The surface of the Moon is a critical boundary that shapes our understanding of the Moon as a whole. All geologic mapping and remote sensing techniques utilize only the outermost portion of the Moon. Before leaving the Moon for study in our laboratories, all lunar samples that have been studied existed at or very near the surface. With the exception of the deeply probing geophysical techniques, our understanding of the interior of the Moon is derived from surficial, but not superficial, information, coupled with boundary of the lunar crust, it is the lower boundary layer of the tenuous lunar atmosphere and constitutes both a source and a sink for atmospheric gases. The surface is also where the Moon interacts with the space environment, causing changes in the physical nature of lunar materials, and provides a laboratory for the study of processes that occur on all airless bodies.

The data obtained remotely by the Galileo, Clementine, and Lunar Prospector missions, as well as data derived from lunar meteorites, have resulted in major changes to our understanding of global distributions of chemistry and rocks. This chapter summarizes the current understanding of this critical interface, the surface of the Moon, in its role as the lower boundary of the lunar atmosphere, the upper boundary of the crust, and the window through which we view, through remote sensing, the composition of the crust and the history of the Moon. In this post-Lunar Prospector time, the view of the Moon has changed, lending new perspectives to lunar samples and lunar processes. But the New View will likely remain in flux as we continue to digest the results from these recent space missions and move forward to a new era of lunar exploration.

Despite the freshness of our perspective, this is an important moment to capture, …

Understanding the Lunar Surface and Space-Moon Interactions

Within the last 15 years, approximately 80 high-quality, three-dimensiona l structure re­ finements of terrestrial, lunar, and meteoritic pyroxenes were published. The majority of the refinements involve Cl/c clinopyroxenes and Pbca orthopyroxenes, but P2,/c structures are also well represented, Few data are available on the Pbcn pyroxenes that constitute the fourth major structure type. The topology of the four space groups can be described with idealized models composed of tetrahedral-octa hedral-tetrahedr al "I-beam" units that lie parallel to [001]. The different symmetries are a result of different stacking sequences of the octahedral layers and/or of symmetrically-di stinct tetrahedral chains in adjacent layers. In all of the py­ roxenes refined, the Ml cation site is coordinated by six oxygens arranged in a regular pseudo-octahedra l configuration. The M2 site is irregularly coordinated by six, seven, or eight oxygens. The M2 coordination depends upon the size of the cation occupying the site: higher coordination numbers are usually associated with larger cations. maximum devia­ tion of the tetrahedral chains from an extended configuration (03-03-03 = 180°) occurs in the B chains of several Pbca structures where 03-03-03 � 136 o. The A tetrahedral chains in most P21/c structures are S-rotated, but all other chains are 0-rotated. Studies of pyroxene structures at elevated temperatures and pressures revealed that the cation polyhedra expand and compress differentially. The high temperature studies documented a P21/c !:::+ Cl/c tran­ sition in the Fe-Mg pyroxenes, and showed that the temperature of the transition decreases with increasing ferrosilite content. In addition, these studies provided further insight into the miscibility between the high-calcium and low-calcium pyroxenes and produced cell parame­ ter data that are basic to geothermometry studies involving exsolution lamellae. Site occu­ pancy refinements confirmed the preference of the Fe2+ for the larger, more distorted M2 site in the Fe-Mg pyroxenes. Cations in synthetic pyroxenes show a preference of Mn > Zn > Fe2+ >Co> Mg for the M2 site. T-0 distances in the Pbca orthopyroxenes indicate that Al concentrates in the TB tetrahedron. Examination of the chemistry of 175 naturally-occurr ing pyroxenes from a variety of lith­ ologies confirms complete solid solution between diopside and hedenbergite and extensive solid solution between enstatite and ferrosilite under crustal P-T conditions. In this limited set of samples the number of Ca atoms per formula unit does not exceed 1.0, which is consis­ tent with its occurrence only in the M2 site. The range in total AI between 0 and 1.0 is smaller than expected, and the maximum amount of Iv Al substitution is 55% of the T site occupancy. Na, when present as a jadeite or acmite component, is responsible for the highest non-quadri­ lateral contents of the pyroxenes examined. The most important substitutional couples in ter­ restrial Fe-Mg pyroxenes and augites are viFe3+ _,v AI and IvAI- v'Al. Detailed statistical analysis of 1200 high-quality pyroxene analyses from 11 planetary basalt suites revealed that the v'Ti-'v AI couple is one of the two most important couples for essentially all suites consid­ ered. Fe3+ is important in all of the terrestrial suites, but is virtually absent in the lunar and

Structural and chemical variations in pyroxenes

The paper attempts a synthesis of the major-element chemistry, petrography, mineral chemistry, and crystal chemistry of the mare basalts returned by Apollo and Luna missions A classification of the mare basalts based on major-element chemistry is given, and textural sequences within each major-element group are identified The mineral chemistry and crystal chemistry of each mineral group are considered within the framework of the major-element groups and the textural sequences The various classes of models for the origin of the mare basalts and the nature of their source regions are discussed in the context of the major- and trace-element chemistries and experimental investigations

Mare basalts: Crystal chemistry, mineralogy, and petrology

Ion-thinned samples of lunar rock 12052 have been examined in the electron microscope. Exsolution textures have been observed in the pyroxene on a finer scale than those found in Apollo 11 rocks, indicating a faster cooling rate. The early stages of the reaction involve modulations on both (001) and (100) which suggest that decomposition may occur by a spinodal mechanism. A two-stage exsolution structure in the augite rim of a phenocryst is consistent with a sudden increase in the cooling rate—possibly when the magma was extruded onto the surface of the moon.

An electron microscopic study of a lunar pyroxene

Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa 032 (NWA 032), an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole-rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro-faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole-rock oxygen isotopic composition and ratios of Fe:Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe-metal, troilite, and ilmenite; zoning to extremely Fe-rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare-earth element pattern with a well-defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light rare-earth elements and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in rare-earth element patterns, but have higher concentrations of Al2O3. Ar-Ar step release results are complex, but yield a whole-rock age of ca. 2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite-spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/hr based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe-Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe-rich rims on the phenocrysts formed during rapid crystallization (cooling rates ~ 20 to 60 °C/hr) after eruption.

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C.T. Prewitt

Papers

Apollo 12 clinopyroxenes: Chemical trends

Cummingtonite: A reversible, nonquenchable transition from P21/m to C2/m symmetry

Apollo 12 clinopyroxenes - Exsolution and epitaxy

Apollo 12 clinopyroxenes: Subsolidus relations.