AUTOCORRELATION ANALYSIS OF THE IR SPECTRA OF SYNTHETIC CARBONATES ALONG THE CALCITE-DOLOMITE JOIN

Synthetic carbonates along the calcite-dolomite join were investigated by autocorrelation analysis to test the usefulness of this method in modeling the mixing properties (ΔH^mix) of solid solutions in a system with a well-defined miscibility gap. Carbonates were synthesized at 5 mol% increments of MgCO₃ (MC) between calcite and dolomite using mixtures of synthetic CaCO₃ and MC sealed in Pt capsules with either oxalic acid dihydrate or oxalic acid. Single-phase carbonates up to 25% MC were made at 1000°C/0.6 GPa/22 h in the presence of 10% oxalic acid dihydrate, while single-phase carbonates from 30-50% MC were made at 1150°C/2.5GPa/6-22h in the presence of 10% oxalic acid. Compositions were confirmed by Rietveld refinements of X-ray diffraction powder patterns with NaCl as an internal standard using the calibration data of Titschack et al. (2011, Am Min). Mid-infrared spectra were obtained in the range of 350-4000 cm^-1 from samples pressed in KBr disks with sample:KBr ratios of 1:200 while spectra in the far-infrared range of 50 – 650 cm^-1 were obtained from samples pressed in polyethylene (PE) disks with sample:PE ratios of 1:80 or 2:80 (25-250 cm^-1 range). Autocorrelation analysis was done on the CO₃ stretching/bending modes from 700-1400 cm^-1 (= MIR), the CO₃ translation/libration modes from 200 – 520 cm^-1 (= FIR1), and the Ca-Mg translations in the 75-175 cm^-1 range (= FIR2) yielding corresponding ΔCorr values.

Results indicate that the MIR, rather than the FIR region, agrees best with the enthalpy of mixing measurements (Navrotsky & Capobianco 1987 Am Min; Bischoff 1998 Aq Geochem) in the 0-15% MC range and supports decreased solubilities of Mg-calcites with 0-6% MC (Bishoff et al. 1993, Carb Evap), though volumes appear to vary linearly. Above 15% MC, the ΔCorr_MIR values increase rapidly and vary in a manner consistent with the asymmetric solvus of this join. The FIR regions both indicate strong positive ΔH^mix across the entire join. Assuming that the ΔCorr values in the MIR region most accurately reflect ΔH^mix, these results suggest that carbonates with 0-10% MC are particularly stable and may explain why most bio- and non-biogenic carbonates are between 5-20% MC. These results also suggest synthetic calcites with up to 10% MC may be an optimal host composition for carbon sequestration.