Band 98, Heft 5-6

With the advent of techniques that preclude mineral separation, ages from specific compositional domains in monazite [(Ce,La,Th)PO 4 ] have provided a wealth of information regarding the timing of the geologic evolution of numerous regions. However, confusion can arise when single grains show large differences in age that fail to correlate to chemistry or location within the monazite. Generalizations that lead to incorrect age interpretations include that monazite zoning in Y, Th, and/or the rare earth elements (REE) always identify: (1) distinct tectonic events; (2) environment of crystallization; and (3) provenance of detrital grains. Increasing Th contents in monazite do not always reflect: (1) increasing grade in metamorphic grains; (2) changes in silicate melt composition in igneous grains; (3) make the mineral more susceptible to alteration; nor (4) control the mineral’s uptake of REE. Metamorphic monazites from Himalayan garnet-bearing rocks with coexisting allanite show no relationship between Th content and REE. Instead, chondrite-normalized REE patterns of the allanite mirror those of the monazite, indicating the variations are related to the reactant that formed the mineral. Generalizations about Pb behavior in monazite remain problematic. Incorporation of Pb into monazite has thus far been precluded by experimental studies, yet common Pb has been measured in many studies of natural monazite. A clear understanding about controls of monazite composition and the role of the chemical and/or pressure-temperature (P-T) environment of the rocks in which it forms is required to correctly interpret the meaning of the mineral’s age(s).

Monazite (Ln 3+ PO 4 ) and related solid solutions are a well-known source of rare earth elements on earth. They may also accommodate large amounts of thorium and uranium without sustaining damage to the structure by self-irradiation. Such observations led to monazite-type structures being proposed as a potential host matrix for sequestering long-lived radionuclides produced during the nuclear fuel cycle and/or plutonium and americium from dismantled nuclear weapons. Monazite has two main advantages as a matrix for the containment of radioactive waste (or “radwaste”). The first is a highly flexible structure that permits accommodation of high concentrations of actinides. The incorporation of trivalent elements may be achieved by direct synthesis of An 3+ PO 4 (An 3+ = plutonium, Pu to einsteinium, Es), while tetravalent cation incorporation requires coupled substitutions, either on the anionic site (leading to monazite-huttonite solid solution) or on the cationic site (monazite-cheralite solid solution). Various methods developed for the preparation of such compounds are summarized here, as well as the experimental conditions required for the production of sintered pellets, with a particular focus on plutonium-bearing compositions. The second highly favorable property of monazite is its high chemical durability. Several experimental procedures developed to determine normalized leaching rates are reviewed, as well as results obtained from natural and synthetic monazite. Potential phases formed during dissolution were considered because they also partially control the concentration of actinides in the media. A preliminary list for such phases of interest, as well as corresponding thermodynamic data, is presented.

Ferrihydrite is the main form of ferric iron in surficial environments and a key reactive nanoparticle that regulates nutrient availability and the mobility of metal(loid) contaminants, yet its structure is not completely elucidated. Two models exist to date: the “f-phase” in which Fe is fully octahedral and the “akdalaite-model” possessing 20% of the Fe atoms in tetrahedral coordination. In this study, germanium was used as a structural probe to re-examine the validity of the latter model. Germaniumbearing ferrihydrites containing 0.2, 0.6, 1.4, 2.2, 2.9, 3.8, 12, and 15 wt% Ge were synthesized in the laboratory at 25 and 65 °C. X-ray diffraction analyses showed all the precipitates to be six-line ferrihydrite. Semi-quantitative energy-dispersive X-ray microanalyses (TEM) indicate that the precipitates made from solutions having Fe/Ge molar ratios of two and four have Fe/Ge atomic ratios of 3.8-3.9 and 4.4-5.1, respectively, which suggest a limit of Ge uptake in ferrihydrite of about 20 at% relative to total cations. Based on TEM examinations, these high Ge-bearing ferrihydrites are homogenous and consist of equant and plate-like crystallites about 5-6 nm in size. Furthermore, it appears that higher Ge concentrations in solution have no significant effect on the crystallite size, supporting the incorporation of Ge in the ferrihydrite structure. Extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that the Fe atoms in both the low and high Ge-bearing ferrihydrites are in octahedral coordination and that Ge occurs in the ferrihydrite structure by filling the empty tetrahedral sites and coordinating to 4 edge-sharing FeO 6 trimers through sharing a common oxygen (Ge-O-Fe linkage). Incorporation of the Ge tetrahedra in the ferrihydrite structure requires redistribution of Fe occupancy along the alternating O/OH layers while forming an ordered distribution of octahedral Fe and tetrahedral Ge. The local structure around Ge mimics a Keggin-like motif in two different, yet equivalent, orientations. It appears that the split diffraction peak at 1.46 and 1.51 Å is a characteristic feature of Ge-rich ferrihydrite and suggests that it is a fingerprint of increased order due to significant Ge incorporation in the tetrahedral sites. The findings can be rationalized in terms of the incorporation of Ge in the so-called “f-phase” of the classical ferrihydrite model, and demonstrate the flexibility of the model in terms of accommodating a Keggin-like cluster without the need of imposing unrealistic constraints as in the akdalaite model. Direct comparison of the imaginary parts of the Fourier transforms for ferrihydrite and maghemite further confirms the absence of tetrahedral Fe in ferrihydrite. The absence of tetrahedral Fe substantiates the use of goethite-like or akaganeite-like models to describe the polyhedral structure of ferrihydrite used in modeling sorption reactions at the ferrihydrite-water interface.

Taiban is a heavily shocked L6 chondrite showing opaque melt veins. Raman spectroscopy was used to characterize the high-pressure silicate assemblages in a thin section crossed by a shock-created 4 mm wide melt vein. Raman spectra using different excitation wavelengths allowed identification of mineral phases such as olivine, wadsleyite, ringwoodite, high-Ca clinopyroxene, majorite-pyrope, jadeite, maskelynite, and lingunite. Olivine is Fe depleted in contact with the ringwoodite, which suggests chemical fractionation during a solid-state olivine-ringwoodite transformation. Raman imaging revealed a close correlation between the blue ringwoodite color and the peak observed at 877 cm -1 ; this signal shows strong near-resonance Raman enhancement when measured with near-IR excitation lines (785 and 830 nm) close to the optical absorption bands of the ringwoodite. We propose that the blue color of the ringwoodite is due to a small amount of iron in fourfold coordination inside the spinel structure, and that yields the observed spectral features in differently colored ringwoodite. Under the formation conditions of the studied silicate pocket, all enstatite transformed to a majorite-pyrope solid solution, whereas the high-Ca clinopyroxene likely remained unchanged. Maskelynite grains in the margins of the pocket often contain lingunite or are totally transformed to jadeite. Based on static high-pressure results, the mineral assemblages in the pocket suggest peak pressure in the 17-20 GPa range with maximum temperature (T max ) in the range 1850-1900 K as the formation conditions for this Taiban chondrite during shock.

Kangite (IMA 2011-092), (Sc,Ti,Al,Zr,Mg,Ca,⃞) 2 O 3 , is a new scandia mineral, occurring as micrometer-sized crystals with REE-rich perovskite and spinel in a davisite-dominant ultra-refractory inclusion from the Allende CV3 carbonaceous chondrite. The phase was characterized by SEM, EBSD, synchrotron micro-Laue diffraction, micro-Raman, and EPMA. The mean chemical composition of the type kangite is (wt%) TiO 2 36.6, Sc 2 O 3 26.4, ZrO 2 11.3, Al 2 O 3 7.0, Y 2 O 3 5.4, CaO 3.9, MgO 3.14, Dy 2 O 3 1.8, SiO 2 1.7, V 2 O 3 1.31, Er 2 O 3 0.92, FeO 0.8, Gd 2 O 3 0.60, Ho 2 O 3 0.40, Tb 2 O 3 0.18, Cr 2 O 3 0.09, ThO 2 0.04, O -0.3, sum 101.28, which leads to an empirical formula calculated on the basis of 3 O atoms of [(Sc 0.54 Al 0.16 Y 0.07 V 0.03 Gd 0.01 Dy 0.01 Er 0.01 ) 3+ ∑0.83 (Ti 0.66 Zr 0.13 ) 4+ ∑0.79 (Mg 0.11 Ca 0.06 Fe 0.02 ) 2+ ∑0.19 ⃞0.19]∑ 2.00 O 3 . Synchrotron micro-Laue diffraction (i.e., an energy scan by a high-flux X-ray monochromatic beam) on one type domain at submicrometer resolution revealed that kangite has a cation-deficient Ia3 bixbyite-type cubic structure. The cell parameters are a = 9.842(1) Å, V = 953.3(1) Å 3 , Z = 16, which leads to a calculated density of 3.879 g/cm 3 . Kangite is a new ultra-refractory mineral, likely originating through low-temperature oxidation of a Sc-,Ti 3+ -enriched high-temperature condensate oxide dating to the birth of the Solar System.

Diagenetic illite from the Proterozoic Chuanlinggou Formation, China, has been investigated using the techniques of transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometer (EDS). Polytypes of 1M d and 1M illite coexist in the diagenetic illite. The 1M d polymorph is predominant in the <0.2 μm size fraction, which has high Al and low Mg contents and irregular crystal shapes. Lattice-fringe imaging and one-dimensional structural imaging show abundant structure defects. The 1M d illite has both layer and sheet terminations caused by edge dislocations and layer displacements along c* caused by screw dislocations. These observations indicate that the disordered 1M d structure of the diagenetic illite resulted from abundant dislocations. The dislocation serves as nucleation sites for the illite nano particles to spontaneously nucleate in the initial stage of illite formation. Such a nucleation process obeys the Ostwald ripening rule. The illite only shows a 1.0 nm thickness for each layer in high-resolution TEM images, which suggests that the illite crystals have no mixed smectite layer, and the 1M d illites are authigenic and not illitized from smectite or illite/smectite. Heterogeneous nucleation led to directly crystallization of the 1M d illite during the initial growth of illite in shale under a low-temperature and tectonic-stress-free environment. The results have implications for interpreting the distribution of 1M d illite in sedimentary rocks.

Trapped liquid in an igneous cumulate solidifies over a range of time and temperature that can be retrieved by use of the lever rule in binary solutions applied to plagioclase using the range in the An content found for the individual rock studied. The An range, so called, when measured in sufficient detail, defines the solidification history. The instantaneous solid composition along the solidus defines the zoning of the plagioclase as it follows the trapped liquid on the liquidus. The reference bulk composition of the trapped liquid is given by an intercept on the initial solid-liquid lever, defined by the fraction of plagioclase in the trapped parent magma times the residual porosity. The mafic fraction is assumed to solidify by reaction independently of the plagioclase zoning. The residual porosity is estimated from the content of evolved components in the trapped liquid and can also be calculated from the An range itself when that is calibrated to a value independently determined from the evolved components. Examples from a recent treatment of residual porosity are given for the solidification of selected rock compositions from the Kiglapait and Skaergaard intrusions. The same principles apply to the solidification of melt inclusions, with the difference that the latter tends to sample an evolved sheath by capture, rather than a parent magma trapped by closure of a cumulate, because only the cumulate has had time to exchange the evolved rejected solute owing to its slow solidification. Experimental examples of melt inclusions trapped at liquidus conditions demonstrate the highly evolved composition of the sheath compared to the bulk glass composition. The application of the solidification principle to presumed liquid immiscibility in the Skaergaard intrusion suggests that the melt inclusions are evolved during the capture process and then solidified as given by the An range in the plagioclase. The hypothesis of liquid immiscibility is not needed in this analysis. The very different end-stage feldspar histories of the two intrusions is attributed to the sequestering of low-melting feldspar components in the trapped liquids of the Skaergaard cumulates, compared to the lesser amounts trapped in the Kiglapait cumulates.

In the search for habitable environments on Mars, sites that have been altered by liquid water represent an important target. Quickly dissolving and precipitating carbonates, found in multiple locations on Mars, represent a key potential mineral indicator of locations with a history of aqueous alteration, and therefore possible habitability. Recent liquid surface water on Mars may differ significantly from the fluids, either sedimentary, hydrothermal, or pedogenic, that formed carbonate minerals on Mars. Interactions of carbonate rocks with such recent surface water may therefore cause changes in chemistry and mineralogy with depth that form weathering profiles recording the conditions of alteration. To test the prediction that martian carbonates may record a signature of aqueous alteration on Mars, we performed reactive transport modeling of putative carbonate outcrops on Mars using the reactive transport code CrunchFlow. Model inputs included two parent rocks chosen based on the carbonate-rich rock at Gusev Crater, Comanche, one consisting of 60% magnesite, 20% siderite, and 20% porosity, and one consisting of 50% magnesite, 20% siderite, 9% calcite, 1.6% rhodochrosite, and ~20% porosity. Reacting solutions consisted of four scenarios: acidic, sulfate-containing solutions, perhaps resulting from an acid vapor scenario; dilute solutions resulting from martian analog basalt dissolution; concentrated solutions resulting from modeled evaporation of the dilute solutions; and solutions based on the results of the Phoenix Lander. Modeling was performed under low oxidation and high oxidation conditions. Based on the modeling results, a carbonate rock would become enriched in siderite upon interaction with acidic water under low oxidation conditions; the same rock would become enriched in calcite after interaction with Ca-enriched solutions. A carbonate rock reacting under more oxidizing conditions would preferentially dissolve siderite, leaving a surface enriched in magnesite and ferric oxide. Therefore, the Mg-rich carbonates on Mars are consistent with possible interaction with liquid water under oxidizing conditions, similar to those measured at the Phoenix landing site. Future detailed chemical and mineralogical profiles through weathered rock into the unaltered interior of carbonate rocks may therefore allow the quantitative interpretation of the aqueous history and possible habitability of locations on Mars.

The large scale Targejia hot spring type Cs deposit in Tibet is unique and still active today with the distribution of hot springs being controlled by tectonic movements of the Tibetan Plateau. The ore bodies of the Targejia Cs deposit are mainly composed of geyserite, a Cs-containing opal. In this study, a combination of X-ray diffraction, infrared spectroscopy, and 29 Si, 1 H, 133 Cs magic angle spinning nuclear magnetic resonance (MAS NMR) were used to study the location and mobilization of Cs in geyserites. 29 Si NMR spectra indicate that the relative amounts of Q 2 , Q 3 , and Q 4 species vary in geyserite samples of different mineralization stages. Based on the 133 Cs chemical shift as well as the change in 29 Si chemical shift ranges (especially for Q 2 ), cesium is inferred to associate with Q 3 and Q 2 silanol groups, where it is coordinated by O 2- , OH - , and H 2 O as a network modifying cation. As Cs-bearing geyserite ages and dehydrates, Q 2 and Q 3 polymerize giving an increase in Q 4 . This is accompanied with a decrease in Cs content indicating that Cs may be leached out from the opals as it loses its original coordination environment in the silicate framework.

Phyllosilicates are among the most important stable minerals within the Earth’s crust. Their use as geo-thermometer bears great potential for application to the thermal history of rocks within the stability range of layered silicates and was tested here. A high-resolution analytical technique combining focused ion beam (FIB) milling and analytical electron microscopy (AEM) analysis has been applied to a series of sandstone core samples from the Gulf Coast (Texas, U.S.A.). The nanoscale compositional variations of K-deficient mica and chlorite flakes show that rim compositions are the most likely to approach equilibrium compositions, whereas core compositions may be relict, especially for illite-like phases. These rim analyses were used to test existing empirical or thermodynamically formulated thermo(baro)meters against maximum temperatures, which are perfectly constrained for the selected samples as they were measured in situ during drilling (100-230 °C and 300-1200 bars). The results show that most of the empirical models overestimate the temperature, while thermodynamic models yields reasonable estimates for diagenetic to anchizonal conditions, especially if the Fe 3+ content is taken into account. This study clearly shows that phyllosilicates thermometry is reliable when combined with an analytical technique giving access to the fine-scale compositional variations that may represent local equilibration, whereas using micrometric compositional analysis precludes trustworthy application of such thermometers.

This work reports on the thermal decomposition of muscovite within a granite powder doped with 8.5 wt% ZnO and heated during 10 min to 68 h at 1175 °C, and the implications for the sequestration of Zn, and other heavy metalts in such decomposition products. Samples were characterized using analytical scanning and transmission electron microscopy. After 10 min, muscovite is completely pseudomorphosed by Si-rich glass, spinel structure phases, and minor mullite. Spinel phases incorporate Zn, but their compositions depend on their position within the muscovite pseudomorphs. Al-rich oxides crystallize at the core of the pseudomorphs while Zn-Al spinels are located at the rims. The most Al-rich spinels have compositions close to γ-Al 2 O 3 , a metastable transition alumina, with up to 5 wt% MgO, 2 wt% Fe 2 O 3 , 4 wt% ZnO, and 9 wt% SiO 2 . The most Zn-rich spinels show compositions intermediate between Al2O3 and gahnite (ZnAl 2 O 4 ), with up to 31 wt% ZnO and significant contents of MgO (3 wt%), Fe 2 O 3 (5 wt%), and SiO 2 (10 wt%). After 68 h, stable spinels are gahnite close to the end-member composition with MgO and Fe 2 O 3 contents below ca. 5 wt%, and SiO 2 contents ca. 1 wt%. These results support the existence of a metastable solid solution between γ-Al 2 O 3 and gahnite. This experimental work shows that Zn can be incorporated in spinel structures after heating at 1175 °C during short durations and Zn is preferentially incorporated in the muscovite pseudomorphs as opposed to the Qtz-Fds glass. Consequently, the thermal breakdown of phyllosilicates can be a viable process to immobilize heavy metals such as Zn.

A series of hydrothermal diamond-anvil cell experiments have been conducted to evaluate the role of supercritical water on the isotopic equilibrium between H/D methane isotopologues at 600-800 °C and 409-1622 MPa. Raman spectroscopy was deployed to investigate the distribution of H/D isotopic molecules formed during hydrothermal decomposition of Si 5 C 12 H 36 in H 2 O-D 2 O aqueous solutions. To this end, the intensities of the fundamental vibrational C-H and C-D modes of deuteromethanes were employed to determine the thermodynamic properties of isotope exchange reactions between H/D isotopologues and to constrain the methane D/H molar ratios. By adjusting the initial volume ratios of silane/H 2 O-D 2 O, reactions in the CH 4 -D 2 O-H 2 O system were monitored for gaseous and supercritical-water phases. Discreet differences between the equilibrium constants, describing the relationship between the CH 3 D-CH 2 D 2 -CHD 3 -CH 4 species dissolved in supercritical water or present as a homogeneous gas phase, are revealed. The bulk D/H methane composition in the liquid-system is also twice that of the D/H molar ratios recorded in the gas-bearing system. Accordingly, condensedphase isotope effects are inferred to play a key role on the evolution of H/D isotopologues, likely induced by differences in the solubility of the isotopic molecules driven by the excess energy/entropy developed during mixing of non-polar species in the H 2 O-D 2 O structure. Our experiments show that isotope fractionation effects need to account for the presence of condensed matter (e.g., melts, magmatic fluids), even at conditions at which theoretical models suggest minimal (or nonexistent) isotope exchange, but comparable to those within the Earth’s interior.

The Ca-Sr fractionation between zoisite and, respectively, lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D(Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/ aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K zo A1 -af (Sr-Ca) = 3.38 to 41.08 for the A1 site and K zo A2 -af (Sr-Ca) = 0.45 to 6.51 for the A2 site. Assuming γ Ca af = γ Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields Δμ 0 = -29 kJ/mol and W zo A1 Sr-Ca = 5.5 kJ/mol for the A1 site and Δμ 0 = -1.1 kJ/mol and W za A2 Sr-Ca = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P2 1 /m form. Calculated lawsonite-aqueous fluidexchange coefficients indicate overall preference of Ca over Sr in the solid and are K law Cmc-af D(Sr-Ca) = 1.12 to 11.32 for orthorhombic and K law P21m-af D(Sr-Ca) = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming γ af Ca = γ af Sr and a symmetric mixing model yields similar values of Δμ 0 = -9 kJ/mol and W law Cmcm Sr-Ca = 10 kJ/mol for orthorhombic and Δμ 0 = -10 kJ/mol and W law P21/m Sr-Ca = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D zo-af Sr = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D law Cmcm-af = 0.6 ± 0.2 for orthorhombic lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and lawsonite-bearing rocks. For low Sr-concentrations, x zo Sr directly reflects x af Sr and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.

Infrared spectroscopy is a powerful technique for the characterization of minerals, permitting insights into their structural and thermodynamic properties. The intrinsic complexity of mineral solid solutions makes the interpretation of their spectroscopic data a challenging task. In this work, the IR vibrational spectra of andradite-grossular (Ca 3 Fe 2 Si 3 O 12 -Ca 3 Al 2 Si 3 O 12 ) solid solutions were simulated at the ab initio level with the CRYSTAL09 code by using a large allelectron Gaussian-type basis set and the B3LYP hybrid functional. All the 23 symmetry-independent configurations resulting from the substitution of 1 to 8 Fe atoms with Al atoms in the 16a octahedral site of the andradite primitive cell were considered. The IR active transverse optical frequencies and their intensities were computed. Graphical representation of the spectra, animation of the modes and isotopic substitution of the cations were used as additional interpretation tools. The dominant highfrequency modes, corresponding to Si-O stretching motions, show a simple linear behavior of both frequencies and intensities with respect to the binary composition; this trend is related to the linear behavior of the mean lattice parameter. Also the frequencies of the low-energy bands show, roughly speaking, a linear dependence on composition; however, the behavior of the dominant intensities is more complicated and strongly connected to the Al and Fe atomic fraction. When considering different possible structures at fixed composition, some spectral features display a dependence upon short-range Y cation ordering. Overall, we show how ab initio calculations permit to analyze complex systems such as solid solutions, establishing relations among structure and properties and providing critical and robust interpretations to the experimental findings.

Cristobalite-tridymite blade nanocrystals cemented by SiO 2 -glass and tridymite nanocrystals fill separately SiO 2 -glass spherules released by explosive volcanism, in rhyolitic tuffs from the Province of the Mesa Central, Mexico. This paper presents the mineralogy of silica polymorphs, occurrence, and process of formation in the Province of the Mesa Central. The understanding of the origin of these pure SiO 2 -polymorphs, their association with high-temperature minerals, fractionation of magmas, and role of volatiles contribute to our knowledge on silica minerals, their technological implications and damaging health effects. Results indicate that a precursor magma, from which kyanite crystallized, partitioned into an immiscible Fe-rich magmatic liquid that crystallized Fe-cordierite, Fe-amphiboles, and fayalite and into a siliceous melt that led to low-temperature glasses of 78.22-80.01 wt% SiO 2 and Si/Al ratio 4.07-5.65. Presence of amphiboles, sulfur in cristobalite-tridymite crystals and alunite suggest association of volatiles. The crystallization of silica polymorphs is associated with the dissolution of water vapor and volatiles in the precursor magma, establishing a silicate melt-water system of two critical points, one of them at pressure and temperature near the critical point of water and another close to the critical point of SiO 2 , and defining a critical curve between them and supercriticality at pressures and temperatures lower than the critical point of SiO 2 . Decreasing the ambient conditions from supercritical to subcritical would have allowed the separation of liquid and gases and the crystallization of cristobalite-tridymite and tridymite nanocrystals from the vapor phase. Cristobalite single crystals were not formed and transformation of cristobalite and trydimite to quartz did not occur. We conclude that glasses did not crystallize cristobalite or tridymite. Components in excess of the pure phases precipitated as nanoparticles of siliceous glass forming agglomerates, some containing iron hydroxides and alunogen.

Raman spectroscopy has been employed to investigate possible compositional effects on the high-pressure phase transition of Mg-rich orthoenstatite to a newly discovered P2 1 /c phase. Three natural orthoenstatite (OEN) samples were used in this study: near end-member Mg orthoenstatite (Zabargad Island, Egypt), Al-free, Fe-bearing orthoenstatite (Morogoro, Tanzania) and Al-rich, Fe,Ca-bearing orthoenstatite (Kilbourne Hole, New Mexico). Experiments were carried out at room temperature. For all samples, the high-pressure phase transition is characterized by a splitting of the 660-680 cm -1 doublet in the Raman spectrum into a triplet, with a corresponding change of peak intensities. These spectral changes are caused by the lowered symmetry of the high-pressure phase, as indicated by structural refinement from single-crystal X-ray diffraction results. The high-pressure phase of all samples appears to have space group P2 1 /c. No evidence for a C2/c phase was observed. Our results indicate that upon compression, the presence of 10 mol% Fe decreases the onset pressure of formation of the high-pressure P2 1 /c phase by about 1 GPa. Results for the Kilbourne Hole OEN show that upon compression, a combined enrichment of Al and Ca contents increases the onset pressure of formation of high-pressure clinoesntatite (HPCEN2) by over 3 GPa relative to Tanzania OEN. Upon decompression, all samples revert to single crystals of the orthoenstatite starting phase. Our measurements suggest that orthoenstatite is the prevalent phase of Mg-rich pyroxene throughout the uppermost mantle, whereas the newly discovered P2 1 /c phase might be present near the bottom of uppermost mantle, slightly shallower than the top of the transition zone.

The high-pressure and high-temperature equation of state of rock salt-structured cobalt oxide was measured up to 65 GPa and 2600 K using synchrotron X-ray diffraction in conjunction with the laser heated diamond-anvil cell. Fitting a Mie-Grüneisen-Debye model to the data we find best-fit parameters V 0 = 77.4 (fixed) Å 3 , K0 = 190 (1) GPa, K′ = 3.49 (4), γ 0 = 1.54 (4), q = 2.87 (15), and θ 0 = 517.8 K (fixed). We use this newly determined equation of state in conjunction with existing measurements of the thermoelastic parameters of cobalt metal to calculate the Gibbs free-energy difference between the cobalt oxide and cobalt metal phases as a function of pressure and temperature. A comparison of the energetics of the Co/CoO system with the Ni/NiO system predicts that below 58 GPa CoO+Ni is stable relative to NiO+Co, while above 58 GPa the reverse is true. This tipping point in energy can be mapped as a crossing point in the electrochemical potential of the two metal ions, suggesting that cobalt becomes more siderophile than nickel with increasing pressure. This result is in qualitative agreement with existing measurements of nickel and cobalt partition coefficients between mantle and core materials.

Kanemite belongs to the group of naturally occurring sodium silicate minerals that was first found in Kanem, at the edge of the Lake Chad, and has been synthesized in different ways from NaOH-SiO 2 mixtures and used as precursor for the design of microporous and mesoporous materials. The fluoride route to the synthesis of microporous materials is based on the substitution of OH - anions by fluoride anions, which may subsequently also play a mineralizing role, and gives rise to materials with higher hydrophobicity and thermal and hydrothermal stability. Moreover, F - plays an important role in the incorporation of framework heteroatoms, thereby affecting the activity of the final material. The aim of this study was to synthesize fluorokanemite using different synthetic routes and different F- source. The final product was characterized by a combination of methods that provided information regarding the incorporation of fluorine into the framework and the short- and long-range structural order of the fluorosilicate. Kanemite with water content close to ideal was obtained in all cases. The washing process was found to have no effect in the long- or short-range structural order of the layer framework, although it did affect the structure of the cation in the interlayer space of kanemite. The mineralizing agent therefore appears to be the key to the synthesis. Furthermore, it governs the resulting kanemite structure by controlling the formation of hydrogen bonds in the framework, and therefore the degree of lamellar structure condensation.

The crystal structure of halloysite, despite being one of the most commonly occurring clay minerals on the Earth’s surface, remains elusive. This paper reports on a multi-methodological study to shed more light on the atomic arrangement of halloysite from Olkhon Island, Lake Baikal, Russia, which has a prismatic morphology. It has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), selected-area electron diffraction (SAED) and, in particular, high-resolution transmission electron microscopy (HRTEM), to reveal its atomic structure and formation process. XRD analysis indicated a basal spacing of ca. 7.2 Å and two characteristic peaks with d = 4.28 and 4.03 Å on the tail of the 02,11 band. The halloysite grain cross sections displayed various prismatic forms, ranging from a rectangle to a regular 18-sectored polygon in SEM and TEM. The SAED pattern from a sector of the polygonal prisms with the incident beam parallel to the prism axes showed a regular one-layer oblique reciprocal lattice, similar to that of kaolinite along Yi-directions. HRTEM imaging performed with the new computer-assisted minimal-dose system and an incident beam perpendicular to the prism axis showed stacking of most dioctahedral 1:1 layers with their pseudo-mirror plane perpendicular to the prism axis and an almost random, or rather, alternating lateral stagger to the right or left from the preceding layer, which corresponds to interlayer displacements of τ + and τ - in Zvyagin symbols, or layer displacements of t 1 and t 2 used to describe the stacking in kaolinite. This stacking feature explains the SAED pattern from the side of the prismatic grains and the two characteristic peaks on the tail of the 02,11 band in the XRD pattern. Based on these results, it is proposed that tubular halloysite initially forms as a hydrated one with the pseudo-mirror plane of the kaolinite layers perpendicular to the tube axis, then dehydrates with, possibly, partially hydrogen-bonded interlayers, and finally transforms to a prismatic one consisting of sectored flat layers with the complete hydrogen-bonded interlayers. During this transformation, stacking with comparable ratio and frequent alternation of τ + and τ - is formed, to minimize morphological change of the tubes.

Fluorine is an important proxy for magmatic differentiation processes in the shallow parts of volcanic plumbing systems. Fluorphlogopite is one of the more important fluorine carriers in magmatic rocks. In the present study, a full crystal chemical investigation of fluorophlogopite 1M from Piano delle Concazze, Mt. Etna volcano, Italy, is carried out. The fluorophlogopite occurs in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the “Ellittico” eruptive center; ~15 ka BP). It is primarily associated with fluorapatite covered with amorphous SiO 2 and crystallized during syn/post-eruption pneumatolytic stages. The mica sample studied here is among the most Fe- and Ti-rich fluorophlogopite found in nature. EPMA data yielded the following mean chemical formula for this mineral (K 0.83 Na 0.13 )(Fe 2+ 0.44 Fe 3+ 0.09 Mg 2.18 Al 0.05 Ti 0.23 Mn 0.01 )(Al 0.92 Si 3.08 )O 10.64 (Cl 0.01 F 1.35 ). Structure refinements on four fluorophlogopite crystals, performed in space group C2/m, converged at R = 0.03-0.04, with cell parameters in the ranges a = 5.323-5.324, b = 9.219-9.222, c = 10.116- 10.119 Å, β = 100.1-100.3°. Major substitutions are OH - ↔ F - ,M 3+ -oxy ( VI M 2+ +OH- ↔ VI M 3+ +O 2- ) and Ti-oxy substitution: VI M 2+ +2(OH) - ↔ VI Ti 4+ +2O 2- . The fluorophlogopite from Piano delle Concazze exhibits the shortest c-parameter with respect to other fluorophlogopites found in nature. The short c parameter is essentially due to the absence of the hydroxyl group in favor of F - and especially of O 2- and to the thus increased attractive interaction between the interlayer cation and the anion content (F - , O 2- ) located at the O4 site. A comparison with other natural fluorophlogopites (namely from Biancavilla, Etna and Presidente Olegario, Brazil) show intermediate crystal-chemical features for the Piano delle Concazze fluorophlogopite. Particularly at Etna, differences in the chemical composition of the crystallized fluorophlogopites could be related to the various extent of enrichments by transfer of a gas phase achieved in specific parts of the volcanic plumbing system.

Pyroxenes have the capacity to incorporate both hydrogen and fluorine in their structures, and accurate measurement of these volatile elements can be used to constrain geophysical and petrologic processes in planetary bodies. For example, pyroxenes may be used to constrain the volatile contents of melts from which they crystallized. However, the experimental determination of H and F in pyroxenes is difficult, particularly at the relatively low levels present in natural samples. Here we evaluate methods for determining both H and F in orthopyroxene. We measured trace concentrations of H (~40-400 ppm H 2 O) and F (<1-17 ppm) in a suite of nine orthopyroxenes from varying geological environments, using secondary ion mass spectrometry (SIMS). The SIMS data for H (measured as 16 O 1 H, referenced to 30 Si and 18 O) are cross calibrated against Fourier transform infrared (FTIR) spectra, in turn calibrated against either manometry (Bell et al. 1995) or the frequency-dependent molar absorption coefficient derived by Libowitzky and Rossman (1997). Despite the fact that our samples exhibit a wide range of IR band structures, with varying percentages of absorbance split among low (2600-3350 cm -1 ) and high (3350-3700 cm -1 ) wavenumber bands, the SIMS data are fit with the same precision and virtually the same regression slope regardless of which IR calibration is used. We also confirm previous suggestions that the matrix effect for SIMS analyses between orthopyroxene and olivine is small (≤20%). Anomalously high yields of 16O1H in some analyses can be attributed to the presence of amphibole lamellae, and these analyses must be filtered out with different criteria than for olivine due to differences in the geometrical relationship of host to inclusion. For F, our derived values are highly dependent on analytical uncertainties related to the use of silicate glasses as standards. Regardless of the accuracy of our calibration, we see systematic differences in F concentrations in orthopyroxenes and olivines depending on their geological context. Samples derived from crustal environments and from Colorado Plateau minette diatremes have very low F (≤3 ppm), while higher contents can be found in megacrysts from South African kimberlites (up to 17 ppm in orthopyroxene and 47 ppm in olivine) and in xenocrysts from the Rio Grande Rift (Kilbourne Hole, 7-9 ppm in orthopyroxene).

Studies of coexisting, nominally anhydrous minerals in mantle samples show that clinopyroxene is an especially important host for hydrogen. Recent experimental studies have also shown that clinopyroxene may contain significant amounts of fluorine, which has implications for the F budget of the mantle. More accurate quantification of H and F is therefore a desirable goal. We measured H in 13 natural clinopyroxenes using Fourier transform infrared (FTIR) spectroscopy. 16 O 1 H/ 30 Si and 19 F/ 30 Si were also measured in the samples using secondary ion mass spectrometry (SIMS). H data were compared between the two techniques and F was calculated with reference to F-bearing silicate glass standards. Four of the clinopyroxenes are used as standards for SIMS calibration in multiple laboratories, and three have been measured previously using hydrogen manometry and/or elastic recoil detection analysis. Compared to clinopyroxenes in previous surveys comparing FTIR and SIMS, the 13 samples cover a broader range in chemistry and band positions in the O-H vibrational spectrum. They also all lack detectable amphibole lamellae, which are otherwise commonly present in this mineral group. In contrast to orthopyroxene, the SIMS and FTIR data for clinopyroxene show significantly better correlations (r 2 = 0.96-0.98) when the frequency-dependent IR calibration of Libowitzky and Rossman (1997) is applied, as opposed to the Bell et al. (1995) calibration (r 2 = 0.92-93). We derive a frequency-dependent molar absorption coefficient with parameters different from those of Libowitzky and Rossman’s calibration, which was established using data on stoichiometric hydrous phases and gives poor agreement with the manometrically determined value for PMR-53. Comparison of data for PMR-53 to our SIMS calibrations for orthopyroxene and olivine suggests that the matrix effect among these phases is less than 20% relative. Fluorine concentrations vary depending on geological context, with the highest concentrations (up to 214 ppm) found in diopsides from crustal metamorphic environments. Mantle samples follow similar geographic trends as olivines and orthopyroxenes, with higher F in xenocrysts from Kilbourne Hole (46 ppm) and South African kimberlites (up to 29 ppm) compared to the Colorado Plateau (8 ppm). On the basis of chemical correlations, we propose two different incorporation mechanisms for F: (1) coupled subsititution with Al 3+ and/or Fe 3+ in tetrahedral sites; and (2) coupled substitution with monovalent cations (Na and K) in the M2 site. The second substitution is more relevant to mantle augites than crustal diopsides. Our measured F concentrations are much lower than those in some clinopyroxenes synthesized in recent high P-T studies. Nevertheless, our data support suggestions that the F budget of the mantle can be entirely accommodated by incorporation in nominally anhydrous/fluorine-free minerals.

We propose a microscopic model of the dominant carbonate for phosphate substitution in fluorapatite. A well-crystallized sedimentary fluorapatite sample containing ~2.3 ± 0.8 wt% of carbonate was investigated using Fourier transform infrared spectroscopy (FTIR) and 13C and 19F magic angle spinning nuclear magnetic resonance (MAS NMR). About 75% of the carbonate groups replace the phosphate group (“B-site”), whereas a lesser contribution from carbonate groups located in the structural channels (“A-site”) is observed. Beside the dominant 19 F NMR signal of channel ions at ~ -102 ppm, an additional signal corresponding to ~8% of fluoride ions is observed at -88 ppm. 19 F double quantum-single quantum (DQ-SQ) MAS NMR and 13 C{ 19 F} frequency-selective Rotational Echo DOuble Resonance (REDOR) experiments prove that this additional signal corresponds to isolated fluoride ions in the apatite structure, located in close proximity of substituted carbonate groups. Density functional theory (DFT) calculations allow us to propose a composite carbonate-fluoride tetrahedron defect model accounting for these experimental observations. The planar carbonate ion lies in the sloping face of the tetrahedron opposite a fluoride ion occupying the remaining vertex, together replacing the tetrahedral phosphate ion. This “francolite-type” defect leads to a diagnostic narrow IR absorption band at 864 cm -1 that could be used as a guide to, e.g., detect the incipient transformation of fossil bone and teeth samples.

We report the first occurrence of kumdykolite in a meteorite (Sahara 97072, EH3). This orthorhombic form of albite occurs in the core of a concentrically zoned metal-sulfide nodule. In contrast to the terrestrial kumdykolite, the meteoritic sample has a domain structure that is consistent with either orthorhombic (Pmnn) or monoclinic (P2 1 ) space groups. The two symmetries are indicated by the presence or lack, respectively, of h + k = 2n + 1 reflections in [001] selected-area electron diffraction patterns, effects that likely result from different Si-Al ordering. Pmnn kumdykolite has only one tetrahedral site for Si and Al, whereas P2 1 kumdykolite would have three tetrahedral sites for Si and one for Al. We propose that kumdykolite formed above 1300 K and cooled rapidly enough to preserve its unique structure. Apparently, the cooling rate varied on the scale of nanometers allowing the local development of Si-Al ordering.

We describe the first observation of primary magmatic aragonite in carbonatite and carbonated syenite, occurring as xenoliths in a Pliocene basaltic diatreme located near the Hungary-Slovakia border. The aragonite-hosting matrix consists of disordered P-rich calcite, occasionally associated with trachyte glass. We interpret the aragonite growth as evidence of supra-lithostatic overpressure in the magmatic plumbing system that connected the crustal basaltic reservoir with the partial melting zone of the lithospheric mantle, and the disordered calcite ± trachyte as quenched residual, immiscible melts, generated close to the solidus of the carbonated alkali basalt differentiated in the crustal reservoir. The quenching event was a phreato-magmatic eruption within the stability field of the low-pressure calcite; this was triggered by advective overpressure, caused by expanding gas bubbles in a quasiincompressible silicate melt system. The high-pressure, pre-eruption origin of aragonite is indicated by enrichment in 13 C compared to the associated calcite interpreted as a record of CO 2 degassing at T > 500 °C. The oxygen (δ 18 O ranges of 22.1-24.5‰ V-SMOW in aragonite, 21.6-22.7‰ in calcite) and carbon (δ 13 C ranges of -4.4 to -5.9‰ V-PDB in aragonite, -11.9 to -12.7‰ in calcite) isotope signatures are consistent with a degassed carbonatite melt primarily derived from a subduction zone.