Masahiro Kayama, et al., report that silica polymorphs, such as quartz, tridymite, cristobalite, coesite, stishovite, seifertite, baddeleyite-type SiO2, high-pressure silica glass, moganite, and opal, have been found in lunar and/or martian rocks by macro-microanalyses of the samples and remote-sensing observations on the celestial bodies. Because each silica polymorph is stable or metastable at different pressure and temperature conditions, its appearance is variable depending on the occurrence of the lunar and martian rocks. In other words, types of silica polymorphs provide valuable information on the igneous process (e.g., crystallization temperature and cooling rate), shock metamorphism (e.g., shock pressure and temperature), and hydrothermal fluid activity (e.g., pH and water content), implying their importance in planetary science.
This article focused on reviewing and summarizing the representative and important investigations of lunar and martian silica from the viewpoints of its discovery from lunar and martian materials, the formation processes, the implications for planetary science, and the future prospects in the field of “micro-mineralogy”. Silica is one of the constituent minerals in the Earth’s crustal rocks, and is one of the minor minerals that is often distributed in extraterrestrial materials such as lunar and martian meteorites,
returned samples (Apollo and Luna collections), and the parent bodies (e.g., the Moon and Mars). Over the past ten years, various types of silica polymorphs (e.g., quartz, tridymite, cristobalite, moganite, coesite, stishovite, seifertite, and baddeleyite-type SiO2) and non-crystalline silica (high-pressure (HP) silica glass and opal) have been reported in lunar and martian rocks using microanalyses and studying the celestial bodies through remote-sensing observations, although a lack of the diversity and wide distribution was believed before. For example, several previous investigations have supported the hypothesis that few high-pressure phases are included in lunar meteorites, despite the existence of many craters and regoliths, because a part of them (e.g., the high-pressure SiO2 phases) may be volatilized during impact collisions under the high vacuum condition of the Moon. However, recent studies have revealed that coesite, stishovite, and seifertite lie in several lunar meteorites and an Apollo collection. Hydrothermal silica was believed to be absent from Minerals 2018, 8, 267; doi:10.3390/min8070267 www.mdpi.com/journal/minerals Minerals 2018, 8, 267 2 of 31
extraterrestrial materials, but moganite and opal-A, originating from the fluid activity of the parent bodies, have recently been discovered from lunar and martian meteorite.
Opaline silica deposits have also been observed in association with martian volcanic materials by the Mars rover “Spirit and Opportunity”, In addition, silica nanoparticles were detected in Enceladus by the Cassini spacecraft. Of course, for decades, many researchers have reported igneous silica such as quartz, tridymite, and cristobalite in various types of extraterrestrial materials, especially in lunar samples (lunar meteorites and Apollo and Luna collections) and martian meteorites [11,12]. From the latest viewpoint of planetary science, there seems to be a wide variety of silica polymorphs in the solar system.
According to the pressure–temperature phase diagram for SiO2, Quartz, tridymite, and cristobalite, as well as coesite and stishovite, are stable at different pressure and temperature regions. The high-temperature and high-pressure SiO2 phases are quenchable down to ambient conditions, and thus exist in extraterrestrial materials via rapid cooling processes during magmatic eruption, decompression after impact events, or aqueous fluid activity. For example, seifertite is thermodynamically stable at more than ~100 GPa, which is not realistic for an impact event on the Moon and Mars. However, seifertite can be converted as a metastable phase from cristobalite at ~11 GPa, and can consequently be quenched down to an ambient condition. As a result of this process, seifertite remains present in some of the lunar and martian meteorites that have experienced the host bodies. Thus, the occurrence of these high-temperature and high-pressure SiO2 phases allows us to set constraints for the crystallization temperature, cooling rate, and shock pressure and temperature, which provide valuable information on magma eruption and the sizes of the crater and impactor that collided on the parent bodies.
In addition, the metastable SiO2 phase moganite has been found to be distributed in a lunar meteorite, which is thought to have precipitated from high-pressure alkaline fluid activity on the Moon. Based on the moganite precipitation model, the bulk content of H2O ice in the Moon’s subsurface can be theoretically calculated. Thus, igneous, shock metamorphic, and hydrothermal events of various celestial bodies have been interpreted by a mineralogical description of such extraterrestrial silica.
Minerals 2018, 8, x FOR PEER REVIEW 2 of 31 believed to be absent from extraterrestrial materials, but moganite and opal-A, originating from the fluid activity of the parent bodies, have recently been discovered from lunar and martian meteorites. Opaline silica deposits have also been observed in association with martian volcanic materials by the Mars rover “Spirit and Opportunity”. In addition, silica nanoparticles were detected in Enceladus by the Cassini spacecraft. Of course, for decades, many researchers have reported igneous silica such as quartz, tridymite, and cristobalite in various types of extraterrestrial materials, especially in lunar samples (lunar meteorites and Apollo and Luna collections) and martian meteorites.
For additional information on silica present on the Moon: http://i2mportal.com/resource_detail.php?resource_id=7743
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