A chemical method based on potassium fixation was developed for quantitative determination of vermiculite in soils. The cation-exchange capacity (CEC) of a sample is determined by washing with CaCl 2 and replacement of Ca with MgClu. The sample is then washed with KC1, heated to HOC overnight to dehydrate and collapse the layers of vermiculite and to fix K. The K remaining exchangeable is determined by NHiCl washings. The difference between these two CEC values gives the interlayer charge of vermiculite. A number of vermiculite standards and soil samples of widely different clay mineral compositions give remarkably near 100% totals when the vermiculite content is based on the average value of interlayer charge of 154 meq/100 g of vermiculite and when the other minerals present are appropriately determined (mica by KzO, chlorite by ignition loss, montmorillonite by CEC measured by nonfixed K, quartz and feldspar by Na 2 S 3 O7 fusion, allophane, kaolinite, and halloysite by selective dissolution in NaOH). The total CEC of vermiculite is 159 meq/100 g when the allocated external surface charge of 5 meq is included.

A B S T R A C TT H E objective of this paper is to present flow sheets for a system of quantitative mineralogical analysis of clays of soils and sediments and to show representative results. Selective dissolution analysis by the Na2S207-HCl-NaOH procedure yields the quartz and feldspar contents (0 to 63%) and differentiates feldspar K from mica K. The NaOHthermal system of selective dissolution yields the allophane plus gibbsite, kaolinite plus halloysite, and dickite contents (0 to 84% for the sediments; 1 to 2 5 % for soil clays) Mica contents (0 to 92% for the rock specimens, 7 to 4 3 % for soil clays) are determined by nonfeldspathic K (and Na). Vermiculite contents (1 to 9 7 % of specimens; 3 to 2 1 % for soil clays) are measured by blocking of interlayer CEC by drying at 110°C while K saturated and replacing with NH4CI. Montmorillonite (and palygorskite) contents (0 to 85% of specimens; 3 to 36% of soil clays) are determined by the CEC not blocked by the K and NH4 sequence for vermiculite. Chlorite contents (0 to 8 5 % for specimens; 0 to 37% for soil clays) are determined by thermal gravimetric analysis, after allocation of OH water lost between 300 and 950°C to other hydrous minerals determined.The best evidence of the accuracy of the system of analysis lies in the consistent total recovery of 24 standard mineral samples averaging 100.4 i 1·3 ( ± standard error of means) and of 22 soil clay samples averaging 99.5 i 0.8. The different constituents were present in widely different proportions in the various samples, and were determined by independent methods. The complementary total of near 100% (maximum range 95 to 105% for specimens; 95 to 103% for soil clays) for the analyses is a significant measure of the specificity of the several determinations.

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THE objective of this paper is to present flow sheets for a system of quantitative mineralogical analysis of clays of soils and sediments and to show representative results. Selective dissolution analysis by the NasSsOv-HC1-NaOH procedure yields the quartz and feldspar contents (0 to 63%) and differentiates feldspar K from mica K. The NaOHthermal system of selective dissolution yields the allophane plus gibbsite, kaolinite plus halloysite, and dickite contents (0 to 84~o for the sediments; 1 to 25% for soil clays) Mica contents (0 to 92~o for the rock specimens, 7 to 43% for soil clays) are determined by nonfeldspathie K (and Na). Vermiculite contents (1 to 97~o of specimens; 3 to 21% for soil clays) are measured by blocking of interlayer CEC by drying at ll0~ while K saturated and replacing with NH4C1. Montmorfllonite (and palygorskite) contents (0 to 85~o of specimens; 3 to 36~o of soil clays) are determined by the CEC not blocked by the K and NH4 sequence for vermiculite. Chlorite contents (0 to 85~o for specimens; 0 to 37% for soil clays) are determined by thermal gravimetric analysis, after allocation of OH water lost between 300 and 950~ to other hydrous minerals determined. The best evidence of the accuracy of the system of analysis lies in the consistent total recovery of 24 standard mineral samples averaging 100.4 4-1.3 (4-standard error of means) and of 22 soft clay samples averaging 99.5 4-0.8. The different constituents were present in widely different proportions in the various samples, and were determined by independent methods. The complementary total of near 100~o (maximum range 95 to 105% for specimens; 95 to 103% for soil clays) for the analyses is a significant measure of the specificity of the several determinations.

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