Spectral reflectance properties of carbonaceous chondrites: 7. CK chondrites

The reflectance spectra of 15 CK chondrites have been measured as part of an ongoing study of carbonaceous chondrite reflectance spectra. The available sample suite includes multiple grain sizes and samples with petrologic grades varying from CK4 to CK6. CK reflectance spectra are all characterized by an olivine-associated absorption band in the 1.05 μm region. Compared to pure olivine, CK spectra are darker, have a more subdued olivine absorption band, and are often more blue-sloped. Reflectance at 0.56 μm varies from 9.6% to 22.5%, and olivine band depth varies from 6.7% to 31.0%, for powders that include the finest fraction. With increasing grain size, and exclusion of the finest fraction, CK spectra become darker and more blue sloped, while the olivine absorption band initially becomes deeper and then shallower. The presence of calcium–aluminum inclusions (CAIs), whose abundance varies widely in CKs, does not normally lead to the appearance of a well-defined absorption band in the 2.1 μm region, although the overall blue slope of many CKs is likely attributable to Fe-bearing spinel in CK CAIs. The only consistent spectral feature that relates to metamorphic grade is that CK6 spectra have uniformly deeper olivine absorption band than CK4–5.5 spectra. This could be related to various factors such as loss/aggregation of opaques that may accompany metamorphism. Comparison of CV and thermally metamorphosed carbonaceous chondrite to CK spectra suggests that metamorphism to between ∼1000 and 1200 °C is required for CV spectra to match CK spectra; CV spectra are uniformly darker and have shallower olivine absorption bands than CK spectra.