Quantitative Determination of Clinoptilolite in Soils by a Cation-exchange Capacity Method

-A cation-exchange capacity (CEC) method based on ion-sieving properties was developed for the quantitative determination of clinoptilolite in soils. In this method, both zeolitic and non-zeolitic exchange sites in the soil sample are saturated with Na t. The CEC of the non-zeolitic exchange sites is determined by replacing the Nain these sites with tert-butylammonium ions. The tert-butylammonium ion cannot be exchanged into the zeolitic exchange sites because it is too large to pass through the channels in the clinoptilolite structure. The sample is next washed with NH4OAc to replace the Na + in the zeolitic exchange sites. The amount of soil zeolite is then estimated by comparing the CEC of zeolitic exchange sites to the total zeolite CEC ( 175 meq/100 g for pure clinoptilolite). Prior to the CEC analyses, carbonates and organic matter must be removed to minimize interference with the exchange process. A high correlation (r-' = .96) was observed between the abundance of clinoptilolite estimated using the CEC method and the abundance estimated by semiquantitative X-ray powder diffraction analysis. The CEC procedure was used to quantify clinoptilolite in an Aridic Calciustoll soil from south Texas. About 2-5% clinoptilolite occurs in the A and B horizons, and concentrations progressively increase with soil depth to as much as 20% in the CBk2 horizon. Key Words--Cation-exchange capacity, Clinoptilolite, Quantitative mineralogy, Smectite, Soil, Tertbutylammonium, Zeolite. I N T R O D U C T I O N Few procedures deal with the direct quantitative determination of minerals in soils and sediments, primarily due to the heterogeneity of the system and the time required to conduct such analyses. Most quantitative mineralogy is accomplished by X-ray powder diffraction (XRD) methods. X R D procedures are protracted if the analysis of each soil separate (i.e., sand, silt, clay fractions) must be made to minimize the effects of particle size on crystal orientation. Also, it becomes difficult to establish an accurate estimate of the mineral in question in the whole soil sample. Thus, a procedure that quantifies a mineral directly on a whole-soil basis would vastly improve the accuracy of the estimate and probably save time. The occurrence of natural zeolites in soils is not well known and has only recently gained the attention of soil scientists. Therefore, very little has been done to quantify these minerals in soils. Of the more than 40 natural zeolite species, clinoptilolite, [(Na,K)6_2xCax]. (A16Si3oO72)'24H20, seems to be the most abundant zeolite in soils and sediments (Ming and Mumpton, 1987). Clinoptilolite has been reported in slightly acid to strongly alkaline soils (Ming and Dixon, 1986; Speirs et al., 1984; Jacob, 1984; Graham and Southard, 1983; Fanning et al., 1983; Southard and Kolesar, 1978; Asvadurov et al., 1978; Travnikova et al., 1973; Gorbunov and Bobrovitskiy, 1973). Present address: Mail Code SN 12, NASA Johnson Space Center, Houston. Texas 77058. Copyright ~ 1987. The Clay Minerals Society The zeolite of primary interest in this study is clinoptilolite, which is widespread in soils derived from the Oligocene Catahoula Formation of southern Texas (Ming and Dixon, 1986). Most of the clinoptilolitebearing soils are Mollisols that contain calcite. To aid in determining the distribution and stability of the zeolite in soils, a rapid, quantitative method of determining the amount of clinoptilolite in unfractionated soils was developed.