Original paper

Relationship between composition and lattice parameters of some sedimentary dolomite varieties

McCarty, Douglas K.; Drits, Victor A.; Sakharov, Boris

Abstract

A representative collection of iron-poor non-stoichiometric sedimentary dolomite samples of various composition, location, and age was studied by X-ray qualitative and quantitative phase analysis as well as by coupled plasma-atomic emission (ICP-AES) and X-ray fluorescence (XRF) spectroscopy. The main task is to determine relationships between the lattice parameters and Ca content in non-stoichiometric dolomite.It is found that the unit cell parameters as well as the lattice hkl spacings of the studied dolomite samples are related by the following linear equations with strong linear correlation coefficients:c = 7.3097a − 19.135 R = 0.9988a = 1.0306d(104) + 1.8337 R = 0.9991c = 7.6284d(104) − 6.0059 R = 0.9984ceff = 7.4038aeff − 19.59 R = 0.9991aeff = 1.0251d(104) + 1.8499 R = 0.9989aeff = 1.8215d(113) + 0.8157 R = 0.9990ceff = 7.5244d(104) − 5.7065 R = 0.9983where aeff = d(110) + √3d(030) and ceff = 6d(006).These almost perfect interdependences exist because in the studied dolomite structures not only unit cell parameters (as expected by Vergard's Law), but also corresponding hkl spacings co-vary in a linear manner with Ca content as follows:c = 0.8632nCa + 15.14 R = 0.9971a = 0.1168nCa + 4.6903 R = 0.9967d(104) = 0.119nCa + 2.7658 R = 0.9981aeff = 0.11967nCa + 4.6872 R = 0.9975ceff = 6d(006) = 0.8852 nCa + 15.1146 R = 0.9958.The validity of these equations is confirmed by a comparison of Ca contents determined by the regressions and chemical analysis for monophase dolomite samples for which 1.00 < nca 1.09 consist of two dolomite phases with different amounts of excess Ca. Quantitative determination of proportions of each of these phases in the two-phase dolomite samples was carried out using the Rietveld software program Autoquan®. The lattice parameters and hkl spacings are used to determine Ca content in each of these phases using the regressions obtained for the monophase samples. Comparison of the calculated average Ca content with that determined from the chemical analysis shows that in average the difference between the compared values are equal to ± 0.012 atoms of Ca per formula or ± 0.06 mol % CaCO3.The results are in accordance with a structural model where only the B sites have a mixed occupancy confirming the results of a previous work.

Keywords

excess-ca dolomitelattice parametersdolomite phase compositionx-ray diffractionunit cell refinement