M. V. Gerasimov

Valence state of iron in a condensate from the Luna 16 regolith

This paper reports the results of an X-ray photoelectron spectroscopic study of the condensate phase of regolith sample L1639 returned by the Luna 16 mission. The reduced Si0, Si2+, Al0, Ti2+, and Ti3+ forms were detected in the sample. Iron occurs in all valence states, and Fe3+ species were detected for the first time in the condensate. Minor Fe3+ concentrations were observed in the upper layers of the sample containing the maximum amounts of condensate products. The fraction of ferric Fe is 22%, and the Fe0: Fe2+: Fe3+ proportion is 33: 45: 22. The appearance of ferric Fe in the lunar condensate is explained by the reaction of FeO disproportionation occurring either at the stage of the expansion and cooling of impact-related vapor or directly in the condensed phase on the surface of regolith particles. This interpretation is supported by the results of a model experiment on augite vaporization and condensation. The experiment simulating impact vaporization was carried out on a laser set-up at a temperature of ∼3000–4000 K and a pulse duration of ∼10−3 s in a He atmosphere (P = 1 atm). The results of analyses provided compelling evidence that the condensate produced after augite vaporization contains Fe in all oxidation states, and the proportions of different valence forms approach the stoichiometry of the disproportionation reaction.

Experimental data on the thermal reduction of phosphorus and iron and their significance for the interpretation of the impact reworking of lunar materials

Experiments simulating impact melting and vaporization were performed to study variations in the valence state of P on the Moon. It was shown that P changes its valence and is partially transformed from oxidized (phosphate) to reduced (P0) species at high temperatures typical of the impact process. The reduction of P occurs concurrently with Fe reduction from silicates. The metallization of Fe and P is due to the effect of thermal reduction only. This supports the hypothesis of R. Hunter and L. Taylor that P0 produced from lunar phosphates (apatite and whitlockite) interacted with reduced Fe to form typical P-bearing minerals of lunar highland rocks, schreibersite and Fe-P alloys.

Estimation of temperature conditions for the formation of HASP and GASP glasses from the lunar regolith

Impact cratering on the Moon’s surface was accompanied by the high-temperature melting of rocks, melt evaporation, and silicate vapor condensation. Evidence for the extensive evaporative fractionation of melts was found in HASP (High-Alumina Silica-Poor) glasses from the lunar regolith. Numerous objects of condensation origin were found in the Apollo 14 regolith breccia. They are referred to as GASP (Gas-Associated Spheroidal Precipitates). With respect to chemical characteristics, namely FeO and SiO2 contents, GASP were subdivided into Fe-rich (FeGASP) and Si-rich (SiGASP) condensates. Based on experimental data on the evaporation of aluminous basalt sample 68415.40 from the Apollo 16 collection and the calculated compositions of residual melts and complementary vapors at various temperatures, we compared the obtained compositions with the chemical analyses of the HASP glasses and GASP condensates. The comparison was aimed at estimating the temperature conditions of HASP and GASP formation. The comparison showed that the compositions of the HASP glasses and GASP condensates are consistent with the compositions obtained in the equilibrium experiment. In accordance with the experiment, the temperature range of the evaporation of HASP glasses was estimated as ∼1750–1870°C. The temperature interval of condensation, with allowance for the effect of vapor supercooling, is ∼1700–1500°C for FeGASP and no higher than 1700–1750°C for SiGASP. This paper discusses the problems of establishing interphase thermodynamic equilibrium during the dispersion of a vapor-melt cloud, vapor supercooling during its condensation, and the influence of the curvature of melt and condensate particles on the character of evaporation and condensation.

Effect of the disproportionation reaction of ferrous iron in impact-evaporation processes

Evidence for the disproportionation of iron was found in model experiments imitating impact melting, evaporation, and condensation. The experiments were carried out using a laser system at a characteristic temperature of ∼3000–4000 K and a pulse duration of ∼10−3 s in a He atmosphere (P = 1 atm). Augite and mixtures of peridotite with MnO2 and WO3 were used as starting target materials. Experimental products (condensed vapor phase) were analyzed by X-ray photoelectron spectroscopy. The results of condensate analysis provided compelling evidence for the presence of iron in three oxidation states (Fe0, Fe2+, and Fe3+). In an experiment with augite, the proportions of iron species of different valences were similar to the stoichiometry of the disproportionation reaction. Similar evidence for this reaction was first found in a condensate from the samples of the fine fraction of the Luna 16 regolith. In the layers of the lunar condensate, the proportions of the valence states of iron were on average Fe0:Fe2+:Fe3+ = 1.2: 1.9: 0.7.

Cluster type of silicate vaporization: Newly obtained experimental data

Impact cratering is usually associated with the partial or complete vaporization of the high-temperature impact melts. According to its chemical characteristics, the vaporization of major oxides, silicate minerals, and rock melts can be classified into the following four types: (1) congruent vaporization without decomposition of the compound in the vapor phase, (2) congruent vaporization with the decomposition of the compound in the vapor phase, (3) incongruent vaporization, and (4) cluster vaporization. The latter type of vaporization pertains to the transfer of material into vapor phase in the form of complicated atomicmolecular groups (clusters) of certain stoichiometry. Cluster vaporization takes place at superhigh temperatures typical of impact processes. The clusters can comprise compounds of different individual volatility, and this often results in the enrichment of the vapor phase in elements traditionally thought to be refractory. Examples of cluster vaporization are offered by lately obtained experimental results on laser-pulse vaporization of larnite, merwinite, and wollastonite. Condensed vapor generated at the vaporization of orthosilicates (larnite and merwinite) was proved to be dominated by chain bonds of Si-O tetrahedrons and to contain molecular groups of wollastonite and pseudowollastonite stoichiometry.

Conditions of condensate rim formation on the surface of lunar regolith particles

Condensate objects observed in the lunar regolith are distinctly separated on the basis of morpho-logical and chemical characteristics into droplets condensed during the expansion of an impact-generated vapor cloud and films condensed on the relatively cold surface of mineral particles. Using the analyses of both condensate forms and experimental data on the evaporation of melt corresponding to a typical lunar highland rock of the gabbro-anorthosite composition from Apollo 16 sample 68415.40, the temperature conditions of vapor condensation during lunar impact events were estimated. The comparison of condensate compositions with the analyses of vapors from the evaporation experiment showed that, compared with the compositions of droplet-type condensates, the condensate rims were formed from a vapor with high contents of refractory CaO and Al2O3 and at very different condensation temperatures. The enrichment of vapor in CaO and Al2O3 could be attained only at high temperatures of melt evaporation (higher than ∼ 1850°C according to experimental data). The estimated condensation temperatures of droplets are significantly lower, ∼1750–1500°C. Rim-type condensates were produced by vapor quenching on the relatively cold surface of a solid mineral particle, which resulted in almost complete precipitation of all major components of the silicate vapor without fractionation in accordance with their individual volatilities.

New experimental evidence on cluster-type vaporization of feldspars

This paper reports experimental data on the investigation of the chemical composition of condensed silicate matter produced during the high-temperature pulse vaporization of feldspars. The experiments simulated the conditions of vaporization accompanying a high-velocity impact. Samples of albite, bytownite, calcic and sodic labradorite, and sanidine were used in the experiments. The investigation of the condensate layers obtained in the experiments included the determination of element distribution and structural characteristics of the materials using layer-by-layer X-ray photoelectron spectroscopy. It was shown that the vaporization of the samples occurred mainly through the release of complex atom–molecule groups referred to as clusters. “Nepheline,” “wollastonite,” and “sillimanite” clusters were identified as characteristic groups. The thermodynamic evaluation of melt composition at temperatures up to 5000 K performed using the Magma program confirmed high activities of these components in feldspar melts.