Ferromagnesian

Geochemical Nature and Types of Deposits. The cmst of the earth contains approximately 2—3 ppm uranium. AlkaHc igneous rock tends to be more uraniferous than basic and ferromagnesian igneous rocks (10). Elemental uranium oxidizes readily. The solubiHty and distribution of uranium in rocks and ore deposits depend primarily on valence state. The hexavalent uranium ion is highly soluble, the tetravalent ion relatively insoluble. Uraninite, the most common mineral in uranium deposits, contains the tetravalent ion (II). 

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

Bums, R.G. (1993) Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars. Geochim. Cosmochim. Acta 57 4555-4574... 

Ganguly J. (1982) Mg-Fe order-disorder in ferromagnesian silicates, 11 thermodynamics, kinetics and geological applications. In Advances in Physical Geochemistry, Vol. 2 (ed. S. K. Saxena), pp. 58-99. Springer-Verlag. 

Ferromagnesian and Al-rich chondrules from type 3.0-3.1 ordinary chondrites typically... 

Kita, N. T., Nagahara, H., Togashi, S. and Morshita, Y. (2000) A short duration of chondrule formation in the solar nebula evidence from 26A1 in Semarkona ferromagnesian chondrules. Geochimica et Cosmochimica Acta, 64, 3913-3922. 

McKeegan, K. D., Greenwood, J. P., Leshin, L. A. and Cosarinsky, M. (2000) Abundance of 26 A1 in ferromagnesian chondrules of unequilibrated ordinary chondrites (abstr.). Lunar and Planetary Science XXXI, CD 2009. 

Mostefaoui, S., Kita, N., Togashi, S. et al. (2002) The relative formation ages of ferromagnesian chondrules inferred from their initial aluminum-26/aluminum-27 ratios. Meteoritics and Planetary Science, 37, 421-438. 

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. 

Inactive Coupon Experiments. A fresh cut surface of Lac du Bonnet granite contains both altered and unaltered ferromagnesian... 

Pm and Am, using a combination of autoradiographic and petrographic techniques, indicate a clear preference for sorption on ferromagnesians, opaques and their alteration products (4,5),... 

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. 

Fractional crystallisation has been dominated by separation of various proportions of clinopyroxene and olivine in the mafic magmas, and of cli-nopyroxene and feldspars in the felsic melts. These generated decrease in ferromagnesian elements (FeO, MgO, Ni, Co, Cr, etc.) and increase in incompatible elements (e.g. Th, Ta, Nb, REE), with ongoing evolution. In contrast, ratios of incompatible trace elements were not affected by fractionation processes, and can be used to infer compositions of mantle-equilibrated melts. 

Major and trace elements of the Ischia rocks define continuous trends on variation diagrams, although with scattering (Fig. 6.16). Ti02, MgO, FeOto-tai, CaO, P2O5, ferromagnesian trace elements, Sr and Ba decrease with in-... 

Major elements show a vertical trend for potassium in the mafic rocks (Fig. 6.16c). These have moderate enrichments in incompatible elements, which are lower than in other Campanian volcanoes ferromagnesian trace element contents are high in the mafic rocks (Ni up to 230 ppm, Cr up to 600 ppm). REE are fractionated, with small or no Eu anomalies (Fig. 6.17a). Incompatible element patterns of mafic rocks are fractionated and contain small negative anomalies of HFSE and positive spikes of Pb (Fig. 6.17b). Sr and Nd isotopic ratios respectively display lower and higher... 

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