QUARTZ DISTRIBUTION IN A-HORIZON SOILS OF THE CONTERMINOUS U.S.—ITS EFFECT ON MAJOR AND TRACE ELEMENT DISTRIBUTION PATTERNS

As an adjunct to geochemical data for soils produced by the Conterminous U.S. Soil Geochemical Landscapes project of the USGS we determined the mineralogy, including the quartz content, of the < 2mm size fraction of 4,791 A-horizon soils by Rietveld refinement calculations from x-ray diffraction data. The average quartz content is 49.4 wt. % and varies from 100% to < 0.2%. Thus, quartz content is one of the most significant variables in this large data set. Because of the chemical simplicity of quartz (it contains essentially no elements other than Si and O) it causes variable dilution of all other elements, which reside entirely in the non-quartz fraction of soil. Quartz shows very prominent regional patterns, being very high, for instance, in the Atlantic and Gulf of Mexico Coastal Plains, where it commonly forms more than 95% of soil, and very low in much of the Pacific Northwest, where it commonly makes up less than 20% of soils. The reciprocal of this pattern is shown by many other elements, especially lithophile elements, indicating that their concentrations reflect, in large part, variable dilution by quartz of the minerals that contain them. Interpreting the cause of national- and regional-scale variations of element concentrations should take into account the degree to which these different concentrations reflect only variable quartz dilution. Quantitative data on the quartz content of our analyzed soils allows us to calculate the chemical composition of the non-quartz soil fraction by assigning all elements to that portion of the soil not composed of quartz. National maps of the chemical composition of the non-quartz fraction of soil generally differ greatly from maps of the composition of bulk soils and reveal significant patterns of enrichment and depletion that commonly are masked by variable quartz dilution in the chemistry of the bulk soil. These maps may be particularly significant in studies of agriculture and plant growth because plants derive nutrients and potentially deleterious elements solely from the non-quartz fraction of soil and presumably respond to the composition of that fraction more than to the composition of the bulk soil