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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-colour photomicrograph of fumarole minerals from Mutnovsky, a volcano on the Kamchatka Peninsula in Russia, was taken using a scanning electron microscope and colour-enhanced by optical microscopy. 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
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