Manganese amorphous forms

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. 

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. 

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. 

Some metals are irreversibly adsorbed, probably via incorporation into the mineral phases, such as amorphous iron oxyhydroxides, as shown in Figure 11.6d. Some of these amorphous phases form by direct precipitation from seawater. As noted earlier, hydrothermal fluids are an important source of iron and manganese, both of which subsequently precipitate from seawater to form colloidal and particulate oxyhydroxides. Other metals tend to coprecipitate with the iron and manganese, creating a polymetallic oxyhydroxide. It is not clear the degree to which biological processes mediate the formation of such precipitates. Since the metals are incorporated into a mineral phase, this type of scavenging is better referred to as an absorption process. 

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

The form of nickel in particles from different industries varies. The mineralogical composition, chemical content, and form of dusts from nine industries in Cracow, Poland, were examined (Rybicka 1989). The chemical form of a particle-associated heavy metal that was assessed by a five-step extraction scheme classified the metal as exchangeable, easily reducible (manganese oxides, partly amorphous iron oxyhydrates and carbonates), moderately reducible (amorphous and poorly crystallized iron oxyhydrates), organically bound or sulfidic, and residual. Dusts from power plants had a silicate characteristic with quartz and mullite predominant. Approximately 90% of the nickel from these... 

Phosphorus extracted from sediment by NaOH has been related to non-occluded, surface-exchangeable, bioavailable forms (22). Hydrochloric acid extraction yields occluded phosphorus incorporated in hydrous metal oxides, carbonate and phosphate minerals of sediment. Hydroxylamine reagent specifically removes hydrous manganese oxides, while amorphous hydrous oxides of iron and aluminijm are removed by the oxalate reagent. Total available sediment phosphorus analyses includes sediment organic phosphorus components in addition to the inorganic portion determined by the selective extraction procedures. 

About 35% of the iron and 75% of the manganese in soils and sediments is in the form of free oxides (Canfield, 1997 Cornell and Schwertmann, 1996 Thamdrup, 2000). The remainder occurs as a minor constituent of silicate minerals. The lattice stmcture of Fe(III) oxide minerals varies widely. Freshly oxidized Fe(III) precipitates rapidly as ferrihydrite (Fe(OH)3), a reddish-brown, amorphous, poorly crystalline mineral. Ferrihydrite is the dominant product of Fe(II) oxidation whether it occurs by abiotic oxidation, aerobic microbial oxidation, or anaerobic microbial oxidation (Straub et al., 1998). Over a period of weeks to months, amorphous ferrihydrite crystals undergo diagenesis to yield well-ordered, strongly crystalline, stable minerals such as hematite(a-Fe203) and goethite (a-FeOOH) (Cornell and Schwertmann, 1996). 

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). 

---