A photoelectron spectroscopy study of lava stones

In areas surrounding volcanos the atmospheric agents remove from the emitted stones surface layers that go directly in the air and groundwater. The chemical composition of magmatic stones is currently studied by techniques that do not differentiate between bulk and surface composition. To our knowledge there are no XPS investigations of magmatic stones. Therefore, in this study we report on the first surface chemical investigation, using the X-ray photoelectron spectroscopy technique, of magmatic stones and in particular, of lapilli emitted by the Etna volcano (Sicily). Results show important differences between bulk and surface composition of major oxides. We have found that Si, Fe, Ca and K segregate into the bulk whilst, Al, Na, P and Mn segregate on the surface. For example, the Mn surface concentration is 266.7 % with respect to that observed in the bulk and is due to the surface, water soluble MnOOH species. The present study highlights the potentiality of the XPS technique in the investigation of the surface composition of magmatic materials.