Manganese oxide minerals have been used for thousands of years-by the ancients for pigments and to clarify glass, and today as ores of Mn metal, catalysts, and battery material. More than 30 Mn oxide minerals occur in a wide variety of geological settings. They are major components of Mn nodules that pave huge areas of the ocean f loor and bottoms of many fresh-water lakes. Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions that affect groundwater and bulk soil composition. Their typical occurrence as fine-grained mixtures makes it difficult to study their atomic structures and crystal chemistries. In recent years, however, investigations using transmission electron microscopy and powder x-ray and neutron diffraction methods have provided important new insights into the structures and properties of these materials. The crystal structures for todorokite and birnessite, two of the more common Mn oxide minerals in terrestrial deposits and ocean nodules, were determined by using powder x-ray diffraction data and the Rietveld refinement method. Because of the large tunnels in todorokite and related structures there is considerable interest in the use of these materials and synthetic analogues as catalysts and cation exchange agents. Birnessite-group minerals have layer structures and readily undergo oxidation reduction and cation-exchange reactions and play a major role in controlling groundwater chemistry.

Hydrous manganese oxides are an important class of minerals that help regulate the geochemical redox cycle in near-surface environments and are also considered to be promising catalysts for energy applications such as the oxidation of water. A complete characterization of these minerals is required to better understand their catalytic and redox activity. In this contribution an empirical methodology using X-ray photoelectron spectroscopy (XPS) is developed to quantify the oxidation state of hydrous multivalent manganese oxides with an emphasis on birnessite, a common layered structure that occurs commonly in soils but is also the oxidized endmember in biomimetic water-oxidation catalysts. The Mn2p 3/2 , Mn3p, and Mn3s lines of near monovalent Mn(II), Mn(III), and Mn(IV) oxides were fit with component peaks; after the best fit was obtained the relative widths, heights and binding energies of the components were fixed. Unknown multivalent samples were fit such that binding energies, intensities, and peak-widths of each oxidation state, composed of a packet of correlated component peaks, were allowed to vary. Peak-widths were constrained to maintain the difference between the standards. Both average and individual mole fraction oxidation states for all three energy levels were strongly correlated, with close agreement between Mn3s and Mn3p analyses, whereas calculations based on the Mn2p 3/2 spectra gave systematically more reduced results. Limited stoichiometric analyses were consistent with Mn3p and Mn3s. Further, evidence indicates the shape of the Mn3p line was less sensitive to the bonding environment than that for Mn2p. Consequently, fitting the Mn3p and Mn3s lines yielded robust quantification of oxidation states over a range of Mn (hydr)oxide phases. In contrast, a common method for determining oxidation states that utilizes the multiplet splitting of the Mn3s line was found to be not appropriate for birnessites.

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Precise single-crystal X-ray structure refinements of three hollandite-type minerals have allowed a detailed study of the hoUandite structure to be made. The minerals hollandite [(Bao.75Pbo.16Nao.loKo.04)(Mn,Fe, A1)s(O,OH)16], cryptomelane [(Ko.94Nao.25Sro.13-Bao.lo)(Mn,Fe,A1)s(O,OH)16], and priderite [(Ko.9o-Ba0.35) (Ti,Fe,Mg)sO16] were refined to residuals of R = 0.0165 (599 observations, 48 parameters), R = 0.0299 (623 observations, 53 parameters), and R = 0.0096 (316 observations, 29 parameters) respectively. The first two structures are monoclinic (I2/m) and priderite is tetragonal (I4/m). The symmetry of hollandite compounds depends on the ratio of the average ionic radius of the octahedral cations to that of the tunnel cations. Structures in which this ratio is >0.48 distort, reducing the tunnel volume, and thereby lowering the symmetry from tetragonal to monoclinic. The position occupied by a tunnel cation is determined primarily by the size of the cation. Relatively small cations, such as Ba 2+ in priderite and Pb 2+ in hollandite, displace from the special position, 2(a), to more stable sites that are at the sum of the ionic radii from the nearest O atoms. This study also indicates that the reduced form of Mn in hoUandite and cryptomelane is Mn3+; bond lengths calculated from the refinements suggest that Mn 3+ is more easily accommodated in the structures than the larger Mn 2+.

Previous research suggests that the complex symbolic, technological, and socioeconomic behaviors that typify had roots in the middle Pleistocene<200,000 years ago, but data bearing on human behavioral origins are limited. We present a series of excavated Middle Stone Age sites from the Olorgesailie basin, southern Kenya, dating from ≥295,000 to ~320,000 years ago by argon-40/argon-39 and uranium-series methods. Hominins at these sites made prepared cores and points, exploited iron-rich rocks to obtain red pigment, and procured stone tool materials from ≥25- to 50-kilometer distances. Associated fauna suggests a broad resource strategy that included large and small prey. These practices imply notable changes in how individuals and groups related to the landscape and to one another and provide documentation relevant to human social and cognitive evolution.

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An x-ray examination of more than 150 specimens of fine-grained quartz varieties from around the world has revealed that more than 10% and as much as 80% of the silica in many samples is actually moganite, a little-known silica polymorph. Rietveld refinements of 50 powder x-ray diffraction patterns produced by fibrous quartz (agate, chalcedony) and nonfibrous quartz (chert, flint) indicate that the concentrations of moganite within each subgroup are widely distributed. The large amount of moganite (>30%) found in cherts from arid, alkaline environments may resurrect length-slow silica as an indicator of evaporitic regimes, and the absence of moganite in weathered and hydrothermally altered silica samples may be a useful measure of fluid-rock interaction.

Raman spectra were collected for an extensive set of well-characterized tunnel-structure Mn oxide mineral species employing a range of data collection conditions. Using various laser wavelengths, such as 785, 633, and 532 nm at low power levels (30–500 µW), as well as the comprehensive database of standard spectra presented here, it is generally possible to distinguish and identify the various tunnel structure Mn oxide minerals. The Raman mode relative intensities can vary significantly as a function of crystal orientation relative to the incident laser light polarization direction as well as laser light wavelength. Consequently, phase identification success is enhanced when using a standards database that includes multiple spectra collected for different crystal orientations and with different laser light wavelengths. For the hollandite-group minerals, the frequency of the Raman mode near 630 cm–1 shows a strong linear correlation with the fraction of Mn3+ in the octahedral Mn sites. With the comprehensive Raman database of well-characterized Mn oxide standards provided here (and available online as Supplemental Materials1), and use of appropriate data collection conditions, micro-Raman is a powerful tool for identification and characterization of biotic and abiotic Mn oxide phases from diverse natural settings, including on other planets.