School of Engineering and Science - Jacobs University

7.5. Carbonate rocks
A common type of rock analyzed at JUB are carbonates, which are chemical
precipitates generally containing low trace metal contents, similar to cherts. Carbonate
rocks are dominated by (Ca,Mg)CO 3 minerals, and Ca and Mg may exist in solid
solution within the carbonate mineral structure. As a result, carbonate rock samples
typically analyzed may range from limestones (CaCO 3 ), through dolomites
((Ca,Mg)CO 3 ), to pure magnesites (MgCO 3 ). All of these carbonate rock types are
effectively decomposed using the HNO 3 carbonate decomposition method, though, as
noted previously, any accessory aluminosilicate minerals and refractory organic
carbon are relatively unaffected by dissolution with HNO 3 .
This section discusses the application of both decomposition methods (HF-
HClO 4 and HNO 3 ) to carbonate rocks, and describes the applicability and accuracy of
these methods with respect to specific elements of interest. As the literature reference
values are generally determined by analytical methods capable of thoroughly
characterizing the whole-rock abundance of elements, the inability of the carbonate
decomposition method to dissolve refractory silicate minerals results in ‘poor’
analytical results for elements typically hosted within these minerals (e.g., Zr, Nb, Hf,
Ta, Th, among others). It is therefore necessary to first examine data obtained using
the HF-HClO 4 whole-rock decomposition method, as these data are most comparable
to the average reference value calculated from literature sources.
The most commonly utilized carbonate CRM is the dolomite JDo-1, issued by
the Geological Survey of Japan (GSJ). The JDo-1 dolomite is 34.0% CaO and 18.5%
MgO, with only trace amounts of Al, Mn, or Fe. Therefore, major element
interferences would be expected from the high Ca and Mg contents, and should
primarily affect determinations of Co, Ni, Zr, and Nb. Figure 19 presents comparisons
between concentrations determined at JUB using the HF-HClO 4 decomposition
method with the average reference values. Determinations of Co and Ni are not shown
in Fig. 19, as the elements are severely compromised in HCl acid matrices by
interferences generated from Mg and Ca (Table 4). The JUB measured Co of 1.46
mg/kg is several times higher than the average reference value of 0.234 mg/kg, and
measured Ni is 11.1 mg/kg, also several times higher than the reference value of 2.9
mg/kg.
Of the remaining elements Ti, Sr, Y, the REE, and W show good agreement
with the average reference values, and the reference values themselves are consistent
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