Beschreibung:
A study using optical microscope, electron microprobe, X-ray diffraction and Mössbauer spectroscopy has been performed on volcaniclastic rocks from the Hilârio Formation in the region of Volta Grande (RS) -Brazil. These rocks have been altered by hydrothermal events evidenced either as vein deposits or in massive rock. Deposition of chlorite in veins was followed by corrensite -f calcite and the alteration events do not seem to be determined by the temperature but variations in f(O₂) and Р(СО₂) might have controlled the crystallization of these phases. If so, corrensite must be considered as an independent phase and not a transitional step in the reaction saponite > chlorite.