Quantifying carbon fixation in trace minerals from processed kimberlite: A comparative study of quantitative methods using X-ray powder diffraction data with applications to the Diavik Diamond Mine, Northwest Territories, Canada

The capacity of mine waste to trap CO2 is, in some cases, much larger than the greenhouse gas production of a mining operation. In mine tailings, the presence of secondary carbonate minerals that trap CO2 can therefore represent substantial fixation of this greenhouse gas. The abilities of three methods of quantitative phase analysis to measure trace nesquehonite (MgCO3 center dot 3H(2)O) in samples of processed kimberlite have been assessed: the method of reference intensity ratios (RIR), the internal standard method, and the Rietveld method with X-ray powder diffraction data. Tests on synthetic mixtures made to resemble processed kimberlite indicate that both the RIR and Rietveld methods can be used accurately to quantify nesquehonite to a lower limit of approximately 0.5 wt.% for conditions used in the laboratory. Below this value, estimates can be made to a limit of approximately 0.1 wt.% using a calibration curve according to the internal standard method. The RIR method becomes increasingly unreliable with decreasing abundance of nesquehonite, primarily as a result of an unpredictable decline in preferred orientation of crystallites. For Rietveld refinements, structureless pattern fitting was used to account for planar disorder in lizardite by considering it as an amorphous phase. Rietveld refinement of data collected from specimens that were serrated to minimize preferred orientation of crystallites gives rise to systematic overestimates of refined abundances for lizardite and underestimates for other phases. The resulting pattern of misestimates may be mistaken for the effect of amorphous and/or nanocrystalline material in samples. This effect is mitigated by collecting data from non-serrated specimens, which typically give relative errors on refined abundances for major and minor phases in the range of 5-20%. However, relative error can increase rapidly for abundances less than 5 wt.%. Nonetheless, absolute errors are sufficiently small that estimates can be made for the amount of CO2 stored in secondary nesquehonite using the RIR method or the Rietveld method for abundances >= 0.5 wt.% and a calibration curve for abundances <0.5 wt.%. The extent to which C is being mineralized in an active mine setting at the Diavik Diamond Mine Northwest Territories, Canada, has been investigated. Rietveld refinement results and calibrated abundances for trace nesquehonite are used to estimate the amount of CO2 trapped in Diavik tailings. Results of quantitative phase analysis are also used to calculate neutralization potentials for the kimberlite mine tailings and to estimate the contribution made by secondary nesquehonite. (C) 2009 Elsevier Ltd. All rights reserved.