207 Advances in Mineralogy and Petrology

Wednesday, 7 November 2012
8:00 AM - 12:00 PM
208A

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

Paper number: 207-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

Synthetic carbonates along the calcite-dolomite join were investigated by autocorrelation analysis to test the usefulness of this method in modeling the mixing properties (ΔHmix) 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 ΔCorrMIR values increase rapidly and vary in a manner consistent with the asymmetric solvus of this join. The FIR regions both indicate strong positive ΔHmix across the entire join. Assuming that the ΔCorr values in the MIR region most accurately reflect ΔHmix, 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.