Nesbitt & Young (1982, 1984) used the general acid-based reactions (usually H 2CO 3) during weathering of feldspars and volcanic glass and investigated the kinetic behavior of these minerals.
Conversely, low CIA values suggest low weathering effects on these cations. High CIA values reflect the removal of labile cations, such as Ca 2+, Na 2+, and K + in relation to more stable cations, such as Al 3+ and Ti 4+, from the rock during chemical weathering. The Chemical Index of Alteration (CIA) is a good measure of the degree of chemical weathering. The cations Ca, Na, and K are released during chemical weathering into weathering solutions. With quartz being a stable mineral, the other components are rather unstable. The composition of upper-crust rocks is dominated by plagioclase, quartz, K-feldspar, volcanic glass, biotite, and muscovite (Nesbitt & Young, 1984). The chemical weathering of siliciclastic rocks has a strong effect on the major element composition and the associated minerals. Mg is thus relatively abundant in mafic/basic igneous rocks, but also in granite and granodiorite in form of biotite. olivine and (ortho-)pyroxene, or under ‘wet’ conditions (high water and oxygen fugacity) as amphibole and mica. In magmatic rocks, Mg has crystallized relatively early as high-temperature minerals, e.g. However, calcite (CaCO 3) can incorporate Mg into its crystal lattice to a certain amount leading to the differentiation of low-Mg calcite ( 5 mol% MgCO 3) (which is reflected in the ‘calcite – aragonite sea’ cycles, e.g., Sandberg, 1983, Stanley et al., 2010). Mg-bearing carbonate minerals are mainly dolomite (CaMg(CO 3) 2), ankerite (Ca(Fe,Mg,Mn)(CO 3) 2), and magnesite (MgCO 3). Authigenic minerals may include carbonate minerals, such as dolomite, ankerite, and magnesite.ĭue to its strong affinity with phyllosilicates (clay and mica minerals) and the instability of most of the igneous minerals (olivine, amphibole, pyroxene), Mg concentrations are commonly higher in ‘shales’ than in sandstones. Mg is commonly associated with clay minerals, such as chlorite and smectite, mica e.g., biotite and glauconite (the latter an indicator for marine origin), as well as olivine (e.g., forsterite), amphibole, pyroxene, spinel, and garnet (e.g., pyrope). In siliciclastic sediments, the mineral assemblage can comprise detrital and authigenic minerals. Most common element – mineral associations: Sedimentary rocks can have a diverse range of Mg-bearing minerals. Mg has one main oxidation stage, +2, and is the eighth-most abundant element in the Earth’s crust. Magnesium (symbol Mg atomic number 12 relative atomic mass 24.305) In most siliciclastic sediments Ti is present in only low concentrations, with TiO 2 concentrations commonly 1%) and ultramafic (> 2%) igneous rocks, compared to felsic (TiO 2 ~0.2%) rocks. Ti can also (beside others) be incorporated in traces into the crystal lattice of amphibole, pyroxene, garnet, biotite, illite, and chlorite. Most common element – mineral associations: heavy minerals, such as rutile/anatase/brookite TiO 2), ilmenite (FeTiO 3), titanite (or sphene) (CaTiSiO 5), and titanomagnetite (Fe”(Fe”’,Ti) 2O 4). Ti has three main oxidation states, +2, +3, +4, with +4 being the most common one. Titanium (symbol Ti atomic number 22 relative atomic mass 47.867)
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