Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.206b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 206
Mathematics
Logic
1
Scientific paper
Introduction: Kaidun is a CR-type chondrite (Kaidun I) that abundantly contains clasts of CI (Kaidun II), EH, and EL chondrites (1,2). Despite extensive mineralogical- petrological studies (2) some mineralogical aspects, especially of the CI clasts, remain unexplored. Zolensky et al. (3) described three CI clasts with emphasis on matrix bulk composition and phyllosilicate mineralogy. The abundant anhydrous minerals have so far not been studied in detail. Here we report the results of our SEM and EMP study of olivines, pyroxenes, and carbonates in Kaidun II. Results: Kaidun II consists mainly of phyllosilicates dominated by serpentine and saponite (3) and magnetite and sulfide (mostly pyrrhotite) with morphologies typical for CIs. Less abundant but not rare are grains (<~10 micrometers - ~30 micrometers) of anhydrous silicates and carbonates. Among these silicates olivine is most abundant followed by enstatite. Also a few Ca-rich pyroxenes have been encountered. Most silicates are of irregular shape. The dominant carbonate is calcite, less common is dolomite. In addition we have found several phosphate grains (~20 micrometers) of unusual composition. The averaged compositon of 2 grains is Na2O 3.4, MgO 21, P2O5 37.9, and FeO .98 wt%. The composition of this hydrated (?) phase does not correspond to any known phosphate mineral. Olivines: Most olivines are forsteritic (Fo(sub)96-Fo(sub)99), a few grains are Fe-rich down to Fo(sub)42. The forsterites have variable but generally high minor element contents. The contents of CaO and MnO vary from <0.02- 0.62 and <0.02-2.5 wt%, respectively, and Cr2O3 contents are usually below 0.5 wt% (range 0.13-1.30). Contents of Al2O3 and TiO2 are usually low (on average <0.05 wt%). Pyroxenes: Enstatite is the dominant pyroxene variety. It is also rich in minor elements. The ranges for CaO, MnO, Al2O3, and TiO2 are 0.37-2.15, 0.05-0.51, 0.10- 1.86, and 0.05-0.18 wt%, respectively. Cr2O3 (range 0.02-0.97 wt%) is positively correlated with FeO (0.80- 3.50 wt%). The compositions of two high Ca-pyroxenes were En(sub)62Fs(sub)17Wo(sub)21 and En(sub)62.5Fs(sub)2.5Wo(sub)35. Carbonates: Dolomites in Kaidun II have relatively high FeO (4.1- 4.7 wt%) and MnO (2.1-4.5 wt%) contents. Calcites are rich in FeO(1.1-1.3 wt%) and poor in MnO(<0.02-0.05 wt%). Discussion: The minor element contents of the forsteritic olivines in Kaidun II are similar to those of forsterites in other CI chondrites (e.g., 4) except for the generally high MnO contents. The MnO contents of Kaidun forsterites by far exceed those reported from olivines in Orgueil and Alais (4). However, low-Fe, Mn-enriched olivines, and pyroxenes have been described from micrometeorites (5) and chondritic porous IDPs and UOCs (6). Dolomite (7) or subequal amounts of dolomite and ferroan magnesite (8) are the most abundant carbonates in CIs, while in CMs calcite is most abundant (7). In contrast, in Kaidun II calcite is the most abundant carbonate. In addition, it is clearly richer in Fe and poorer in Mn as compared to calcites previously described from CIs and CMs (7). Conclusion: The unique character of the Kaidun meteorite is at least to some extent reflected in the abundance and composition of isolated mineral grains in the Kaidun II clast. In particular, the Mn-enriched Fe-poor olivines are distinct from olivines in other CCs. Some resemblance to olivines in micrometeorites and IDPs exists-a feature also supported by the high abundance of enstatite in MMs, IDPs, and Kaidun. References: (1) Ivanov A. V. et al. (1986) Meteoritika 45, 3-19. (2) Ivanov A. V. (1989) Geochemistry International 26, 84-91. (3) Zolensky M. E. et al. (1991) Lunar Planet. Sci. (abstract) 22, 1565. (4) Steele I. M. (1990) Meteoritics 25, 301-307. (5) Presper T. et al. (1992) this volume. (6) Klock W. et al. (1989) Nature 339, 126-128. (7) Johnson J. A. and Prinz M. (1991) Lunar Planet. Sci. (abstract) 22, 643. (8) Kerridge J. F. et al. (1980) Meteoritics (abstract) 15, 313.
Brandstatter Frans
Ivanov Alexei V.
Kurat Gero
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