Mineralogical description and hypothesis on the formation of menilites from Galera, Granada (Spain)

Menilites, intriguing botryoidal rocks found in Galera, Granada, Spain, have been examined through a multidisciplinary approach integrating mineralogical analysis and advanced imaging techniques. Characterized as opal and dolomite-bearing rocks, their complex morphologies and diverse internal structures prompted an investigation into their origin. Employing microfocus X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray computed tomography, we present a detailed study of the menilites, revealing opal-A, opal-CT, dolomite and quartz as primary constituents. Notably, the internal homogeneity contrasts with the diverse external shapes. The proposed hypothesis suggests a seismic influence in menilite formation. Seismic events in porous environments above the water table may induce fluidization, resulting in the distinctive menilite structures. Osmotic pressure differences between nodules and the surrounding rock, coupled with fluidization during seismic events, could explain the observed morphologies. Validation of the proposed hypothesis requires further fieldwork and analogue experimentation. This study contributes valuable insights into the mineralogical composition, internal structures and potential formation mechanisms of menilites, laying the groundwork for future research in the field of sedimentary geology and mineralogy.