Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..379k&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 379
Mathematics
Logic
1
Condensation, Diamonds, Dust, Interplanetary, Plasma
Scientific paper
We report here the results of an experiment designed to determine whether a group of silicate, oxide, sulfide, and metal substrates would remain stable or decompose under the highly energetic conditions of a plasma in which diamonds can form metastably. Using a microwave plasma reactor, we exposed the substrates to a plasma consisting of 99.5% hydrogen and 0.5% methane for 24 hr. The temperature of the substrate was approximately 950 degrees C, the pressure of the vacuum chamber was approximately 45 mbar, and the flow rate of the gas mix was approximately 200 cm^3/s. The silicate substrates were olivine, plagioclase, augite, nepheline, pyrope, hornblende, serpentine, and phlogopite; oxide substrates were magnetite and hematite; sulfide substrates were troilite, pentlandite, and pyrrhotite; and metal substrates were iron, nickel, kamacite, and taenite. Metastable diamonds formed on all the silicate substrates except nepheline. There was no diamond formation on any of the oxide, sulfide, or metal substrates. In every instance where diamonds did not grow the substrates were covered with thick secondary deposits. A substantial amount of pitting was observed on the surfaces of all the silicate substrates except nepheline. There is evidence for the loss of a substantial amount of silicon from at least three silicate substrates (e.g., plagioclase, anorthite, and pyrope in a Ca-rich groundmass). A thick layer of platelike Al-rich secondary deposits (possibly metallic Al and/or corundum) formed on the nepheline substrate. The magnetite, hematite, troilite, pentlandite, pyrrhotite, iron, kamacite, and taenite substrates decomposed significantly. Thick, dark, highly porous secondary iron-rich deposits formed on the surfaces of these substrates. These iron-rich deposits may consist of metallic Fe, FeC, and/or FeS and may be reaction products resulting from the interaction of the plasma and substrates. Other secondary deposits common to the silicates were iron (possibly in the form of metallic Fe, iron carbide, and/or iron sulfide) and calcium (possibly in the form of elemental Ca and/or oldhamite). Secondary iron typically formed porous, loosely bound aggregates, often containing small (0.1-micrometer) spheroids. Fibers approximately 0.05 micrometers in diameter and 10-30 micrometers in length were observed on substrates containing a substantial amount of Ca (e.g., plagioclase, augite, anorthite, and pyrope in a Ca-rich groundmass). The initial presence of Ca in the substrates, loss of Si from the substrates, and growth of fibers on substrates may be related. Samples were run in groups of seven and secondary mineral formation, at least in part, resulted from cross contamination. We can infer from this cross contamination that Fe, Si, Ca, and S were fractionated from some substrates, incorporated into the plasma, and then deposited in different morphologic forms. These forms include fibers, fibrous networks, iron-rich, porous, loosely bound aggregates, and spheroids. The pressure and temperature conditions used in this experiment can be found in the atmospheres of red giant stars. The results of this experiment suggest that most of the minerals used as substrates would not be stable under conditions that favor the metastable growth of diamonds. However, morphologic structures similar to those produced on the substrates in this experiment may also exist in the atmospheres of red giant stars.
Cassidy William A.
Kern C. M.
Witkowski R. E.
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