CINECA IRIS Institutional Research Information System
IRIS è il sistema di gestione integrata dei dati della ricerca (persone, progetti, pubblicazioni, attività) adottato dall'Università degli Studi di Torino.
AperTO è l'archivio istituzionale Open Access destinato a raccogliere, rendere visibile e conservare la produzione scientifica dell'Università degli Studi di Torino.
EnglishThe 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 of both ramsdellite (a = 4.5131(6), b = 9.2689(13), c = 2.8610(4) Å, V = 119.69(3) Å3; S.G. Pbmn) and pyrolusite (a = 4.4030(2), c = 2.87392(16) Å, V = 55.715(5) Å3; S.G. P42/mnm), along with a smaller amount of “groutellite”. A mixed powder was used to study the ramsdellite→pyrolusite transformation by in situ high-temperature X-ray powder diffraction. Our results reveal that this transformation is not a direct transition, but it occurs in two steps, as a function of temperature; ramsdellite transforms into an amorphous phase, which then recrystallizes into pyrolusite. Amorphization of ramsdellite and crystallization of pyrolusite kinetics were studied by the universal equation for solid–solid reactions. The two activation energies are comparable, but the pre-exponential factor of the ramsdellite amorphization is two orders of magnitude larger than pyrolusite crystallization’s. As a consequence, ramsdellite→pyrolusite transformation implies the formation of an amorphous transition, due to a mismatch between the conversion rates, that reaches its maximum at around 630 K and then decreases at higher T, when pyrolusite crystallization is strongly promoted.
| Titolo: | High-temperature ramsdellite–pyrolusite transformation kinetics | |
| Autori Riconosciuti: | ||
| Autori: | Curetti N.; Bernasconi D.; Benna P.; Fiore G.; Pavese A. | |
| Data di pubblicazione: | 2021 | |
| Abstract: | 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 of both ramsdellite (a = 4.5131(6), b = 9.2689(13), c = 2.8610(4) Å, V = 119.69(3) Å3; S.G. Pbmn) and pyrolusite (a = 4.4030(2), c = 2.87392(16) Å, V = 55.715(5) Å3; S.G. P42/mnm), along with a smaller amount of “groutellite”. A mixed powder was used to study the ramsdellite→pyrolusite transformation by in situ high-temperature X-ray powder diffraction. Our results reveal that this transformation is not a direct transition, but it occurs in two steps, as a function of temperature; ramsdellite transforms into an amorphous phase, which then recrystallizes into pyrolusite. Amorphization of ramsdellite and crystallization of pyrolusite kinetics were studied by the universal equation for solid–solid reactions. The two activation energies are comparable, but the pre-exponential factor of the ramsdellite amorphization is two orders of magnitude larger than pyrolusite crystallization’s. As a consequence, ramsdellite→pyrolusite transformation implies the formation of an amorphous transition, due to a mismatch between the conversion rates, that reaches its maximum at around 630 K and then decreases at higher T, when pyrolusite crystallization is strongly promoted. | |
| Volume: | 48 | |
| Fascicolo: | 11 | |
| Pagina iniziale: | 1 | |
| Pagina finale: | 11 | |
| Digital Object Identifier (DOI): | 10.1007/s00269-021-01166-2 | |
| Parole Chiave: | High-temperature transformation; MnO; 2; polymorphs; Pyrolusite; Ramsdellite; Reaction kinetics | |
| Rivista: | PHYSICS AND CHEMISTRY OF MINERALS | |
| Appare nelle tipologie: | 03A-Articolo su Rivista |
File in questo prodotto:
| File | Descrizione | Tipologia | Licenza | |
|---|---|---|---|---|
| HT-MnO2.pdf | PREPRINT (PRIMA BOZZA) | Open Access Visualizza/Apri |
http://hdl.handle.net/2318/1816307
Copyright © 2022