Volume 103, Issue 3

Saturn’s moon Titan has a surface that is dominated by molecular materials, much of which are photochemically produced in the moon’s atmosphere. This outlook reviews the potential minerals that would be expected to form on the surface and subsurface of Titan from these molecular solids. We seek to classify them and look toward how the future study of these minerals will enhance our understanding of this planetary body. The classification uses the basis of intermolecular interactions, with the materials grouped into “Molecular solids,” “Molecular co-crystals,” and “Hydrates” classes alongside speculation on other possible classes of potential Titan minerals.

Mesoporous silica particles of type MCM-41 (Mobile Composition of Matter No. 41), exhibiting highly ordered mesoporosity (pores with diameter between 2 and 50 nm) and surface roughness, are developed and used as a functional coating on bioactive glass-based scaffolds for bone tissue engineering. The degradability and the mesostructure stability of these novel MCM-41 particles were evaluated. The particles are immersed in simulated body fluid (SBF) for up to 28 days at 37 °C, and the variation of the ordered porosity, surface characteristics, and chemical composition of the particles are assessed by SEM-EDX, HRTEM, FTIR, ICP-OES, and pH measurements. The results indicate that the MCM-41 particles are affected by immersion in SBF only during the first few days; however, the surface and the mesopore structure of the particles do not change further with increasing time in SBF. The pore channel diameter increased slightly, confirming the stability of the developed material. The release of dissolved Si-species, which reached a maximum of 260 mg SiO 2 per gram of material, could play a key role in gene activation of osteoblast cells and in inducing new bone matrix formation.

We present a numerical model of trace-element oscillatory zoning patterns formed when zircon crystallizes from silicate melt, which is also appropriate for other accessory phases with known partition and diffusion coefficients and saturation conditions. The model accounts for diffusion-controlled accessory mineral growth and the equilibrium crystallization of major mineral phases. Consideration of recent, experimentally determined dependencies of partition coefficients on temperature, we find that thermal changes provide the simplest explanations of oscillatory zoning in accessory minerals. Numerical experiments with different cooling rates explore different crystallization scenarios with and without the precipitation of other phases and/or the interface reaction of phosphorus (P) and yttrium (Y) to form xenotime. However, these processes are monotonically related to growth rate and do not cause oscillations. Minor 3–10 °C variations in temperature do not result in zircon dissolution, but strongly influence zircon growth and lead to variations in coeval Y, Hf, and rare earth element (REE) concentrations of up to a factor of two, comparable to those observed in nature. Such temperature variations may be very common in any igneous body in response to external factors such as replenishment by hotter magmas or convection. More significant temperature fluctuations may result in initial minor dissolution at higher temperatures during a mafic recharge event but with continuous growth afterward. At high temperature (>~850 °C) the amplitude of oscillations is relatively small that confirms observations of both less common oscillatory zoning in hot and dry volcanic rhyolites and abundant oscillations in plutonic zircons and in zircons in cold and wet crystal-rich mushes. Additional oscillations in zircon are modeled in response to oscillations of pressure on the order of ±35–50 bars, causing water concentration fluctuations of ±0.1 wt% in water-saturated melt cells with a gas bubble. These variations cause variations of Zr diffusion and zircon growth rates. Such fluctuations could result from pressure oscillations due to recharge and convection in the magma chamber. All simulated runs generate smoothed oscillatory growth zoning; similar patterns found in nature may not necessarily require post-growth intracrystalline diffusion.

The strong dependence of vanadium partitioning between olivine and silicate melt (DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $) on redox conditions ( f O 2 ) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ~6.5 wt% H 2 O. The data demonstrate a strong negative correlation between DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $ and oxygen fugacity, similar to the behavior observed previously at higher temperatures and in MgO-rich compositions. The correlation between DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $ and ΔQFM in the range from –0.5 to +3.2 is described for melts with MgO < 12 wt% and Na 2 O < 4 wt% at temperatures ≤1250 °C by the empirical equation: ΔQFM = − 3.07− 0.29+0.26log⁡ DVO1-M− 3.34− 0.49+0.40$\begin{array}{} -3.07_{-0.29}^{+0.26}\,\log D_{\text V}^{\text{O1-M}}-3.34_{-0.49}^{+0.40} \end{array} $ with the standard error (SE) as a function of log DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $: 2SE(ΔQFM) = –0.275log DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $ + 0.4. We suggest that this equation can be used as an oxybarometer, which is particularly well applicable to the hydrous island-arc magmas at relatively low temperature. Application of the equation to the composition of melt inclusions and their host olivine phenocrysts from basalts of Mutnovsky volcano, containing vanadium concentrations in the range of 250–370 and 4–6 ppm, respectively, reveals an oxygen fugacity in the range ΔQFM +1.9 to +2.3. The estimates are in a good agreement with olivine-spinel oxybarometry for Mutnovsky basalts and may be typical for moderately evolved island-arc magmas.

There has been much research undertaken on structural OH − in various nominally anhydrous minerals including the common silicate garnets (i.e., X 3 Y 2 Si 3 O 12 , where X = Mg, Fe 2+ , Mn 2+ , and Ca and Y = Al, Fe 3+ , and Cr 3+ ). However, it is still largely not understood where small concentrations of H atoms are incorporated in the garnet crystal structure. In this work, the IR single-crystal spectra of end-member or approaching end-member composition andradite, pyrope, and almandine are measured. Both a natural and synthetic andradite sample show a broad, asymmetric OH − -stretching mode at 3563 cm −1 that splits into two narrower modes at lower temperatures. They are located at 3575 and 3557 cm −1 at 80 K with the higher wavenumber mode being considerably more intense compared to that at lower energy. These results are analyzed together with published IR spectra of synthetic end-member katoite, pyrope, and almandine also recorded at low temperature. These garnets show similar IR behavior with a broad OH − band at room temperature that splits into two narrower bands at lower temperatures and with a similar intensity relationship as shown by andradite. This behavior is indicative of the hydrogarnet substitution. The measured IR spectra of natural almandine- and pyrope-rich (Dora Maira, Italy) crystals, on the other hand, show different spectroscopic features with several OH − modes that are not consistent with the hydrogarnet mechanism. An analysis of the energy of the OH − -stretching mode is made for various composition hydrogarnet clusters [i.e., X 3 Y 2 (O 4 H 4 ) 3 , where X = Mg, Fe 2+ , Mn 2+ and Ca and Y = Al and Fe 3+ ] in terms of crystal-chemical properties and local atomic configurations. The OH − mode energy, which lies roughly between 3660 and 3550 cm −1 at RT for various end-member garnets, is a function of the mass of the X- and Y-cations due to mode coupling and/or mixing. In addition, the strength of the chemical bonding between the X- and Y-cations and the O 2- anion of the OH − dipole plays a role in affecting the wavenumber of the OH − -stretching vibration. OH − mode broadening, observed in the spectra of end-member garnets, is primarily a result of thermal anharmonic disorder, especially with regard to the light H cation. OH mode broadening in intermediate solid-solution composition garnets is a function of both thermal effects and variations in local cation configurations around the OH − dipole(s). Published IR spectra of certain high-pressure pyrope-rich garnets, both synthetic and natural, are analyzed and arguments made that OH − can often be incorporated as the hydrogarnet or hydropyrope substitution. IR spectra similar in appearance, having multiple relatively narrow OH − modes that are distinct from those indicating the hydrogarnet substitution, can be observed for certain synthetic end-member and various composition natural pyropes from Dora Maira and some natural spessartines. This indicates that other common OH − substitution mechanisms, which have yet to be determined, can also occur in different silicate garnets.

The search for extinct and extant life on Mars is based on the study of biosignatures that could be preserved under Mars-like, extreme conditions that are replicated in different terrestrial analog environments. The mineral record in the subsurface of the Río Tinto system is one example of a Mars analog site that has been exposed to weathering conditions, including the biogeochemical activity of Fe and S chemolithotrophic bacteria, for millions of years. The SEM-EDAX analysis of different samples recovered in the Peña de Hierro area from four boreholes, ranging from 166 to 610 m in depth, has provided the identification of microbial structures that have affected a suite of hydrothermal minerals (~345 Ma) as well as minerals likely produced by biological activity in more recent times (<7 Ma). The hydrothermal minerals correspond to reduced sulfur or sulfate-bearing compounds (e.g., pyrite and barite) that are covered by bacilli- or filamentous-like microbial structures and/or secondary ferrous carbonates (e.g., siderite) with laminar to spherical structures. The secondary iron carbonates can be in direct contact or above an empty interphase with the primary hydrothermal minerals following a wavy to bent contact. Such an empty interphase is usually filled with nanoscale, straight filamentous structures that have a carbonaceous composition. The occurrence of a sulfur and iron chemolithotrophic community in the Río Tinto basement strongly suggests that the association between sulfur-bearing minerals, dissolution scars and secondary minerals of biological origin is a complex process involving the microbial attack on mineral surfaces by sulfur reducing bacteria followed by the precipitation of iron-rich carbonates. In this scenario, iron sulfide compounds such as pyrite would act as electron donors under microbial oxidation, while sulfate minerals such as barite would act as electron acceptors through sulfate reduction. Furthermore, the formation of siderite would have resulted from carbonate biomineralization of iron chemoheterotrophic organims or other microorganisms that concentrate carbonate through metabolic pathways. Although the distribution of the mineral biosignatures at depth clearly follows a redox gradient, they show some irregular allocation underground, suggesting that the geochemical conditions governing the microbial activity are affected by local changes associated with the fracturing pattern of the Río Tinto basement. The abundance of sulfur- and iron-bearing minerals in the Mars crust suggests that the Río Tinto mineral biosignatures can be useful in the search for extant and extinct subsurface life on the red planet.

Water diffusion in silicate melts is a fundamental process controlling physical and chemical consequences for magmatism, but mechanisms of diffusion in silicate glasses and melts are not fully understood. In this study, water diffusion experiments in silica glass were performed at temperatures of 650–850 °C and water vapor pressure of 50 bar, with the aim of improving our understanding of the mechanism of water diffusion in a simple SiO 2 -H 2 O system, and to construct a general water diffusion model for multi-component silicate glasses. Hydrogen diffusion profiles in silica glass were measured by secondary ion mass spectrometry (SIMS) down to a water concentration of ~10 ppm. Water diffusion profiles indicate that water diffusion becomes slower with decreasing water concentration in silica glass, with the water concentration dependence being greater than in multi-component silicate glasses, particularly at low concentrations (e.g., Doremus 1969, 2000; Zhang and Behrens 2000). A new water diffusion model is proposed for silica glass, where the greater concentration dependence is attributed to the limited number of diffusion pathways in silica glass, formed by breaking Si–O–Si bonds through hydroxyl formation. The model was applied to multi-component silicate glasses, taking into account the effects of metal cations that act as network modifiers by providing additional diffusion pathways for water molecules. The lower water concentration dependence in multi-component silicate glasses and melts is explained by little dependence of the number of diffusion pathways on water concentration because it is controlled extrinsically by network modifier cations. It is concluded that the number of diffusion pathways is an essential controlling factor for water diffusion in silica and silicate glasses.

Sulfur is an important component in volcanic gases at the Earth surface but also present in the deep Earth in hydrothermal or magmatic fluids. Little is known about the evolution of such fluids during ascent in the crust. A new optical cell was developed for in situ Raman spectroscopic investigations on fluids allowing abrupt or continuous changes of pressure up to 200 MPa at temperatures up to 750 °C. The concept is based on a flexible gold bellow, which separates the sample fluid from the pressure medium water. To avoid reactions between aggressive fluids and the pressure cell, steel components in contact with the fluid are shielded by gold foil. The cell was tested to study redox reactions in fluids using aqueous ammonium sulfate solutions as a model system. During heating at constant pressure of 130 MPa, sulfate ions transform first to HSO4− $\begin{array}{} \displaystyle \text{HSO}_{4}^{-} \end{array}$ ions and then to molecular units such as H 2 SO 4 . Variation of pressure shows that the stability of sulfate species relies on fluid density, i.e., highly charged species are stable only in high-density fluids. Partial decomposition of ammonium was evident above 550 °C by the occurrence of a nitrogen peak in the Raman spectra. Reduced sulfur species were observed above 700 °C by Raman signals near 2590 cm –1 assigned to HS – and H 2 S. No clear evidence for the formation of sulfur dioxide was found in contrary to previous studies on aqueous H 2 SO 4 , suggesting very reducing conditions in our experiments. Fluid-mineral interaction was studied by inserting into the cell a small, semi-open capsule filled with a mixture of pyrite and pyrrhotite. Oxidation of the sample assembly was evident by transformation of pyrite to pyrrhotite. As a consequence, sulfide species were observed in the fluid already at temperatures of ~600 °C.

We report the results of a comparative study to explore the usefulness of 3D measurements of confined fission track lengths (TINTs) relative to horizontal confined track length measurements (dips ≤10°), and evaluate their suitability for thermal history modeling. Confined fission track lengths were measured in 10 annealed Fish Canyon Tuff apatites containing synthetic mixtures of different length components, and two Durango apatites containing spontaneous fission tracks. Measurements were primarily carried out using a digital image-based microscope system, and they were compared to those from a regular optical drawing tube-digitizing tablet setup, and a confocal laser scanning microscope. The results indicate that 3D measurements of confined track lengths are closely comparable to conventional horizontal track measurements, and the mean track lengths of inclined (dips >10°) and horizontal (dips ≤10°) confined tracks from the one sample are equivalent within the measurement uncertainty. A strong dip-bias was observed, so that almost all the confined tracks measured were dipping at <30°, and the great majority (~70%) were dipping at ≤10°, thereby qualifying as “horizontal” confined tracks. Our results suggest that a useful increase of more than 40% in sample size can be achieved from including dip-and refraction-corrected 3D track length measurements. Some evidence was seen for a small bias in favor of shorter tracks at higher dip angles but this has very little influence on the mean lengths or length distributions up to the practical limit of dips (~30°) observed in these measurements. Results obtained using the same measurement system by a single analyst over time and between six different observers in the one laboratory show good reproducibility. These results also agree well with conventional horizontal confined track length measurements in the same samples in the second laboratory involved. We conclude that 3D measurements of confined track lengths, including both horizontal and inclined tracks, are suitable for use in current fission track annealing models derived from experiments using horizontal confined tracks.

Subsolidus and melting relationships for the system CaCO 3 –MgCO 3 have been reexamined using a Kawai-type multi-anvil apparatus at 3 and 6 GPa in graphite capsules. Phase boundaries were delineated according to the chemical composition of phases measured by electron microprobe in energy dispersive mode and identification of crystal phases by Raman spectroscopy. At 3 GPa, the dolomite-magnesite solvus intersects the melting loop at about 1250 °C, and the isothermal three-phase line so produced represents the peritectic reaction: dolomite (Ca# 43) = magnesite (Ca# 13) + liquid (Ca# 48), where Ca# = 100·Ca/(Ca+Mg). The melting loop for the CaCO 3 –MgCO 3 join extends from 1515 °C (CaCO 3 ) to 1515 °C (MgCO 3 ) through a liquidus minimum at 1230 °C (near 53 mol% CaCO 3 ). Starting from 1425 °C at ≤ 30 mol% CaCO 3 in the system, the liquid quenches to dendritic carbonate and periclase and contains rounded voids, indicating an incongruent melting reaction: MgCO 3 (magnesite) = MgO (in liquid) + CO 2 (fluid and/or liquid). At 6 GPa, aragonite + magnesite assemblage is stable up to 1000 °C. The reaction aragonite + magnesite = dolomite locates between 1000 and 1050 °C. The presence of dolomite splits the system into two partial binaries: aragonite + dolomite and dolomite + magnesite. The dolomite-magnesite solvus intersects the melting loop between 1400 and 1450 °C, and the isothermal three-phase line so produced represents the peritectic reaction: dolomite (Ca# 31) = magnesite (Ca# 21) + liquid (Ca# 57). The melting loop for the CaCO 3 –MgCO 3 join extends from 1660 °C (CaCO 3 ) to 1780 °C (MgCO 3 ) through a liquidus minimum at 1400 °C and 62 mol% CaCO 3 . The compositions of carbonate crystals and melts from the experiments in the carbonated eclogite (Yaxley and Brey 2004) and peridotite (Dalton and Presnall 1998) systems are consistent with the geometry of the CaCO 3 –MgCO 3 melting loop at 3 and 6 GPa: Ca-dolomite melt coexists with Mg-calcite in eclogite and peridotite at 3 GPa and dolomite melt coexists with magnesite in peridotite at 6 GPa.

Monazite is an important host of rare earth elements in the lithosphere including redox-sensitive Ce, which may occur as trivalent and tetravalent in terrestrial environments. Here, monazite solubility is explored as a function of oxygen fugacity through a series of dissolution experiments in alkali-rich and H 2 O fluids at 925 °C and 1.5 GPa. The oxygen fugacity was controlled with seven different solid-state buffers and ranged from about the iron-wüstite to above the magnetite-hematite equilibrium reactions. The solubility of natural monazite increases monotonically at oxygen fugacities equal to or higher than the fayalite-magnetite-quartz equilibrium. Electron microscopy reveals incongruent dissolution at Ni-NiO and above, where Ce-oxide is observed with monazite as a stable phase. Solubility experiments were also conducted with synthetic crystals (CePO 4 , LaPO 4 , and Th+Si-doped monazite). End-member CePO 4 exhibits profound changes to the surface of the crystal under oxidized conditions, with erosion of the crystal surface to depths of ~100 μm or greater, coupled with precipitation of Ce-oxide. In contrast, the solubility of LaPO 4 shows no sensitivity to the redox state of the experiment. The addition of Th (~3 wt%) and Si (~0.3 wt%) to monazite promotes crystal stability under oxidizing conditions, though small ThO 2 -CeO 2 (5–10 μm) crystals are present on the surfaces of these crystals, whose abundance increases at higher oxygen fugacities. In aggregate, these experiments show that the stability and solubility of monazite is affected by oxygen fugacity, and that the redox state of a fluid may be partially responsible for redistribution of rare earth elements and phosphorus in the crust. Lithospheric fluids with oxygen fugacities at or above the fayalite-magnetite-quartz equilibrium may contribute to some of the complex textures, variable chemistry, and age relationships observed in natural monazite.

To find the thermodynamically stable crystal structure of CaSiO 3 perovskite (CaPv) at high pressure and 300 K, we have conducted synchrotron X-ray diffraction (XRD) on thermally stress-annealed samples in a Ne pressure medium in the diamond-anvil cell at 28–62 GPa. Rietveld refinements of the diffraction patterns are significantly improved in fitting the positions and intensities of the split lines of CaPv if the starting model is a tetragonal perovskite-type structure with the SiO 6 octahedral rotation around the tetragonal c -axis. The result is in contrast with other previous experiments, but is consistent with first-principles calculations, reconciling the discrepancy between computations and experiments on the crystal structure of CaPv. We attribute the observed difference to the formation of the thermodynamically more stable phase under improved stress conditions in our experiments. Our fitting shows that the bulk modulus of CaPv is 223 ± 6 GPa when its pressure derivative fixed to 4, which is also consistent with first-principles calculations. The previous observations of the diffraction patterns of CaPv inconsistent with the first-principles studies could be due to the formation of a metastable crystal structure of CaPv under elevated deviatoric stresses.

A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not “accidental” xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly “metamorphic” trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be “magmatic” rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate “metamorphic” from “magmatic” gem corundum ought to be used with caution.

In this paper, we describe a unique occurrence of submicrometer polycrystalline zircon and arborescent aeschynite microcrystals on anatase with truncated bipyramidal habit, which were formed during low-temperature hydrothermal alteration of miarolitic cavities in a granophyre from Cuasso al Monte (Italy). The results suggest that temperatures in the range 100–250 °C and acidic conditions characterized the formation of anatase and that the preservation of this metastable phase was due to the presence of Nb (Nb/Ti atomic ratio ~0.032). We also detected an increase in the Nb content along the anatase rim. This is related to the incipient segregation of Nb driven by the transition toward the thermodynamically more stable rutile phase, which is possibly arrested by cooling below the diffusion blocking temperature. In these samples, zircon clearly postdates anatase and predates aeschynite. The resulting possible scenario is that F-rich hydrothermal fluids altered primary minerals at relatively high temperature and then, on cooling below 250 °C, deposited in the following sequence: anatase, zircon, and aeschynite, which reflects both elemental saturation and fluid fractionation with decreasing temperature. Finally, the aeschynite identified in this study through TEM-EDS shows an unusual chemical composition [(Fe 0.35 Ca 0.22 Th 0.15 U 0.13 Y 0.10 Pb 0.11 ) Σ1.06 (Nb 1.05 Ti 0.69 Al 0.14 As 0.06 ) Σ1.94 O 6 ], which cannot be reconciled with any known aeschynite sub-species. Conventional classification suggests that it should be called niobioaeschynite-(Y), although Y is present at only 0.10 apfu, and Fe (0.35 apfu) predominates in the A-site. This suggests that the aeschynite-group mineral classification system should be re-considered.