Quantitative Mineralogy of Fine-grained Sedimentary Rocks: A Preliminary Look at QEMSCAN® Richard I. Grauch,* Dennis D. Eberl,** Alan R Butcher, *** and Pieter W.S.K. Botha *** * U. S. Geological Survey, Denver Federal Center, MS 973, Denver, CO 80225 ** U. S. Geological Survey, 3215 Marine Street, Suite E-127, Boulder, CO 80303 *** Intellection Pty Ltd., 27 Mayneview Street, Milton, Queensland, 4064, Australia This is a collaborative effort to examine the practicality of using QEMSCAN®, a method utilizing an automated scanning electron microscope equipped with 4 energy dispersive x-ray detectors and proprietary software, to efficiently and rapidly quantify the mineralogy of a large number of mudstone core samples. We compared QEMSCAN® modal analyses to chemical analyses of the samples and RockJock (X-ray diffraction) generated quantitative mineralogy [1]. The 27 samples used in this preliminary study are a subset of 207 samples obtained from 2¼ inch diameter drill core. The subset was selected to represent a range between inhomogeneous and relatively homogenous material taken from quarter sections of 1 to 3 inch long intervals of core. A thin-section billet was cut from the sample and the remainder was powdered and used for chemical and X-ray diffraction analyses. Part of the thin-section billet was used for a polished thin section and the remainder as a polished mount for QEMSCAN® analysis (Fig. 1A, B, C). Repeated modal analyses of the same 9 mm2 area using different pixel spacing demonstrate very good precision and the practicality of using short analytical times (Table 1). Those results compare well, despite some sample heterogeneity, with analyses of a larger portion, ~ 250 mm2, of the same sample (Table 1). Although the Species Identification Protocol (SIP) used for identifying individual grains is very similar to a list of minerals developed from non-quantitative optical petrography and SEM examination of the polished thin section, it requires some modification to reflect the true mineralogy of the samples. The “K Al Silicates” mineral category (Table 1) is a chemical category for volumes of analysis that contain grains too small to be resolved by QEMSCAN®. Modal analyses of the 27 samples were used to construct a mineralogic/stratigraphic sequence with lithologic breaks which are in general agreement with our megascopic core logging. Comparing electron-beam derived modal analyses of small volumes (even large portions of a thin section) to quantitative mineralogy or chemistry of the powders is, at best, an approximation. However, that approach may be the best way to independently test the validity of the modal analyses. QEMSCAN® and RockJock values of modal carbonate-bearing phases compare very well with the chemically determined carbonate content of the rocks, r2 = 0.86 and 0.98, respectively (Fig. 2). Differences between RockJock and QEMSCAN® may be explained by the heterogeneity of the samples, but the correlation between total carbonate minerals determined by the two methods is good, r2 = 0.82. Comparison of quartz content determined by both methods yields a fairly good correlation (r2 = 0.82, 2 extremes excluded) but very different values (Fig. 3), suggesting that QEMSCAN® missed some quartz. The missing quartz may be in the general category of “K Al Silicate” and in the Micrite category. Our preliminary results suggest that QEMSCAN® offers a precise, rapid, and potentially accurate method for determining quantitative mineralogy of fine-grained sedimentary rocks. However,

Microsc Microanal 14(Suppl 2), 2008Copyright 2008 Microscopy Society of America

532DOI: 10.1017/S1431927608082858

further testing and refinement of the experimental set-up is required and major challenges to be addressed include: (1) analyzing different portions of even the most homogenous samples, (2) simultaneously analyzing multiple grains, and (3) possibly misidentifying individual grains or not having all appropriate minerals in the SIP. References [1] D.D. Eberl, U.S. Geological Survey Open-File Report 03-78 (2003).

Fig.1. Calcareous mudstone CL-37.9. A. Transmitted light photograph of thin section – note groove indicating stratigraphic up. B. Ambient light photograph of thin-section billet used for QEMSCAN® analysis. C. QEMSCAN® mineral map, square is location of 9 mm2 area analyzed (Table 1). TABLE 1. QEMSCAN® modal mineralogy of calcareous mudstone CL 37.9. Area of Scan ~ 250 mm2 ~ 9 mm2 Pixel Spacing (microns) 20 20 10 5 2 Quartz 6.1 6.9 6.8 7.0 7.0 Alkali Feldspar 0.7 0.8 0.8 0.9 0.9 Plag Feldspar 16.5 17.1 17.0 17.0 17.0 K Al Silicates 19.6 17.1 16.7 16.8 16.7 Micrite (fine-grained carbonate, ± quartz, clay) 27.5 26.9 26.9 26.9 26.9

Calcite 15.2 17.7 17.9 17.8 17.9 Dolomite 1.7 2.3 2.3 2.3 2.3 Kaolinite 3.1 2.9 3.2 3.1 3.1 Pyrite 6.8 6.1 6.1 6.0 6.0 Apatite 0.1 0.2 0.2 0.2 0.2 Gypsum 0.5 0.6 0.6 0.6 0.6 Biotite 1.9 1.3 1.3 1.2 1.2 Rutile 0.1 0.1 0.1 0.1 0.1 Others 0.1 0.1 0.0 0.0 0.0 Time Required for Scan (minutes) 3 12 57 516

Fig.2. Comparison of % carbonate minerals (QEMSCAN® and RockJock) vs % carbonate Carbon (chemical analysis). N=27.

Fig.3. Comparison of % quartz determined by QEMSCAN® and RockJock.

533Microsc Microanal 14(Suppl 2), 2008