals from a core of lava to a first altered cortex and then in sandy and clayey arene of weathering. Then we compare the alteration in the B and A horizons of an overlying andosol, deriving from tephrite pyroclasts under a perhumid climate. We can observe also the leu-cite alterationin a ((petriscos (leucotephrite) under a xeric climate. The clay minerals composition was obtained from a powder of-whole material and the less 2fim fraction, by X Ray diffraction and TEM. The alteration of leu-cite to analcime in the first cortex is inherited from a former hydrothermal process. While clay minerals are forming by further weathering in relation with drainage intensity, e.g.: allophane and imogolite in andosols under perhumid climate and with high drainage; halloysite under less humid cli-mate or with slower drainage