The deep regolith of the southeastern United States has undergone rapid erosion in the last two centuries due to intensive agricultural practices, which has altered the landscape and its inherent fertility. Parent material, landscape position, and land use are important factors in controlling the mineral and elemental composition of soil profiles. Independent quantitative X-ray diffraction (QXRD) and whole-rock chemical analysis of eight weathering profiles agreed well and allow mineral reaction pathways to be constrained as particles are conveyed in the subsurface. QXRD analysis of saprolite, argillic, and soil A-horizons in the profiles highlights the imprint of bedrock on the regolith, which includes Neoproterozoic meta-tonalitic to meta-granodioritic and Paleozoic meta-granitic to biotite- and amphibolite-gneissic lithologies. Also, aeolian input slightly influenced A-horizon composition. The clay mineral assemblage is dominated by kaolinite, but profiles differ in the amount of interstratified clay minerals, halloysite, hematite, goethite, and gibbsite. Rare-earth element totals vary between 30 and 1048 ppm and are generally correlated positively with clay and clay mineral content. Eu and Ce anomalies reflect parent rocks and subsequent hydrolysis and redox history, with trends depending upon landscape position and clay content in the weathering profile. Weathering profiles on a high-order interfluve and those that were actively cultivated have thick argillic horizons (as defined by clay mineral abundance) and are depleted in alkali and alkaline-earth elements. Profiles proximally developed on old-field pine and never-cultivated hardwood forest land do not show large differences in mineral composition trends, whereas profiles on old-field sites with ongoing cultivation exhibit assemblages enriched in clay minerals and (oxyhydr)oxides. Old-field pine sites that were historically eroded by previous cultivation tend to have shallower and thinner argillic horizons, which may well impact critical-zone processes involving gas and water fluxes. This study highlights that mineral compositions of deep regolith, saprolite, and shallow soil horizons are dependent on local geomorphology (i.e. watershed- and hillshed-orders). Quantifying soil and regolith compositional trends across the landscape is a prerequisite for determining rates of chemical and physical erosion on human and geologic time scales.

Under closed geochemical conditions, the weathering of a serpentinite rock composed of serpentine (70–85%) and magnesian chlorite (10–15%) associated with magnetite and chromite leads to the complete replacement of serpentine and chlorite by 2:1 layer silicates and produces new Fe oxides. The serpentine minerals crystallize under different habits issued from the serpentinization processes: mesh and hourglass pseudomorphic textures were formed from olivine, and thin-bladed pseudomorphic textures from pyroxene and amphibole crystals. Serpentine veins crosscut the whole rock with locally non-pseudomorphic interpenetrating and interlocking serpentines.

Specific weathering microsystem habits with specific clay mineral crystallizations originate from these different habits: a poorly aluminous saponite in thin-bladed textures, two Fe-rich montmorillonites in mesh and hourglass (MH) textures, and in veins (V) which differentiate on Al, Mg and Fe contents. Magnesian chlorites, isolated from serpentine by hand-picking under a stereomicroscope, are found to weather to trioctahedral vermiculite. Magnetite and chromite extracted from the bulk samples are replaced by newly formed Fe oxides, maghemite, goethite and hematite, which give way to specific Fe accumulation habits in the regolith zone of the weathering profile.

Halloysite is a 1:1 dioctahedral clay mineral that has been studied widely for applications in nanotechnology and as a mineral exploration guide for recognizing regolith-hosted heavy rare earth element (HREE) deposits. In Brazil, pegmatites from the state of Rio de Janeiro have been catalogued, but their potential to host halloysite deposits has never been studied. After a mineral exploration programme, one pegmatite with considerable halloysite contents and economic potential was discovered. This study reports the mineralogical and chemical characterization of the halloysite of this pegmatite and evaluates the possibility of clay-adsorbed HREE deposits, like that in the Zudong (China) regolith-hosted HREE deposit. Seven samples were collected in horizontal channels. Bulk samples and clay fractions (<2 μm) were analysed by quantitative mineral analysis (X-ray diffraction/Rietveld method), chemical analysis (major elements by X-ray fluorescence and Y, U, Th and rare earth elements by inductively coupled plasma mass spectrometry), scanning electron microscopy, Fourier-transform infrared spectroscopy, particle-size analysis, nitrogen physisorption and cation-exchange capacity. Mixed polygonal/cylindrical halloysite-7Å in concentrations between 6.3 and 35.4 wt.% in bulk samples and between 58.0 and 89.8 wt.% in the clay fractions were identified in the pegmatite. The clay fractions presented an average chemical composition of 45.46 wt.% SiO2, 36.10 wt.% Al2O3, 14.62 wt.% loss on ignition and 1.04 wt.% Fe2O3, as well as technological properties close to those observed in world-class halloysite deposits such as Dragon Mine (USA) and Matauri Bay (New Zealand). The clay minerals did not present significant HREE contents.

The surfaces of most of the atmosphereless solar system bodies are referred to as regolith or layers of usually loosely connected fragmentary debris, produced by meteorite impacts. Measurement of light scattered from such surfaces provide information about the composition and structure of the surface. In the present work, the effect of porosity and particle size, on reflectance is studied for regolith like samples. For modelling the experimental data Hapke 2008 is used and found to be in good agreement with laboratory data. From the present study, it can be concluded that the physical properties of a regolith, such as porosity, particle size etc are effectively represented by albedo.

C. R. M. Butt comments: In their recent paper, Lacinska & Styles (2013) described in detail the geological setting, mineralogy and petrology of ‘silicified serpentinite’ in the Hajar Mountains, United Arab Emirates (UAE). They note that their ‘silicified serpentinite’ is essentially the same unit as ‘birbirite’ and other informally named quartz-rich outcrops that overlie ultramafic rocks in this and other regions. Some authors have suggested such silicification to be hydrothermal in origin, but it is now generally accepted to be due to weathering. Lacinska & Styles (2013) concluded that ‘silicified serpentinite’ is a silcrete; this is correct, but it is a purely descriptive term and, without qualification, has no specific genetic implications other than being silica-cemented regolith (Butt & Zeegers, 1992; Eggleton, 2001). The materials illustrated are more correctly termed a silicified saprolite as they preserve the fabric of the protolith.

Synthetic geomicrobiology is a potentially new branch of synthetic biology that seeks to achieve improvements in microbe–mineral interactions for practical applications. In this paper, laboratory and field data are provided on three geomicrobiology challenges in space: (1) soil formation from extraterrestrial regolith by biological rock weathering and/or the use of regolith as life support system feedstock, (2) biological extraction of economically important elements from rocks (biomining) and (3) biological solidification of surfaces and dust control on other planetary surfaces. The use of synthetic or engineered organisms in these three applications is discussed. These three examples are used to extract general common principles that might be applied to the design of organisms used in synthetic geomicrobiology.

The issue of the variation of the surface ultraviolet (UV) environment on Mars was investigated with particular emphasis being placed on the interpretation of data in a biological context. A UV model has been developed to yield the surface UV irradiance at any time and place over the Martian year. Seasonal and diurnal variations were calculated and dose rates evaluated. Biological interpretation of UV doses is performed through the calculation of DNA damage effects upon phage T7 and Uracil, used as examples for biological dosimeters. A solar UV ‘hotspot’ was revealed towards perihelion in the southern hemisphere, with a significant damaging effect upon these species. Diurnal profiles of UV irradiance are also seen to vary markedly between aphelion and perihelion. The effect of UV dose is also discussed in terms of the chemical environment of the Martian regolith, since UV irradiance can reach high enough levels so as to have a significant effect upon the soil chemistry. We show, by assuming that H2O is the main source of hydrogen in the Martian atmosphere, that the stoichiometrically desirable ratio of 2:1 for atmospheric H and O loss rates to space are not maintained and at present the ratio is about 20:1. A large planetary oxygen surface sink is therefore necessary, in contrast with escape to space. This surface oxygen sink has important implications for the oxidation potential and the toxicology of the Martian soil. UV-induced adsorption of {\rm O}_{2}^{-} super-radicals plays an important role in the oxidative environment of the Martian surface, and the biologically damaging areas found in this study are also shown to be regions of high subsurface oxidation. Furthermore, we briefly cover the astrobiological implications for landing sites that are planned for future Mars missions.