The original mineralogical and geochemical compositions of andesitic volcanic rocks from SE of Spain have
been modified by different processes, under hydrothermal environment. In particular, silicification process
increases the initial silica concentration of the volcanic rock from 69 % to 97%. One of the characteristics
of this alteration is the breakdown of mafics minerals and feldspars with supply of Al, Fe, Mg…etc to the
interstitial fluids which can be later introduce as traces in the new quartz formed. The silica rocks created
in the process are mainly constituted by quartz with different crystal sizes and textures. The spectral
analysis of the cathodoluminiscence (CL) emission in combination with the Scanning Electron microscopy
observations show that the igneous quartz crystals of the volcanic rock are characterized by a dominant
band at 2,95 eV (420 nm) while the quartz crystals formed by silicification have a dominant band at 2,54
eV (~ 500 nm). Although it is difficult to identify the causes of these spectral differences, it is considered
that the 420 nm emission band is due to native defects of the igneous quartz and the ~ 500 nm emission
band is due to the substitutional incorporations of impurity ions in the hydrothermal quartz crystals. The
SEM-CL imagines show a non uniform luminescent emission. This is interpreted in terms of either
microsilicifications affecting the quartz crystals and the groundmass of the volcanic rocks or as consequence
of the pseudomorphic replacements by quartz of the primary minerals. The hydrothermal silicification is
an early process that increase the amount of silica of the initial andesitic rock making other petrological
and geochemical types of volcanic rock