Reflectance spectra of iron meteorites: Implications for spectral identification of their parent bodies

Abstract– The 0.35–2.5 μm reflectance spectra of iron meteorite powders and slabs have been studied as a function of composition, surface texture (for slabs), grain size (for powders), and viewing geometry (for powders). Powder spectra are invariably red‐sloped over this wavelength interval and have a narrow range of visible albedos (approximately 10–15% at 0.56 μm). Metal (Fe:Ni) compositional variations have no systematic effect on the powder spectra, increasing grain size results in more red‐sloped spectra, and changes in viewing geometry have variable effects on overall reflectance and spectral slope. Roughened metal slab spectra have a wider, and higher, range of visible albedos than powders (22–74% at 0.56 μm), and are also red‐sloped. Smoother slabs exhibit greater differences from iron meteorite powder spectra, exhibiting wider variations in overall reflectance, spectral slopes, and spectral shapes. No unique spectral parameters exist that allow for powder and slab spectra to be fully separated in all cases. Spectral differences between slabs and powders can be used to constrain possible surface properties, and causes of rotational spectral variations, of M‐asteroids. The magnitude of spectral variations between M‐asteroids and rotational and spectral variability does not necessarily imply a dramatic change in surface properties, as the differences in albedo and/or spectral slope can be accommodated by modest changes in grain size (for powders), small changes in surface roughness (for slabs), or variations in viewing geometry. Since metal powders exhibit much less spectral variability than slabs, M‐asteroid spectral variability requires larger changes in either powder properties or viewing geometry than for slabs for a given degree of spectral variation.     

The Substitution of Al and F in Titanite at High Pressure and Temperature: Experimental Constraints on Phase Relations and Solid Solution Properties

Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO₃–Al₂SiO₅–TiO₂–CaF₂. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and X_Al ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO²⁺]_–1[AlF²⁺]₁.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in X_Al, whereasthe maximum X_Al of titanite solid solution occurs with fluoriteand either anorthite or Al₂SiO₅. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO₄O is very close to the ideal molecular activity model(X_Ti) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain