X-RAY DIFFRACTION AND PETROGRAPHIC ANALYSES OF THE LARGE DIABASE DIKE INTRUDING THE SILVERMINES GRANITE IN SE MISSOURI

In a large diabase dike that intrudes the Silvermines granite on the east side of the old dam along the St. Francois River in the St. Francois Mountains of southeastern Missouri, various minerals and features has been identified. Twelve thin sections, spanning the width of the dike were evaluated for mineral and textural variability. As expected, the texture is finer near the chill margin and coarsens near the center of the dike. Circular features, 0.5-1.0 mm in diameter and rimmed with magnetite, are evident across the dike with some variation in mineralogy. In thin section analysis, opaque minerals such as magnetite, ilmenite or hematite, as well as sulfide minerals have been identified within the matrix and circular features for approximately 20-30% of the mineralogical mode. Based on reflected light characteristics, the sulfide mineral is most likely pyrite. Analysis of feldspars showed that plagioclase was the dominant mineral in thin section. The character of these plagioclase crystals was determined by measuring the size and shape of selected crystals on digital photographs as well as comparing the matrix material and phenocrysts. Larger phenocrysts of plagioclase were often altered while smaller phenocrysts were elongated and contained distinct cores when viewed perpendicular to the long axis. In addition to plagioclase, orthoclase crystals are found occasionally as altered xenocrysts. Thin section analyses also identified quartz, calcite within the circular features, as well as minor amounts of chlorite, limonite, and altered pyroxene. In order to run X-Ray diffraction, the sample first had to be crushed and sieved to <63 ìm. From there, it was separated into magnetic (0.3g) and nonmagnetic (0.7g) fractions using a hand magnet. The sample was run on a Rigaku MiniFlex with a Cu-radiation source and MDI Jade 8.0 software was used to analyze the powder diffraction files. After removing the background, the primary peaks were identified and reported with intensities, 2-theta, and d-spacings. The minerals that were identified in the nonmagnetic fraction included andesine plagioclase, quartz, calcite, and chlorite (baileychlore). In the whole rock, magnetite, labradorite/andesine, quartz, calcite, and chlorite were identified as a result of peak analysis for the powder diffraction pattern.