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Terrestrially occurring iron silicide spherules, described in the geological literature for 160 years as cosmogenic and approved as "extraterrestrial" minerals by IMA CNMMN in 1984, so far have escaped any serious examination by meteoriticists. Our isotopic and REE data, obtained for silicide spherules for the first time, disagree with the meteoritic origin of gupeiite (Fe 3 Si) and xifengite (Fe 5 Si 3 ) spherules from two continents. Despite departures from terrestrial norms ( 87 Rb/ 86 Sr-0.0174; 87 Sr/ 86 Sr-0.700181; 3 He/ 4 He-7.57 × 10 −6 ; 40 Ar/ 36 Ar-325.9), the compositions of 143 Nd/ 144 Nd (0.512034) and 147 Sm/ 144 Nd (0.06357), as well as REE abundances, clarify provenance from upper crust sediments for samples with U/Pb age of 121-314 ka from the Ala-Tau range in the Urals. However, the morphology of flanged button shapes, ring waves, and eccentro-radiating ridges reliably constrains the origin of silicide spherules to distal meteoritic impact ejecta. Arc jet ablation experiments have previously demonstrated that similar morphologies, observed on australite tektites, reflect aerodynamic ablation rates corresponding to flight velocities well into orbital range. These features are generally accepted as conclusive evidence for hypervelocity atmospheric entry from space. Internal structure, consistent with accretion through the coalescence of 3-5 µm droplets, and composition, closely corresponding to 1893-1154 K span of C-type condensation sequences, indicate a high probability of processing through recondensation of ejecta vapor.
Terrestrially occurring iron silicide spherules, described in the geological literature for 160 years as cosmogenic and approved as "extraterrestrial" minerals by IMA CNMMN in 1984, so far have escaped any serious examination by meteoriticists. Our isotopic and REE data, obtained for silicide spherules for the first time, disagree with the meteoritic origin of gupeiite (Fe 3 Si) and xifengite (Fe 5 Si 3 ) spherules from two continents. Despite departures from terrestrial norms ( 87 Rb/ 86 Sr-0.0174; 87 Sr/ 86 Sr-0.700181; 3 He/ 4 He-7.57 × 10 −6 ; 40 Ar/ 36 Ar-325.9), the compositions of 143 Nd/ 144 Nd (0.512034) and 147 Sm/ 144 Nd (0.06357), as well as REE abundances, clarify provenance from upper crust sediments for samples with U/Pb age of 121-314 ka from the Ala-Tau range in the Urals. However, the morphology of flanged button shapes, ring waves, and eccentro-radiating ridges reliably constrains the origin of silicide spherules to distal meteoritic impact ejecta. Arc jet ablation experiments have previously demonstrated that similar morphologies, observed on australite tektites, reflect aerodynamic ablation rates corresponding to flight velocities well into orbital range. These features are generally accepted as conclusive evidence for hypervelocity atmospheric entry from space. Internal structure, consistent with accretion through the coalescence of 3-5 µm droplets, and composition, closely corresponding to 1893-1154 K span of C-type condensation sequences, indicate a high probability of processing through recondensation of ejecta vapor.
A glassy fulgurite, formed recently on a morainal ridge in southeastern Michigan, contains micrometer- to centimeter-sized metallic globules rich in native silicon, which unmixed from a silica-rich liquid. The unusual character of these globules and their potential for elucidating conditions of fulgurite formation prompted further study. Thermodynamic calculations indicate that temperatures in excess of 2000 K and reducing conditions approaching those of the SiO(2)-Si buffer were needed to form the coexisting metallic and silicate liquids. The phases produced are among the most highly reduced naturally occurring materials known. Some occurrences of other highly reduced minerals may also be due to lightning strike reduction. Extreme reduction and volatilization may also occur during high-temperature events such as lightning strikes in presolar nebulae and impacts of extraterrestrial bodies. As a result of scavenging of platinum-group elements by highly reduced metallic liquids, geochemical anomalies associated with the Cretaceous-Tertiary boundary may have a significant terrestrial component even if produced through bolide impact.
This review systematically presents all finds of geogenic, impact-induced, and extraterrestrial iron silicide minerals known at the end of 2021. The respective morphological characteristics, composition, proven or reasonably suspected genesis, and possible correlations of different geneses are listed and supported by the available literature (2021). Artificially produced iron silicides are only dealt with insofar as the question of differentiation from natural minerals is concerned, especially regarding dating to pre-industrial and pretechnogenic times.
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