Manganese polymorphism

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. 

No binary xanthate structures of manganese, technetium, or rhenium have been reported. Indeed, there are relatively few crystal structures known with only a single manganese structure (two polymorphs), two phosphine adducts of technetium and three structures containing rhenium, two of which are dinuclear. 

When subjected to high temperatures and pressures, polymorphs have been prepared for some of the difluorides. These all have distorted variants of the fluorite structure, with cubic, or pseudocubic, tetragonal cells. Manganese difluoride has been most studied (28-30) and similar polymorphism reported for cobalt and zinc difluorides (30). Recently, palladium and silver difluorides have been shown to behave in a similar way forming cubic metastable phases (31). In all cases there is a decrease in volume for the structure change. 

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. 

Fritsch S, Post JE, Navrotsky A (1997) Energetics of low temperatrrre polymorphs of manganese dioxide and oxyhydroxide. Geochim Cosmochim Acta 61 2613-2616 Fritsch S, Post JE, Strib SL, Navrotsky A (1998) Thermochemistry of framework and layer manganese dioxide related phases. Chem Mater 10 474-479... 

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). 

During the past few years, a significant effort has been directed towards the synthesis of low-dimensional Mn02 nanostructures with controlled morphologies. For instance, a-, P-, y-, 6-, k-, and E-Mn02 polymorphs were synthesized in the shape of nanorods [141-145], nanowires [144, 146-148], nanofibers [149, 150], nanoneedles [151, 152], and nanotubes [153, 154]. Lately, the attention of a considerable number of chemists and materials scientists has also been oriented towards the self-assembly of 1-D nanostructured manganese dioxides into 2- and 3-D ordered microstructures [144, 155-158]. 

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