ABSTRACT

The need for quantitative analysis of the major mineral constituents in carbonate rocks resulted from studies of carbonate aggregates used in highway concrete.

Three quantitative X-ray methods for calcite and dolomite are investigated. The first, a comparatively simple X-ray method, determines the relative percent of the carbonate fraction that is calcite and dolomite by measuring the intensity ratios of the largest diffraction maxima of the respective minerals. The sample preparation and X-ray procedures were changed and the altered method experimentally shown to be reliable. The second, a combination of chemical and X-ray procedures, involves the X-ray determination of the structural formulas of calcite and dolomite and an EDTA titration procedure for determining the proportion of calcium and magnesium ions in the sample. Two sources of error were found: (1) non-carbonate minerals in the sample liberating "extra" calcium and magnesium ions during the EDTA titration procedure, and (2) incorrect structural formulas of dolomite due to inaccurate X-ray calibration curve. The third method, an internal standard technique, was established and calibration curves with confidence belts constructed. This method gives absolute--not relative--values for not only calcite and dolomite but also quartz.

Two methods for determining the clay mineral fraction are discussed. One involves clay size separation and X-ray procedures, the other is simply a subtraction of the previously determined carbonate and quartz fractions from the total rock sample. The latter is considered comparatively free of possible error from assumptions and therefore more reliable than the former technique.

Two techniques employed in determining the structural formulas of the carbonate minerals were investigated. Both are X-ray procedures and involve measuring the variation of the lattice spacings in the crystal structure as related to compositional changes. The Harker and Tuttle method, which measures the variation of the largest diffraction line in relation to an internal standard is the more applicable to natural rock samples.

Four procedures are recommended at the present time: the internal standard; the subtraction; the clay separation--X-ray; and the corrected Harker and Tuttle methods. These are considered the most applicable to the four respective problem areas--the quantitative determination of calcite, dolomite, and quartz; the quantitative and the qualitative evaluation of the clay mineral fraction; and the composition of the carbonate minerals calcite and dolomite.