Volume 101, Issue 5

It is now widely accepted that Earth’s transition zone, located at depth between 410 and 670 km, is most likely hydrated. However, a definite conclusion has yet to be reached regarding the nature of the hydrous phase or phases that have the capacity to efficiently transport water down to such depths. In the April issue of American Mineralogist , Nishihara and Matsukage (2016) show that (FeH) 1–x Ti x O 2 can be stable in wet basalts and sediments at high pressures and temperatures. These phases allow subducting lithosphere to transport far more water to the mantle transition zone than previously thought possible.

Eruption forecasting is a central goal in volcanology. In recent years, eruption forecasts have achieved great success due to the increased monitoring of active volcanoes. However, understanding the physical processes responsible for volcanic unrest remains a challenge. In the January issue of American Mineralogist , Viccaro et al. (2016) linked signals of seismic unrest to magma mixing events responsible for the 2010 eruption at Eyjafjallajökull in Iceland. Their study represents a multi-disciplinary effort in which integration of petrological and geophysical observations leads to a better understanding of how volcanoes work, by providing a look into Eyjafjallajökull’s magmatic plumbing system and estimates of its magmatic ascent rates. This information is key to interpreting monitoring data and successfully forecasting eruptions.

The short-lived 182 W- 182 Hf isotope system is useful for constraining early mantle and core differentiation processes (e.g., Klein et al. 2002), if its siderophile behavior is understood. With the publication of the bulk thermoelastic properties of the high-pressure polymorphs of WO 2 in the January issue of American Mineralogist , Shofner et al. (2016) determine the relative energetics of the W-WO system as a function of pressure and temperature, thus defining the W-WO redox equilibrium relative to Fe-FeO, Ni-NiO, and Co-Co, at deep Earth conditions.

Charles Darwin analogized the diversification of species to a Tree of Life. This metaphor aligns precisely with the taxonomic system that Linnaeus developed a century earlier to classify living species, because an underlying mechanism—natural selection—has driven the evolution of new organisms over vast timescales. On the other hand, the efforts of Linnaeus to extend his “universal” organizing system to minerals has been regarded as an epistemological misfire that was properly abandoned by the late nineteenth century. The mineral taxonomies proposed in the wake of Linnaeus can be distinguished by their focus on external character (Werner), crystallography (Haüy), or chemistry (Berzelius). This article appraises the competition among these systems and posits that the chemistry-based Berzelian taxonomy, as embedded within the widely adopted system of James Dwight Dana, ultimately triumphed because it reflects Earth’s episodic but persistent progression with respect to chemical differentiation. In this context, the pioneering work of Hazen et al. (2008) in mineral evolution reveals that even the temporal character of the phylogenetic Tree of Life is rooted within a Danan framework for ordering minerals.

This decade marks the centenary of the discovery of X-ray diffraction. The development of mineralogy as a scientific discipline in which the properties of minerals are understood in terms of their atomic-scale structures has paralleled the development of diffraction crystallography. As diffraction crystallography revealed more precise details of mineral structures, more subtle questions about mineral properties could be addressed and a deeper understanding of the relationship between the two could be attained. We review the developments in X‑ray single‑crystal diffraction crystallography over the last century and show how its power to provide fundamental information about the structures of minerals has evolved with the improvements in data quality and the increased technological capacity to handle the data. We show that modern laboratory X‑ray diffraction data are of the quality such that mineralogical results are no longer limited by the data quality, but by the physical validity of the refinement models used to interpret the data.

The high-cosmic abundance of sulfur is not reflected in the terrestrial crust, implying it is either sequestered in the Earth’s interior or was volatilized during accretion. As it has widely been suggested that sulfur could be one of the contributing light elements leading to the density deficit of Earth’s core, a robust thermal equation of state of iron sulfide is useful for understanding the evolution and properties of Earth’s interior. We performed X-ray diffraction measurements on FeS 2 achieving pressures from 15 to 80 GPa and temperatures up to 2400 K using laser-heated diamond-anvil cells. No phase transitions were observed in the pyrite structure over the pressure and temperature ranges investigated. Combining our new P-V-T data with previously published room-temperature compression and thermochemical data, we fit a Debye temperature of 624(14) K and determined a Mie-Grüneisen equation of state for pyrite having bulk modulus K T = 141.2(18) GPa, pressure derivative KT′=5.56(24)$K_{\rm{T}}^\prime = 5.56\left( {24} \right)$, Grüneisen parameter γ 0 = 1.41, anharmonic coefficient A 2 = 2.53(27) × 10 −3 J/(K 2 ·mol), and q = 2.06(27). These findings are compared to previously published equation of state parameters for pyrite from static compression, shock compression, and ab initio studies. This revised equation of state for pyrite is consistent with an outer core density deficit satisfied by 11.4(10) wt% sulfur, yet matching the bulk sound speed of PREM requires an outer core composition of 4.8(19) wt% S. This discrepancy suggests that sulfur alone cannot satisfy both seismological constraints simultaneously and cannot be the only light element within Earth’s core, and so the sulfur content needed to satisfy density constraints using our FeS 2 equation of state should be considered an upper bound for sulfur in the Earth’s core.

Telica volcano in Nicaragua currently exhibits persistent activity with continuous seismicity and degassing, yet it has not produced lava flows since 1529. To provide insight into magma chamber processes including replenishment and crystallization, crystal size distribution (CSD) profiles of plagioclase feldspar phenocrysts were determined for Quaternary Telica basalts and basaltic andesites. Textural analysis of 14 highly crystalline lavas (>20 vol% phenocrysts) indicates that the samples are dominated by sieve-textured plagioclase feldspar phenocrysts whose origin requires thermochemical disequilibrium within the magmatic system. The CSD curves display an inverse relationship between phenocryst length and population density. Concave-up patterns observed for the Telica lava samples can be represented by linear segments that define two crystal populations: a steeply sloping segment for small crystals (<1.5 mm) and a gently sloping segment for crystals >1.5 mm in length. The two crystal populations may be explained by magma replenishment and a mixing model in which a mafic magma is introduced to a stable chamber that is petrologically and geochemically evolving. Residence times calculated using the defined linear segments of the CSD curves suggest these magmatic processes occur over timescales on the order of decades to centuries. The crystal size distribution and textural analysis advocate for the current persistent activity as being consistent throughout Telica’s historic and prehistoric eruptive periods and driven by replenishment of mafic magma.

The Howard’s Pass district (HPD) comprises 14 Zn-Pb sedimentary exhalative (SEDEX) deposits and is located within the Selwyn basin, Yukon, Canada. Although the HPD is renowned for its large accumulation of base-metal sulfides, in places the Late Ordovician to Early Silurian host rocks also contain abundant carbonate-bearing fluorapatite (CBFA). This mineral is present stratigraphically below, within, and above the SEDEX deposits and occurs as fine-grained layers that are interbedded with cherty carbonaceous mudstone. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometric analysis reveal that mineral compositions and rare earth element-yttrium (REE-Y) systematics, respectively, are remarkably similar throughout the stratigraphic succession. North American Shale Composite (NASC)-normalized La/Sm and La/Yb ratios indicate that the original REE compositions in CBFA have undergone only minor compositional modification subsequent to deposition. Uniformly negative Ce anomalies indicate that the mineral formed in analogous manner to modern and ancient sedimentary phosphorites under suboxic bottom-water conditions. Europium anomalies are mostly absent, indicating that reduced, slightly acidic high-temperature hydrothermal fluids were not a major source of REE-Y to CBFA. The chemical homogeneity of the mineral irrespective of its stratigraphic position indicates that a common process was responsible for its deposition within the sedimentary rocks of the HPD. On the basis of the similarity of the REE patterns to modern and ancient phosphorites, and the absence of positive Eu anomalies, we conclude that the CBFA is of hydrogenous origin, and not hydrothermal as suggested by previous workers. As such, phosphorite formation in the HPD is casually related to SEDEX Zn-Pb deposit formation.

The frequent occurrence of iron slag in excavation layers of the Hellenistic to Byzantine city of Sagalassos (SW Turkey), indicates continuous iron working from the 1st to 7th century A.D. The slag samples are identified as smithing hearth bottoms and are found in dump fills, building foundations, and road constructions. Although several smelting sites are present in the 1800 km 2 large territory of the city, the chemical signature of the smelting slag does not correspond to the characteristics of the smithing slag found in the city itself. The smithing slag does offer insight in the smithing techniques applied, which in turn provides information about the objects produced and production chain or “chaîne opératoire” followed for iron production in Sagalassos. Textural and mineralogical analyses reveal two main smithing techniques that were applied throughout the 1st to 7th century A.D. The most commonly applied smithing technique is used for the production of simple bulk materials, like hammers or anvils. Microscopic textures in the slag show mainly mechanical deformation at relatively high temperatures. The second, more sophisticated smithing technique shows the blacksmith treated the iron at different temperatures. The presence of high-lime contents indicates the use of a flux and/ or a protective agent against oxidation. This technique was applied to make more complex objects or objects with cutting edges. Based on the reconstruction of the smithing techniques, the iron production of Sagalassos produced mainly everyday bulk material. The current study demonstrates the importance of using the mineralogy of smithing slag in reconstructing the technological evolution of iron production. Microtextural and mineralogical studies and comparison with modern smithing techniques offer greater insight in the complex world of Roman blacksmithing.

Iron-bearing periclase is thought to represent a significant fraction of Earth’s lower mantle. However, the concentration of iron in (Mg,Fe)O is not well constrained at all mantle depths. Therefore, understanding the effect of iron on the density and elastic properties of this phase plays a major role in interpreting seismically observed complexity in the deep Earth. Here we examine the high-pressure behavior of polycrystalline (Mg,Fe)O containing 48 mol% FeO, loaded hydrostatically with neon as a pressure medium. Using X-ray diffraction and synchrotron Mössbauer spectroscopy, we measure the equation of state to about 83 GPa and hyperfine parameters to 107 GPa at 300 K. A gradual volume drop corresponding to a high-spin (HS) to low-spin (LS) crossover is observed between ~45 and 83 GPa with a volume drop of 1.85% at 68.8(2.7) GPa, the calculated spin transition pressure. Using a newly formulated spin crossover equation of state, the resulting zero-pressure isothermal bulk modulus K 0T,HS for the HS state is 160(2) GPa with a K0T,HS′$K_{0{\rm{T,HS}}}^\prime $ of 4.12(14) and a V 0,HS of 77.29(0) Å 3 . For the LS state, the K 0T,LS is 173(13) GPa with a K0T,LS′$K_{0{\rm{T,LS}}}^\prime $ fixed to 4 and a V 0,LS of 73.64(94) Å 3 . To confirm that the observed volume drop is due to a spin crossover, the quadrupole splitting (QS) and isomer shift (IS) are determined as a function of pressure. At low pressures, the Mössbauer spectra are well explained with two Fe 2+ -like sites. At pressure between 44 and 84, two additional Fe 2+ -like sites with a QS of 0 are required, indicative of low-spin iron. Above 84 GPa, two low-spin Fe 2+ -like sites with increasing weight fraction explain the data well, signifying the completion of the spin crossover. To systematically compare the effect of iron on the equation of state parameters for (Mg,Fe)O, a spin crossover equation of state was fitted to the pressure-volume data of previous measurements. Our results show that K 0,HS is insensitive to iron concentration between 10 to 60 mol% FeO, while the spin transition pressure and width generally increases from about 50–80 and 2–25 GPa, respectively. A key implication is that iron-rich (Mg,Fe)O at the core-mantle boundary would likely contain a significant fraction of high-spin (less dense) iron, contributing a positive buoyancy to promote observable topographic relief in tomographic images of the lowermost mantle.

Allanite-fluorapatite reaction coronas around monazite are abundant in metamorphic rocks. We report here special cases where a new generation of “satellite” monazite grains formed within these coronas. Using examples from different P-T regions in the eastern Alps, we examine the origin and the petrological significance of this complex mineralogical association by means of the electron microprobe utilizing Th-U-Pb monazite dating and high-resolution BSE imaging. Satellite monazite grains form when a monazite-bearing rock is metamorphosed in the allanite stability field (partial breakdown of first generation monazite to fluorapatite plus allanite), and is then heated to temperatures that permit a back reaction of fluorapatite plus allanite to secondary satellite monazite grains surrounding the remaining original first generation monazite. Depending on the whole-rock geochemistry satellite monazites can form under upper greenschist- as well as amphibolite-facies conditions. In each of the three examples focused on here, the inherited core monazite was resistant to recrystallization and isotopic resetting, even though in one of the samples the metamorphic temperatures reached 720 °C. This shows that in greenschist- and amphibolite-facies polymetamorphic rocks, individual grains of inherited and newly formed monazite can and often will occur side by side. The original, inherited monazite will preferentially be preserved in low-Ca, high-Al lithologies, where its breakdown to allanite plus fluorapatite is suppressed. Conversely, a medium- or high-Ca, monazite-bearing rock will become particularly fertile for secondary monazite regrowth after passing through a phase of strong retrogression in the allanite stability field. Based on this knowledge, specific sampling strategies for monazite dating campaigns in polymetamorphic basement can be developed.

We investigated the effect of Mg 2+ on calcite hillock growth over a broad range of solution conditions in terms of supersaturation (Ω calcite ) and Mg/Ca ratios using atomic force microscopy and secondary ion mass spectrometry. We found that both the incorporation pattern/incorporated Mg 2+ quantity in the hillock structure and the Mg 2+ -induced morphological change of the hillock surface showed strong dependence of the growth conditions. Specifically, when Mg/Ca was high (i.e., >5) and Ω calcite was low (i.e., ~0.45), Mg 2+ was predominantly incorporated into the negative sectors of the hillock structure, resulting in gradual loss of step structure and morphological amorphism on these vicinal surfaces. When Mg/Ca and Ω calcite were in intermediate ranges (i.e., Mg/Ca < 5, and 0.45 < Ω calcite < 1), the originally straight edges of the hillock steps exhibited curvatures of varying degrees and formed “tear-drop” morphologies. It is noted that such “tear-drop” morphology was stable within the duration of the experiments and did not evolve into other surface patterns. By contrast, when both Mg/Ca and Ω calcite were high (i.e., Mg/Ca > 5, and Ω calcite > 1.1), the growing hillocks experienced two phases of morphological changes, initiated with the formation of “tear-drops” followed by the development of linear ruptures along [ 4 81] and [44 1 ] directions. And the occurrence of these ruptures segmented the hillock surface effectively into multiple isolated plateaus. Significantly, we revealed the underlying mechanisms for these condition-specific effects of Mg 2+ on calcite growth, which mainly resulted from the interplay among three major factors: (1) the size-mismatch between Mg 2+ and Ca 2+ that causes structural strains in magnesian calcite and leads to morphological amorphism in high-Mg carbonate; (2) the asymmetry of the calcite crystal structure that sets a physical limitation for Mg 2+ incorporation patterns in the hillock structure; and (3) the step advancing rate (i.e., the calcite growth kinetics) that affects both Mg 2+ incorporation and the accommodation of Mg 2+ -induced structural strains in the hillock structure. Detailed discussions were given for each growth scenario. The results of our study provide a theoretical base to decipher the roles of Mg 2+ in CaCO 3 mineralization, and thus, have important implication for a range of processes that involve the growth of Mg-Ca-CO 3 systems, such as biomineralization, carbon capture and storage, and scale controls in industrial settings.

Understanding carbon speciation in Earth materials is important to unravel the geochemical evolution of the Earth’s atmosphere, the composition of mantle partial melts, and the overall distribution of carbon in the deep mantle. In an effort to provide the systematic protocols to characterize carbon-bearing fluid inclusions and other carbon-bearing species using high-resolution 13 C solid-state NMR, one of the element-specific probes of local structure around carbon, we explore the atomic configurations around the carbon species formed during the reaction between 13 C-enriched amorphous carbon and MgSiO 3 enstatite synthesized at 1.5 GPa and 1400 °C using 13 C MAS NMR spectroscopy and Raman spectroscopy. The Raman spectra for the fluid inclusion show the presence of multiple molecular species (e.g., CO 2 , CO, CH 4 , H 2 O, and H 2 ) and reveal heterogeneous distribution of these species within the inclusion. 13 C MAS NMR results show that the sharp peak at 125.2 ppm is dominant. While the peak could be assigned to either molecular CO 2 in the fluid phase or fourfold-coordinated carbon ( [4] C), the peak is likely due to fluid CO 2 , as revealed by Raman analyses of micrometer-sized fluid inclusions in the sample. The peaks at 161.2, 170.9, and 173.3 ppm in the 13 C NMR spectrum correspond to the carbonate ions (CO32−${\rm{CO}}_3^{2 - }$) and additional small peak at 184.5 ppm can be attributed to carbon monoxide. Based on the established relationship between 13 C abundance and peak intensity in the 13 C MAS NMR, the estimated 13 C amounts of CO 2 , CO32−${\rm{CO}}_3^{2 - }$, and CO species are much larger than those estimated from carbon solubility in the crystals, thus, indicating that those carbon species are from external phases. The 13 C NMR spectrum for amorphous carbon showed a peak shift from ~130 to ~95 ppm after compression, thereby suggesting that the amorphous carbon underwent permanent pressure-induced densification, characterized by the transition from sp 2 to sp 3 hybridization and/or pressure-induced changes in sp 2 carbon topology. While direct probing of carbon species in the crystalline lattice using NMR is challenging, the current results and method can be utilized to provide quantitative analysis of carbon-species in the fluid-inclusions in silicates, which is essential for understanding the deep carbon cycle and volcanic processes.

High-temperature oxide melt solution calorimetry using sodium molybdate (3Na 2 O·4MoO 3 ) solvent at 973 K was performed for the Na 3 x RE 0.67– x TiO 3 (RE = La, Ce) perovskite series. The enthalpies of formation of lanthanum perovskites from oxides (La 2 O 3 , Na 2 O, TiO 2 ), are –107.25 ± 2.56, –93.83 ± 6.06, –80.68 ± 5.93, and –33.49 ± 4.26 kJ/mol and enthalpies of formation from elements are –1614.05 ± 5.37, –1596.44 ± 7.68, –1594.03 ± 7.58, and –1577.56 ± 6.36 kJ/mol for Na 0.459 La 0.522 Ti 0.999 O 3 , Na 0.454 La 0.523 Ti 0.994 O 3 , Na 0.380 La 0.567 Ti 0.980 O 3 , and La 0.692 Ti 0.979 O 3 , respectively. The enthalpies of formation of cerium perovskites are –99.98 ± 5.78 and –45.78 ± 3.30 kJ/mol from oxides (Ce 2 O 3 , Na 2 O, TiO 2 ), and –1611.34 ± 6.90 and –1602.06 ± 2.72 kJ/mol from elements for Na 0.442 Ce 0.547 Ti 0.980 O 3 and Ce 0.72 Ti 0.96 O 3 . The A- site defect perovskites become more stable relative to oxide components as sodium contents increase. Na 0.5 Ce 0.5 TiO 3 and Na 0.5 La 0.5 TiO 3 could be considered as thermodynamically stable end-members in natural loparite minerals, in which these end-members are in solid solution with CaTiO 3 and other components.

Bastnaesite, [RE-CO 3 -OH/F] (RE = rare earth) is one of the major sources of rare earth elements found in commercial deposits at Mountain Pass, California, Bayan Obo, China, and elsewhere. Synthetic forms of bastnaesite have been explored for applications including optical devices and phosphors. Determination of thermodynamic properties of these phases is critical for understanding their origin, mining, and processing. We report the first experimental determination of formation enthalpies of several OH and F bastnaesites based on high-temperature oxide melt solution calorimetry of well-characterized synthetic samples. The formation enthalpies from binary oxides and fluorides for all the bastnaesite samples are highly exothermic, consistent with their stability in the garnet zone of the Earth’s crust. Fluoride bastnaesite, which is more abundant in nature than its hydroxide counterpart, is thermodynamically more stable. For both OH and F bastnaesite, the enthalpy of formation becomes more negative with increasing ionic radius of the RE 3+ cation. This periodic trend is also observed among rare earth phosphates and several other rare earth ternary oxides. For a given RE, the formation enthalpies from binary oxides are more negative for orthophosphates than for bastnaesites, supporting the argument that monazite could have formed by reaction of bastnaesite and apatite at high temperature. The difference in formation enthalpy of monazite and bastnaesite provides insight into energetics of such reactions along the rare earth series.

Studying the reason for the formation of two structural sub-types, seven arsenate and vanadate compounds of descloizite and adelite groups [specifically ( 1 ) CdCo(OH)(AsO 4 ), ( 2 ) CdCu(OH) (AsO 4 ), ( 3 ) SrCo(OH)(AsO 4 ), ( 4 ) SrZn(OH)(AsO 4 ), ( 5 ) SrCu(OH)(VO 4 ), ( 6 ) CdCo(OH)(VO 4 ), and ( 7 ) CdCu(OH)(VO 4 ) (bold numbers throughout paper refer to these compounds)] were synthesized under low-temperature hydrothermal conditions. 1 – 2 and 6 – 7 are isostructural with descloizite, and 3 – 5 are with adelite-group minerals and several synthetic compounds. Together with a sample of conichalcite, (8) CaCu(OH)(AsO 4 ), they were investigated using single-crystal X-ray diffraction [ R ( F ) = 0.0153–0.0283 for 1 – 5 and 8 ; for 6 and 7 , R ( F ) = 0.0603 and 0.0444, respectively] and Raman spectroscopy. Although crystallizing in different orthorhombic space groups, the atomic arrangements of descloizite-( Pnam ) and adelite-( P 2 1 2 1 2 1 ) type compounds adopt the same topology: the atomic arrangement is characterized by M 2O 6 octahedrons ( M 2 = Mg 2+ , Al 3+ , Mn 2+,3+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) edge-linked into chains. These chains are interconnected by X O 4 tetrahedrons ( X = Si 4+ , P 5+ , V 5+ , As 5+ , Mo 6+ ) into a three-dimensional framework. Cavities host M 1 atoms ( M 1 = Na + , Ca 2+ , Cd 2+ , Hg 2+ , Pb 2+ ); their coordination varies from 7 for descloizite-type representatives to 8 for adelite-type structures. The OH stretching frequencies in the Raman spectra are in good agreement with the observed O–H…O donor-acceptor distances. A detailed discussion of the crystal chemistry of these compounds and their influence on the space-group symmetry indicate a distinct dependence of the structural changes on the average ionic radii ( r M 1 + r X )/2.

The cores of the Earth and other differentiated bodies are believed to be comprised of iron and various amounts of light elements. Measuring the densities and sound velocities of iron and its alloys at high pressures and high temperatures is crucial for understanding the structure and composition of these cores. In this study, the sound velocities ( v P and v S ) and density measurements of body-centered cubic ( bcc )-Fe were determined experimentally up to 6.3 GPa and 800 K using ultrasonic and X-ray diffraction methods. Based on the measured v P , v S , and density, we obtained the following parameters regarding the adiabatic bulk K S and shear G moduli of bcc -Fe: K S0 = 163.2(15) GPa, ∂ K S /̸ P = 6.75(33), ∂ K S /∂ T = –0.038(3) GPa/K, G 0 = 81.4(6) GPa, ∂ G /̸ P = 1.66(14), and ∂ G /∂ T = –0.029(1) GPa/K. Moreover, we observed that the sound velocity–density relationship for bcc -Fe depended on temperature in the pressure and temperature ranges analyzed in this study and the effect of temperature on v S was stronger than that on v P at a constant density, e.g., 6.0% and 2.7% depression for v S and v P , respectively, from 300 to 800 K at 8000 kg/m 3 . Furthermore, the effects of temperature on both v P and v S at a constant density were much greater for bcc -Fe than for ε-FeSi (cubic B20 structure), according to previously obtained measurements, which may be attributable to differences in the degree of thermal pressure. These results suggest that the effects of temperature on the sound velocity–density relationship for Fe alloys strongly depend on their crystal structures and light element contents in the range of pressure and temperature studied.

Majindeite (IMA 2012-079), Mg 2 Mo 3 O 8 , is a new mineral, occurring as submicrometer-sized crystals with Ni-Fe and Ru-Os-Ir alloys, ± apatite and Nb-oxide. The observed assemblages are partially or wholly enclosed by MgAl 2 O 4 spinel in a Type B1 Ca-Al-rich inclusion, ACM-2 , from the Allende CV3 carbonaceous chondrite. The type majindeite has an empirical formula of (Mg 1.57 Fe 0.43 )Mo 3.00 O 8 , and a nolanite-type P 6 3 mc structure with a = 5.778 Å, c = 9.904 Å, V = 286.35 Å 3 , and Z = 2, leading to a calculated density of 5.54 g/cm 3 . Majindeite likely formed during the subsolidus oxidation of Mo-rich precursor phase(s) included in Fe-Ni rich alloys in a system that was open to O, Mg, and Ca, which were derived externally and introduced via cracks, subgrain boundaries, and/or surfaces exposed at the exterior of the spinel. If magnetite existed in the phase assemblage, it was lost due to Fe volatilization prior to the formation of majindeite. The immediate precursor to majindeite was likely kamiokite. Majindeite formed during an oxidation event contemporaneous with or postdating the formation of grossular-rich veins in melilite. Kamiokite, the Fe-rich analog of majindeite, also occurs in ACM-2 but only within phase assemblages that contain magnetite and which are entirely enclosed in melilite ± alteration products. Here, grossular-rich veins are not observed and the coexisting awaruites are more Fe-rich than those observed with majindeite. As with majindeite, the precursors for kamiokite grains were also likely to have been Mo-rich alloys, but the Mo-oxide remained magnetite-saturated throughout the alteration process and therefore remained Fe-rich.

Microanalysis of Fe 3+ /ΣFe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe 3+ were employed based on (1) area-normalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R 2 correlation with Fe 3+ /ΣFe, (3) the ratio of spectral intensities at two energies determined by highest R 2 correlation with Fe 3+ /ΣFe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe 3+ /ΣFe show significant improvements in accuracy over the former four types of univariate models. Fe 3+ /ΣFe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5–6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe 3+ vs. Fe 2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe 3+ /ΣFe.

The crystal structure and equation of state of coesite (space group C 2 /c ) and its high-pressure polymorph coesite-II (space group P 2 1 /n ) under pressure have been studied using X-ray powder diffraction in a diamond-anvil cell (DAC) up to 31 GPa at room-temperature and first-principles calculations at 0 K up to 45 GPa. New diffraction peaks appear above 20 GPa, indicating the formation of coesite-II structure. The calculated enthalpies provide theoretical support for the pressure-induced phase transformation from coesite to coesite-II at ~21.4 GPa. Compared with coesite, the coesite-II structure is characterized by a “doubled” b -axis and the breakdown of the linear Si1-O1-Si1 angle in coesite into two distinct angles—one is ~176°, close to linear, whereas the other decreases by 22 to 158°. Coesite is very anisotropic with the a -axis the shortest and twice more compressible than the b- and c -axis. By comparison, coesite-II is not so anisotropic with similar compressibilities in its a -, b -, and c -axis. As analyzed by a third-order Eulerian finite strain equation of state, the bulk modulus of coesite at 21.4 GPa is 182.3 GPa, and that of coesite-II is 140.8 GPa, indicating that coesite-II is much more compressible than coesite. The existence of coesite-II in the coldest subduction zone will change the elasticity and anisotropic properties of the subducting materials dramatically.

The accuracies of two different approaches to model thermodynamic mixing properties of solid solutions are explored using the rutile-cassiterite solid solution as an example. Both methods employ an expansion of the configurational enthalpy in terms of pairwise interactions energies. In the first method the partition function is directly computed from the excess energies of all Ti/Sn configurations within a 2 × 2 × 4 supercell. In the second method the free energy of mixing is calculated by a thermodynamic integration of the thermally averaged enthalpies computed with the Monte Carlo method using an 8 × 12 × 16 supercell. The phase relations derived from Monte Carlo simulations agree well with the available experimental data, under the condition that the free energy is corrected for the effect of the excess vibrational entropy. The direct calculation of the partition function provides reasonable phase relations only when the configurational entropy is corrected to be consistent with the ideal mixing in the high-temperature limit. Advantages and drawbacks of the both approaches are discussed. The findings are generally applicable to models of isostructural solid solutions.

The muscovite–paragonite (Ms–Pg) series [K 1– x Na x Al 2 (Si 4–y Al y )O 10 (OH) 2 ] is a group of micas with end-members of Ms ( x = 0, y ≍ 1) and Pg ( x = 1, y ≈ 1). This mineral series is found in the Earth’s crust and upper mantle. The series shows a wide immiscibility gap between the end-members. Density functional theory (DFT) is used to show the compression in five models of the 2 M 1 polytype Ms-Pg series to 6 GPa. Bulk moduli and cell-parameter moduli were obtained from a least-square fitting of pressures and volumes to a third-order Birch-Murnaghan equation of state. Bulk-modulus values of the end-members of the series agree with the range of experimental values. Bond lengths and atomic-group geometries were studied as a function of the pressure and composition of the series by determining the moduli. Compression mechanism has been determined. The excess volumes, V ex , were higher for the Na-rich members than for the K-rich members. V ex follow a Redlich-Kister behavior. The excess free energy, G ex , was calculated isobarically in a semiempirical way: the DFT excess volume data were calculated in one experimental model (A from Roux and Hovis 1996) in a Redlich-Kister function. The G ex as a function of the composition of the Ms-Pg join of the A model show two minima with constant composition to 0.75 GPa, evolving to richer end-member compositions at greater pressures. Therefore, the solvus should increase the gap of immiscibility at high pressure.

Joegoldsteinite, a new sulfide mineral of end-member formula MnCr 2 S 4 , was discovered in the Social Circle IVA iron meteorite. It is a thiospinel, the Mn analog of daubréelite (Fe 2+ Cr 2 S 4 ), and a new member of the linnaeite group. Tiny grains of joegoldsteinite were also identified in the Indarch EH4 enstatite chondrite. The chemical composition of the Social Circle sample determined by electron microprobe is (wt%) S 44.3, Cr 36.2, Mn 15.8, Fe 4.5, Ni 0.09, Cu 0.08, total 101.0, giving rise to an empirical formula of (Mn 0.82 Fe 0.23 )Cr 1.99 S 3.95 . The crystal structure, determined by electron backscattered diffraction, is a Fd 3 m spinel-type structure with a = 10.11 Å, V = 1033.4 Å 3 , and Z = 8.

The eruption and storage temperatures of rhyolitic magmas are critical factors for understanding the mechanisms of their eruption and petrogenesis. Temperatures are particularly important when comparing the magmatic histories of hot-dry rhyolites from the Yellowstone-Snake River Plain (YSRP) and Iceland to cold-wet rhyolites such as the Bishop Tuff. Here we employ mineral pair oxygen isotope fractionations for estimating rhyolite temperatures independent of pressure and other compositional factors. We report high precision oxygen isotope analyses of quartz, pyroxene, magnetite, and zircon that we use to estimate crystallization and storage temperatures. Temperatures for YSRP and Icelandic rhyolites are highest for quartz-magnetite and quartz-clinopyroxene (~950 °C), with lower quartz-zircon (850 °C) temperatures that are similar to estimates of zircon saturation. The magnitude and pattern of these temperatures is consistent with crystallization from near-liquidus rhyolites. In contrast, oxygen isotope temperatures calculated for the Bishop and other “cold-wet” type tuffs define low ~760 °C temperatures for all three mineral pairs consistent with prolonged mineral residence at near-solidus conditions. Preservation of a down-temperature crystallization sequence of hot magnetite and clinopyroxene with colder zircon in hot-dry YSRP and Icelandic rhyolites suggest <1000 yr magma residence, where magnetite does not have sufficient time to diffusively equilibrate oxygen in a lower temperature melt. This is consistent with recently determined high precision U-Pb crystallization ages zircons from the same units indicating magma generation shortly before eruption.

The full elastic tensors of two K-rich monoclinic alkali feldspars, Or 83 Ab 15 sanidine and Or 93 Ab 7 orthoclase, have been determined by using the Impulse Stimulated Light Scattering technique to measure surface acoustic wave velocities. The new data confirm that alkali feldspars exhibit extreme elastic anisotropy, so the bounds of their isotropic average properties span a wide range. The measured adiabatic moduli are, for Or 83 Ab 15 and Or 93 Ab 7 , respectively, K Reuss = 54.7(7), 54.5(5) GPa; K Voigt = 62.9(1.1), 64.4(0.6) GPa; G Reuss = 24.1(1), 24.5(1) GPa; and G Voigt = 36.1(5), 36.1(7) GPa. The small differences in moduli between the samples suggests that variations in composition and in state of Al, Si order only have minor effects on the average elastic properties of K-rich feldspars. The new measurements confirm that the earliest determinations of elastic wave velocities of alkali feldspars, widely used to calculate wave velocities in rocks, resulted in velocities systematically and significantly too slow by 10% or more.

In this study, Ca K -edge X-ray absorption spectroscopy (XAS) has been employed to study the Ca structural role in a set of silicate glasses. For the first time the accurate analysis of the Ca pre-edge peak has been performed, providing information about the different Ca local environments, and on its structural role. This approach was tested on Ca-aluminosilicate glasses (CAS system) with constant SiO 2 content (50 and 76 mol%), ranging from percalcic to peraluminous domains. In alkali-earth aluminosilicate glasses Al and Si are network formers in a large range of compositions, whereas calcium should act either as network modifier and charge compensator. The results obtained show a different Ca behavior as a function of Al content, which in turn is related to the Al local environment. For example, we observe changes in the pre-edge peak centroid energies depending on the changes in the role of Ca as a network modifier (Al = 0 mol%) to charge compensator (aluminosilicate join) to a more complex role in the peraluminous domain, depending on the fraction of [4],[5],[6] Al. This approach was further applied to study Fe-bearing glasses and, a diopside (CaMgSi 2 O 6 ) composition at different temperatures. It was possible to correlate the variations occurring in the Ca pre-edge peak centroid positions with both redox state and crystal nucleation mechanism.

Coupled substitution of Fe 3+ and H + for Si in wadsleyite was studied by polarized infrared and Mössbauer spectroscopies and single-crystal X-ray diffraction. Single crystals of Fe-bearing hydrous wadsleyite were synthesized at 16 GPa and 1870 K using a Kawai-type multi-anvil apparatus. Water and Fe contents of the sample were 0.19–0.26 wt% H 2 O and Fe/(Mg+Fe) of 0.099(2), respectively. Mössbauer spectra showed 13(4)% Fe 3+ /ΣFe and Fe 3+ at the tetrahedral site with 5(3)% IV Fe 3+ /ΣFe. Crystal structure refinement by single-crystal X-ray diffraction indicated that Fe (presumably Fe 3+ ) occupied 4.9(5)% of the tetrahedral site. Infrared light polarized with the electric vector E // a and c was absorbed at 3477(2) cm –1 while no absorption was observed at the region in spectra with E // b . The pleochroic behavior of the 3477 cm –1 band can be interpreted as protonation of silicate oxygen O3 in Fe-bearing hydrous wadsleyite. The protonation of O3 together with the presence of Fe 3+ at the tetrahedral site confirms the coupled substitution of Fe 3+ and H + for Si in Fe-bearing hydrous wadsleyite.