ANALYSIS OF GEOLOGIC MATERIALS USING RIETVELD QUANTIATIVE X-RAY DIFFRACTION

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1 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume ANALYSIS OF GEOLOGIC MATERIALS USING RIETVELD QUANTIATIVE X-RAY DIFFRACTION Robin M. Gonzalez, Thomas E. Edwards, Timothy D. Lorbiecke, Ryan S. Winburn, and John R. Webster* Division of Science, Minot State University, 500 University Ave. W. Minot, ND ABSTRACT Rietveld quantitative X-ray diffraction (RQXRD) was used to quantify the mineralogy of a volcanic rock (dacite). To assess the success of the RQXRD method, the bulk mineralogy of the dacite was determined through a combination of optical study of phenocrysts (plagioclase, augite, orthopyroxene, and Fe-Ti oxide) and detailed study of the groundmass using a scanning electron microscope with an energy dispersive X-ray (SEM/EDX) analysis system. The groundmass consisted of distinct crystals of feldspar, pyroxene, and Fe-Ti oxide, and patches of very fine-grained mixtures (referred to as funkalite) of feldspar, SiO 2 phase, and Fe-Ti oxide. Groundmass volume percentages were determined by point counting high magnification SEM photomicrographs. These data were combined with phenocryst percentages and converted to weight percentages. The optical/sem and RQXRD results compared rather well, suggesting RQXRD can provide a relatively rapid method for quantifying the mineralogy of volcanic rocks. INTRODUCTION The Rietveld method has been determined to provide relatively accurate results when applied to coal combustion by-products [1,2,3]. The Rietveld method has been applied to igneous rocks [4], including volcanic rocks. Volcanic rocks in general have a porphyritic texture (having two distinct crystal size populations) that reflects a two-stage cooling history. Slow cooling at depth results in phenocrysts (larger crystals) and rapid cooling upon ascent and eruption results in small crystals and/or glass that comprise the groundmass. Phenocrysts can be identified and quantified by optical microscopy, but the groundmass is too fine-grained to be studied optically. Use of a scanning electron microscope (SEM) to quantify the mineralogy, while suitable, is not feasible for routine work because of the time involved. Rietveld quantitative X-ray diffraction (RQXRD) is well suited for quantifying the mineralogy of volcanic rocks. However, evaluating the success of the Rietveld method for volcanic rocks is difficult because the groundmass (typically the majority of the rock) is too fine-grained to study by optical microscopy. RQXRD data have only been compared previously with normative mineralogy [4]. To obtain the "observed" bulk mineralogy needed to assess the RQXRD method, the groundmass mineralogy must be quantified and combined with phenocryst abundances determined by optical microscopy. A major focus of this study was to characterize the groundmass of a volcanic sample using a SEM with an energy dispersive X-ray (EDX) analysis system. This study involved testing RQXRD using a dacite sample (TL-11) from the central Oregon High Cascades. This sample exhibited clear distinction between phenocrysts and groundmass. The phenocrysts were previously quantified using optical microscopy (point counting). They consisted of plagioclase feldspar, less abundant augite, orthopyroxene, and Fe-Ti oxide, and a trace amount of olivine. Numerous phenocrysts were analyzed using an electron microprobe. SEM/EDX METHOD The SEM (JEOL SM-35CF) was used to obtain digital backscatter electron (BSE) photomicrographs at high magnifications (4000X). BSE images (Figure 1) are best for distinguishing phases because intensity varies with the average atomic weight of the materials. These photomicrographs were used as a guide during EDX microanalysis. Microanalysis was carried out in both spot mode (electron beam stationary) and in area mode (scanning at high

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3 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume magnification). Five sites in the groundmass of TL-11 were studied to obtain representative analyses of the groundmass components. Once they were identified using EDX microanalysis, the volume percentages of the minerals in the 4000X BSE photomicrographs were determined. This was done by drawing the borders of the minerals using a computer drawing program (Figure 2). A grid was superimposed on each "digitized" image and was used to point count the volume percentage of each constituent of the groundmass. The constituents consisted of distinct crystals of feldspars, pyroxenes, and Fe-Ti oxides, and patches of very fine-grained mixed phases informally referred to here as "funkalite". A Fe-Ti oxide groundmass Area of B plagioclase feldspar phenocrysts B funkalite Fe-Ti oxide alkali feldspar augite phenocrysts Figure 1. SEM backscatter electron photomicrographs of TL-11. (A) View showing basic nature of the sample. (B) Enlarged view of the groundmass. Patches of funkalite, a mottled mixture of alkali feldspar, SiO 2 -phase, and minor Fe-Ti oxide, are found between microcrysts of alkali feldspar and Fe-Ti oxide. A B C D Figure 2. SEM backscatter electron photomicrographs of TL-11. (A) Site 2. (B) Site 2 digitized. (C) Site 4. (D) Site 4 digitized. Yellow: feldspar; red: Fe-Ti oxide; blue: pyroxene; white: funkalite; green: voids. Because the funkalite consisted of a mixture, its volume percentage had to be distributed among its component phases. This was done using recalculations of EDX analyses of funkalite. A feldspar recalculation spreadsheet was used to determine weight percentages of feldspar, SiO 2 phase, and Fe-Ti oxide. Fe- and Ti-oxide were assigned to Fe-Ti oxide. Then the SiO 2 in the analysis was manually adjusted (reduced) until the remaining SiO 2 (and other oxides) yielded good feldspar stoichiometry. The weight percentage of the subtracted SiO 2 was assigned to the SiO 2 phase, and the remaining (appropriate) oxides to the feldspar component of the funkalite. The weight percentages of the funkalite components were converted to volume percentages using appropriate densities. The volume percentages were then combined with volume percentages of the discrete groundmass minerals. These total groundmass percentages were then combined with

4 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume the phenocryst percentages to obtain the bulk sample volume percentages. Finally, all volume percentages were converted to weight percentages using appropriate densities. OPTICAL AND SEM/EDX RESULTS The abundances of phenocrysts and groundmass quantified through previous optical point counting (500 points) are presented in Table 1. Figure 3 is a typical thin section view showing the four minerals that were quantified: plagioclase, augite, orthopyroxene, and Fe-Ti oxide. Olivine is also present in trace amounts, but was not encountered during the point counting. Table 1. Volume percentages from optical point counting Volume % Phenocrysts Plagioclase 21.6 Olivine Trace Augite 1.4 Orthopyroxene 0.4 Fe-Ti Oxide 0.8 Groundmass 75.8 Plagioclase Feldspar Orthopyroxene Augite Fe-Ti oxide Augite Figure 3. Plain-light photomicrograph of TL-11 showing representative phenocrysts and abundant groundmass. The area shown is 2 mm wide. High magnification BSE images showed that the groundmass consisted of crystals and patches of funkalite. The boundaries between them varied from sharp to irregular and almost gradational. Figure 2 illustrates the variability in the appearance of the funkalite. It varied from light swirly heterogeneous patches (e.g., Figure 2A) to dark homogeneous patches (e.g., Figure 2C). The heterogeneous funkalite typically appeared to be composed of feldspar, a SiO 2 phase, and trace amounts of Fe-Ti oxide. Electron microprobe analyses of phenocrysts, spot EDX analyses of groundmass crystals, and area EDX analyses of funkalite show that mineral compositions vary among phenocryst and groundmass crystals. Feldspar and pyroxene compositions are plotted in Figure 4. Plagioclase feldspar phenocrysts varied from An 33 to An 67 (Figure 4A). Groundmass feldspar crystals were dominantly Na-rich (albite-rich), while the feldspar component of funkalite was alkali feldspar that was generally more K-rich. Augite phenocrysts had a rather limited compositional range, while orthopyroxene had a wider range (Figure 4B). Groundmass pyroxene crystals consisted of augite that is more Fe-rich and less calcic than augite phenocrysts, and pigeonite. Fe-Ti oxides were dominantly magnetite-ulvospinel solid solutions (Mt 47 Usp 53 to Mt 56 Usp 44 ), but there was

5 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume some hematite-ilmenite solid solution (Hm 08 Ilm 92 ). Groundmass Fe-Ti oxide compositions (Mt 63 Usp 37 and Hm 15 Ilm 85 ) were slightly different than phenocryst compositions. A Anorthite CaAl 2 Si 2 O 8 B Wollastonite Ca 2 Si 2 O 6 Plagioclase Feldspar Bytownite Labradorite Andesine Diopside CaMgSi 2 O 6 Hedenbergite CaFeSi 2 O 6 Oligoclase Augite Albite NaAlSi 3 O 8 KAlSi 3 O 8 Alkali Feldspar Orthoclase Mg 2 Si 2 O 6 Fe 2 Si 2 O Orthopyroxene Pigeonite 6 Enstatite Ferrosilite Figure 4. Plots of phenocryst and groundmass mineral compositions. (A) An-Ab-Or ternary showing the compositions of feldspar phenocrysts (red), groundmass crystals (yellow), and the feldspar component in the heterogeneous funkalite patches (green). The purple shaded field shows feldspar components based on analysis of dark patches. (B) Wo-En-Fs ternary for pyroxene phenocrysts (red) and groundmass crystals (yellow). Area EDX analyses carried out on the two types of funkalite showed significant differences in the abundances of feldspar, SiO 2 phase, and Fe-Ti. The dark patches contained more SiO 2 phase (52-97 wt.%) compared to the normal heterogeneous funkalite (8-53 wt.%), which contained more feldspar component. The compositional ranges of the feldspar components of the two types of funkalite are quite similar, except for the somewhat more limited range and slightly lower Ca contents exhibited by the dark homogeneous funkalite patches. Volume percentages determined from point counting of the digitized images of each groundmass site are presented in Table 2. The funkalite components were determined by recalculation of an average EDX chemical analysis for each of the five groundmass sites. The groundmass volume percentages were normalized to the percentage of groundmass in the bulk sample and then combined with phenocryst percentages. The combined bulk mineralogy is presented in Table 3. The calculated weight percentages comprise the known values with which the RQXRD results are compared. Table 2. Groundmass mineralogy (vol. %) Site 1 Site 2 Site 3 Site 4 Site 8 Average Feldspar G F Orthopyroxene G Fe-Ti Oxide G F SiO 2 Phase F G = distinct groundmass crystals, F = funkalite

6 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Table 3. Bulk Mineralogy of TL-11 Volume % Density (g/cm 3 ) Weight % of Bulk Sample Weight % by Mineral Type Feldspars Plagioclase P Na-rich G Alkali F Pyroxenes Augite P Orthopyroxene P Pigeonite + Augite G Fe-Ti Oxides P G F SiO 2 Phase F P = phenocrysts, G = distinct groundmass crystals, F = funkalite RIETVELD METHODS AND RESULTS The XRD data was collected using a Philips X Pert MPD with 1 fixed divergence and antiscatter slits and a 0.2 mm receiving slit. The data was collected from θ counting for 2 seconds per step. The data were analyzed using the software program General Structure Analysis System (GSAS) [5] using a previously described refinement protocol [1-3]. Multiple refinements were performed due to the large number of phases believed to be present within the sample as outlined previously [2, 3]. Weight percentages for ten different minerals were determined with an eleventh phase (rutile) being the internal standard. Due to limitations in the analysis software, only nine phases could be analyzed at a time. Five individual refinements were completed so that a representative average could be taken. Through analysis of the final refinement plots it was observed that a few smaller peaks were left unaccounted for. Since all phases are not accounted for in each refinement, every plot will have peaks that remain unaccounted for. Only those peaks that remained after all refinements were completed can be representative of an additional phase. The results of five refinements are given in Table 4, along with the average weight percentages for each phase. Although the weight percents vary among the refinements, the relative amounts are consistent. Table 4. Weight percent data from five separate refinements Average Totals by Mineral Type Feldspars Anorthite Albite Sanidine Microcline Pyroxene Augite Fe-Ti Oxides Ulvospinel Magnetite Illmenite Hematite SiO 2 Phase Cristobalite Amorphous/Unaccounted* * calculated by subtracting crystalline phase percentages from 100%

7 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume DISCUSSION Refinement number 3 (Table 4) seems to deviate the most from its counterparts. This deviation is most apparent in the low values for the phases sanidine, microcline, anorthite and cristobalite. Variability among the refinements is due to peak overlaps and the choice of phases within each refinement. There is also some uncertainty in the optical/sem data. Additional optical point counting and detailed study of groundmass sites might improve the results. Also, it is unclear at this time whether the dark type funkalite patches are completely crystalline. Their homogeneous nature suggests they could be patches of SiO 2 phase, with other oxides in EDX analyses coming from surrounding materials, rather than a mixed-phase material. Despite uncertainties, the RQXRD results compare quite well with optical/sem results. Table 5 summarizes the bulk mineralogy as determined by the two methods. These results suggest that it is feasible to quantify the mineralogy of volcanic rocks using RQXRD. Table 5. Comparison of optical-sem and RQXRD results Optical-SEM RQXRD Feldspars Pyroxenes Fe-Ti Oxides SiO 2 Phase Amorphous/Unaccounted 0.1 Future optical/sem work on TL-11 will involve additional optical point-counting and detailed study of additional groundmass sites, with attention to the nature of the dark homogeneous funkalite patches. Future RQXRD work may involve determining the additional phase(s) that presently appears to be missing in the refinements. Modifying structures and site occupancies used in the refinements might also improve results. In addition, refinement of the compositions of solid solution phases will be investigated. Initial studies will focus on the feldspar minerals, followed by investigation of the iron-containing phases. If RQXRD can routinely be used to successfully quantify the mineralogy of volcanic rocks and the compositions of solid solution phases, it will provide a powerful tool in the study of volcanic rocks. While this use of RQXRD will not replace detailed microanalysis of minerals in igneous rocks, it could provide a rapid screening tool for selection of samples that are best suited for detailed microanalysis. Future work will involve additional volcanic rocks, including samples that contain glass in the groundmass and samples that have different phenocryst compositional ranges. Of particular interest is plagioclase and whether RQXRD could be used to distinguish samples that have a limited plagioclase phenocryst compositional range (i.e., non-mixed) from those that have a wide and/or bimodal range (i.e., mixed). ACKNOWLEDGEMENTS A Minot State University Small Grant provided partial funding for this research. The Materials Characterization Lab at North Dakota State University provided the XRD scan. REFERENCES [1] Winburn, R.S., Lerach, S.L., Jarabek, B.R., Wisdom, M.A., Grier, D.G., and McCarthy, G.J. (2000). Adv. X-Ray Anal., 42, [2] Winburn, R.S. (1999). Ph.D. Dissertation, North Dakota State University [3] Winburn, R.S., Grier, D.G., Peterson, R.B., McCarthy, G.J., and Grier, D.G. (2000). Powd. Diff., 15, [4] Hill R.J., Tsambourakis, G., and Madsen, I.C. (1993). J. Pet., 34, [5] Larson, A.C., and Von Dreele, R.B. (1994). Los Alamos National Laboratory Report LAUR