Silicate iron minerals

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

The Fe content of sediments varies greatly with the type of rock (Wedepohl, 1969 a). Sandstones contain ca. 10 g kg Fe, claystones ca. 50 g kg and carbonatic rocks ca. 4 g kg Fe. In recent deep sea sediments Fe contents are low in carbonates (9 g kg ), but high in clays (65 g kg ). Sedimentary iron minerals belong to the groups of oxides, carbonates, clay silicates and sulphides. In addition, Fe is a common impurity in other sedimentary minerals. 

Almost all rocks contain at least some iron. The more important minerals, in which Fe is a major constituent, are given with their Fe contents in Table 16.1. In all of these minerals except magnetite, iron is exclusively or predominantly in the bivalent state. During weathering, the iron is released from these minerals and secondary , pedogenic iron minerals are formed. The most important ones are Fe-contain-ing clay silicates and Fe oxides but under reducing conditions, Fe carbonates, sulphides and phosphates may also be formed. 

Faye, G. H. (1968b) The optical absorption spectra of iron in six-coordinate sites in chlorite, biotite, phlogopite and vivianite. Some aspects of pleochroism in the sheet silicates. Canad. Mineral., 9,403-25. 

White, W. B. Keester, K. L. (1966) Optical absorption spectra of iron in the rockforming silicates. Amer. Mineral., 51,774—91 [see Bancroft Bums (1967a)]. 

The formation of other primary iron mineral compounds is determined by the presence in solution of active forms of silica, carbon, and sulfur. The sediments can be subdivided provisionally into four groups oxide, silicate, carbonate, and sulfide, formally corresponding to the sedimentary facies of the iron-formation, according to James (1954). For each group of sediments the relationships between iron compounds were determined by calculation, in a wide range of pH (from 0 to 14) and Eh (from -I-1.0 to —1.0 V). 

Fig. 76. Diagrams of mineral equilibria in silicate iron-formations in the absence of carbon dioxide (isothermal sections) / = actual boundaries of stability fields of minerals 2 = boundaries unrealistic under the given conditions S = isobars of fluid pressure (P, = jO + kbar) 4 = isobars of log...

Fig. 93. Equilibrium of magnesian-iron minerals in rocks with excess silica. A. In silicate iron-formations (aqueous fluid. Cum + Px -t- OH- Q association). B. In carbonate iron-rich rocks (carbonic acid fluid. Car -(- Px -I- 01 -t- Q association). Figures indicate maximum iron content of orthopyroxene in association with quartz and olivine.

The industrially most important minerals for aluminum manufacture are the bauxites (laterite). Bauxite is not a uniform material, but comprises a mixture of different aluminum oxide hydrates and hydroxides [e.g. boehmite, y-AIO(OH), diaspore, a-AlO(OH)] with aluminum silicates, iron and titanium oxides etc. with between 35 and 70% (mostly 50 to 65%) aluminum oxide. 

Many common substances—notably silicates, some mineral oxides, and a few iron alloys—are attacked slowly, if at all, by the methods just considered. In such cases, recourse to use of a fused-salt medium is indicated. Here, the sample is mixed with an alkali metal salt, called the jliLX, and the combination is then fused to form a water-soluble product called the melt. Fluxes decompose most substances by virtue of the high temperatures required for their use (300°C to 1000 C) and the high con-eentrations of reagents brought into contact with the sample. 

Aluminium, on the other hand, accumulates in the clay mineral fraction because it forms insoluble aluminosilicates and hydroxyoxides. The AI remains behind in the soil as other ions leach away. Iron also accumulates in soils but this is not apparent from Table 7.3 because the silicate clay minerals, with the exception of hydrous mica, are low in Fe. Iron precipitates in soils only as hydroxyoxides. Hydrous mica is altered parent material and is not reconstituted from the soil solution as are kaolinite, montmorillonite, and allophane. The <105° C water in Table 7.3 is, roughly speaking, adsorbed water the >105° C water is hydroxyl ions and water within crystal structures. 

Table 7.1 Reactivity of iron minerals towards sulfide (1000 iM OHjS, pH 7.5, 25°C) according to 1 Poulton et al. (2004), 2 Canfield et al. (1992), and 3 Raiswell and Canfield (1996). The poorly-reactive silicate fraction was...

---