Rhyolite-dacite-trachyandesite association: a Moessbauer spectroscopy study

Three volcanic dykes, rhyolite, dacite and trachyandesite cutting a radioactive granite, located between Latitudes 22o 47' 396''-22o 47' 884'' N and Longitudes 31o 54' 883''-31o 54' 894'' E in the south Western Desert of Egypt were sampled and analyzed by X-ray diffraction, 57Fe Moessbauer spectroscopy and chemical method. They are consisted of feldspar and quartz together with some paramagnetic minerals including aegirine plus minor riebeckite in the rhyolite; aegirine plus some riebeckite in the dacite; and riebeckite plus trace aegirine in the trachyandesite, respectively. The bulk content of iron in each dyke has characteristic ferric-quadrupole splitting and oxidation values: 0.29 millimeters per second (mm/s) and 100% for rhyolite; 0.31 mm/s and 82% for dacite; and 0.35 mm/s and 0.69% for trachyandesite. Variations in the quadrupole splitting have been attributed to changes from the local crystal chemistry, while the oxidation variations are source-related.