Clay sized fraction and powdered whole-rock X-ray analyses from alluvial basin deposits in central and southern New Mexico

Abstract

As part of the study of the geochemistry of the Southwest Alluvial Basins Regional Aquifer-System Analysis, X-ray analyses were performed on powdered whole rock and clay-size fraction of selected samples of alluvial basin deposits. The samples were analyzed to determine the minerals and clay types that may be reacting with ground water and thus affecting water quality. Nineteen samples from the outcrop of Tertiary and Quaternary alluvial basin deposits in the central and southern Rio Grande rift were collected and analyzed. The analys*es of the samples consisted of particle-size analysis (sand, silt, and clay percentages), powdered whole-rock X-ray analysis, claysize fraction X-ray analysis, and semiquantitatlve analysis of the relative abundance of different clay-mineral groups present. The results indicate that calcium smectite and mixed layer illite-smectite are the most abundant clay groups in the samples. Quartz, calcite, plagioclase, and orthoclase are the most common nonclay minerals. The zeolite mineral found in the samples probably is stilbite but could be heulandite. INTRODUCTION In 1978, Congress appropriated money to begin a Regional Aquifer-System Analysis (RASA) program. This program was designed to study the hydrology of large areas of the country that are underlain by a regional aquifer system. The Southwest Alluvial Basins (SWAB) regional aquifer study is one of these RASA studies. The SWAB study area consists of basins along the Rio Grande from its headwaters in southern Colorado to Preside, Texas, and several closed basins in southwestern and southern New Mexico. Twenty-two basins were identified in this study area. The objectives of the study of the geochemistry for the SWAB study were to define the areal water-quality distributions in selected basins and to investigate the chemical reactions or processes that occur in the ground-water system. In conjunction with the study of the geochemistry, X-ray analyses of powdered whole rock and clay-size fraction of selected samples from alluvial basin deposits were performed to examine the minerals and clay types present in the aquifers that may be reacting with ground water. The purpose of this report is to present the data resulting from the analyses. No interpretations of the data are presented in this report. Method of Study Samples from Tertiary and Quaternary alluvial basin deposits in the southern and central Rio Grande rift area were collected from unsaturated outcrops at land surface. In general, the sample sites were chosen to correspond with outcrops of rock units described or mapped in previously published reports or road logs. Sample sites also were chosen so that many of the different ages and rock types of alluvial basin deposits in the southern and central Rio Grande rift were represented. An attempt was made to dig into the outcrop a sufficient distance to get a fresh or unweathered sample. The difference in clay mineralogy or whole-rock mineralogy between unsaturated samples at the surface and saturated aquifer material at depth is unknown. At land surface, there may be leaching of minerals from the rock, concentration or precipitation of minerals in the rock due to evaporation of precipitation, or chemical reactions and thus changes in mineralogy caused by the oxidizing surface environment. The samples were analyzed by personnel of the New Mexico Bureau of Mines and Mineral Resources. The analyses consisted of particle-size analysis (sand, silt, and clay percentages), powdered whole-rock X-ray analysis, claysize fraction X-ray analysis, and semiquantitative analysis of the relative abundance of different clay groups present. The Supplemental Information section at the end of this report explains the procedure used in the X-ray examination of the clay-size fraction mineralogy. The seraiquantitative relative abundance of different clay groups is based on X-ray peak heights rather than peak areas (George Austin, New Mexico Bureau of Mines and Mineral Resources, written commun., 1981). Acknowledgments Charles Kaehler of the U.S. Geological Survey and John Hawley of the New Mexico Bureau of Mines and Mineral Resources collected some of the samples. George Austin of the New Mexico Bureau of Mines and Mineral Resources was helpful throughout discussions concerning the results of the X-ray analysis.