Ferromagnesian minerals

Table on p. 75). Lithium tends to occur in ferromagnesian minerals where it partly replaces magnesium it occurs to the extent of about... 

FEMIC. This term is used by penologists to designate ihe more common ferromagnesian minerals such as pyroxene and olivine. Rocks relatively rich in lemic minerals are said lo he urafic. 

The application of chemical kinetics to weathering processes of soil minerals first appeared in the work of Wollast (1967). He concluded that the rate-limiting step for weathering of feldspars was diffusion (Chapter 7). This work touched off a lively debate that is still raging today about whether weathering of feldspars and ferromagnesian minerals is controlled by chemical reaction (CR) or diffusion. 

Often fractional orders best describe soil chemical processes. For example, the reaction order for dissolution of oxides, calcite, feldspars, and ferromagnesian minerals is often <1 (Stumm et al., 1985 Bloom and Erich, 1987). 

As mentioned earlier, the dissolution of oxides and hydroxides, like feldspars and ferromagnesian minerals, appears to be a surface-controlled reaction. One indication of this is the high E values found by several investigators. Bloom and Erich (1987) obtained E values ranging from 59 4.3 to 67 0.6 kj mol"1 for gibbsite dissolution in acid solutions (pH 1.5-4.0). These values are much higher than for diffusion-controlled reactions reported earlier. 

The sorption on newly formed iron hydroxides is known to be reversible on short time scales (8). The differences in autoradiographs (B) and (C) of Figure 3 indicate that a large amount of exchangeable activity is associated with the ferromagnesian minerals however, areas other than those of high iron content also show a decrease in activity after the CaCl2 extraction. 

Fig. 5.10. Conceptual model of carbonic acid weathering of ferromagnesian minerals on Mars. Reprinted from Marion et al. (2003a) with permission...

Matsui, Y. Banno, S. (1969) Partition of divalent transition metals between coexisting ferromagnesian minerals. Chem. Geol., 5, 259-65. 

Pitt, G. D. Tozer, D. C. (1970) Radiative heat transfer in dense media and its magnitude in olivines and some other ferromagnesian minerals under typical upper mantle conditions. Phys. Earth Planet Interiors, 2,189-99. 

High concentrations of chromium may occur naturally in groundwater in areas with mafic or ultramafic volcanic or metamorphic rocks (i.e. rocks that consist mainly of ferromagnesian minerals with no quartz). 

Nickel has a similar chemical behaviour to iron and cobalt, and commonly substitutes for iron in ferromagnesian minerals. 

Oxidation. For those elements that can exist in more than one valence state oxidation, and indeed reduction, may be a major reaction in the chemical-weathering process. Iron and manganese are the most important elements that behave in this way. For example, iron in the ferromagnesian minerals is in the Fe(II) state, which is oxidized to Fe(III) when released from the mineral. This can cause changes in the charge balance, requiring other ions to be lost. Formation of iron oxide can cause physical disruption to the mineral. 

The minerals in a coal clearly reflect its geochemical environment of deposition and diagenesis rather than the composition of the source rocks. Ferromagnesian minerals such as pyroxenes, amphiboles, and ohvines are very rare in coal feldspars are also uncommon. These minerals are unstable in low-pH peat-swamp environments and are apparently destroyed in the early stages of coalihcation. In contrast, more resistant accessory minerals such as rutile, zircon, and rare-earth phosphates are relatively common in coal. 

Clays are volumetric ally the most abundant mineral group in coal. They can be authigenic or detrital in origin. Kaolinite is the most common clay and the most common authigenic mineral in coals. The silicon and aluminum in kaolinite are, perhaps, residual from the dissolution of ferromagnesian minerals and feldspars. Illite and mixed layer clays in coal are almost exclusively detrital in origin. Chlorites, smectites, and other clay minerals may be abundant locally. 

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

The presence of iron in the weathering reactions is fundamental to lat-erite development and also derives from the breakdown of iron silicates. One of the simplest ferromagnesian mineral structures is that of fayalite (Fe-rich olivine), which is a common silicate in many mafic rocks according to Curtis (1976), ferric iron (Fe3+)may be produced directly from a simple oxidation and hydration reaction ... 

Weathering in the saprolite zone causes the destruction of feldspars and ferromagnesian minerals. Here, the mobile elements Na, Ca, K and Sr are leached and evacuated from the system (although K and Ba can be retained... 

The alteration of all but the most resistant primary minerals occurs in the mid- to upper saprolite zones in addition, less stable secondary minerals such as smectite are also destroyed. Serpentine, magnetite, ilmenite and chlorite are progressively weathered through the zone. Ferromagnesian minerals are the principal hosts for transition metals such as Ni, Co, Cu and Zn in mafic and ultramafic rocks they become leached from the upper horizons and reprecipitate with secondary Fe-Mn oxides in the mid- to lower-saprolite. 

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