Mineralogy and chemistry of incrustations resulting from the 2014-2015 eruption of Fogo volcano, Cape Verde

The last eruption of the Fogo volcano, in the Cape Verde Archipelago, occurred in 2014-2015. A mineralogical and chemical study was undertaken on fumarole incrustations resulting from this event and compared with results obtained from the previous 1995 eruption. The mineralogical constitution of the fumarole deposits was assessed by X-ray diffraction and the chemical characterization was performed through X-ray fluorescence spectrometry with a wavelength dispersive system and by energy dispersive X-ray fluorescence at the European Synchrotron Radiation Facility. The most common compounds/minerals in solid deposits were sulfur, sodium chloride, and calcium sulphates with variable degrees of hydration, sodium sulphate, hydrated sulphates of sodium aluminum, potassium magnesium, or sodium magnesium and a fluorine-bearing mineral. Thenardite (Na₂SO₄) and its polymorph (phase III) were found simultaneously for the first time in incrustations, to the best of our knowledge. A large span of minor and trace elements present in incrustations (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ba, Ce, Tl, Pb) were also identified, some of them potentially hazardous to animal and human health. This study reveals that low temperature incrustations, allied to the atmospheric conditions of Fogo volcano, constitute a natural laboratory to observe the process of mineral formation—namely the Na₂SO₄ phase III considered metastable.