Agglutinitic Glass Research Articles

Influencing factors for the preparation of Fe0 in lunar soil simulant using high-temperature carbothermic reduction

Metallic iron (Fe0) within lunar soil grains can affect their visual and near-infrared spectra, adhesion, biological toxicity, and electrostatic migration characteristics. The Fe0 particle therefore is an important component of lunar soil. This study devised a high-temperature carbothermic reduction method for preparing Fe0 via graphite reduction of Chinese Lunar Regolith Simulant (CLRS-2) in an argon atmosphere, which is similar to the Fe0 in the agglutinitic glass of lunar samples. The X-ray diffraction and electron microscopy data show that the mixture of lunar soil simulants and graphite heated at high temperature rapidly quenched to form a glassy phase with dispersed Fe0 particles, the microstructure of which is consistent with the α-Fe (bcc) in lunar soil. This paper also discusses the main influencing factors for preparing Fe0, such as the ratio of raw materials, heating temperature, and holding time. The optimal experimental conditions are a graphite-to-CLRS-2 mass ratio of 1.0: 27.0 (Fe/C = 2.0: 2.6), reduction temperature of ∼1600 °C, and holding time of ∼4 h, which produce single-phase α-Fe with an average particle size of ∼180 ± 10 nm and no residual impurities (e.g., graphite).

An Empirical Model to Estimate Abundance of Nanophase Metallic Iron (npFe0) in Lunar Soils

Lunar soils gradually become mature when they are exposed to a space environment, and nanophase metallic iron (npFe0) generates within them. npFe0 significantly changes the optical properties of lunar soils and affects the interpretation of the remotely sensed data of the lunar surface. In this study, a correlation analysis was conducted between npFe0 abundance and reflectance spectra at short wavelengths for lunar soil samples in four size groups based on their spectral and compositional data, collected by the Lunar Soil Characterization Consortium (LSCC). Results show that 540 nm single scattering albedo (SSA) of lunar soils correlates well with their corresponding npFe0 abundance for each size group of lunar soil samples. However, it is poorly correlated with npFe0 abundance when all size groups were considered because of the strong interference from grain size variation of lunar soils. To minimize the effect of grain size, the correlation of npFe0 abundance with the spectral ratio of 540 nm/810 nm SSA of all size groups for LSCC samples was calculated and results show that a higher correlation existed between them (R2 = 0.91). This ratio can serve as a simple empirical model for estimating npFe0 abundance in lunar soils. However, bias could be introduced to the estimation result when lunar soils possess a high content of agglutinitic glass and ilmenite. Our future work will focus on improving the model’s performance for these lunar soils.

An improved radiative transfer model for estimating mineral abundance of immature and mature lunar soils

An improved Hapke’s radiative transfer model (RTM) is presented to estimate mineral abundance for both immature and mature lunar soils from the Lunar Soil Characterization Consortium (LSCC) dataset. Fundamental to this improved Hapke’s model is the application of an alternative equation to describe the effects of larger size submicroscopic metallic iron (SMFe) (>50nm) in the interior of agglutinitic glass that mainly darken the host material, contrasting to the darkening and reddening effects of smaller size SMFe (<50nm) residing in the rims of mineral grains. Results from applying a nonlinear inversion procedure to the improved Hapke’s RTM show that the average mass fraction of smaller and larger size SMFe in lunar soils was estimated to be 0.30% and 0.31% respectively, and the particle size distribution of soil samples is all within their measured range. Based on the derived mass fraction of SMFe and particle size of the soil samples, abundances of end-member components composing lunar soil samples were derived via minimizing the difference between measured and calculated spectra. The root mean square error (RMSE) between the fitted and measured spectra is lower than 0.01 for highland samples and 0.005 for mare samples. This improved Hapke’s model accurately estimates abundances of agglutinitic glass (R-squared=0.88), pyroxene (R-squared=0.69) and plagioclase (R-squared=0.95) for all 57 samples used in this study including both immature and mature lunar soils. However, the improved Hapke’s RTM shows poor performance for quantifying abundances of olivine, ilmenite and volcanic glass. Improving the model performance for estimation of these three end-member components is the central focus for our future work.

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3–1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35–70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20–45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40–50 wt.% SiO 2, 10–35 wt.% Al 2O 3, 1–8 wt.% FeO, and <25 wt.% TiO 2; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.

Minerals mapping of the lunar surface with Clementine UVVIS/NIR data based on spectra unmixing method and Hapke model

The distribution of minerals on the lunar surface is information which could contribute to studying lunar origin and evolution. In this paper, the distribution of clinopyroxene, orthopyroxene, olivine, ilmenite, and plagioclase on the lunar surface has been mapped based on Hapke radiative transfer model and linear unmixing of spectra with Clementine UVVIS/NIR data. The results have been validated on the basis of minerals modal abundance data of the Apollo samples, and problems in the minerals abundance mapping have been analyzed. The validation based on analysis data of Apollo samples indicates that plagioclase mapped in this paper represents the total abundance of plagioclase and agglutinitic glass. The minerals mapping results show that the lunar surface is mainly composed of pyroxene, plagioclase, agglutinitic glass, and ilmenite. Basalt in the lunar mare is mainly composed of clinopyroxene and ilmenite, and lunar highland is mainly composed of plagioclase and agglutinitic glass. Orthopyroxene is mainly distributed on the north of Mare Imbrium, on the south of Maria and Aitken Basin. According to our results, there is probably no large area of olivine distribution on the lunar surface which is different from earlier published results. Therefore, emphasis should be put on the olivine distribution in the minerals mapping using hyperspectral data such as M 3 of Chandrayaan-1 and IIM of ChangE-1.

Mineralogical and chemical characterization of lunar highland soils: Insights into the space weathering of soils on airless bodies

With reflectance spectroscopy, one is measuring only properties of the fine‐grained regolith most affected by space weathering. The Lunar Soil Characterization Consortium has undertaken the task of coordinated characterization of lunar soils, with respect to their mineralogical and chemical makeup. It is these lunar soils that are being used as “ground truth” for all airless bodies. Modal abundances and chemistries of minerals and glasses in the finest size fractions (20–45, 10–20, and &lt;10 μm) of four Apollo 14 and six Apollo 16 highland soils have been determined, as well as their bulk chemistry and IS/FeO values. Bidirectional reflectance measurements (0.3–2.6 μm) of all samples were performed in the Reflectance Experiment Laboratory. A significant fraction of nanophase Fe0 (np‐Fe0) appears to reside in agglutinitic glasses. However, as grain size of a soil decreases, the percentage of total iron present as np‐Fe0 increases significantly, whereas the agglutinitic glass content rises only slightly; this is evidence for a large contribution to the IS/FeO values from the surface‐correlated nanophase Fe0, particularly in the &lt;10 μm size fraction. The compositions of the agglutinitic glasses in these fine fractions of the highland soils are different from the bulk chemistry of that size; however, compositional trends of the glasses are not the same as those observed for mare soils. It is apparent that the glasses in the highland soils contain chemical components from outside their terrains. It is proposed that the Apollo 16 soils have been adulterated by the addition of impact‐transported soil components from surrounding maria.

Unique properties of lunar impact glass: Nanophase metallic Fe synthesis

Lunar regolith contains important materials that can be used for in-situ resource utilization (ISRU) on the Moon, thereby providing for substantial economic savings for development of a manned base. However, virtually all activities on the Moon will be affected by the deleterious effects of the adhering, abrasive, and pervasive nature of lunar dust (<20 {micro}m portion of regolith, which constitutes {approx}20 wt% of the soil). In addition, the major impact-produced glass in the lunar soil, especially agglutinitic glass (60-80 vol% of the dust), contains unique nanometer-sized metallic Fe (np-Fe{sup 0}), which may pose severe pulmonary problems for humans. The presence of the np-Fe0 imparts considerable magnetic susceptibility to the fine portion of the lunar soil, and dust mitigation techniques can be designed using these magnetic properties. The limited availability of Apollo lunar soils for ISRU research has made it necessary to produce materials that simulate this unique np-Fe{sup 0} property, for testing different dust mitigation methods using electromagnetic fields, and for toxicity studies of human respiratory and pulmonary systems, and for microwave treatment of lunar soil to produce paved roads, etc. A method for synthesizing np-Fe{sup 0} in an amorphous silica matrix is presented here. This type of specific simulantmore » can be used as an additive to other existing lunar soil simulants.« less

Microwave Sintering of Lunar Soil: Properties, Theory, and Practice

The unique properties of lunar regolith make for the extreme coupling of the soil to microwave radiation. Space weathering of lunar regolith has produced myriads of nanophase-sized Fe 0 grains set within silicate glass, especially on the surfaces of grains, but also within the abundant agglutinitic glass of the soil. It is possible to melt lunar soil ~i.e., 1,200- 1,500° C! in minutes in a normal kitchen-type 2.45 GHz microwave, almost as fast as your tea-water is heated. No lunar simulants exist to study these microwave effects; in fact, previous studies of the effects of microwave radiation on lunar simulants, MLS-1 and JSC-1, have been misleading. Using real Apollo 17 soil has demonstrated the uniqueness of the interaction of microwave radiation with the soil. The applications that can be made of the microwave treatment of lunar soil for in situ resource utilization on the Moon are unlimited.

Nanophase Fe0 in lunar soils

Back scattered electron and transmission electron imaging of lunar soil grains reveal an abundance of submicrometer-sized pure Fe0 globules that occur in the rinds of many soil grains and in the submillimeter sized vesicular glass-cemented grains called agglutinates. Grain rinds are amorphous silicates that were deposited on grains exposed at the lunar surface from transient vapors produced by hypervelocity micrometeorite impacts. Fe0 may have dissociated from Fe-compounds in a high temperature (>3000°C) vapor phase and then condensed as globules on grain surfaces. The agglutinitic glass is a quenched product of silicate melts, also produced by micrometeorite impacts on lunar soils. Reduction by solar wind hydrogen in agglutinitic melts may have produced immiscible droplets that solidified as globules. The exact mechanism of formation of such Fe0 globules in lunar soils remains unresolved.

A statistical approach to estimate the 3D size distribution of spheres from 2D size distributions

Size distribution of rigidly embedded spheres in a groundmass is usually determined from measurements of the radii of the two-dimensional (2D) circular cross sections of the spheres in random flat planes of a sample, such as in thin sections or polished slabs. Several methods have been devised to find a simple factor to convert the mean of such 2D size distributions to the actual 3D mean size of the spheres without a consensus. We derive an entirely theoretical solution based on well-established probability laws and not constrained by limitations of absolute size, which indicates that the ratio of the means of measured 2D and estimated 3D grain size distribution should be π/4 (≈.785). Actual 2D size distribution of the radii of submicron sized, pure Fe 0 globules in lunar agglutinitic glass, determined from backscattered electron images, is tested to fit the gamma size distribution model better than the log-normal model. Numerical analysis of 2D size distributions of Fe 0 globules in 9 lunar soils shows that the average mean of 2D/3D ratio is 0.84, which is very close to the theoretical value. These results converge with the ratio 0.8 that Hughes (1978) determined for millimeter-sized chondrules from empirical measurements. We recommend that a factor of 1.273 (reciprocal of 0.785) be used to convert the determined 2D mean size (radius or diameter) of a population of spheres to estimate their actual 3D size.

Systematic global mixing and melting in lunar soil evolution

We present a revised model of soil evolution constrained by the recent LSCC data for lunar soils. In addition to an observed universal increase in feldspathic components in the finer fractions of soils [Taylor et al., 2001, 2003a, 2003b], both differential melting and lateral mixing processes appear to be required during the evolution of lunar soils. We propose mare‐highland mixing of a significant glass component along with a preferential melting sequence for agglutinitic glass formation of: glass &gt; plagioclase &gt; pyroxene ≫ ilmenite.

Heterogeneous agglutinitic glass and the fusion of the finest fraction (F3) model

Abstract— Evidence in favor of the model fusion of the finest fraction (F3) for the origin of lunar agglutinitic glass has been accruing. They include (1) theoretical expectations that shock pulses should engulf and melt smaller grains more efficiently than larger grains, (2) experimental results of impact shock, albeit at lower than presumed hypervelocity impacts of micrometeorites on the lunar regolith, and (3) new analyses confirming previous results that average compositions of agglutinitic glass are biased towards that of the finest fraction of lunar soils from which they had formed. We add another reason in support of the F3 model. Finer grains of lunar soils are also much more abundant. Hence, electrostatic forces associated with the rotating terminator region bring the finest grains that are obviously much lighter than courser grains to the surface of the Moon. This further contributes to the preferential melting of the finest fraction upon micrometeoritic impacts. New backscattered electron imaging shows that agglutinitic glass is inhomogeneous at submicron scale. Composition ranges of agglutinitic glass are extreme and deviate from that of the finest fraction, even by more than an order of magnitude for some components. Additionally, we show how an ilmenite grain upon impact would produce TiO2‐rich agglutinitic glass in complete disregard to the requirements of fusion of the finest fraction. We propose an addition to the F3 model to accommodate these observations (i.e., that micrometeorite impacts indiscriminately melt the immediate target regardless of grain size or grain composition). We, therefore, suggest that (1) agglutinitic glass is the sum of (a) the melt produced by the fusion of the finest fraction of lunar soils and (b) the microvolume of the indiscriminate target, which melts at high‐shock pressures from micrometeoritic impacts, and that (2) because of the small volume of the melt and incorporating cold soil grains, the melt quenched so rapidly that it did not mix and homogenize to represent any preferential composition, for example, that of the finest fraction.

Lunar Mare Soils: Space weathering and the major effects of surface‐correlated nanophase Fe

Lunar soils form the “ground truth” for calibration and modeling of reflectance spectra for quantitative remote sensing. The Lunar Soil Characterization Consortium, a group of lunar sample and remote sensing scientists, has undertaken the extensive task of characterization of lunar soils, with respect to their mineralogical and chemical makeup. This endeavor is aimed at deciphering the effects of space weathering of soils from the Moon, and these results should apply to other airless bodies. Modal abundances and chemistries of minerals and glasses in the &lt;45 μm size fractions of nine selected mare soils have been determined, along with the bulk chemistry of each size fraction, and their IS/FeO values. These data can be addressed at http:/web.utk.edu/∼pgi/data.html. As grain size decreases, the bulk composition of each size fraction continuously changes and approaches the composition of the agglutinitic glasses. Past dogma had it that the majority of the nanophase Fe0 resides in the agglutinitic glasses. However, as grain size of a soil decreases, the percentage of the total iron present as nanophase‐sized Fe0 increases dramatically, while the agglutinitic glass content rises only slightly. This is evidence for a large contribution to the IS/FeO values from surface‐correlated nanophase Fe0, particularly in the &lt;10 μm size fraction. This surficial nanophase Fe0 is present largely as vapor‐deposited patinas on the surfaces of almost every particle of the mature soils. It is proposed that these vapor‐deposited, nanophase Fe0‐bearing patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe0.

The effects of space weathering on Apollo 17 mare soils: Petrographie and chemical characterization

Abstract— The lunar soil characterization consortium, a group of lunar‐sample and remote‐sensing scientists, has undertaken the extensive task of characterization of the finest fractions of lunar soils, with respect to their mineralogical and chemical makeup. These compositional data form the basis for integration and modeling with the reflectance spectra of these same soil fractions. This endeavor is aimed at deciphering the effects of space weathering of soils on airless bodies with quantification of the links between remotely sensed reflectance spectra and composition. A beneficial byproduct is an understanding of the complexities involved in the formation of lunar soil.Several significant findings have been documented in the study of the &lt;45 μm size fractions of selected Apollo 17 mare soils. As grain size decreases, the abundance of agglutinitic glass increases, as does the plagioclase, whereas the other minerals decrease. The composition of the agglutinitic glass is relatively constant for all size fractions, being more feldspathic than any of the bulk compositions; notably, TiO2 is substantially depleted in the agglutinitic glass. However, as grain size decreases, the bulk composition of each size fraction continuously changes, becoming more Al‐rich and Fe‐poor, and approaches the composition of the agglutinitic glasses. Between the smallest grain sizes (10–20 and &lt; 10 μm), the IS/FeO values (amount of total iron present as nanophase Fe0) increase by greater than 100% (&gt;2x), whereas the abundance of agglutinitic glass increases by only 10–15%. This is evidence for a large contribution from surface‐correlated nanophase Fe0 to the IS/FeO values, particularly in the &lt;10 μm size fraction. The surface nanophase Fe0 is present largely as vapor‐deposited patinas on the surfaces of almost every particle of the mature soils, and to a lesser degree for the immature soils (Keller et al., 1999a). It is reasoned that the vapor‐deposited patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe0.

X-Ray Digital Imaging Petrography of Lunar Mare Soils: Modal Analyses of Minerals and Glasses

It is essential that accurate modal (i.e., volume) percentages of the various mineral and glass phases in lunar soils be used for addressing and resolving the effects of space weathering upon reflectance spectra, as well as for their calibration such data are also required for evaluating the resource potential of lunar minerals for use at a lunar base. However, these data are largely lacking. Particle-counting information for lunar soils, originally obtained to study formational processes, does not provide these necessary data, including the percentages of minerals locked in multi-phase lithic fragments and fused-soil particles, such as agglutinates. We have developed a technique for modal analyses,sensu stricto, of lunar soils, using digital imaging of X-ray maps obtained with an energy-dispersive spectrometer mounted on an electron microprobe. A suite of nine soils (90 to 150 μm size fraction) from the Apollo 11, 12, 15, and 17 mare sites was used for this study. This is the first collection of such modal data on soils from all Apollo mare sites.The abundances of free-mineral fragments in the mare soils are greater for immature and submature soils than for mature soils, largely because of the formation of agglutinitic glass as maturity progresses. In considerations of resource utilization at a lunar base, the best lunar soils to use for mineral beneficiation (i.e., most free-mineral fragments) have maturities near the immature/submature boundary (Is/FeO ≅ 30), not the mature soils with their complications due to extensive agglutination. The particle data obtained from the nine mare soils confirm the generalizations for lunar soils predicted by L. A. Taylor and D. S. McKay (1992,Lunar Planet Sci. Conf. 23rd,pp. 1411–1412 [Abstract]).

Anatomy of individual agglutinates from a lunar highland soil

Abstract— We report results of our investigation of the relationship between values of Is/FeO (relative concentration of nanophase Fe0 divided by total FeO content), glass abundance, total Fe content, and degree of digestion of &lt;20 μm clasts for 22 individual agglutinates (250–1000 μm) from the mature Apollo 16 soil 61181 (Is/FeO = 82 units in the &lt;250 μm fraction). Agglutinates are important products of space weathering on the Moon, and they influence spectral observations at visible and near‐IR wavelengths. Values of Is/FeO for individual agglutinates (250–1000 μm) within this single soil span a range from 3 to 262 units which is larger than the range observed for all Apollo 16 bulk soils (∼0 to 110 units). No correlation was observed between Is/FeO and glass abundance and FeO concentrations for either agglutinitic glass or whole agglutinate particles under investigation. Our results suggest that the variation in Is/FeO for agglutinates from a single soil may be in part a consequence of natural mixing processes on the Moon that produce highly‐variable environments (with respect to surface exposure) for agglutinate formation and in part to variable kinetics of reactions in an agglutinate melt, which are influenced by a variety of factors including melt composition, temperature, impactor velocity, and quench rate. We cannot exclude but do not see evidence for other processes including addition of exotic agglutinates, micrometeoritic bombardment into compositionally‐diverse microtargets, recycling of agglutinates, preferential melting of very fine soil particles, and production of nanophase Fe0 in amorphous rims of very fine irradiated lunar grains contributing to the observed variation of Is/FeO.

Quantitative mineralogical characterization of lunar high‐Ti mare basalts and soils for oxygen production

Efficient lunar resource utilization requires accurate and quantitative evaluation of mineral and glass abundances, distribution, and extraction feasibility, especially for ilmenite. With this in mind, true modal analyses were performed on high‐Ti mare basalts and soils with X ray/backscattered electron signal digital‐imaging techniques, and these data indicate that (1) ilmenite concentrations are similar for basalts and immature‐submature soils with similar TiO2 content; (2) ilmenite liberation of crushed mare basalts and immature‐submature mare soils are comparable (i.e., both contain similar amounts of free ilmenite); and (3) because of impact melting and agglutination of primary minerals, mature mare soils contain less ilmenite (both free and attached). Modal analyses of magnetic separates of high‐Ti mare basalts and soils show that (1) ilmenite was concentrated by a factor of ≥3.3 and (2) soil ilmenite was concentrated to factors of 1.7–2.3. The lower soil ilmenite separation efficiency is attributed to Fe°‐bearing agglutinitic glass and amorphous rinds adhered to soil particles. Mass yields of magnetically generated feedstocks were generally less than 5 wt % in most cases. Calculation of oxygen yield (as released by hydrogen gas reduction of ilmenite) show that (1) beneficiated basalt will provide the most oxygen (8–10%), because of higher ilmenite concentration; (2) reduction of raw immature‐submature mare soils and basalts will produce similar amounts of lunar liquid oxygen (LLOX) (2.1–3.1%); and (3) raw Fe‐rich pyroclastic soil, 74220, will provide more oxygen (5.4%) than beneficiated high‐Ti mare soils and half that of beneficiated high‐Ti mare basalts. High‐Ti mare soils are attractive resources for lunar liquid oxygen (LLOX) production because of their unconsolidated nature, high ilmenite abundance, and widespread occurrence. Energy‐intensive excavation and comminution likely prohibits the basalt mining during early lunar occupation. Orange soils are important resources for LLOX and various volatile elements, but slower reaction kinetics and glass sintering pose potential difficulties for large‐scale operations.

Chemical variability and origin of agglutinitic glass

Seemingly pure agglutinitic glass is riddled with many submicrometer‐sized vesicles and clasts, as shown by scanning electron microscopy of ion‐etched polished grain mounts. Electron microprobe analyses of agglutinitic glasses that have to be identified with relatively low power optical microscopes are difficult and tend to be biased by very fine grained contaminants. Energy dispersive X ray analysis (EDXA) of small spots (1 μm × 1 μm) of pure agglutinitic glass, carefully selected to be free of clasts or vesicles as determined by secondary electron imagery, can overcome some of the analytical problems, We have performed over 600 EDXA of pure glass in 32 large agglutinates from five different Apollo landing sites. Analyses adjacent to or away from clasts do not show any difference in chemical composition and suggest that secondary assimilation of clasts by the primary impact melt has not been a major process affecting the final composition. The average glass composition of several agglutinates in any single soil is different from the bulk composition of the soil; it is biased toward that of the finest fraction of the soil. This is consistent with agglutination models advocating the fusion and/or incorporation of the finest soil fraction. Most of the large multigenerational agglutinates contain small glassy areas or domains that are remarkably homogeneous in chemical composition. They may represent the melt from single impact events, and their homogeneity indicates that total melting of a microtarget in lunar soils upon micrometeoritic bombardment might be the origin of agglutinitic glass. The compositions of these domains are different in different agglutinates, and even within single multigenerational agglutinates, and bear no systematic relationship to the composition of either the bulk soils or their finest fractions. It seems likely that random small targets in lunar soils (grains ∼100 μm or less) can only produce very small volumes of melt that are not representative of the soil itself. The total melting of microtargets is not contradictory to models invoking incorporation or fusion of the finest fraction because such models consider average compositions of several agglutinates with respect to the parent soils; they therefore refer to larger scale effects of agglutination. A large soil grain, if hit directly by a micrometeorite smaller than the grain itself, is likely to behave more like a solid target; a collection of very fine soil grains, if hit by a micrometeorite larger than the collection of the grains, will behave as a porous target. Because porous targets produce relatively more melt than solid targets, the average melt composition from a large number of random micrometeoritic impacts is expected to be biased to that of the finest fraction of lunar soils.

Chemical Fractionation of the Lunar Regolith by Impact Melting

Impact-produced agglutinitic glass in both lunar highland and mare soils is enriched in mafic elements, in potassium, phosphorus, and sulfur, and in most lithophile elements, whereas it is depleted in plagioclase components including europium. It is proposed that the chemical fractionation is the result of a multistage partial-melting mechanism that accompanies micrometeoroid impacts into soils. The process would be expected to occur on solar system bodies that have an impact-produced regolith.