Multidisciplinary characterization of the "Ghiara" volcanic paleosoil (Mt. Etna volcano, Italy): Petrologic characters and genetic model

The large expanses of lava field and soils spread out around Mt. Etna volcano (Italy), make unique the landscape of the whole area. In this work, we investigated a sample set of a peculiar aggregate locally named with the term "Ghiara" collected in different outcrops of Mt. Etna volcano southern flank. A distinctive character of this material is that it is found exclusively beneath solidified lava flows. In past centuries, Ghiara was widely employed in construction field for its reddish hue and the high pozzolanic activity; for the same reasons it has now raised the interest of the scientific community involved in the formulation and production of eco-friendly materials as alternative to the traditional ones. The aim of this paper is to define its chemical and mineralogical characters and to provide a model for its genesis. We conducted a multidisciplinary analytical approach, complementary to the few investigations reported in literature, in order to define the chemical compositions by means of X-ray fluorescence (XRF), the mineralogy through synchrotron radiation X-ray diffraction (XRD) analyses and spectroscopic features by Raman and Fourier-transform infrared (FT-IR) methods. The analytical results show that the original Etnean paleosoils, mainly given by pyroclastic fall and accumulation, underwent marked modifications that produced the unique mineral assemblage of the Ghiara, characterized by the presence of hematite, a mineral responsible for its reddish appearance. The heterogeneity of the samples, however, does not allow us to define a univocal genetic model. On the basis of the outcrops geometry and the environmental conditions we propose a range of models that fit the characters of each sample. In detail, the genesis of this material can be influenced by one or all the following phenomena: i) chemical oxidation, ii) biological oxidation and iii) hydrothermal transformation by interaction of meteoric waters with volcanic volatiles from lavas. This work represents a first contribute in the knowledge of Ghiara characters and genesis, which on one side will help to a better use in its employment for construction purposes, on the other will contribute to constrain the hematite formation processes in different environments.