ELECTRON-MICROPROBE ANALYSIS OF GEOLOGICAL CARBONATES

Electron microprobe analysis (EMPA) of geological materials is often c arried out assuming stoichiometry for one unanalyzed element to calcul ate matrix correction (ZAF) factors. Stoichiometric ZAF routines for m ultielement groups such as carbonate (CO32-) are not commonly availabl e. Consequently, carbonates are commonly analyzed by means of the 0 st oichiometry techniques applied to silicate and oxide minerals. Using r eal and simulated analytical data, we show that errors of up to 20% oc cur if ZAF corrections are made with the assumption of metal valence a nd stoichiometric O. Two techniques that allow the input of complete c ompositional data from CO32- analyses to ZAF factor calculations and t hat give accurate results are discussed. First, a new technique uses c arbonate standards with compositions represented by the metal oxide MO 4. The results are then recalculated as metal carbonate. The second me thod assigns incorrect valences to the elements, C is assumed to be st oichiometric, and 0 is analyzed directly. We show that an analytical t otal of 100% +/- square-root N, where square-root N represents errors due to X-ray generation statistics, provides a powerful indication of analysis quality. The use of normalized and difference analysis method s. which necessarily assume totals of 100%, are to be avoided for this reason.