Fumarole minerals are minerals that are deposited by
fumarole exhalations. They form when gases and compounds desublimate or precipitate out of condensates, forming mineral deposits. They are mostly associated with volcanoes (as volcanic sublimate or fumarolic sublimate), following
deposition from volcanic gas during an eruption or discharge from a volcanic vent or fumarole, but have been encountered on
burning coal deposits as well. They can be black or multicoloured and are often unstable upon exposure to the atmosphere. This natural-color
photomicrograph of fumarole minerals from
Mutnovsky, a volcano on the
Kamchatka Peninsula in Russia, was taken using a
scanning electron microscope. Yellow and red crystals of
thallium(I) iodide are visible, with a gradual transition between the two
polymorphs. The crystals are located on a substrate of altered rock. This image is 700 micrometres (0.028 in) across on the long side.
Photograph credit: Mikhail Zelensky
