Manganese crystalline forms

Among five crystalline forms of manganese dioxide electrochemical y-Mn02 is known to be the most electrochemically active. 

Manganese tris(acetylacetonate) exists in two crystalline forms, ft and y. High quality structures of both have been published.99,100 Each exhibits definite evidence of Jahn-Teller distortion. In the jS-form the average Mn—O distances are 1.95 and 2.00 A in a compressed M06 octahedron the... 

S02 and MoOa but manganese also forms 03Mn(0K), termed potassium permanganate, which is analogous in formula as well as in crystalline form with potassium perchlorate, OgCl(OK). It is convenient, however, also to include chromium and manganese in the iron group of elements. 

Lovley and Phillips, 1986). Minerals such as ferrihydrite and lepidocrocite (y-FeOOH) are generally reduced more rapidly than relatively stable minerals such as goethite and hematite (Postma, 1993). Amorphous manganese oxides such as vernadite are more easily reduced than strongly crystalline forms such as pyrolusite, but the overall influence of crystallinity on reduction kinetics appears to be weaker for manganese and iron oxides (Burdige et ai, 1992). 

The difference between the forms involves either (1) crystalline structure (2) the number of atoms in the molecule of a gas or (3) the molecular structure of a liquid. Carbon is a common example of (1), occurring in several crystal forms (diamond, carbon black, graphite) as well as several amorphous forms. Diatomic oxygen and diatomic ozone are instances of (2) and liquid sulfur and helium of (3). Uranium has three crystalline forms, manganese four, and plutonium no less than six. A number of other metals also have several allotropic forms which are often designated by Greek letters, e.g., a-, y-, and A-iron. 

Oxides of manganese and iron. Acidified hydroxylamine hydrochloride releases metals from the manganese oxide phase with minimal attack on the iron oxide phases. Amorphous and crystalline forms of iron oxides can be discriminated between by extracting with acid ammonium oxalate in the dark and under UV light, respectively. 

And here also another alum is obtained by substituting Am for K. A good many more alums may be procured by substituting Na (sodium) for K, and Mn for Al, that is, manganese for aluminum and all these salts have the same crystalline form and the same general properties. Here, as in the former more simple group, the analogy in constitution is at once obvious. Every alum is... 

Wheli we consider the facts above noticed, the circumstance that the sulphates of copper and manganese, with 4 eq. of water of crystallisation, those of iron and cobalt, with 5 eq., and those of magnesia, zinc, and nickel, with 6 eq. respectively crystallise in the same forms, different for each group and when we reflect on the two remarkable groups of double sulphates just described, we cannot avoid the conclusion, that similarity of constitution is one main cause of similarity of crystalline form and this constitutes the doctrine of isomorphism, as at present understood and admitted. By similarity of constitution is meant, not only a likeness in the nature of the elements present, but a similar airangement of those elements. 

Oxidized forms of iron and manganese are present as insoluble solid phases either as amorphous or as crystalline forms (Figure 10.7). However, at low pH both oxidized and reduced forms of iron and manganese are present in the soluble phase (Figure 10.7). Some of the common insoluble... 

Amorphous forms of Fe(III) and Mn(IV) are reduced preferentially, leaving more resistant crystalline forms. Thus, Fe(III) and Mn(IV) reduction rates will decrease on removal of respective bioavailable iron and manganese. In redox zones where Mn(IV) reduction occurs, lack of bioavail-able Mn(IV) can promote the reduction of Fe(III) oxides in the presence of crystalline forms of Mn(IV) oxides. Similarly, in redox zones where Fe(III) reduction occurs, depletion of bioavailable Fe(III) oxides can promote the reduction of electron acceptors of lower reduction potentials. 

Mobile pools of iron and manganese are present in water-soluble or dissolved forms in soil pore water. Immobile forms include solid phases such as insoluble precipitates and mineral phases (amorphous and crystalline forms) present both in aerobic and anaerobic soil layers. The flux of dissolved iron and manganese is typically from anaerobic soil layers to aerobic soil layers, where it is oxidized to insoluble precipitates. This results in the establishment of concentration gradients across the aerobic-anaerobic soil interface. Mobilization is also regulated by pH and CEC. Manganese is more soluble in moderately acidic conditions (between pH 5 and 6) than iron. 

Several studies have shown that the more crystalline the Fe(III) and Mn(IV) oxides, the slower the rate of their reduction (reviewed by Lovley, 1991, 2004). In laboratory experiments, Ottow and Klopotek (1969, cited by Lovley, 1987) reported that the reduction capacity of select iron minerals was found to be in the order of FeP04 > Fe(OH)j > FeOOH > Fe203. Amorphous and poorly crystalline forms of Fe(III) and Mn(IV) oxides dominate wetlands that undergo frequent wet and dry cycles. These systems are dynamic, and repeated oxidation-reduction reactions involving iron and manganese will not allow time for stable crystalline forms of Fe(III) and Mn(IV) oxides to form in these systems. Strong relationships are observed between Fe(III) reduction rates and poorly crystallined forms of Fe(III) oxides (as determined by hydroxylamine extraction) in fresh and brackish water sediments (Lovley and Philips, 1987). 

Pyrolusite is a black, opaque mineral with a metallic luster and is frequendy soft enough to soil the fingers. Most varieties contain several percent water. Pyrolusite is usually a secondary mineral formed by the oxidation of other manganese minerals. Romanechite, a newer name for what was once known as psilomelane [12322-95-1] (now a group name) (7), is an oxide of variable composition, usually containing several percent water. It is a hard, black amorphous material with a dull luster and commonly found ia the massive form. When free of other oxide minerals, romanechite can be identified readily by its superior hardness and lack of crystallinity. 

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