DETERMINING A VIABLE X-RAY-DIFFRACTION TECHNIQUE FOR ROUTINE QUANTIFICATION OF THE QUARTZ CONTENT OF POWDERED CARBONATES

Powdered carbonate samples are digested in hydrochloric acid, and the resulting residue is analyzed using an internal standard method. Because the acid-insoluble residues of carbonates are mixtures of many different minerals, severe overlap problems occur with most commonly used standards. Spinel (synthetic) having a simple cubic pattern with 2THETA (Cu) peaks at 19.04-degrees, 31.30-degrees and 36.88-degrees is generally free of overlap problems. Not only are the primary and secondary quartz peaks between the first two spinel peaks, but the spinel peaks can be used to determine and correct for inadequate thickness of the x-ray diffraction samples when necessary. The peak intensity of the quartz and of the standard are applied to an experimentally determined intensity ratio vs. percent quartz curve in the usual way. In addition, however, the intensities of the three spinel peaks are measured for each sample. Apparent increases in intensity of the low 2THETA peaks compared with the peak at 36.88-degrees (I = 100) indicate a sample of insufficient thickness. The sample can be re-prepared or a correction made. Changes of intensity ratio of the spinel peaks as mu*g varies vs. intensity correction (in percent) to the 20.88-degrees and 26.66-degrees quartz peaks had been graphed. Knowing the spinel intensity ratios, a correction can be read directly from the graph without determining g or mu* and is generally no greater than 1-2% for the 20.88-degrees peak intensity. The technique of acid digestion followed by percentage determination with an internal standard permits, in most cases, measurement to levels such that the upper confidence limit is well below 0.1%.