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  • 1
    Publication Date: 2019-01-25
    Description: Altitude-dependent, high radar-reflectivity surfaces on Venus are observed on most mountainous volcanic terranes above a planetary radius of about 6054 km. However, high radar-reflectivity areas also occur at lower altitudes in some impact craters and plain terranes. Pyrite (FeS2) is commonly believed to be responsible for the high radar reflectivities at high elevations on Venus, on account of large dielectric constants measured for sulfide-bearing rocks that were erroneously attributed to pyrite instead of pyrrhotite. Pentlandite-pyrrhotite assemblages may be responsible for high reflectivities associated with impact craters on the Venusian surface, by analogy with Fe-Ni sulfide deposits occurring in terrestrial astroblemes. Mixed-valence Fe(2+)-Fe(3+) silicates, including oxyhornblende, oxybiotite, and ilvaite, may contribute to high radar reflecting surfaces on mountain-tops of Venus.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F; p 233-234
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  • 2
    Publication Date: 2013-08-31
    Description: Experiments demonstrated that oxidation of ferromagnesian silicates and magnetite occurs when these minerals are heated at 800 C in 1 atmosphere of CO2, under which conditions hematite is thermodynamically stable. The 30 ppm oxygen impurity in CO2 presumably facilitates the oxidation of some of the ferrous iron initially present in the crystal structures of the minerals. Mossbauer spectral measurements reveal, however, that only CO2 degraded olivine and pigeonite is hematite formed as a magnetically ordered phase at ambient temperatures. In orthopyroxene, some of the ferric iron produced by oxidation is present as nanophase hematite which, because it remains superparamagnetic until 4.2 K, must exist as particles less than or equal to 4 nm in diameter. In the calcic pyroxenes much of the oxidized ferrous iron may still remain as structural Fe3(+) in the host silicates. Some ferric iron may also be present as unit cell sized Fe2O3 inclusions in the pyroxenes, or be segregated along cleavage planes, or be coating mineral grains. In these states of aggregation, the Fe2O3 is unidentifiable by x ray diffraction and in low temperature Mossbauer spectra. Applications of this research to the surface of Venus are discussed.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990; p 207-209
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  • 3
    Publication Date: 2013-08-31
    Description: Inferences from these investigations are that Fe(3+)-bearing minerals such as hematite magnesioferrite, acmite, and epidote are thermodynamically unstable, and that magnetite is the predominant mixed-valence iron oxide mineral on venus. Recently, the Fe(2+)-Fe(3+) silicate mineral laihunite was proposed to be a reaction product of olivine with the venusian atmosphere. This possibility is discussed further here. We suggest that other mixed-valence Fe(2+)-Fe(3+)-Oz-OH(-) silicates could also result from surface-atmosphere interactions on Venus. Topics discussed include the following: (1) conversion of hematite to magnetite; (2) stability of laihunite; (3) the possible existence of oxy-amphiboles and oxy-micas on Venus; and (4) other mixed-valence Fe(2+)-Fe(3+) silicates likely to exist on Venus.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the International Colloquium on Venus; p 15-17
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  • 4
    Publication Date: 2019-06-28
    Description: An enigma of Venusian mineralogy is the suggestion that Fe(3+)-bearing minerals exist under the reducing conditions of the Venusian atmosphere. Analysis of the spectrophotometric data from the Venera 13 and 14 missions, combined with the laboratory reflectance spectral measurements of oxidized basalts at elevated temperatures, led to the suggestion that metastable hematite might exist on Venus. Heating experiments at 475 C when f(sub O2) approximately 10(exp -24) demonstrated that the hematite to magnetite conversion is rapid indicating metastable hematite is not present on Venus. In addition to hematite, several other ferric oxide and silicate minerals have been proposed to occur on Venus, including laihunite or ferrifayalite, Fe(3+)-bearing tephroite, oxyamphiboles, and oxybiotites. Heating experiments performed on these Fe(3+)-bearing minerals under temperature-f(sub O2) conditions existing on Venus suggest that only oxyamphiboles and oxybiotites may be stable on the surface of Venus.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 3: N-Z; p 1369-1370
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  • 5
    Publication Date: 2019-06-28
    Description: The stability of pyrite has become the focal point of study, since regions of low radio thermal emissivity were detected during the Pioneer Venus and Magellan radar mapping missions of Venus. Thermodynamic calculations and experimental studies have been performed assuming CO2 is a major contributor to the weathering of sulfides, as CO2 is the major gas species in the Venusian atmosphere. The purpose of the present study, however, is to demonstrate that CO2 is a non-reactive gas and that a minor species, S2, is the most important gas in determining whether pyrite is stable in the high radar reflectivity regions.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 3: N-Z; p 1369-1370
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