MODAL ANALYSIS: PIXEL COUNTING VS. POINT COUNTING USING DATA FROM THE TIESCHITZ H/L3.6 METEORITE

Pixel counting, a method of modal analysis, is a quantifiable way to determine the ratios between different components in chondritic meteorites, as an alternative to the traditional method of point counting in the petrographic microscope. Point counting fundamentally relies on optical identification and how many points on a grid are counted. Grid size is often not recorded in the literature. Pixel counting relies on the physical area analyzed, the assumption that the area is representative of the whole sample, and the accuracy to which all components are outlined for image analysis. Pixel counting was applied to the H/L3.6 chondrite meteorite Tieschitz to acquire accurate ratios of chondrules to matrix and opaque minerals present in the sample. Previous work (J. Grossman 1988) states the modal abundances of components in ordinary chondrites to be 65%-75% chondrule, 10%-15% metals, and the remainder fine-grained matrix. Our analysis of Tieschitz reveals that the percentages of chondrules (including homogenous silicate grains) and opaques are much lower than previously documented in Tieschitz. A 180 mm² area was x-ray mapped at 10 μm/pixel, a resolution acceptable to resolve different components, but not ideal. Region maps of smaller areas (3.37 mm²) and higher resolution (3 μm/pixel) allowed more precise identification of opaques, chondrules and matrix. Preliminary analysis indicates that the sum of the area of all the chondrules in Tieschitz is less than the 65%-75% reported by G.R. Huss et al. (1981). Pixel counting offers accountability in the sense that researchers can include their images in their work, and the image analysis can be scrutinized accordingly. For point counting, details such as definition of the target area, number of points counted, grid size used and optical images of the whole area would be necessary to evaluate modal analysis, but these details have rarely been recorded. This work serves as a preliminary comparison of pixel counting in x-ray element maps with the traditional method of point counting.