High-temperature ramsdellite–pyrolusite transformation kinetics
نویسندگان
چکیده
The two most common polymorphs of MnO2, ramsdellite and pyrolusite, are often found in natural association. Our starting sample is from the Mistake mine (Arizona) containing macroscopic crystals both (a = 4.5131(6), b 9.2689(13), c 2.8610(4) Å, V 119.69(3) Å3; S.G. Pbmn) pyrolusite 4.4030(2), 2.87392(16) 55.715(5) P42/mnm), along with a smaller amount “groutellite”. A mixed powder was used to study ramsdellite→pyrolusite transformation by situ high-temperature X-ray diffraction. results reveal that this not direct transition, but it occurs steps, as function temperature; transforms into an amorphous phase, which then recrystallizes pyrolusite. Amorphization crystallization kinetics were studied universal equation for solid–solid reactions. activation energies comparable, pre-exponential factor amorphization orders magnitude larger than crystallization’s. As consequence, implies formation due mismatch between conversion rates, reaches its maximum at around 630 K decreases higher T, when strongly promoted.
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ژورنال
عنوان ژورنال: Physics and Chemistry of Minerals
سال: 2021
ISSN: ['1432-2021', '0342-1791']
DOI: https://doi.org/10.1007/s00269-021-01166-2