Summary:
Gozo (Għawdex) Isl., in the Maltese Archipelago, is generally composed of Neogene (23 -2.5 Ma) and Oligocene (33-23 Ma) sedimentary rocks. These sedimentary rocks are mostly limestone, marl, clays and conglomerates which are fundamentally poor in As and therefore no elevated concentrations would be expected in this area. However, recent analyses (personal communication) have warned for a substantial concentration of As(V) in groundwater -liquid- samples. Thus, solid samples, from all geological formations of Gozo Isl., were collected in the frame of the present study, in order to determine the source of this potential contaminant. The solid samples were characterized by means of diffraction (powder-XRD), microscopic (SEM-EDS) and analytical (bulk XRF, bulk ICP-OES/MS) techniques. Selected samples were also treated with acetic acid (CH3COOH) in order to remove calcite and examine the residual minerals. According to the data, it is confirmed that As is particularly accumulated (ca. 23 ppm) in the matrix of the phosphate-bearing conglomerates -phosphorite "hardground" (C1)- that are intercalated into the so-called Globigerina Limestone Formation (Mlg & Mug). These materials are rich in apatite-group minerals (ca. 16 wt.% P2O5 in bulk of rock) and therefore they can be classified as phosphorites. On the other hand, the so-called "blue clay" formation (Mbc), which is full in pyrite framboid pseudomorphs in microscale, contains normal/low amounts of As (ca. 2.5 ppm). Moreover, relatively high concentrations of U (ca. 108 ppm) were revealed into the phosphorites, whereas V, Zn, and Ni are relatively common in both formations. According to geochemical diagrams, except As and U, the Gozo Isl. phosphorites show positive geochemical anomalies -as compared to UCC- with regard to Cd, Se, Sb, and Mo, but they are depleted in LILE (e.g. Cs, Rb, K). The evaluation of REE chondrite-normalized diagrams, together with discrimination diagrams (Th/Sc vs. Zr/Sc), indicate continental margin phosphorites, deposited likely in deep water, due to weathering of rather felsic rocks. The environmental enrichment factors (EF) and geo-accumulation indices (Igeo) confirmed that the concentrations of As and, to a certain degree, U and Cd are comparably higher than the average concentrations in the geological background. Arsenic content is among the highest measured globally, in respect of sedimentary phosphorite rocks. A subsequent investigation, by Synchrotron-based X-ray spectroscopic techniques (SR-μ-XRF and AsK-& FeK-edge μ-XANES), indicated that As occurs, like in the case of groundwater, in the form of As(V) -i.e., as arsenate (AsO43-), whereas itis related to Fe, implying Fe-rich phases, probably Fe-bearing clays and Fe-oxyhydroxides. That means potential environmental particles (As-rich micro- and nano-particles) occur in the coastal region of Gozo. Besides, U4+ is also associated with Fe-rich microareas according to the relevant SR-μ-XRF and UL3-edge μ-XANES. Subsequent TEM-EDS investigation showed the Gozo phosphorite consists, in fact, of various crystalline and potentially amorphous phases forming intergrowths in nanoscale. Particularly, the material contains carbonate apatite nanocrystals (together with microcrystals shown by SEM-EDS), as also proved by complementary FTIR spectra, in association with Fe-bearing clays and Fe-oxyhydroxides, as had implied by μ-XANES spectroscopy.
Keywords:
Gozo, Malta, As, Arsenic, micro, nano, particles, environmental, analytical, techniques, nanominerals, minerals, XRD, XRF, SEM, TEM, XANES, Synchrotron, ICP-MS,
