HEAVY MINERAL ANALYSIS OF SANDSTONES BY RIETVELD ANALYSIS

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1 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume HEAVY MINERAL ANALYSIS OF SANDSTONES BY RIETVELD ANALYSIS John R. Webster*, Roy P. Kight, Ryan S. Winburn, and Christopher A. Cool Division of Science, Minot State University, 500 University Avenue West, Minot ND ABSTRACT Rietveld analysis of heavy minerals has been tested using an Eocene sandstone from the Medicine Pole Hills south of Rhame, ND. Heavy minerals were separated from three size fractions covering the range of 0.18 to 0.30 mm. Polished grain mounts prepared from each size fraction were used to identify and measure the size of 1,517 grains by optical microscopy. Mineral compositions were determined by microanalysis of 101 grains using a scanning electron microscope (SEM) with an energy dispersive X-ray analysis system. Powdered sample mixed with a rutile internal standard (10%) was used to collect an X-ray diffraction pattern ( θ). Rietveld refinements were carried out using GSAS and an averaging technique. Based on optical/sem work, heavy minerals in the sandstone consist of diopside (56.1%), edenite (19.9%), epidote (6.1%), augite (5.3%), paragasite (4.2%), almandine-rich garnet (3.6%), and ten minor phases (4.8%). Results of Rietveld refinements completed thus far compare reasonably well with the optical/sem results for phases of at least modest abundance. One notable difference from optical/sem results was the abundance of diopside (46.0%). Other differences included higher abundances for two minor phases (4.2% plagioclase and 2.9% talc) and inclusion of goethite (1.1%) and quartz (0.7%), which were not identified as grains in the optical/sem work (although quartz was seen as inclusions within epidote). The greatest difficulty with the refinements was severe preferred orientation of several phases. The spherical harmonics corrections applied seem reasonable, but additional work on this problem is needed. INTRODUCTION The mineralogy of clastic sediment found in a sandstone provides important clues about its origin, including source rock lithologies and transportation history. In addition to the major constituents of sandstones (quartz, feldspars, and lithic fragments), sandstones contain minor abundances of other mineral grains, including heavy minerals (densities greater than 2.85 g/cm 3 ). Heavy minerals reflect the source area because different rock types contain different heavy mineral assemblages [1]. They can be very useful, particularly when interpretation of the major constituent grains is ambiguous. Heavy mineral analysis begins with disaggregation of grains and sieving to obtain a particular size range, typically in the fine to medium sand size range. Heavy minerals are separated using a heavy liquid with a density of 2.85 g/cm 3. In traditional analysis, the heavy mineral grains are mounted on a glass slide and then identified and counted using transmitted light microscopy [2, 3]. Problems associated with analysis using transmitted light microscopy include: (1) identification can be difficult, even for experienced petrographers; (2) opaque minerals cannot be identified; and (3) percentages based on counting grains of each mineral may not equate with actual volume or weight percentages. Because traditional heavy mineral analysis is difficult, we are investigating the feasibility of using Rietveld refinement of X-ray diffraction (XRD) data to quantify mineral percentages. The method has been applied to geologic materials, including sedimentary rocks [4], but apparently not to quantifying heavy minerals. If Rietveld refinement can be used to successfully quantify heavy mineral abundances, future work will focus on using refinements to obtain compositional information about solid solution minerals. It is hoped that the Rietveld method will provide more detailed information about the heavy minerals, and thus the source rocks, allowing more detailed comparisons of sandstones and more confident correlations.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume METHODS This study was carried out using a poorly consolidated medium-grained Eocene sandstone exposed in the Medicine Pole Hills (MPH) of southwestern North Dakota. A 4-kg sample of the sandstone was collected seven miles south of Rhame, ND. The MPH sandstone sample was disaggregated and divided into 16 equal splits. One 242-g split was separated by grain size using 1/4-phi interval sieves. Three size fractions chosen for heavy mineral separation: mm, mm, and mm. Heavy minerals were separated from each of the three size fractions in a glass funnel using a heteropolytungstate solution (density of 2.85 g/cm 3 ). Rubber tubing with a pinch clamp was used to collect the heavy minerals, and then the light minerals in a second funnel. The collected minerals were filtered, washed, and dried. Optical and SEM/EDX Characterization Heavy mineral grains from the three size fractions were glued onto separate one-inch glass slides and then ground to partially expose the grains. They were then polished using diamond and alumina polishes, and coated with carbon using a sputter coater. The grain-mount sections were studied by optical microscopy to identify the mineral types present. A JEOL JSM-35CF scanning electron microscope (SEM) equipped with a Tracor Northern 5200 energy dispersive X-ray (EDX) analysis system was used to analyze 40 typical grains in the mm section in order to confirm optical identifications. Optical microscopy was then used to identify each grain on the three sections (1,517 total), noting grains of questionable identification. A 20 x 20 gridded reticle (ocular lens) was used to measure the area of each grain to the nearest half square. An approximate volume of each grain was calculated from the area (volume = area 1.5 ). Of the 244 grain identifications that were questionable, 61 grains (from the mm fraction) were analyzed by SEM/EDX. These results were used to re-categorize the remaining 183 grains of questionable optical identification. For each section, the total area and volume of each mineral were calculated, and weight percentages were determined from both the total area and total volume using densities based on SEM/EDX analyses. Bulk sample weight percentages were calculated based on the weight contributions of the three size fractions to the XRD sample. XRD Analysis and Rietveld Refinement A 1.01-g sample of heavy mineral grains was ground by hand in an agate mortar and pestle g of powdered sample was mixed with g of powdered rutile (R900 grade, from DuPont). X-ray diffraction analysis was carried out at North Dakota State University using a Philips X Pert diffractometer. The scan covered a 2-theta range of 20 to 80 with a step size of 0.03 degrees and a 2-second count time per step. The slits used consisted of 1-degree fixed divergence and anti-scatter slits and a 0.2-mm receiving slit. Rietveld refinements were carried out using General Structure Analysis System (GSAS) [5]. The refinement protocol used is one previously outlined by members of this group [6] except that the orientation functions were introduced earlier in the refinement. Preferred orientation was corrected using spherical harmonics. Because GSAS is limited to nine phases in any one refinement, an averaging technique was used to determine the composition of the heavy mineral sample. In this method, a number of refinements are completed with differing phases in each refinement. The five refinements completed thus far focused on the major phases in the sample. Diopside, edenite, augite, kaersutite, and rutile (internal standard) were included in each refinement. The other phases were included in at least two different refinements with the exception of epidote (one refinement). The results of the five refinements were then averaged. RESULTS From a 203-g split of MPH sandstone, a total of 16.4 g of grains (8.1%) was recovered in the mm size range. From this sand, 1.37 g (8.4%) of heavy minerals was recovered. After grain mounts for each of the three size fractions were prepared, most of the remaining heavy mineral grains were combined to form a single sample for XRD analysis. The weight

4 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume contributions from the three size fractions were: g from the mm fraction, g from the mm fraction, and g from the mm fraction. Optical and SEM Characterization Initial optical identification of 1,517 grains was largely based on those minerals encountered during the first round of SEM/EDX microanalysis. Most grains (84%) were identified without uncertainty. Of the 244 identifications that were categorized as questionable (including those labeled as unknown), most were altered or partially altered ( grungy ) grains. Approximately half (124) occurred in the mm size fraction, which contained 521 grains. SEM/EDX microanalysis of the 61 questionable grains in the mm size fraction revealed that only 18% had been correctly identified. The most significant problems were grungy grains initially identified as plagioclase feldspar or diopside. Many were initially identified as plagioclase because an altered plagioclase grain was encountered during the first set of SEM/EDX analyses. Most of these turned out to be epidote, alkali feldspar, or augite. As a specific example, 13 questionable grains identified originally as diopside were analyzed by SEM/EDX. Six were diopside, and seven were found to be other minerals (4 epidote, 2 augite, and 1 sphene). These identifications were corrected as needed. The percentage of all other questionable grains tentatively identified as diopside in each of the three sections was assigned to appropriate minerals based on the results of the 13 grains analyzed (46% to diopside, 31% to epidote, 15% to augite, and 8% to sphene). The same procedure was followed for each type of questionable heavy mineral. After all of the 244 questionable grains were corrected or reassigned, heavy mineral weight percentages were determined for each size fraction, and the overall weighted averages were calculated (Table 1). Mineral Table 1. Mineral weight percentages based on optical/sem work and Rietveld refinements Optical -SEM Results From Grain Areas From Grain Volumes Average from all Refinements Rietveld Refinement Results Phases Used (times each phase was included) Alkali Feldspar Albite (2) Almandine Garnet Almandine (5) Aluminosilicate Apatite Augite Augite (5) Brown Kaersutite (5) Diopside Diopside (5) Epidote Epidote (1) Green Edenite (5) Grossular Garnet Magnetite (Fe-Ti ox) Orthopyroxene Plagioclase Feldspar Bytownite (5) Pyrope Garnet Sphene Talc Talc (2) 1.1 Goethite (3) 0.7 Quartz (1) 3.0 Unaccounted

5 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Compositions of nearly all solid solution phases based on SEM/EDX microanalysis are plotted on appropriate diagrams in Figure 1. The only solid solution minerals not plotted in Figure 1 are talc and grossular-rich garnet. Two analyses show that talc grains are Mg-rich talc-minnesotaite A Wollastonite Ca 2 Si 2 O 6 B 2.0 Tschermakite Tschermakite Paragasite Paragasite 6.0 Diopside CaMgSi 2 O 6 (Al) IV Hedenbergite 1.0 CaFeSi 2 O 6 Tschermakitic Edenitic Edenite Paragasitic Edenite 7.0 Si Augite Mg 2 Si 2 O 6 Fe 2 Si 2 O Orthopyroxene Pigeonite 6 Enstatite Ferrosilite 0 Tremolite (Na+K) Green Brown 8.0 C Ca 2 Al. Al 2 O. OH[Si 2 O 7 ][SiO 4 ] Clinozoisite Ca 2 Fe 3+. Al 2 O. OH[Si 2 O 7 ][SiO 4 ] Epidote Pyrope D Mg 3 Al 2 Si 3 O 12 E Anorthite CaAl 2 Si 2 O 8 Plagioclase Feldspar Bytownite Labradorite Andesine Oligoclase Albite Fe 3 Al 2 Si 3 O 12 Mn 3 Al 2 Si 3 O 12 Almandine Spessartine NaAlSi 3 O 8 KAlSi 3 O Alkali Feldspar 8 Orthoclase Figure 1. Compositions of more abundant solid solution minerals as determined from SEM/EDX microanalysis. (A) Compositions of diopside (green), augite (red), and orthopyroxene (blue) on a Wo-En- Fs ternary. (B) compositions on a classification diagram based on the calcic amphibole classification of Hawthorne [7]. (C) Epidote grain compositions on a clinozoisite-epidote binary. (D) Garnet compositions on a Mg-Fe-Mn ternary. (E) Feldspar compositions on an An-Ab-Or ternary; feldspar grains plotted in red; feldspar inclusion in epidote plotted in green.

6 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume solid solutions (Mg 2.22 Fe 0.78 Si 4 O 10 (OH) 2 - Mg 2.07 Fe 0.93 Si 4 O 10 [OH] 2 ). Two grossular-rich garnet grains consist of one grossular-almandine solid solution ([Ca 2.14 Fe 0.83 Mn 0.03 ][Al 1.98 Fe ]Si 3 O 12 ) and one grossular-andradite solid solution (Ca 3 [Al 1.14 Fe Ti 0.03 ]Si 3 O 12 ). XRD Analysis and Rietveld Refinement Rietveld refinement of the XRD data was difficult because of the large number of phases involved, many overlapping peaks (Figure 2), and preferred orientation. The preferred orientation was corrected using spherical harmonics for diopside (14 th order), edenite (12 th order), kaersutite (8 th order), and augite (4 th order). The results of averaging five refinements are presented in Table 1. DISCUSSION Figure 2. XRD pattern of the MPH sandstone heavy mineral sample. There is minimal difference between the mineral abundances based on measured grain areas and those based on volumes estimated from the grain areas. This suggests that use of grain areas to calculate weight percentages is probably adequate. Assignment of mineral identifications of questionable grains based only on SEM/EDX analyses of approximately half of those that were present in the mm fraction assumes the 61 grains analyzed are representative of the 183 questionable grains that were not analyzed. Thus, the abundances of some phases should be considered preliminary. This is particularly true for epidote, augite, feldspars, and talc. More confidence could be placed in the optical-sem/edx results if they included microanalysis of all questionable grains present in each of the three size fractions.

7 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume The greatest problem in the Rietveld refinements was the preferred orientation exhibited by several phases. The levels of correction applied were determined to be adequate to minimize orientation effects in the refinement results without over-correcting, which our group has found to occur if excessive spherical harmonics corrections are used. Slight changes in spherical harmonics corrections for certain phases might lead to better results. Refinement results should also be improved by including additional refinements in the averaging technique. It is unclear at this point how many of the minor phases might be reasonably quantified. Despite the preferred orientation problem, and the need for additional refinements to include in the averaging, there is reasonable agreement between mineral abundances determined from the optical/sem work and Rietveld refinements. The most significant difference between the two methods is in the abundance of the dominant phase, diopside. Rietveld refinements yield a lower percentage (46%) compared to the optical/sem work (56-57%). This difference is probably greater than uncertainty in optical identifications or the limited number of refinements can explain. It is most likely due to the effects of preferred orientation. There is fairly good agreement among results for other minerals with at least modest abundances (edenite, kaersutite, epidote, augite, and almandine garnet). Among low-abundance phases, the XRD-Rietveld results yielded higher percentages for two, plagioclase feldspar and talc. Additional refinements and possibly including corrections for preferred orientation for these two phases may improve their results. The results to date in this study suggest that the XRD-Rietveld method promise to yield good results in characterizing phases of at least modest abundance in heavy mineral suites. This particular heavy mineral suite may be (hopefully is) somewhat unusual in terms of the number of phases, and the number of abundant phases that have good cleavage (and cause preferred orientation problems). While additional refinement work will likely yield improved comparisons with optical/sem results, the eventual goal is to be able to apply Rietveld refinements without the need for detailed optical and SEM work. A priority in future work will be to find sample preparation protocol that will avoid orientation problems. The MPH sandstone heavy minerals will provide a good sample for investigating techniques that can be used to minimize preferred orientation during sample preparation, such that orientation corrections will not be necessary. ACKNOWLEDGEMENTS A grant from the Minot State University Small Grants Program helped make this work possible. The authors thank the Materials Characterization Lab at North Dakota State University for providing the XRD scan. REFERENCES [1] Hubert, J. F., Procedures in Sedimentary Petrology, Wiley-Interscience: New York, 1971, [2] Carver, R. E., Procedures in Sedimentary Petrology, Wiley-Interscience: New York, 1971, [3] Lewis, D. W.; McConchie, D., Analytical Sedimentology, Chapman & Hall: New York, 1994, 197 p. [4] Mumme, W. G.; Tsambourakis, G.; Hill, R. J., J. Sed. Res., 1996, 66, [5] Larson, A. C.; Von Dreele, R. B.; Los Alamos National Laboratory Report LAUR , [6] Winburn, R. S., Ph.D. Dissertation, North Dakota State University, [7] Hawthorne, F.C., Reviews in Mineralogy, Mineralogical Society of America: Blacksburg, VA, 1981, 9A,