Abstract: Hydrothermal alteration of andesitic and dacitic rocks in the Goldfield District, Nevada, has produced regularly interstratified chlorite-vermiculite and tobermorite. X-ray diffraction and oscillating-heating data indicate the regular interstratification of the chlorite-vermiculite. This mineral is found in the zone of least alteration and was formed by the alteration of hornblende phenocrysts. Penninite is an intermediate stage in this reaction. Increased intensity of alteration resulted in the disappearance of chlorite-vermiculite and the development of montmorillonite. Tobermorite, found only in the dacite, is associated with alunite as pseudomorphs after plagioclase phenocrysts in the most intense zone of alteration. The intermediate stage of hydration for tobermorite is indicated by the 11 Å spacing of the 002 diffraction peak. Oscillating-heating X-ray data at atmospheric pressure show that tobermorite decomposes thermally in two stages. The (00l) planes collapse at approximately 520 °C and crystal planes having an 8 Å periodicity collapse at about 760 °C. Wollastonite develops from tobermorite above 700 °C under static heating conditions.