Samples of asteroid (101955) Bennu delivered by the OSIRIS-REx mission offer the opportunity to study pristine planetary materials unchanged by exposure to the terrestrial environment. Here we use a combination of X-ray diffraction and various electron microscopy techniques to explore the detailed mineralogy of Bennu samples and determine the alteration history of the planetesimal protolith from which they originated. The samples consist largely of hydrated sheet-silicate minerals, namely nanoscale serpentine and saponite of varied grain size, which are decorated with micro- to nanoscale Fe-sulfides, magnetite and carbonates. We observe sheet silicates parallel and normal to sulfide surfaces and as inclusions in sulfides; sulfur-rich veins transecting the sheet-silicate matrix; zoned carbonates and phosphates and sulfide and magnetite grains exhibiting embayment. The mineralogical evidence indicates alteration of accreted minerals by a fluid that evolved with time, leading to etching, dissolution and reprecipitation. Sulfide compositions indicate alteration at ~25 °C, similar to conditions inferred for asteroid (162173) Ryugu and Ivuna-type (CI) chondrite meteorites. The fluid probably evolved from neutral to alkaline, culminating with the precipitation of highly soluble salts. We conclude that Bennu’s protolith comprised mainly nanometre to micrometre silicates, with fewer chondrules and calcium–aluminium-rich inclusions than those of most chondrite groups.

The Jadar deposit (Serbia) is a unique end member of the volcano-sedimentary Li deposit class, where the main ore mineral is jadarite (LiNaSiB3O7(OH)), to date only recorded at the Jadar locality. We provide the first account of the features of the Jadar deposit based on the study of drill hole material, complemented by petrographic analysis, whole-rock, and isotopic geochemistry. The Li-B mineralization is hosted by sediments interlayered with tuffs that were deposited in a fault-bounded lacustrine basin. Mineralization processes initially involved alteration of volcanic glass through near-neutral and heated meteoric fluids, which resulted in the formation of hectorite. Lithium clays were later overprinted by higher-pH brines, which underwent significant evaporation, combined with conditions of Na+ and SiO2(aq) saturation, resulting in formation of siliceous gels that were the precursors to jadarite formation and extensive zeolitization of the intrabasinal sediments. Formation of jadarite occurred in two stages: (1) early diagenesis accompanied by zeolitization and precipitation of primary dolomite, which resulted in the sediment-hosted Li mineralization, (2) late remobilization of Li(B-Na)–saturated fluids, leading to formation of epigenetic jadarite-albite infills of fractured rocks. Isotopic geochemistry combined with mineral chemistry proxies indicate that early mineralizing processes were largely dominated by diagenetic fluids that experienced a thermal influence from the coeval cooling of the Miocene S-type granite member of the nearby Cer Mountain Complex. Late thermal maturation of organic matter then resulted in acidic fluids that dissolved the early ore assemblage and led to formation of late diagenetic lithiophosphate and low-Na borates.

Bryozoans represent an extremely diversified phylum in terms of biomineralogy. The mineral composition of the bryozoan skeleton is most likely controlled by the organism itself and to some degree by the ambient environmental conditions (e.g., pH, Mg/Ca ratio, temperature). The purpose of this study is to describe the mineral composition of nearly one-quarter of all Antarctic bryozoan species known to date and to discuss, locally and in a global context, the potential factors that control mineralization in bryozoan skeletons. Collections of bryozoans were gathered by divers and with van Veen grabs (0.1 m²) from depths of 6–300 m at King George Island, Antarctica, during the summer seasons of 1985, 2007, and 2011. An x-ray diffraction analysis of the skeletons indicated that all analyzed Antarctic bryozoans (256 individuals, 71 taxa) precipitated monomineral calcite skeletons. The magnesium content ranged from 0.2 to 10.1 mol% MgCO₃. Most of the studied species (76%) consisted of intermediate-magnesium calcite. Yet extreme variations in mol% MgCO₃ were found between individuals of the same species as well as between species and populations. The high variability found between specimens of the same species suggests that the mol% MgCO₃ could be determined by the biology or physiology of an individual. The magnesium content was equally distributed in each of three growth forms (e.g., erect flexible, erect rigid, and encrusting). The mean mol% MgCO₃ in Antarctic species was significantly lower than that in bryozoans from Mediterranean, Chile, and New Zealand but similar to that in Arctic bryozoans. This finding confirms the latitudinal-pattern hypothesis of a decreased Mg content toward polar regions.