Calcium (Ca) is an important element in chemostratigraphy. But what are the element-mineral links of Ca?
Calcium (symbol Ca; atomic number 20; relative atomic mass 40.078) [Link to webelements.com]
It is the fifth most abundant element in the Earth’s Crust.
Ca has one oxidation state, +2, and six naturally occurring isotopes (40Ca, 42Ca, 43Ca, 44Ca, 46Ca, and 48Ca), of which 40Ca is the most common one.
Common substitutions for Ca are Sr, Ba, Mn, Fe, and P; – particularly carbonate minerals are prone to such element substitutions during diagenetic processes.
Common element-mineral links of Ca
Most common element-mineral links: Ca is most commonly associated with carbonate minerals, such as calcite and dolomite, and the sulfates gypsum and anhydrite. However, in siliciclastic sediments, Ca is mostly associated with Ca-plagioclase, but also amphibole, pyroxene, and Ca-fluorite, as well as in some smectites (e.g., montmorillonite).
Ca in siliciclastic sediments
In siliciclastic sediments, Ca is mainly associated with Ca-plagioclase (anorthite, bytownite), and to a lesser extend pyroxene, amphibole, and Ca-fluorite, as well as in some smectites (e.g., montmorillonite). Besides these, Ca may be present as authigenic minerals (cement) in form of calcite, dolomite, gypsum/anhydrite.
Lithic fragments of igneous rocks (see below), if present, may contribute to the overall Ca concentrations in siliciclastic sediments.
Ca in carbonates and evaporites
In carbonate and evaporite rocks, Ca occurs as calcite/aragonite (CaCO3), dolomite (CaMg(CO3)2), ankerite (Ca(Fe,Mg,Mn)(CO3)2), gypsum (CaSO4·2H2O), and anhydrite (CaSO4).
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A Note on Data Quality
Stoichiometric (i.e., pure) calcite contains 56.03% CaO (40.04% Ca), anhydrite 41.19% CaO (29.44% Ca), and gypsum 32.57% CaO (23.28% Ca),while dolomite comprises of 30.41% CaO (21.73% Ca) and 21.86% MgO (13.18% Mg). These values can be used as a data quality check for geochemical analyses. They should not exceed these limits in their related lithology. Subsequently, the maximum CaO concentration in geochemical analyses of (pure) limestone should be below ca. 56% CaO.
Ca in igneous rocks
In basic and intermediate igneous rocks, such as basalt, dolerite, and gabbro, and andesite and diorite, respectively, Ca is mainly associated with pyroxene (e.g., aegirine-augite: +/- (Ca,Na) (Mg,Fe) (Si2O6)) and amphibole (e.g., hornblende: +/- (Ca,Na)2-3 (Mg,Fe,Al)5 (Al,Si)8O22 (OH,F)2). Titanite (a.k.a. sphene: CaTiSiO5), however, may occur as an accessory mineral in granite (an acidic igneous rock).
Therefore, Ca concentrations are higher in mafic rocks, due to the abundance of minerals such as amphibole, pyroxene, and Ca-plagioclase than in felsic rocks.