Iridescence is very often observed on the glassy surface of freshly erupted basaltic tephra like those erupted by Piton de la Fournaise volcano;
On one side, it disappears quickly on samples collected soon after an eruption; on the other sides, it can be observed on clasts erupted centuries ago; multiple physico-chemical processes are possibly responsible of this intriguing phenomenon (chemical attack by acid gases; thin film effects; microvesicularity; density contrasts; quench rate etc); Better understanding that might help making a more quantitative link with fragmentation processes and eruptive dynamics;
Is anyone aware of recent or old studies which could help in understanding the appearance and change of iridescence on basaltic glass surfaces? Thanks in advance,
Best, Andrea
In the 'The Secret Lives of Minerals' By Elisabeth Nadin, it is said that the iridescent sheen of basalt is due to the occurrence of titanium bearing magnetite octahedral crystals arranged in lamellar fashion just below the surface layer. The blue color is due to the charge transfer between iron and titanium.
Dear Anuradha, thank you for your answer! Unfortunately, the Piton de la Fournaise glasses are either microlite free or do not contain oxides among the (few) microlites....may be I can check at nanolite scale.....I'll tell you if they are there; thanks!
It is probably due to micro thin, conchoidal laminae produced during solidification of the lava. Centuries old glasses tend to devitrify with time and the crystallites so formed produce iridescence.
Hi everybody,
I've heard about Siderazot on some lavas, on Etna and Vesuvius volcanics. Siderazot is a iron nitride (Fe5N2), but it seems to be a doubtful species. You can have a look on Mindat.org and the Handbook of minerals (Bideaux et al). Could be interesting to investigate and to solve this enigma:-)
thanks everybody for the answers to my question; it seems that there are several hypotheses not necessarily mutually excluding each other; in case anybody is interested in performing a joint study of that, I'll be glad of test the several possible processes; best and marry christmas!
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