Green opals in hydrothermalized basalts (Tenerife Island, Spain): alteration and aging of silica pseudoglass

Green opals have been identified in veins and vugs in hydrothermalized basalts from the area of Taganana (Tenerife). The aim of this paper is to document the petrology and geochemistry of these opals and to describe the formation process from amorphous components. Green opals were studied using optical microscopy, powder X-ray diffraction, electron microprobe, atomic absorption spectroscopy and induced coupled plasma mass spectrometer. Our textural and X-ray powder diffraction analyses indicate that green opals are composed of a major groundmass fraction of crystalline (opal CT and celadonite) and amorphous components (opal A and Fe-(hydro)oxides), which host minor siderite and dolomite crystals. The results indicate green opals were formed during the hydrothermal process from a Fe-rich silica amorphous phase (pseudoglass) which aging and alteration transformed into a groundmass of celadonite and opaline mixture rich in Fe-(hydro)oxides. The opaline–celadonite groundmass, or its amorphous precursor, served as a substrate for the growth of siderite and dolomite crystals.