Mineralogical and Chemical Composition of International Carbon and Oxygen Isotope Calibration Material NBS 19, and Reference Materials NBS 18, IAEA‐CO‐1 and IAEA‐CO‐8

Abstract

International carbon and oxygen isotope calibration material NBS 19 and reference materials NBS 18, International Atomic Energy Agency (IAEA)‐CO‐1 and IAEA‐CO‐8 are prepared from naturally occurring rock specimens of marble and carbonatite. Mineralogical and chemical analysis showed that only NBS 19 and IAEA‐CO‐1 represent essentially pure samples of calcite containing < and minimal (< 1%) quantities of quartz. In contrast, both NBS 18 and IAEA‐CO‐8, although primarily composed of calcite, are contaminated by a range of additional phases. NBS 18 was estimated to contain 1% Fe‐dolomite and trace (< 1%) quantities of apatite and quartz. IAEA‐CO‐8 was estimated to contain at least 4% non‐carbonate material (including apatite, barite, biotite and magnetite). NBS 18 and IAEA‐CO‐8 are both derived from samples of carbonatite and the calcite component of each material is characterised by appreciable substitution of Mg + Mn + Sr ± Fe ± Ba (Σ ≈ 14000–15000 μg g‐1) for Ca. The observations reported in this study complement data in the literature detailing significant grain‐scale isotopic heterogeneity in NBS 18 and IAEA‐CO‐8. Both data sets highlight the need for careful characterisation of calibration materials prior to distribution.