Rhyolite–dacite–trachyandesite association: a Mössbauer spectroscopy study

Abstract

Three volcanic dykes, rhyolite, dacite and trachyandesite cutting a radioactive granite, located between Latitudes 22 • 47 396 -22 • 47 884 N and Longitudes 31 • 54 883 -31 • 54 894 E in the south Western Desert of Egypt were sampled and analyzed by X-ray diffraction, 57 Fe Mössbauer 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.Keywords Volcanic dykes · Aegirine · Riebeckite · Mössbauer · Egypt The rhyolite, dacite and trachyandesite are granite-rooted dykes with distinctive colors, brown, light green, and dark green, respectively. Each dyke is ∼0.5 m thick and a few meters long of a fine-grain material exhibiting microscopic fibrous structures. The total field gamma (γ )-ray radiation increases from the trachyandesite to the dacite, and to the rhyolite dyke. The latter has anomalously high radioactivity levels, but without any obvious radioactive mineralization. The major source

Characterization of granites by 57Fe Mössbauer spectroscopy

“…D1, with relative spectral areas or RA values of 60-85%, has quadrupole splitting (DE Q ) and isomer shift (d) varying from 0.29-0.35 and from 0.37-0.40 mm/s, respectively. The values of DE Q and d are consistent with Fe 3+ in octahedral (M1) coordinations and agree with those reported for aegirine (Hassan 2009;De Grave et al 1998;Dollase, Gustafson 1982) and aegirine-augite (Baum et al 1997). Aegirine-augite is an intermediate member of the aegirine (NaFe 3+ Si 3 O 8 )-augite (Ca,Mg,Fe)SiO 3 ) series.…”