Identification of some trace metal minerals in recently compacted and stratified beach rocks close to the estuary of the Nerbioi-Ibaizabal river (Bilbao, Bay of Biscay)

Complete text of publication follows. The aim of the work is the identification of some trace metal minerals inside recently compacted and stratified sand of beaches close to the Estuary of the Nerbioi-Ibaizabal River (Bilbao, Bay of Biscay). This characterisation work is the first step of a research programme aiming to understand the processes responsible of the cementation that probably started between XIXth and XXth centuries. Several hypotheses have been formulated till now, most of them related to the high metallic stress which impacted the surroundings of the Estuary during the whole XXth century from uncontrolled spills coming from both mining and industrial activities. In this work samples collected from the first 50 cms of 'beachrocks (stratified and cemented sands)' have been analysed by optical microscopy (31 thin plates), micro-X-Ray Fluorescence (more than 100 determinations) and micro-Raman spectroscopy (more than 250 spots). These analyses indicated the presence of a high and different number of elements and mineral phases. Using the geological information available for the Basin of the Nerbioi-Ibaizabal River, we can propose which minerals are coming from weathering processes of naturally minerals, who is the mineral acting as the cementing material and which are the new mineral phases appearing as a consequence of the mining and industrial uncontrolled spills of the past. The naturally occurring mineral phases detected during the analyses were quartz, clay, calcite, hematite, pyrite, rutile. Other minor natural minerals must be also present, containing elements like Mn, Sr, Ba and Rb. Surprisingly, the cementing material was identified as aragonite being a thin layer rounding both naturally occurring grains (sand, clays, calcite, etc.) as well as anthropic mineral phases of different shape. Among the anthropic trace metal minerals, iron hydroxioxide, calcium arsenate, magnesium arsenate, organic carbon and mineral carbon (both can be distinguished by Raman spectroscopy), zinc carbonate and calcium phosphate have been detected. Other mineral phases must be also present because the micro-X-Ray Fluorescence measurements gave appreciable amounts of other anthropic elements like Pb, Sn, Cu, Ni, Co, Cr, Hg and Au. Acknowledgements. This work has been financially supported by the Basque Government through the BERRILUR III Project (ETORTEK Progr.).