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Manganese (Mn) oxides have been prevalent on Earth since before the Great Oxidation Event and the Mn cycle is one of the most important biogeochemical processes on the Earth's surface. In sunlit natural environments, the photochemistry of Mn oxides has been discovered to enable solar energy harvesting and conversion in both geological and biological systems. One of the most widespread Mn oxides is birnessite, which is a semiconducting layered mineral that actively drives Mn photochemical cycling in Nature. The oxygen-evolving centre in biological photosystem II (PSII) is also a Mn-cluster of Mn4CaO5, which transforms into a birnessite-like structure during the photocatalytic oxygen evolution process. This phenomenon draws the potential parallel of Mn-functioned photoreactions between the organic and inorganic world. The Mn photoredox cycling involves both the photo-oxidation of Mn(II) and the photoreductive dissolution of Mn(IV/III) oxides. In Nature, the occurrence of Mn(IV/III) photoreduction is usually accompanied with the oxidative degradation of natural organics. For Mn(II) oxidation into Mn oxides, mechanisms of biological catalysis mediated by microorganisms (such as Pseudomonas putida and Bacillus species) and abiotic photoreactions by semiconducting minerals or reactive oxygen species have both been proposed. In particular, anaerobic Mn(II) photo-oxidation processes have been demonstrated experimentally, which shed light on Mn oxide emergence before atmospheric oxygenation on Earth. This review provides a comprehensive and up-to-date elaboration of Mn oxide photoredox cycling in Nature, and gives brand-new insight into the photochemical properties of semiconducting Mn oxides widespread on the Earth's surface. [ABSTRACT FROM AUTHOR]

As a common mineral phase on Earth and Martian regolith, natural rutile was reported as a potential candidate for use as a Fenton catalyst in this study. The influences of Fe and V in various chemical states on the generation of reactive oxygen species (ROSs) and the catalytic activity of rutile were examined. A series of rutile samples with various surface and bulk states of Fe and V were obtained initially by hydrogen annealing of natural rutile at ~773–1173 K. X-ray diffraction, electron paramagnetic resonance spectra, and X-ray photoelectron spectroscopy demonstrated that the atomic fractions of Fe(III) and V(V) decreased sharply with increasing temperature, along with the accumulation of surface Fe(II) and bulk V(III). All as-prepared materials showed enhanced Fenton degradation efficiency on methylene blue (MB) compared with P25-TiO2, and the treated samples exhibited up to 3.5-fold improvement in efficiency at pH 3 compared to the untreated sample. The improved efficiency was attributed mainly to Fenton catalysis involving Fe(II) and V(III). The dissolved Fe2+ played a crucial role in the homogeneous Fenton reaction, while the bound V(III) favored adsorption primarily and may have facilitated heterogeneous Fenton reaction and the regeneration of Fe2+. The pH regulated the reaction mechanism among homogeneous (pH = 3) and heterogeneous (pH = 3.7) Fenton catalysis and physical adsorption (pH = 5, 6). The aim of the present study was to improve the understanding of the potential role of natural rutile with advanced oxidation functions in Earth systems and even on Mars, which also provide an inspiration for screening natural rutile and any other similar, Earth-abundant, low-cost minerals for environmental application. [ABSTRACT FROM AUTHOR]

Calcification in cardiovascular aortic atherosclerotic plaque contains Ca-phosphate minerals. However, most research on cardiovascular calcification has focused on its physiological properties rather than its mineralogical features. In this present study, cardiovascular calcification was characterized by collecting samples from patients' tissues and applying mineralogical techniques. Synchrotron radiation-based micro-X-ray diffraction showed the calcification had a similar structure to hydroxylapatite (HAp). Transmission electron microscopy showed some structurally HAp-like spherical particles with a diameter of ~200 nm and acicular crystals ~100 nm × ~20 nm in size. Selected-area electron diffraction indicated that these mineral particles belonged to the hexagonal crystal system. Fourier-transform infrared (FTIR) spectroscopy showed three typical peaks at 1469 cm-1, 1455 cm-1 and 1413 cm-1, indicating that the carbonate group in the calcification plaque substituted for a hydroxyl group to form B-type CHAp (Ca10(PO4,CO3)x(OH)y). The FTIR mapping results illustrated the intergrowth of calcification and organic tissues and the inhomogeneous substitution of phosphate by carbonate in the calcification area. X-ray absorption near-edge structure analysis affirmed that the chemical environments of Ca in the calcification were close to those in HAp. Based on these mineralogical characteristics, the calcification in plaque is identified as a mixture phase of HAp and B-type carbonate HAp, which is similar to the composition of bones. [ABSTRACT FROM AUTHOR]