INTRODUCTION The primary goal of NASA’s Curiosity rover mission is to explore and quantitatively assess a local region on Mars’ surface as a potential habitat for past or present life A necessary component of that assessment involves an investigation of the surface chemical conditions and paleoclimate of ancient Mars Gale crater was selected as the landing site for Curiosity; it hosts a ~5-km-tall mountain of layered sedimentary rock The rocks of Mount Sharp preserve a long-duration record of martian environmental conditions Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment that was sustained for tens of thousands to tens of millions of years by rivers draining into the crater RATIONALE We seek to constrain the chemical environment within the lake in Gale crater, as well as short- and long-term climate variations in and around Gale crater We focus on fine-grained sedimentary rocks that carry information about sediment provenance, the environment of deposition, the conversion of sediment to rock during burial (ie, lithification), and the chemical conditions of later modification (ie, diagenesis) These were investigated during the first 1300 martian solar days (sols) of rover operations in Gale crater using bulk geochemical and mineralogical analysis techniques, combined with high-resolution color imagery at a variety of scales RESULTS Two mudstone units have been recognized, both deposited in lakes: the Sheepbed member of the Yellowknife Bay formation, an older set of strata defining the base of the stratigraphic section; and the Murray formation, of relatively younger age and positioned higher in the stratigraphic section The chemical index of alteration (CIA) paleoclimate proxy increases by up to ~10 to 20 CIA units (expressed in %) from the Sheepbed member to the Murray formation On the basis of mineralogy, geochemistry, textural properties, and stratigraphic relationships, the Murray formation can be subdivided into two sedimentary associations, or facies: the hematite-phyllosilicate (HP) facies and the magnetite-silica (MS) facies The HP facies is characterized by abundant Fe 3+ oxides accompanied by phyllosilicates, as well as indications of Mn oxidation and trace metal concentration These properties are consistent with deposition in an oxidizing environment The MS facies is recognized by a near-complete absence of pure Fe 3+ minerals, and high concentrations of silica accompanied by magnetite, consistent with deposition in an anoxic environment Both facies were affected by a saline overprint after burial and lithification CONCLUSION The observed variations in CIA are consistent with modest short-term fluctuations in the ancient climate between cold, dry conditions and relatively warmer, wetter conditions These changes occurred during the deposition of lake-bed mudstones in an environment that was conducive to the presence of a long-lived lake in Gale crater We propose that the distinct properties of the two Murray facies were developed as a result of (i) fractionation of river-borne detritus into coarser, denser materials in shallow water close to shore and finer, lower density materials offshore in deeper water as a result of deceleration of river flow as it entered the lake; and (ii) redox stratification of the lake water body, caused by depth-dependent variations in the concentration of atmospheric oxidants and dissolved, groundwater-derived solutes, resulting in oxidizing conditions in shallow water and anoxia in deeper water The addition of saline minerals during a later phase of brine migration through the section records longer-term changes in martian climate at Gale crater, perhaps driven by global atmospheric escape processes The recognition of redox stratification in the lake in Gale crater adds new detail to our understanding of ancient martian aquatic environments Previously reported detections of organic carbon compounds, nitrogen, phosphate minerals, and Fe and S minerals in a variety of redox states, combined with the evidence presented here for relatively stable climate conditions and gradients in fluid oxidation state, provide compelling evidence that all of the physical, chemical, and energetic conditions necessary to establish a habitable environment were present on Mars between ~38 billion and 31 billion years ago

https://science.sciencemag.org/content/sci/356/6341/eaah6849.full.pdf

Redox stratification of an ancient lake in Gale crater, Mars

Cristobalite and tridymite are distinct forms of crystalline silica which, along with quartz, are encountered in industrial operations and industrial products. Because the International Agency for Research on Cancer has designated “crystalline silica” as an IARC Group 2A (probable carcinogen) and quartz and cristobalite as a Group 1 (carcinogen), it is important to properly identify and quantify the silica phase in all materials used in production and encountered in products. Opal is a form of hydrated silica which is also encountered in industry. Although some forms of opal mimic cristobalite and tridymite, they are not truly crystalline. The term “silica” in the industrial sense is used to mean any material whose composition is SiO2 whether it is crystalline or noncrystalline. Some people also consider silica to include hydrated SiO2. There are many forms of SiO2 which have both long-range and short-range order and are recognized as crystalline phases among which are quartz, cristobalite, and tridymite. The hydrated silicas, on the other hand, pose an enigma. Only a few forms show sufficient long-range and short-range order to be considered crystalline. The mineral silhydrite is an example. Opal in all its forms lacks sufficient order to be considered crystalline. Even opal-C, which produces a X-ray pattern similar to the diffraction pattern of cristobalite, lacks not only sufficient order to be considered crystalline but also contains water in the structural make-up. This paper discusses a classification and nomenclature for these forms which is critical to proper regulation. It also reviews the recent literature on tridymite, cristobalite, and opal, and provides an extensive bibliography. Modern studies have shown that opal-A is disordered, but opal-CT and opal-C contain ordered domains that mimic stacked sequences of cristobalite and tridymite sheets such that X-ray patterns show features similar to the crystalline cristobalite and tridymite. There is debate on whether the ordered regions have lost the water that characterizes the opals. In fact, heating studies have shown that all opals show changes on heating characteristic of materials that lose water in the process. The TEM evidence showing domains in the range 10–30 nm in a matrix of disordered opal suggest that the proper term for this system is paracrystalline analogous to inorganic and organic polymers.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0885715600009696

Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography

The status of research on the mineralogy of meteorites about 1876 is compared with that of 1976. The ratio of iron meteorites to other meteorites in collections was then, and is now, about 1:2. However, total accumulated mass of iron meteorites exceeds that of stones by a factor 15. Forty-one well-established minerals in iron meteorites are discussed utilizing twelve tables, presenting the elements, carbides, nitrides, phosphides, sulphides, oxides, phosphates and silicates. The detailed compositions of kamacite, taenite and troilite are discussed, and the ranges in composition of several other minerals are given. Unequilibrated phases, such as $\alpha _{2}$-iron, and shock-transformation products, such as diamond, are briefly mentioned. Finally, the products of terrestrial weathering are discussed, and it is shown that lawrencite in iron meteorites is a dubious phenomenon and not a cosmic mineral.

https://www.solosequenosenada.com/meteorito/docs/meteoritos/1997-The-Mineralogy-of-Iron-Meteteorites-Buchwald-1977.pdf

The Mineralogy of Iron Meteorites

[1] The possible existence of silica-rich rocks on Mars requires consideration of all of the candidate forms of silica that might exist there We synthesized coesite and cristobalite, and obtained natural samples of tridymite, opal-A, and opal-CT The thermal emission spectra of these silica phases were analyzed and related to their crystal structure and crystallinity Structural differences between the polymorphs result in unique spectral character and the degree and type of crystal ordering in opal-CT results in cristobalite- or tridymite-like spectral features These spectra will be useful for analysis of Thermal Emission Spectrometer (TES) data from Mars While silica polymorphs are not modeled in deconvolution of the two main TES surface-type spectra, this does not preclude the possible detection of local abundances of these minerals on Mars Spectral detection of silica phases on Mars would have implications for the volcanic and alteration history of the planet

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GL018354

Thermal emission spectroscopy of the silica polymorphs and considerations for remote sensing of Mars

A non-brecciated, relatively coarsely crystalline eucrite was found in 1940 near Moama, southern New South Wales, Australia. The texture and mineralogy (plagioclase 49.1 volume %, pyroxene 48.9%, tridymite 0.72%, chromite 1.0%, troilite 0.24% and metallic iron 0.14%) are characteristic of an adcumulate formed in a terrestrial layered igneous intrusion. Its composition and texture most closely resemble the eucrite Serra de Mage. Moama and Serra de Mage are the best approximations to the pyroxeneplagioclase cumulates which McCarthy et al. (1973) propose have separated from a primary magma to produce the eucrite group of achondrites. Primary (coarse) and secondary (fine) augitic exsolution lamellae in the host hypersthene grains indicate a cooling history for the Moama eucrite similar to that of mafic rocks in slowly cooled, large, layered terrestrial intrusions such as Bushveld.

Bhian Mason

Papers

Lunar Tridymite and Cristobalite