Thin section

Abstract: Quantification of mineral proportions in rocks and soils by Raman spectroscopy on a planetary surface is best done by taking many narrow-beam spectra from different locations on the rock or soil, with each spectrum yielding peaks from only one or two minerals. The proportion of each mineral in the rock or soil can then be determined from the fraction of the spectra that contain its peaks, in analogy with the standard petrographic technique of point counting. The method can also be used for nondestructive laboratory characterization of rock samples. Although Raman peaks for different minerals seldom overlap each other, it is impractical to obtain proportions of constituent minerals by Raman spectroscopy through analysis of peak intensities in a spectrum obtained by broad-beam sensing of a representative area of the target material. That is because the Raman signal strength produced by a mineral in a rock or soil is not related in a simple way through the Raman scattering cross section of that mineral to its proportion in the rock, and the signal-to-noise ratio of a Raman spectrum is poor when a sample is stimulated by a low-power laser beam of broad diameter. Results obtained by the Raman point-count method are demonstrated for a lunar thin section (14161,7062) and a rock fragment (15273,7039). Major minerals (plagioclase and pyroxene), minor minerals (cristobalite and K-feldspar), and accessory minerals (whitlockite, apatite, and baddeleyite) were easily identified. Identification of the rock types, KREEP basalt or melt rock, from the 100-location spectra was straightforward.

Abstract: Texture has already been defined (p. 33) as the intimate mutual relations of the mineral constituents and glassy matter in a rock made up of a uniform aggregate. It is best studied in thin section under the microscope. Microstructures, also studied in the same way, are due to the juxtaposition of two or more kinds of textural aggregates in a rock. Textures and structures are important, as these features are the indices of the geological processes which have been in operation; and their study provides valuable information as to the physical chemistry of the cooling and solidification of igneous rocks.

Abstract: [1] The first dispersive μ-XANES mapping is presented here. The experiments have been conducted at the iron K-edge, on the “Dispersive-EXAFS” beamline of the European Synchrotron Radiation Facility (France). The mapping has been performed on a polished thin section of 30 μm of a natural metamorphic rock including different types of minerals. Because of the high X-ray absorption due to the thickness of the glass sample holder (∼1 mm), the data have been collected in the fluorescence mode using the so-called “Turbo-XAFS” design. The effective spot size was approximately 10 × 10 microns, and an area 390 × 180 microns in size was mapped. Improvements of the acquisition process allowed collection of each XANES spectrum in 1.5 s and with a step size of 5 microns so that 2808 spectra with full XANES information were collected. Then, automatic procedures for data reduction and mapping reconstruction were developed using Matlab®. The results are maps of iron content, oxidation state, and speciation, with a 5 μm spatial resolution after two-dimensional deconvolution. Subsequent analyses of the reconstructed images provide some quantitative calibrations.

Abstract: Staining of thin sections for plagioclase and alkali feldspar has not become a universally applied petrographic tool, probably due to difficulties encountered in effectively staining plagioclase. Modifications of standard techniques are described for rapidly producing stained thin sections of high quality. Critical steps in the procedure include pre-polishing with wet 600-grit abrasive paper, using adequate etching times, and applying a K-rhodizonate solution of 0.02 g per 30 ml water, or weaker.