Analysis of ordinary chondrites using powder X-ray diffraction: 1. Modal mineral abundances

Abstract

Abstract2013Powder X-ray diffraction (XRD) is used to quantify the modal abundances (in wt%) of 18 H, 17 L, and 13 LL unbrecciated ordinary chondrite falls, which represents the complete petrologic range of equilibrated ordinary chondrites (types 420136). The XRD technique presents an effective alternative to traditional methods for determining modal abundances, such as optical point counting and electron microprobe phase (EMP) mapping. The majority of chondrite powders in this study were previously prepared for chemical characterization from 8 to 20 g of material, which is consistent with the suggested mass (10 g) necessary to provide representative sampling of ordinary chondrites. Olivine and low-Ca pyroxene are the most abundant phases present, comprising one-half to three-fourths of total abundances, while plagioclase, high-Ca pyroxene, troilite, and metal comprise the remaining XRD-measured mineralogy. Pigeonite may also be present in some samples, but it is fitted using a high-Ca pyroxene standard, so exact abundances cannot be measured directly using XRD. Comparison of XRD-measured abundances with calculated Cross, Iddings, Pirsson, Washington (CIPW) normative abundances indicates that systematic discrepancies exist between these two data sets, particularly in olivine and high-Ca pyroxene. This discrepancy is attributed to the absence of pigeonite as a possible phase in the CIPW normative mineralogy. Oxides associated with pigeonite are improperly allocated, resulting in overestimated normative olivine abundances and underestimated normative high-Ca pyroxene abundances. This suggests that the CIPW norm is poorly suited for determining mineralogical modal abundances of ordinary chondrites.