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Potassium metasomatism is a common geologic process occurring in a range of environments on Earth. K‐metasomatism can dramatically change the chemistry of large volumes of crust resulting in formation of abundant K‐rich feldspar while preserving original rock textures. Remote sensing of Mars has revealed the presence of rare but striking felspar‐rich terrains as well as K‐rich phyllosilicates such as illite or muscovite. Furthermore, the Curiosity Rover has detected evidence for K‐enrichment in rocks at Gale Crater. In this study, we use spectroscopic and remote sensing analyses of K‐metasomatized rocks on Earth as a comparison to K‐rich and/or possible granitic rocks on Mars. The results suggest that evidence for K‐feldspar rich, quartz‐bearing, and illite‐bearing rocks on Mars is consistent with K‐metasomatism. K‐rich rocks in Gale Crater have themselves likely not been metasomatized, but the abundant feldspar within them might have been derived from metasomatized crust. Plain Language Summary: Mars is a volcanic planet with a surface dominated by basalt and sedimentary rocks derived from basalt, similar to oceanic crust on Earth (though Mars' crust is thicker). For decades, Mars has been considered a "one plate planet" where plate tectonics never occurred and where continental type rocks never formed. Recent observations have, however, revealed enticing evidence that felsic "continental type" rocks containing K‐feldspar and quartz are present. Such rock units were recognized in Gale Crater based on their high potassium abundances. Granite‐type rocks detected from orbit using infrared data would also presumably be rich in potassium. However, rocks of the same composition can be made through aqueous alteration rather than from magma in the crust. This study shows a striking example of potassium enrichment and mineralogical changes in basaltic rocks on Earth caused entirely by low‐temperature hydrothermal brine in the crust. We use analogous remote sensing measurements to study these potassic rocks on Earth and compare the results to Mars. It appears that at least some of the felsic rock compositions on Mars were caused by metasomatism. Key Points: Subsurface brines can deposit significant potassium in volcanic rocks while preserving original texturesSpectral signatures of K‐metasomatism include K‐feldspar and illiteFelsic compositions on Mars might occur because of low‐temperate alteration by subsurface brines [ABSTRACT FROM AUTHOR]

This study investigated soil clay mineralogy of mangrove forests along the Brazilian coast in eight regions of different environmental characteristics, with a focus on the crystallochemical features and genesis of 2:1 phyllosilicates. Samples from two different depths (0-30 and 60-90 cm) and two clay size fractions (<2 and <0.2 mm) were studied. The analytical tools used were x-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS). A complex assemblage was found in both fractions, including high proportions of kaolinite, illite (Fe-illite/glauconite), and smectite (Fe-beidellite, Fe-montmorillonite), minor amounts of gibbsite and quartz, and traces of K-feldspar, halloysite, and amphibole crystals (TEM-EDS data). Evidence of illite-smectite and possibly kaolinite-smectite mixed-layer phases was found in the <0.2-mm fraction. An authigenic transformation process likely taking place, from kaolinite to Fe-illite/glauconite, through transitory kaolinite- smectite and illite-smectite phases. The reaction is triggered by high Fe activity in solution generated by dissolution of Fe oxides and pyrite present in the sediment, at approximately neutral pH and high salt content in the water. Smectite illitization takes place by substitution of Al3+ for Si4+ in the tetrahedral sheet, of Fe and Mg2+ for Al3+ in the octahedral sheet, and progressive Fe reduction, all of which produces an increase in the layer charge and K uptake. The sequential nature of the transformation (kaolinite-smectite-illite) suggests a solid-state transformation process. Two unusual kaolinite-rich mangroves were found, indicating low reactivity or high deposition rate of continental sediments of soils derived from the Barreiras Group, dominated by kaolinite. [ABSTRACT FROM AUTHOR]

Nuclear waste from thermal plants poses a lasting risk to the biosphere because of its long radioactive life. The planned definitive storage place for it is in deeply buried repositories. Such repositories would need to be both impermeable to water, and plastic during deformation, in order to avoid the formation of cracks that may allow water in. One of the clay minerals, smectite, has these two properties and is an ideal candidate as a sealing material or even host rock for nuclear waste repositories. The chemical stability of smectite in the repository environment is sufficient to maintain good sealing properties during the active life of the relevant radionuclides. [ABSTRACT FROM AUTHOR]