Volume 101, Issue 3

Research over the last four decades on carbonate rocks of the Waterville limestone, Maine, U.S.A., has contributed to the development of both concepts and methodologies for understanding fuid-rock interaction during regional metamorphism, including: (1) buffering of fluid composition by mineral reactions, (2) infiltration of carbonate rocks by aqueous fluids, (3) petrologic fluid-rock ratios, (4) infiltration-driven metamorphism, (5) one- and two-dimensional continuum models for coupled fluid flow and mineral reaction, (6) time-integrated fluid fluxes, and (7) channelized, horizontal fluid flow in the direction of increasing temperature within chemically isolated layers. Disagreement between the last concept and both hydrodynamic models for metamorphic fluid flow and empirical evidence for homogenization of fluid composition at a scale much larger than layer thickness motivated development of the latest models for coupled fluid flow and mineral reaction in the Waterville limestone. The new models consider a flow medium composed of layers that differ in the initial amounts and compositions of minerals, both horizontal flow in the direction of increasing temperature and vertical flow in the direction of decreasing pressure and temperature, significant but imperfect homogenization of fluid composition across layering by CO 2 -H 2 O interdiffusion, and infiltration by fluids that are spatially variable in composition on the kilometer scale with CO 2 content increasing with increasing grade of metamorphism. The new models reproduce measured progress of the biotite-forming reaction in the Waterville limestone over a range of spatial scales spanning six orders of magnitude, from differences in reaction progress of up to a factor of ~100 between adjacent centimeter-thick layers to the coexistence of mineral reactants and products over a distance of ~13 km parallel to the metamorphic field gradient. Results imply that the present spatial distributions of reaction progress represent a steady state achieved when rocks closely approached equilibrium with the infiltrating fluid during metamorphism. The new models resolve what for many years appeared to be fundamental discrepancies among petrologic data for regionally metamorphosed carbonate rocks, hydrodynamic models of regional metamorphism, and the length scales of mass transport of volatiles across layering by diffusion.

Some intergrowths between rhombohedral and cubic Fe-Ti oxides show properties of high remanence and stability, greater than can be explained solely by properties of the individual phases. Magnetic experiments demonstrate magnetic coupling across the interfaces between these phases. These have similarities to intergrowths solely of rhombohedral oxides with the properties of lamellar magnetism. Long-known studies indicate the common interface is along (111) octahedral planes of the cubic phase and (001) of the rhombohedral phase. This is confirmed in new TEM results on a synthetic titano-hematite and on a natural ferri-ilmenite, both with reduction-exsolution lamellae of magnetite, where high-resolution lattice-fringe images demonstrate a common orientation of Fe octahedra along the interface. Such information provides a starting point to investigate atomic configurations, ionic charge imbalance, and magnetic moments along these interfaces, and leads toward a new application of the theory of lamellar magnetism.

The flux theory of the chemical bond, which provides a physical description of chemical structure based on classical electrostatic theory, correctly predicts the angles between bonds, to the extent that they depend on the intrinsic properties of the bonded atoms. It is based on the justifiable assumption that the charge density around the nucleus of an atom retains most of its spherical symmetry even when bonded. A knowledge of these intrinsic bond angles permits the measurement and analysis of the steric angular strains that result from the mapping of the bond network into three-dimensional space. The work ends by pointing out that there are better ways of characterizing bonds than describing them as covalent or ionic.

Earth’s inventory of principle volatiles C, H, N, and S is a legacy of its early stages of accretion and differentiation. Elemental ratios (C/H, C/N, C/S) are powerful tools for understanding early processing of Earth’s volatiles, as they monitor relative fractionations through important processes even when absolute concentrations are less well defined. The C/H ratio of the bulk silicate Earth (BSE), defined from surface reservoirs and minimally degassed oceanic basalts is 1.3 ± 0.3, which is 5–15 times lower than the C/H ratio of carbonaceous and enstatite chondrites and 2–5 times lower than ordinary chondrites. The BSE C/N ratio is superchondritic (40 ± 8; Bergin et al. 2015) while the C/S ratio (0.49 ± 0.14) is nearly chondritic. Successful models of volatile acquisition and processing must account for the effects of accretion, core formation, and atmospheric loss on all three of these ratios. Simple models of equilibration between a magma ocean, the overlying atmosphere, and alloy destined for the core are used to explore the influence of core formation and atmospheric loss on major volatile concentrations and ratios. Among major volatile elements, C is most siderophile, and consequently core formation leaves behind a non-metallic Earth with low C/H, C/N, and C/S ratios compared to originally accreted materials and compared to the BSE. Compared to the predicted effect of early differentiation, the relatively high C/X ratios of the BSE argue in part that significant volatile replenishment occurred after core formation ceased, possibly in the form of a late veneer. However, a late veneer with chondritic composition is insufficient to explain the pattern of major volatile enrichments and depletions because BSE C/H and C/N ratios are non-chondritic. The C/H ratio is best explained if an appreciable fraction of H in the BSE predates delivery in the late veneer. Although atmospheric blow-off is an attractive explanation for the high C/N ratio, available data for C and N solubility and metal/silicate partitioning suggest that atmospheric blow-off cannot counter core formation to produce subchondritic C/N. Thus, unless virtually all core-forming metal segregated prior to volatile accretion (or relative C and N solubilities are appreciably different from those assumed here), the BSE C/N ratio suggests that accreting materials had elevated ratios compared to carbonaceous chondrites. One possibility is that a fraction of Earth’s volatiles accreted from differentiated C-rich planetesimals similar to the ureilite parent body. Reconciling C/H, C/N, and C/S ratios of the BSE simultaneously presents a major challenge that almost certainly involves a combination of parent body processing, core formation, catastrophic atmospheric loss, and partial replenishment by a late veneer. The chondritic C/S ratio of the BSE and relatively low S content of the BSE constrains the BSE C concentration, but a potential complicating factor in interpreting the BSE C/S ratio is the possible effect of segregation of an S-rich matte to the core during the later parts of core-mantle differentiation.

With the arrival of Curiosity on Mars, the MSL has started its ground validation of some of the phyllosilicate characterization carried out with remote sensing near-IR spectroscopy from orbital instruments. However, given the limited range of action of the rover, phyllosilicate identification and characterization will have to rely mainly on orbital near-IR data. Investigation of Earth analogs can greatly assist interpretation of martian spectra and enable more robust analyses. In this contribution, Mg/Fe-rich clays from submarine hydrothermal origin that had been thoroughly characterized previously were investigated with near-IR reflectance spectroscopy. The clays are mixed-layer glauconite-nontronite, talc-nontronite, talc-saponite, and nontronite samples. The hydroxyl bands in the range 2.1–2.35 μm were decomposed into their several individual components to investigate correlations between the octahedral chemistry of the samples and the normalized intensity of several bands. Good correlations were found for the samples of exclusive dioctahedral character (glauconite-nontronite and nontronite), whereas poor or no correlations emerged for the samples with one (talc-nontronite) or two (talc-saponite) trioctahedral layer components, indicating a more complex spectral response. Because these bands analyzed are a combination of the fundamental OH stretching and OH bending vibrations, the response of these fundamental bands to octahedral chemistry was considered. For 2:1 dioctahedral phyllosilicates, Fe and Mg substitution for Al displaces both fundamental bands to lower wavenumbers (longer wavelengths), so that their effect on the position of the combination band is coherent. In contrast, for trioctahedral clays, Al and Fe 3+ substitution of octahedral Mg displaces the OH stretching band to lower wavenumber values, and the OH bending band to higher wavenumber values, resulting in partial or total mutual cancelation of their effects. As a result, clays with near-IR spectra indicating Mg-dominated octahedral compositions may in fact contain abundant Fe and some Al substitution. Thus, remote-sensing near-IR mineralogical and chemical identification of clays on Mars appears relatively straightforward for dioctahedral clay minerals but more problematic for trioctahedral clays, for which it may require a more detailed investigation of their near-IR spectra.

Barite inclusions in rock-forming and accessory minerals and in multiphase solid inclusions (MSI) in ultrahigh pressure (UHP) eclogites have been controversially interpreted to record the presence of high-salinity oxidizing fluids In this article we use fluid as a general term for an aqueous fluid phase, a supercritical fluid phase, and a melt phase. Where these phases are distinguished, we use the specific term as appropriate. close to the peak of metamorphism. A detailed petrographic and microstructural study of barite in partially melted UHP eclogite from Yangkou, in the central Sulu belt of China demonstrates that barite precipitation is a multi-stage process. The different generations of barite provide insight into the evolution of fluid systems as P-T conditions evolved through the late prograde stage of subduction to the peak of UHP metamorphism and subsequently during exhumation. Five microstructural types of barite are recognized in this study. Type I barite (variable high to low Sr/Ba) occurs as equant primary inclusions with rutile in garnet and omphacite within coesite-bearing eclogite. Zr-in-rutile thermometry on the primary inclusions of rutile yields T = 658–699 °C at P = 2.5–4.5 GPa. Thus, barite inclusions were likely precipitated from an internally buffered fluid during the late prograde evolution. Type II barite (low Sr/Ba) occurs in MSI located toward the rims of garnet and omphacite. Since peak pressure was above the second critical endpoint for basaltic compositions, the MSI are inferred to represent aliquots of a silica-rich supercritical fluid trapped during the late prograde evolution close to the metamorphic peak. Type III barite (low Sr/Ba) occurs in pseudomorphs after phengite inclusions in garnet. The replacement phase assemblage formed during exhumation, by in situ melting of phengite. Type IV barite (low Sr/Ba) forms coarse-grained patches associated with sub-solidus replacement of omphacite by hornblende and albite symplectites along grain boundaries. Type V barite (low Sr/Ba) occurs as grain-boundary veinlets, intergranular grains, and closed rings around pyrite that is partly replaced by hematite and goethite. We interpret the types IV and V barite to have precipitated from an internally generated grain-boundary aqueous fluid, which is expected to be a response to H 2 O exsolving from garnet and omphacite during low-pressure amphibolite facies conditions. Therefore, barite precipitated during the late prograde subduction and the retrograde exhumation of UHP eclogite yields information about the mobility of Ba, Sr, and S during the metamorphic evolution.

Eight spinel single-crystal samples belonging to the spinel sensu stricto-magnesiocoulsonite series (MgAl 2 O 4 -MgV 2 O 4 ) were synthesized and crystal-chemically characterized by X-ray diffraction, electron microprobe and optical absorption spectroscopy. Site populations show that the tetrahedrally coordinated site (T) is populated by Mg and minor Al for the spinel sensu stricto compositions, and only by Mg for the magnesiocoulsonite compositions, while the octahedrally coordinated site (M) is populated by Al, V 3+ , minor Mg, and very minor amounts of V 4+ . The latter occurs in appreciable amounts in the Al-free magnesium vanadate spinel, T(Mg)M(Mg0.26V1.483+V0.264+)O4$^{\rm{T}}{\left( {{\rm{Mg}}} \right)^{\rm{M}}}\left( {{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.26}}}}{\rm{V}}_{{\rm{1}}{\rm{.48}}}^{{\rm{3 + }}}{\rm{V}}_{{\rm{0}}{\rm{.26}}}^{{\rm{4 + }}}} \right){{\rm{O}}_{\rm{4}}}$, showing the presence of the inverse spinel VMg 2 O 4 . The studied samples are characterized by substitution of Al 3+ for V 3+ and (Mg 2+ +V 4+ ) for 2V 3+ described in the system MgAl 2 O 4 -MgV 2 O 4 -VMg 2 O 4 . The present data in conjunction with data from the literature provide a basis for quantitative analyses of two solid-solution series MgAl2O4-MgV23+O4${\rm{MgA}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{ - MgV}}_{\rm{2}}^{{\rm{3 + }}}{{\rm{O}}_{\rm{4}}}$ and MgV23+O4-V4+Mg2O4${\rm{MgV}}_{\rm{2}}^{{\rm{3 + }}}{{\rm{O}}_{\rm{4}}}{\rm{ - }}{{\rm{V}}^{{\rm{4 + }}}}{\rm{M}}{{\rm{g}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}$. Unit-cell parameter increases with increasing V 3+ along the series MgAl 2 O 4 -MgV 2 O 4 (8.085–8.432 Å), but only slightly increases with increasing V 3+ along the series VMg 2 O 4 -MgV 2 O 4 (8.386–8.432 Å). Although a solid solution could be expected between the MgAl 2 O 4 and VMg 2 O 4 end-members, no evidence was found. Amounts of V 4+ are nearly insignificant in all synthetic Al-bearing vanadate spinels, but are appreciable in Al-free vanadate spinel. An interesting observation of the present study is that despite the observed complete solid-solution along the MgAl 2 O 4 -MgV 2 O 4 and MgV 2 O 4 -VMg 2 O 4 series, the spinel structure seems to be unable to stabilize V 4+ in any intermediate members on the MgAl 2 O 4 -Mg 2 VO 4 join even at high oxygen fugacities. This behavior indicates that the accommodation of specific V-valences can be strongly influenced by crystal-structural constraints, and any evaluation of oxygen fugacities during mineral formation based exclusively on V cation valence distributions in spinel should be treated with caution. The present study underlines that the V valency distribution in spinels is not exclusively reflecting oxygen fugacities, but also depends on activities and solubilities of all chemical components in the crystallization environment.

The El Laco iron deposits in northern Chile consist of magnetite (or martite) and minor hematite, pyroxene, and apatite. The orebodies are situated on a volcanic complex and resemble lavas and pyroclastic deposits, but a magmatic origin is rejected by some geologists who regard the ores as products of hydrothermal replacement of volcanic rocks. This study describes spherules of magnetite in the ore at Laco Sur and outlines a previously unrecognized crystallization process for the formation of spherical magnetite crystal aggregates during volcanic eruption. Mining at Laco Sur, the second largest deposit at El Laco, shows that most of the ore is friable and resembles pyroclastic material; hard ore with vesicle-like cavities occurs subordinately. The friable ore is a porous aggregate of 0.01–0.2 mm magnetite octahedra with only a local stratification defned by millimeter-thin strata of apatite. Films of iron phosphate are common on magnetite crystals, and vertical pipes called gas escape tubes are abundant in the ore. A SEM study reveals that magnetite spherules in the range 0.05–0.2 mm occur in most samples of friable ore from the central-lower part of the deposit. The proportion of spherules in a sample varies from high to nil, but overall the spherule content is low in the ore. The spherules are aggregates of octahedral crystals, or single octahedra, that have been rounded by stepwise, subparallel growth of magnetite with a systematic slight shift in orientation of successive steps. The shape of the spherules demonstrates that they formed unattached to any surface. Growth from hot magmatic gas saturated in iron in a volcanic plume and deposition as ash fall can account for the features of the spherule-bearing friable ore.

The Tundulu and Kangankunde carbonatite complexes in the Chilwa Alkaline Province, Malawi, contain late-stage, apatite-rich lithologies termed quartz-apatite rocks. Apatite in these rocks can reach up to 90 modal% and displays a distinctive texture of turbid cores and euhedral rims. Previous studies of the paragenesis and rare earth element (REE) content of the apatite suggest that heavy REE (HREE)-enrichment occurred during the late-stages of crystallization. This is a highly unusual occurrence in intrusions that are otherwise light REE (LREE) enriched. In this contribution, the paragenesis and formation of the quartz-apatite rocks from each intrusion is investigated and re-evaluated, supported by new electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data to better understand the mechanism of HREE enrichment. In contrast to the previous work at Tundulu, we recognize three separate stages of apatite formation, comprising an “original” euhedral apatite, “turbid” apatite, and “overgrowths” of euhedral late apatite. The crystallization of synchysite-(Ce) is interpreted to have occurred subsequent to all phases of apatite crystallization. The REE concentrations and distributions in the different minerals vary, but generally higher REE contents are found in later-stage apatite generations. These generations are also more LREE-enriched, relative to apatite that formed earlier. A similar pattern of increasing LREE-enrichment and increased REE concentrations toward later stages of the paragenetic sequence is observed at Kangankunde, where two generations of apatite are observed, the second showing higher REE concentrations, and relatively higher LREE contents. The changing REE distribution in the apatite, from early to late in the paragenetic sequence, is interpreted to be caused by a combination of dissolution-reprecipitation of the original apatite and the preferential transport of the LREE complexes by F- and Cl-bearing hydrothermal fluids. Successive pulses of these fluids transport the LREE out of the original apatite, preferentially re-precipitating it on the rim. Some LREE remained in solution, precipitating later in the paragenetic sequence, as synchysite-(Ce). The presence of F is supported by the F content of the apatites, and presence of REE-fuorcarbonates. Cl is not detected in the apatite structure, but the role of Cl is suggested from comparison with apatite dissolution experiments, where CaCl 2 or NaCl cause the reprecipitation of apatite without associated monazite. This study implies that, despite the typically LREE enriched nature of carbonatites, significant degrees of hydrothermal alteration can lead to certain phases becoming residually enriched in the HREE. Although at Tundulu the LREE-bearing products are re-precipitated relatively close to the REE source, it is possible that extensive hydrothermal activity in other carbonatite complexes could lead to significant, late-stage fractionation of the REE and the formation of HREE minerals.

Textural and compositional variations of apatite from rift-related gabbros, syenogabbros, syenites, quartz-syenites, and nepheline syenites of the Mid-Proterozoic Gardar Province (South Greenland) are presented and compared to apatite compositions from other rock suites. The observed zoning textures of apatite are interpreted to represent (1) primary growth zonation (concentric and oscillatory) that formed during magmatic differentiation and (2) secondary irregular overgrowths, patchy zonation, and resorption textures, assigned to metasomatic overprinting due to interaction with fluids/melts and intra-crystalline diffusion. Compositional variation in the apatites is mainly due to coupled substitutions of Ca and P by variable amounts of Si, Na, and REE, which show increasing concentrations during magmatic differentiation. Furthermore, F concentrations in apatites increase from gabbroic through syenogabbroic to syenitic rocks, whereas Cl concentrations show the opposite trend. Compared to apatite compositions from gabbroic, dioritic, and granitic rocks in general, apatites from alkaline rock suites are characterized by exceptionally high contents of REE and Si and in some alkaline rocks they attain Sr contents comparable to those reported from carbonatites. Typical low Mn and S contents are probably a result of low oxygen fugacity during crystallization at relatively high temperatures.

The mixing of magmas of distinct temperature, bulk composition, mineralogy, and physical properties plays a central role in explaining the diversity of magma types on Earth and in explaining the growth of continental and oceanic crust. Magma mixing is also of practical importance. For example, the mixing of distinct magmas has been cited as an important process in creation of economically important horizons in layered intrusions as well as a triggering mechanism for initiation of volcanic eruptions. The motivation for better quantifying the dynamics and thermodynamics of magma mixing and its attendant plutonic and volcanic products is clear. The degree of magma mixing, which spans a continuum from mingling to complete hybridization, depends upon initial and boundary conditions, magma properties, driving forces, and time available for mixing. Magma mingling produces a heterogeneous mixture of discrete clumps of the end-member magmas, whereas complete hybridization involves the thermodynamic equilibration of two distinct magmas to form a third. Qualitatively, mixing occurs via reduction in the size of compositional heterogeneities (i.e., clumps) through stretching and folding by viscous flow, followed by homogenization, once shear has reduced the size of compositional anomalies to diffusive length scales. Quantification of this process relies on two statistical measures: the linear scale of segregation (Λ) defined as the spatial integral of the compositional correlation function related to the size-distribution of the segregated clumps within the mixture, and the intensity of segregation ( I ) a measure that quantifies how much the composition at each location differs from the average. The mixing dynamics of a layered system are analyzed in terms of the parameters governing mixing (Rayleigh, Lewis, and buoyancy numbers and viscosity ratio) to estimate how the timescale for magma hybridization, τ H , compares to solidification, recharge, diffusive, and assimilation timescales. This analysis illustrates that hybridization times can be shorter than or comparable to thermal, solidification, and replenishment timescales; thus, formation of hybridized or nearly hybridized magmas is one anticipated outcome of mixing. The machinery of thermodynamics can be used to compute the hybrid magma state. An exploratory model for the thermochemistry of hybridization is developed based on binary eutectic phase relations and thermodynamics. Eight thermodynamic parameters define the phase diagram and associated energetics, and six parameters (initial temperatures, compositions, mass ratio of mixing magmas, and an enthalpy parameter) are necessary and sufficient to determine the state of hybrid magma uniquely. While relevant combinations of 14 thermodynamic and mixing parameters might suggest that the number of mixing outcomes (i.e., products) is too high to systematize, Monte Carlo simulations using the exploratory model document how millions of arbitrary initial states evolve into five possible final (mixed) states. Such an analysis implies that a magma mixing taxonomy that defines possible mixed product states can be developed and tied to petrologic indicators of mixing. Additional insights gained from this exploratory model that are supported by independent results from a multicomponent, multiphase thermodynamic model of magma mixing (Magma Chamber Simulator) include: (1) the proclivity of invariant point hybrid states, which may explain some instances of compositionally monotonous melts associated with mixed magma eruptions; (2) a surprising thermal effect such that the temperature of hybridized magma can be significantly less than the initial temperature of either of the mixing magmas. This type of magma mixing may result in crystal resorption, thus invalidating an assumption that resorption textures in crystals are typically the result of a magma heating event; (3) illustration of the differing effects of stoped block temperature and composition on hybrid magma temperature and phase state; and (4) illustration of a cessation of crystallization effect that may pertain to the MORB pyroxene “paradox.” Differences between adiabatic or R-hybridization and diabatic or RFC-hybridization are also explored. The model can be used to elucidate the thermodynamic principles underlying magma mixing in the hybridization limit. These principles are of general applicability and carry over to more compositionally complicated systems.

The southern segment of the ancestral Cascades magmatic arc includes numerous volcanic fields; among these, the Bodie Hills volcanic field (BHVF), astride the California-Nevada border north of Mono Lake, is one of the largest (>700 km 2 ) and most well studied. Episodic magmatism in the BHVF spanned about 9 million years between about 15 and 6 Ma; magmatic output was greatest between ca. 15.0 to 12.6 Ma and ca. 9.9 to 8.0 Ma. About two dozen contiguous and coalescing eruptive centers above middle- to shallow-crustal-level reservoirs generated several trachyandesite stratovolcanoes and numerous silicic trachyandesite to rhyolite flow dome complexes whose compositional variations are consistent with fractionation of observed phenocryst phases. BHVF rocks have high-potassium calc-alkaline compositions consistent with generation of subduction-related continental margin arc magmas beneath thick continental crust. Radiogenic isotope ratios in BHVF rocks vary considerably but suggest somewhat enriched, crustal sources; isotopic ratios for some of the more primitive units are consistent with more depleted, mantle sources. Neither age nor whole-rock compositions of BHVF rocks are well correlated with isotopic variations. Textures and compositions of phenocrysts in BHVF rocks are in accord with the associated magma reservoirs evolving via open-system behavior. Reservoir recharge and subsequent incomplete homogenization are evidenced by the broad compositional diversity characteristic of many BHVF eruptive units. Significant compositional diversity among the products of coeval eruptive centers further suggests that centers responsible for BHVF magmatism were underlain by small, discrete, compositionally distinct, and closely spaced reservoirs. Volcanic rocks of the BHVF host quartz-adularia and quartz-alunite epithermal gold-silver deposits, from which about 3.4 Moz. of gold and 28 Moz. of silver have been produced. The volcanic rocks and contained deposits are broadly coeval, which suggests that the associated magmas are the sources of heat, fluids, and metals involved in deposit genesis. Characteristics of the quartz-adularia deposits are consistent with derivation from near-neutral pH fluids at ?250 °C, whereas those of the quartz-alunite systems require more acidic, oxidized, and sulfur-rich fluids at temperatures <250 °C. Economically viable precious metal accumulations are in fault-hosted vein deposits in the Bodie and Aurora districts. Circulation of hydrothermal fluids through permeable pyroclastic deposits but lacking prominent structural conduits resulted in large areas of altered but unmineralized rock.

Orbital and in-situ data from the surface of Mars indicate that nanophase weathering products are important constituents of martian rocks and soils. Nanophase minerals have the capacity to chemisorb anions like sulfate and phosphate onto their surfaces, but it is not known whether chemisorption is an important or even detectable process via orbital and in-situ observations. The detection of chemisorbed sulfate and phosphate anions on nanophase minerals would constrain the speciation of these anions and past aqueous environmental conditions. Here, we synthesized two nanophase weathering products that are common in terrestrial volcanic soils and have been identified on the martian surface: allophane and nanophase ferric oxide as represented by ferrihydrite. We specifically adsorbed sulfate and phosphate separately onto the nanophase mineral surfaces (4.5 and 1.6 wt% SO42–${\rm{SO}}_{\rm{4}}^{{\rm{2}}-}$, and 6.7 and 8.9 wt% PO43–${\rm{PO}}_{\rm{4}}^{{\rm{3}}-}$ on allophane and ferrihydrite, respectively) and analyzed the untreated and chemisorbed materials using instruments similar to those on orbital and landed Mars missions (including X-ray diffraction, evolved gas analysis, Mössbauer spectroscopy, and VNIR and thermal-IR spectroscopy). Evolved gas analysis is the optimum method to detect chemisorbed sulfate, with SO 2(g) being released at >900 °C for allophane and 400–800 °C for ferrihydrite. Chemisorbed sulfate and phosphate anions affect the thermal-IR spectra of allophane and ferrihydrite in the S-O and P-O stretching region when present in abundances of only a few weight percent; S-O and P-O stretching bands are apparent as short-wavelength shoulders on Si-O stretching bands. Sulfate and phosphate anions chemisorbed to allophane have small but measurable effects on the position of the OH-H 2 O bands at 1.4 and 1.9 μm in near-IR spectra. Chemisorbed sulfate and phosphate anions did not affect the X-ray diffraction patterns, Mössbauer spectra, and visible/near-IR spectra of ferrihydrite. These data suggest that sulfate chemisorbed onto the surfaces of nanophase minerals can be detected with the Sample Analysis at Mars (SAM) instrument on the Mars science laboratory Curiosity rover, and subtle signatures of chemisorbed sulfate and phosphate may be detectable by IR spectrometers on landed missions. The combined use of SAM, the Chemistry and Mineralogy (CheMin) instrument, and the Alpha Particle X-ray Spectrometer (APXS) on Curiosity allows for the most detailed characterization to date of nanophase minerals in martian rocks and soils and the potential presence of chemisorbed anionic complexes.

Crystallographic orientation relationships (CORs) between mineral inclusions and their hosts could potentially deliver information about inclusion formation processes and conditions. Most previous studies are based on small numbers of analyses. This paper uses EBSD to study host–inclusion CORs in an inclusion-rich Permian metapegmatite garnet (Koralpe region, Eastern Alps, Austria), demonstrating the importance of large data sets and of EBSD in particular for the analysis of CORs. The distribution of measured orientations reflects host garnet point group symmetry for 89% of inclusions analyzed (total N = 530). Each inclusion phase (rutile, corundum, and ilmenite) shows at least three different CORs to host garnet. “Statistical” CORs are introduced to describe distributions of inclusion orientations that have one or two degrees of freedom with respect to the host, but still reflect host crystal symmetry. Two end-member characteristics of statistical CORs are distinguished: rotation and dispersion. Most statistical CORs observed show a mixture of both. Each inclusion phase shows at least one statistical COR. Multiple coexisting CORs and statistical CORs are not restricted to rutile. Re-examination of previous garnet–rutile COR studies in light of the new results indicates that COR information may have been overlooked when using small data sets. Variation in COR parameters correlates with broad differences in assumed metamorphic conditions for new and literature samples, suggesting that petrogenetic information may be available if COR formation can be understood. The favorability of the detected CORs cannot be explained by a simple model involving minimization of misfit between lattice planes, implying that other interface properties or the inclusion formation mechanism are important controls on COR development.

Oceanic plates contain numerous hydrous phases including amphiboles, which are important carriers of water into subduction zones. The hydroxyl bound within the crystalline structure of hydrous minerals, as well as changes in hydrogen bond symmetry, can impact the bulk properties of these minerals. In this study, 12 natural amphibole samples spanning a range of 10 compositions were probed with synchrotron infrared spectroscopy at room temperature and pressures up to 60 GPa. Infrared spectra were collected at atmospheric pressure and at regular intervals during compression, allowing for the collection of spectra centered on the typical O-H stretching region at 3600–3700 cm –1 as they evolved with pressure for each composition. The number of O-H bands within each sample was found to vary with composition, but the pressure dependence of O-H frequency shifting more closely correlated with mode frequency at ambient pressure than with composition. Combined with earlier results, these data reveal a linear relationship between mode frequency at ambient pressure and the pressure dependence of O-H stretching modes in amphiboles and sheet silicates. Two sample preparation methods utilized in this study allowed for direct comparison between quasi-hydrostatic neon-loaded sample conditions and the conditions achieved with a KBr pressure medium. Samples loaded in neon preserved sharper peaks, allowing greater spectral resolution, especially at higher pressures when peaks are most likely to broaden or disappear due to crystalline disorder and pressure gradients across the sample. This new quasi-hydrostatic loading method proved valuable to tracing O-H stretching behavior in amphiboles to higher pressures than previously obtained and will lend itself to future study of O-H stretching pressure dependence in a wide range of hydrous minerals.

Sepiolite is a type of magnesium-rich fibrous clay mineral. The unique fibrous structure endows it with various applications in many fields. Revealing the behavior of confined water in sepiolite nanopores is crucial to understand the macroscopic properties of sepiolite. Aiming to ascertain the distribution and dynamics of the confined water, molecular simulations using grand canonical Monte Carlo and molecular dynamics methods have been performed. We obtain the water adsorption isotherm, density distribution profiles, and dynamic information of the confined waters in sepiolite tunnels. We find that zeolitic water is very hard to be desorbed from sepiolite under ambient conditions. Based on the distribution profile and trajectories of the water, we reveal the accurate distribution sites and propose a new distribution model of the confined water including one bound water site and four zeolitic water sites, which are determined by sepiolite lattice. The zeolitic water at different sites can exchange freely and frequently, and thus these sites may be energetically similar. This model provides more fundamental understanding of the hydration of sepiolite, and highlights the water behaviors in the tunnel pores of microporous minerals, which are thought being controlled by the crystallographic structure. The much lower mobility of zeolitic water in sepiolite than that in montmorillonite implies that materials with nano-sized tunnel pores could have more efficient fixation on foreign molecules or ions in environmental applications than those layered materials with slit pores.

We have investigated the high-pressure behavior of Fe 3 O 4 by in situ X-ray diffraction measurements from 11 to 103 GPa. Up to 70 GPa, the previous observed high-pressure Fe 3 O 4 phase (h-Fe 3 O 4 ) is stable, with a CaTi 2 O 4 -type structure. The compression curve shows an abnormal volume contraction at about 50 GPa, likely associated with the magnetic moment collapse observed at that pressure. Fitting the compression data up to 45 GPa to the Birch-Murnaghan equation of state yields a bulk modulus, K T0 = 172 GPa, and V 0 = 277 Å 3 , with fixed K ʹ = 4. At a pressure between 64 and 73 GPa, a new structural transition was observed in Fe 3 O 4 , which can be attributed to a martensitic transformation as described by Yamanaka et al. (2008) for post-spinel structural transition. The diffraction data can be best fitted with a Pnma space group. No breakdown of Fe 3 O 4 was observed up to at least 103 GPa. The new high-pressure polymorph is about 6% denser than the h-Fe 3 O 4 phase at 75 GPa.

Visible to near-infrared (V-NIR) remote sensing observations have identified spinel in various locations and lithologies on the Moon. Experimental studies have quantified the FeO content of these spinels (Jackson et al. 2014), however the chromite component is not well constrained. Here we present compositional and spectral analyses of spinel synthesized with varying chromium contents at lunar-like oxygen fugacity ( f O 2 ). Reflectance spectra of the chromium-bearing synthetic spinels (Cr# 1–29) have a narrow (~130 nm wide) absorption feature centered at ~550 nm. The 550 nm feature, attributed to octahedral Cr 3+ , is present over a wide range in iron content (Fe# 8–30) and its strength positively correlates with spinel chromium content [ln(reflectance min ) = –0.0295 Cr# – 0.3708]. Our results provide laboratory characterization for the V-NIR and mid-infrared (mid-IR) spectral properties of spinel synthesized at lunar-like f O 2 . The experimentally determined calibration constrains the Cr# of spinels in the lunar pink spinel anorthosites to low values, potentially Cr# < 1. Furthermore, the results suggest the absence of a 550 nm feature in remote spectra of the Dark Mantle Deposits at Sinus Aestuum precludes the presence of a significant chromite component. Combined, the observation of low chromium spinels across the lunar surface argues for large contributions of anorthositic materials in both plutonic and volcanic rocks on the Moon.

The temperature variations of the ferrous Mössbauer parameters for a synthetic ilmenite (FeTiO 3 ) have been determined and interpreted over a very wide temperature range (5–900 K). The Debye model of the lattice vibrations was used in interpreting the temperature dependence of the center shift, yielding a characteristic Mössbauer temperature of 350 ± 20 K and a zero-Kelvin intrinsic isomer shift of 1.30 ± 0.01 mm/s. The temperature dependence of the ferrous Mössbauer quadrupole splitting was interpreted using crystal feld theory. A most adequate description of the experimental ∆ E Q ( T ) curve was obtained assuming an energy shift of at the most ca. 500 ± 50 cm –1 for the highest orbital T 2g level relative to the lowest level within this T 2g triplet. The temperature dependence of the hyperfine field was interpreted within the molecular field theory of magnetism assuming the magnetic exchange energy being a function of interatomic spacing, indicating a first-order magnetic transition at the magnetic-paramagnetic transition temperature of 59.0 ± 0.5 K. This detailed presentation of Mössbauer parameters as a function of temperature can serve as a basis for easily detecting ilmenite ore at, for example, the lunar surface and for monitoring by means of Mössbauer spectroscopy the reduction process of the mined mineral, for the purpose of supplying a future Moon base site with oxygen and water.

Pre-edge features in the K absorption edge of X-ray absorption spectra are commonly used to predict Fe 3+ valence state in silicate glasses. However, this study shows that using the entire spectral region from the pre-edge into the extended X-ray absorption fine-structure region provides more accurate results when combined with multivariate analysis techniques. The least absolute shrinkage and selection operator (lasso) regression technique yields %Fe 3+ values that are accurate to ±3.6% absolute when the full spectral region is employed. This method can be used across a broad range of glass compositions, is easily automated, and is demonstrated to yield accurate results from different synchrotrons. It will enable future studies involving X-ray mapping of redox gradients on standard thin sections at 1 × 1 μm pixel sizes.