Interstratified vermiculite–mica in the gneiss–metapelite–serpentinite rocks at Hafafit area, Southern Eastern Desert, Egypt: From metasomatism to weathering

The Hafafit vermiculite in the Southern Eastern Desert of Egypt at the contact of the metapelite and serpentinite rocks with the pegmatites and gneisses of the Hafafit uplift is the only known deposit in the Arabian–Nubian Shield (ANS) rocks of the Eastern Desert (ED). It is distinctively interstratified with mica. The mineralogy and mineral chemistry of this vermiculite at four sites (HV1, HV2, HV3 and HV4) were studied to better understand its origin, which might refers to a specific geologic setting retained to Hafafit area. The vermiculite at Hafafit forms with phlogopite, actinolite–tremolite, asbestos–anthophyllite–talc and talc zones that are arranged from pegmatite and gneisses to the metapelite and serpentinite rocks. These zones were probably formed by metasomatism that related to the intrusion of the granitoid rocks and the connected pegmatites in the upper Pan-African. The XRD and EMPA studies of the interstratified vermiculite-mica concluded that vermiculitization took place through a layer-by-layer transformation of original micas. This formed, in decreasing abundance, mixed-layer phases of biotite/vermiculite (hydrobiotite), phlogopite/vermiculite (hydrophlogopite) and chlorite/vermiculite (corrensite) and discrete phases of vermiculite, chlorite and smectite. A model is suggested, in which chemical weathering by the moving downward meteoric water led to replacement of the interlayer K, in biotite from gneiss and in phlogopite from metasomatic zones, by H2O molecules, Fe2+ was oxidized and (OH)− replaced O2− forming hydrobiotite and hydrophlogopite. By more K remove, Fe was replaced by Mg with the introduction of more layers of H2O molecules leading to formation of the vermiculite. Weathering formed corrensite mixed-layer and chlorite expandable minerals on the expense of chlorite. Formation of the incomplete smectite-like layers and Al-hydroxy interlayers (13.97 Ǻ) took place at the expense of vermiculite, replacing the Mg interlayer cations (12.63 Ǻ). Weathering took place mostly by low-pH solutions and in warm environment and the most extensive degree of weathering was at the HV4 site, in which the lode of vermiculite is the biggest. We propose that vermiculitization at Hafafit occurred due to a specific integration between the geologic setting (including rock type and tectonics) of the area and weathering processes producing the only vermiculite deposit in the ANS rocks of the ED of Egypt.