Carbon and hydrogen stable isotope microanalysis and data correction for rare carbonate minerals: Case studies for stichtite (Mg6Cr2[(OH)16|CO3]∙H2O) and malachite (Cu2CO3(OH)2)

In the absence of reliable mineral-specific isotope microanalysis standards, it is still possible to correct microanalysis data using calibration to traditional stable isotope analysis methods. In this study, samples of the rare carbonate minerals stichtite (Mg6Cr2[(OH)16|CO3]∙H2O) and malachite (Cu2CO3(OH)2) were analyzed for C and H isotopic compositions using both traditional stable isotope analysis methods and secondary ion mass spectrometry (SIMS) microanalysis. For stichtite and malachite analysis, whole rock measurements of carbon and hydrogen isotope ratios were used to correct microanalysis results. For C isotopes in malachite, individual bands of malachite were characterized by traditional analysis and SIMS values were corrected using the results. Although this did not result in a significant improvement in this particular data set compared to using a single bulk C value for the sample, it shows how reliable SIMS data for chemically diverse samples with varying sensitivity factors could be obtained. This study reveals variation in δ13C of up to 6‰ in stichtite and up to 5‰ in banded malachite. Variations in δD of ~ 50‰ were resolved for stichtite. The corrections provided by this method should not be taken as universal and applicable to all malachite or stichtite samples. Separate corrections will likely be required for samples from different locations, and in the case of stichtite, with compositions different from end-member stoichiometry. This work bears upon future microanalysis work on rare minerals for which calibration standards have not yet been established, with implications for work in astrobiology and economic geology.