Polymorphism manganese oxide

Ostwald s step rule holds that a thermodynamically unstable mineral reacts over time to form a sequence of progressively more stable minerals (e.g., Morse and Casey, 1988 Steefel and Van Cappellen, 1990 Nordeng and Sibley, 1994). The step rule is observed to operate, especially at low temperature, in a number of min-eralogic systems, including the carbonates, silica polymorphs, iron and manganese oxides, iron sulfides, phosphates, clay minerals, and zeolites. 

Laberty and Navrotsky (1998) determined the enthalpies of formation of a number of iron oxide and oxyhydroxide polymorphs. Data are listed in Table 2 which also compares the enthalpy relations among aluminum, iron, and manganese. It is evident that the Fe oxyhydroxide phases are much less stable relative to the anhydrous ferric phase (hematite) than are the aluminum oxyhydroxides relative to corundum. This is consistent with the much more frequent observation of hematite than of corundum in the field. It is also evident that the iron phases are as rich in polymorphism as the aluminum phases. It is clear that the enthalpy differences for both anhydrous (AI2O3, Fe203, Mn02) and hydrous (AlOOH, FeOOH, MnOOH) polymorphs are small, setting the stage for nanoscale stability crossovers. 

Stewart SF, Leathart JB, Chen Y, Daly AK, Rolla R, Vay D, Mottaran E, Vidali M, Albano E, Day CP (2002) Valine-alanine manganese superoxide dismutase polymorphism is not associated with alcohol-induced oxidative stress or liver fibrosis. Hepatology, 36 ... 

Manganese(IV) oxide is polymorphic and often markedly non-stoichiometric. Only the high-temperature P-form has the stoichiometry Mn02 and adopts the rutile structure Figure 5.21). It acts as an oxidizing agent when heated with concentrated acids (e.g. reaction 21.51). 

---