Quantitative mineralogical analysis of granitoid rocks : a comparison of X-ray and optical techniques

Five igneous rock specimens from the Peninsular Ranges batholith of southern California have been analyzed for mineral content in a comparative study by X-ray diffraction (reference intensity technique) and optical polarizing microscopy. The X-ray method utilizes aerosol suspension of pulverized rock samples, followed by collection and analysis on glass fiber filters by powder X-ray diffractometry. Under some conditions, a standard well mount may be used for sample preparation, eliminating the need for sample transparency and matrix corrections. Optical analysis consists of standard identification procedures using polarized light followed by point counting. Excellent agreement between the two methods is observed for quantities of the major components for all rocks except an olivine gabbronorite. Discrepancies in the gabbronorite analyses are primarily due to sample inhomogeneities as well as the use of calculated reference intensity constants for the several solid solution minerals occurring in this sample. Where both experimental reference intensity constants and sample measurements are obtained using the aerosol suspension technique for sample preparation, relatively high accuracy and relatively rapid data collection may be achieved with the X-rav method. Introduction and background Quantitative analysis by X-ray diffraction methods has received increasing attention in recent years (see the review by Alexander, 1977). Nevertheless, problems associated with preferred orientation, extinction, and micro-absorption have still limited the application of X-ray diffraction quantitative analysis to relatively inaccurate or semi-quantitative results. Point counting by microscopy still remains the basic and most widely used technique for direct mineralogical analysis of rock materials where particle size is above the 10 pm diameter level. However, many X-ray difraction studies using specialized calibration techniques have been described in the literature and include examples such as that of Parker (1978) for analysis of analcite in pumice, Bidjin and Lahodny-Sare (1976) for a study of kaolinite and boehmite in bauxite samples, the calcite study of Garish and Friedman (1973), and the clay mineral work of Cody and Thompson (1976). Additional specialized techniques such as those described by Henslee and Guerra (1977) using PVC membranes and Leroux and Powers (1970) for films deposited on silver membrane filters, were designed for analysis of free SiO2 materials. Many additional specialized studies of these types exist but cannot be enumerated here. In recent years, a general X-ray diffraction technique has emerged, known as the reference intensity method (RIM), which permits quantitative multicomponent analysis of crystalline materials without the requirement for preparation of calibration curves, and more recently without the need for the adding of an internal standard to the sample. Formulation of the RIM process was given by Chung (1974a,1974b,1975) and was applied to compounds used in paint pigments, such as PbO, TiO2, ZnO, 0003-0ux/E2i I I I 2-l | 35$02.00 I 135 l 136 DAVIS AND WALAWENDER: MINERALOGICAL ANALYSrc OF GRANITOID ROCKS etc., with the compounds reduced to a fine powder and mounted in accordance with the National Bureau of Standards' technique (NBS Monograph25, 1971). The technique as applied to these compounds proved highly successful, primarily due to the use of reference intensity constants determined for the same specific batches of compounds that were used in the "unknown" mixtures. Most of the early work used an introduced compound, such as Al2O3 as the reference material, but the latest study (Chung, 1975) made use of one of the compounds in the unknown mixture as the arbitrary reference. This, of course, was made possible by the previous knowledge of the reference intensity constants for all components of the sample so that transformation of the reference base was possible. In response to a need for characterization of compounds in the atmosphere, a comprehensive study was undertaken by one of us (BLD) to develop a quantitative analysis technique for ambient aerosols. In this case, analysis was made directly on the collecting filter media, usually a highvolume glass fiber filter or one of the membrane filters such as Teflon or Nucleopore. The unique filter method of sample preparation was shown by Davis and Cho (1977) to eliminate preferred orientation in calcite aerosols, and it has been presumed that preferred orientation of other inequant shaped materials is similarly eliminated or greatly reduced. These studies are further described by Davis (1980), and Davis and Johnson (1982a,b). Optical techniques for quantitative evaluation of mineral constituents in thin section have long been used by mineralogists and petrologists. It is important in such an analysis, however, to be sure that the volume adjustment of the various species for shape differences is made to the point count data set. This adjustment is vital in order to make a comparison of the traditional optical analytical technique with the recently developed RIM. To avoid making such corrections, we have selected a series of relatively homogeneous even-grained plutonic rocks from the Peninsular Ranges batholith of southern California. Classification of these rocks range from granite through quartz diorite to gabbroic species that have been described previously by Walawender (1976) and Walawender and Smith (1980). Further interest was added to the study by completing the work independently. Data from one laboratory was not examined by the workers of the other laboratory until completion of the experimental work and compilation of the final analytical results. We feel, therefore, that the present results give a true evaluation of the X-ray technique if one considers the optical results to be the "standard" of comparison. To the authors' knowledge, this study represents the only attempt at a direct and independent verification of the RIM technique since the method was successfully applied by Chung. Brief summary of required X-ray parameters A complete description of the X-ray theory and methodology is not possible here. These details can be obtained by review of the Institute of Atmospheric Sciences (IAS) literature cited above. Briefly, the raw intensity data can be obtained from digital or strip chart output of any properly maintained powder diffractometer. They should be integrated intensities (at least by "triangular" approximation) and must be corrected to constant volume conditions in cases where the theta-compensator has been employed. The intensities representing the "infinite" thickness condition for such a sample is obtained from the relation rr: Iipfr f WHPS [l e-zPi'lvta/'wEsinel where Ii is the raw integrated intensity (counts) for component i, WS is the fraction of the sample material in the sample/matrix composite layer, { is the constant volume factor for diffractometers with theta-compensating slit systems, lrfi is the mass absorption coefficient experimentally determined for the sample material alone, pfi is the mass absorption coefficient for the sample/matrix composite, M" is the mass per unit area of sample layer B on the filter substrate, and 0 is the Bragg diffraction angle. This relation corrects the initial intensities for transparency, matrix absorption, and departure from constant volume conditions (if appropriate). The mass absorption coefficient of the filter is obtained from the relation where I1 is the beam intensity attenuated by the filter, Is is the initial beam intensity, Ml is the mass per unit area of the filter substrate, and gf' is the mass absorption coefficient of the filter material alone. A similar relation is used to obtain the mass absorption coefficient of the sample; ( l )