This Major Research Instrumentation (MRI) Program grant supports acquisition of a powder x-ray diffractometer for the Department of Earth Science and Geography at Vassar College, a non-Ph.D. Granting institution. A new table XRD will replace an extant but aging Siemens D5000 XRD (vintage 1989). The XRD will support research that requires rapid phase identification of fine grained materials and crystallographic structure refinement. Research that will be supported includes studies of clay mineral assemblages in low grade metamorphic rocks in order to determine zones of metasomatic activity, studies of phyllosilicates in soils to understand the cation exchange behavior of soils; and studies of the clay mineralogy of lacustrine sediments in order to understand the paleoevironmental conditions of their formation. The instrument will support undergraduate student research training at this four year liberal arts institution with a historically high enrollment of women and underprivileged students.
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With our NSF grant we purchased a new, desk-top X-ray diffractometer. X-ray diffraction (XRD) is used to study the structures of minerals, and we use this instrument to identify individual minerals, and to study changes in the structures of minerals in response to changing geologic conditions. During the grant period we began two projects as part of the thesis research for two graduating seniors. In the first project we are working with colleagues at three different universities to determine the age of the last movement on a fault in the Hudson Valley. The fault is located in an area that was affected by three different mountain building events during the Paleozoic era, and the fault could have been active during one or more of those events. During fault movement, the structure of the mineral illite changes so that, if you can determine the different structures of illite present, you can then date the age of the most recent fault movement. Sample preparation and XRD analysis are time-consuming, but once the data are collected, the illite structures can be estimated using computer modeling. In the second project we are studying the minerals that form when the volcanic rock known as basalt is heated by circulating hot water. The samples are from a mile-long drill core from the southern Idaho in an area known as the Snake River Plain which has experienced the eruption of many basalt lavas flows over the past 20 million years. Heat from the thick sequence of cooling lavas flows, along with heat from the mantle (because the crust is very thin in this area), warm ground water draining from the mountains of central Idaho. The hot water circulates through the lavas causing changes in the minerals that made up the original lava when it was erupted. This hot water is currently being used for geothermal energy in some places, and the amount of heatflow suggests that the area has the potential to be developed into one of the largest geothermal resources in the country. An understanding the mineral changes that occur during alteration by the hot water can be used as a prospecting tool for discovering other geothermal resources. The core we are studying, which was drilled on the Mountain Home Air Force Base, contains the clay minerals smectite, corrensite, and chlorite. Previous work has shown that smectite normally forms at low temperatures (<100°C) in basaltic lavas, and with increasing temperature will transform eventually to chlorite (at around 250°C). Under some conditions, corrensite will form as an intermediate product during this transition. The Mountain Home core is unique in that smectite appears to coexist with both corrensite and chlorite in one section of the core, but in deeper samples (at presumably higher temperatures) only smectite is present. The corrensite zone coincides with a zone of intense fracturing in which the water is at 145°C and is under artesian pressure. Our goal is to understand the complex interacting factors of this zone, including the roles of temperature, water composition, and pressure, by using X-ray diffraction to identify the occurrence of corrensite and to study structural changes that accompany the changing conditions. An abstract of preliminary results was presented by the student at the northeast sectional meeting of the Geological Society of America (GSA), and his complete thesis results will be presented at the national meeting of GSA. In addition to these research projects, the XRD unit is used in Mineralogy to identify mineral unknowns (teaching the students the use of XRD and the skills of sample preparation), and Paleoclimatology to identify minerals present in lake sediments which helps distinguish different climatic regimes that affect the kind of erosion happening in the basin contributing sediment to the lake. Finally, the local gem and mineral club visited the X-ray lab to learn about the use of XRD (to identify minerals by their structures) and X-ray Fluoresence (XRF) which analyzes the elemental composition of a material. They brought unknowns to be identified, and were able to use both instruments hands-on. During its first your of operation at Vassar College, the NSF-sponsored instrument was used to train senior thesis students for future careers in research, to acquaint earth science majors with the theory and practice of XRD in geological research, and to introduce the general public to the uses of X-ray analysis in the study of unknown materials.
