New insights into the mineralogical facies distribution of Campanian Ignimbrite, a relevant Italian industrial material

Industrial minerals represent an important resource for the Italian economy, both in terms of exploitation and transformation, especially in those sectors for which Italy holds a leadership such as ceramics and glass. Among Italian regions Campania is one of the poorest of such kind of resources although some geological formations could be advantageously exploited. An important role is played by the deposits of sedimentary zeolites linked to the activity of different volcanic districts occurring on the Neapolitan territory such as the Campanian Ignimbrite, the most important volcanic episode of the Campi Flegrei (Southern Italy), which blankets a wide area of the Campanian region. The Campanian Ignimbrite has been thoroughly investigated from a volcanological and petrographic point of view. However, little attention was devoted to the attainment of those information useful to verify the potential of the deposit as well as the interpretation of post-depositional mineral-forming processes that have affected this deposit and have led to the formation of several facies, among which the most important are characterized by intense feldspathization (grey facies) and remarkable zeolitization (yellow facies). X-ray diffraction, X-ray fluorescence, inductively coupled plasma emission spectrometry and scanning electron microscopy methods were used to thoroughly characterize the entire set of samples collected from 31 outcrops. Data so far acquired enabled to define the role played by several parameters (i.e., temperature, alkaline and alkaline-earth cations, etc.) in influencing the zeolitization process and the consequent crystallization of phillipsite, chabazite, and analcime. Feldspathization appears to have been controlled mainly by the emplacement temperature of the deposits. The proposed genetic model involves emplacement of the pyroclastic flow in a single episode, producing a deposit with an upward-decreasing temperature gradient. In this model, the central portion had a temperature insufficient to cause significant feldspathization, and the upper portion of the formation was affected by water percolation while still hot, producing progressive zeolitization. Volcano-stratigraphical parameters suggest some constrains on the maximum temperatures affecting the central portion of WGI. Thermodynamic data on zeolites constrain the maximum temperatures of the LYT unit which likely decrease upwards in the unit up to ambient conditions in CPF.