Figure 11. Photomicrograph of quartz-feldspar porphyry (QFXT). This unit forms large masses in the central part of the Aljustrel area and probably represents a subvolcanic intrusion, although locally it may have erupted as thin sheets following deposition of the massive sulfides and rhyolite C. The unit contains feldspar and quartz crystals up to 2 and 0.5 cm across, respectively. It has the silica contents of rhyolite (about 70 % SiO2), but higher TiO2 and Fe2O3 contents that are typical of those in dacites.