Table: Mineralogy of Selected SAL Deposits

Table 2 Mineralogy of selected spring-limestone deposits. Mineralogy was determined only in cases of suspect for magnesian calcite or aragonite, such as in case of fibrous CaCO3. For deposits that obviously consisted of low-magnesian calcite, a few check samples were investigated by XRD (not shown here). The calculated mole percentage of MgCO3 in calcite is indicated as ('X mol%') in parentheses.

Method:

X-ray powder diffractometry for identification of carbonate polymorphs and determination of magnesium content of calcite was done on a Bruker-AXS D8 diffractometer with Bragg-Brentano geometry, CuK radiation, at 40kV and 40mA acceleration voltage, with parallel-beam optics and an energy-dispersive detector. The data was collected between 2 and 70° 2θ, with a step size of 0,02° 2θ, and a detecting time of four seconds per step. Crystalline phase identification was done with the program Eva.Ink and the PDF4 data base 8.0.113 of ICDD. Chunks of actively-forming spring limestones were sampled for XRD measurement at two different positions (two XRD measurements per sample of active limestone). Quantitative phase composition, including relative percentage aragonite/magnesian calcite/calcite, was done by Rietveld-refinement using Bruker-AXS software TOPAS 2-1; the relative standard deviation for this calculation is five wt%. For determination of Mg content in calcite, crystallographic data (cell volume, lattice constant a and c, d-value of [104]-reflection) were extraced by Rietveld-refinement using TOPAS 2.1 and the initial structure model ICSD 20179 (Borodin et al. 1979). The calculation of mol% MgCO3 followed the calibration by Goldsmith et al. (1961).

References:

Borodin VL, Lyutin VI, Ilyukhin VV, Belov NV (1979) Isomorphic series of calcite-otavite. Doklady Akademii Nauk SSSR 245(5):1099-1101

Goldsmith JR, Graf DL, Eard HC (1961) Lattice constants of the calcium-magnesium carbonates. American Mineralogist 46:453-457