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Small, brightly colored and extremely delicate. They form in a few hours but a heavy rain can be enough to wash them away. Fumarole minerals are typical formations of active volcanic systems, whose formation is regulated by the high temperatures of the fumaroles and by the interaction of fumarolic gases with the surrounding rocks.

The Campi Flegrei caldera is rich in these minerals and is a real open-air laboratory for their observation and study. Suffice it to say that here it is possible to find some minerals which, until now, have not been identified anywhere else in the world.

We talked about it with Massimo Russo, an INGV researcher, an expert in fumarolic minerals and the 'father' of some of them, who managed to make the dream of many come true: to make a passion that has accompanied him since he was a child, when he collected small pebbles by the sea or in the mountains, the job of a lifetime. 

Massimo, what kind of minerals can you find in Campi Flegrei?

Forge1Many, but above all sulfur and hydrated sulphates which are characteristic of the temperatures at the emergence of fumaroles, of 100°C or less.

There are currently 45 certain species in the Solfatara-Pisciarelli-Antiniana area. Of these, 5 type locality, i.e. found for the first time in the world at Campi Flegrei (it is about dimorphite, iron eremovite, Russoite - which took my name -, turn around e paradimorphite).

This is the result of a long and painstaking work published in 2017: after which, due to the closure of the site, unfortunately I no longer had the opportunity to go and carry out further surveys.

What characteristics do these minerals have?

The fumarolic minerals of Campi Flegrei are for the most part more or less soluble in water and often a heavy rain is enough to dissolve them (but then it is sufficient to wait for the dry season to witness their formation again). As I said, these are mainly sulfur and hydrated sulphates, but in the highest temperature fumarole (that of the Bocca Grande) arsenic sulphides and ammonium chloride are formed above all.

How are they formed?

Forge2In the case of fumarolic minerals we speak of "species of neoformation" as they can form in a very short time (hours, at most days, are sufficient). They can be formed in two ways: by direct action of the gaseous phase on the basis of the emergency temperature (these are improperly called "sublimated", even if in reality they should be called "frozen", as sublimation is the opposite process, which from solid phase allows the transition to the gaseous phase); or by gas-rock interaction, always conditioned by the temperature (in this second case we are dealing with the so-called “fumarole incrustations”).

Why are they formed right there?

In reality, fumaroles are formed in the Solfatara of Pozzuoli wherever there are: the action of the hydrogen sulphide which is transformed into sulfuric acid, in fact, alters the rock causing the cations to recombine with the sulphate and with the waterfall. At the highest temperatures, as I said, we speak of "sublimates". 

There are areas in the area that are more favorable than others formation of these minerals?

Forge1Not in particular, but certainly in the highest temperature fumarole, the Bocca Grande (with a temperature ranging between 160 and 164°C) there are the most interesting and rare minerals. 

What do these minerals tell us about the Campi Flegrei caldera?

Low-temperature fumarolic minerals don't really tell us much about the state of the volcano. However, the situation changes - for example - in the case of the island of Vulcano or Vesuvius (when it was open conduit). In these cases, in fact, since the minerals have stable temperatures within which they form, as the temperature increases it is possible to see different minerals, the old ones being replaced by the new ones. These, together with the chemistry of minerals (ie the study of their chemical characteristics), can provide very useful information for understanding the evolution of the volcanic system in question. 

How long have fumarolic minerals been studied and with what techniques?

Forge4I'd say at least 300 years. I approached their study in 1985 and it has become a more "serious" analysis since 2006, thanks to the collaboration of researchers from the Chemistry Department of the University of Milan. Among us there is a perfect harmony of interest and research of rare and new mineral species for the greater knowledge of the places in question, whether it is the area of ​​Solfatara, Vesuvius or the island of Vulcano.

"As" these studies are carried out is a complex 'story'. First of all there is the painstaking research work in the field, then the exhausting optical microscope view at high magnifications of what may be of interest: at this point the area of ​​interest of the sample must be "pushed" to be able to find it again subsequently. Then a first "pass" is done with the electron microscope vision which allows us very high magnifications and at the same time the chemical analysis of the mineral is carried out (non-destructive analysis because we work at low pressure and without the need to metallize the sample). In the case of fumarolic minerals it can often happen that the investigation can stop here in the recognition, defining with certainty the mineral species. In case of doubts, or if there is nothing similar in the database, then we move on to the subsequent analyses. Only after these preliminary investigations will it be understood whether we are dealing with something unknown. And we continue with further investigations.

What are you most passionate about studying minerals?

Forge1Well, since I was a child I collected stones on the beach or in the mountains, then growing up it became a real hobby, but making it a profession is everyone's dream: turning a passion into one's job is a feeling that is difficult to describe in words. 

For me it's fun to go looking for minerals and figure out which species they belong to. Obviously, at first glance it is possible to recognize only a few hundred species (the best ones even arrive several hundred!). But the most exciting aspect for me is being able to observe them under a microscope: the smaller they are, the closer to perfection they are. It is as if nature had placed itself there like a sculptor who patiently shapes them into various shapes and like a painter paints them with many different colours. Nature is really crazy.

Link to the in-depth article on the INGVvulcani Blog: https://ingvvulcani.com/2019/06/17/altro-che-diamanti-minerali-rari-alla-solfatara-di-pozzuoli/