Volume 101, Issue 11

The rates of nucleation and crystal growth from silicate melt are difficult to measure because the temperature-time path of magma is often unknown. We use geochemical gradients around spherulites in obsidian glass to estimate the temperature-time interval of spherulite crystallization. This information is used in conjunction with new high-resolution X-ray computed tomography (HRXCT) data on the size distributions of spherulites in six samples of rhyolite obsidian lava to infer spherulite nucleation rates. A large data set of geochemical profiles indicate that the lavas cooled at rates of 10 -2.2 to 10 -1.2 °C/h, and that the spherulites grew at rates that decreased exponentially with time, with values of 10 -0.70 to 10 0.30 µm/h at 600°C. Spherulites are estimated to have begun nucleating when undercooling [ ΔT , = liquidus T (≈800°C) minus nucleation T ] reached 100-277°C, and stopped when ΔT = 203-365°C, with exact values dependent on assumed cooling and growth rates. Regardless of rates, we find that spherulites nucleated within a ~88-113°C temperature interval and, hence, began when ΔT ≈ 0.65-0.88 x T L , peaking when ΔT ≈ 0.59-0.80 x T L . A peak rate of nucleation of 0.072 ± 0.049 cm -3 h -1 occurred at 533 ± 14 °C, using cooling and growth rates that best fit the data set of geochemical profiles. While our inferred values for ΔT overlap those from experimental studies, our nucleation rates are much lower. That difference likely results from experimental studies using hydrous melts; the natural spherulites grew in nearly anhydrous glass.

Magma reservoirs play a key role in controlling numerous processes in planetary evolution, including igneous differentiation and degassing, crustal construction, and volcanism. For decades, scientists have tried to understand what happens in these reservoirs, using an array of techniques such as field mapping/petrology/geochemistry/geochronology on plutonic and volcanic lithologies, geophysical imaging of active magmatic provinces, and numerical/experimental modeling. This review paper tries to follow this multi-disciplinary framework while discussing past and present ideas. We specifically focus on recent claims that magma columns within the Earth’s crust are mostly kept at high crystallinity (“mush zones”), and that the dynamics within those mush columns, albeit modulated by external factors (e.g., regional stress field, rheology of the crust, pre-existing tectonic structure), play an important role in controlling how magmas evolve, degas, and ultimately erupt. More specifically, we consider how the chemical and dynamical evolution of magma in dominantly mushy reservoirs provides a framework to understand: (1) the origin of petrological gradients within deposits from large volcanic eruptions (“ignimbrites”); (2) the link between volcanic and plutonic lithologies; (3) chemical fractionation of magmas within the upper layers of our planet, including compositional gaps noticed a century ago in volcanic series (4) volatile migration and storage within mush columns; and (5) the occurrence of petrological cycles associated with caldera-forming events in long-lived magmatic provinces. The recent advances in understanding the inner workings of silicic magmatism are paving the way to exciting future discoveries, which, we argue, will come from interdisciplinary studies involving more quantitative approaches to study the crust-reservoir thermo-mechanical coupling as well as the kinetics that govern these open systems.

The Trans-Mexican Volcanic Belt (TMVB) is known for the chemical diversity in its erupted products. We have analyzed the olivine, pyroxene, and plagioclase mineral chemistry of 30 geochemically well-characterized mafic eruptives from Isla Maria at the western end of the arc to Palma Sola in the east. The mineral major oxide data indicate the dominance of open system processes such as antecryst uptake, and the scarcity of mineral-mineral and mineral-melt equilibria suggests that apart from forming microlites, erupted melts do not significantly crystallize during ascent. A combination of plagioclase antecryst chemistry and MELTS thermodynamic modeling of H 2 O-saturated isobaric fractional crystallization was employed to develop a pressure sensor aimed at determining the ponding depths of the co-genetic magmas from which the erupted plagioclase crystal assemblage originates. We show that the depth of magma-mush reservoirs increase eastward along the TMVB. We suggest that magma ponding is triggered by degassing-induced crystallization during magma ascent, and that the pressure sensor can also be regarded as a degassing sensor, with more hydrous melts beginning to degas at greater depths.Modeled initial magma H 2 O contents at the Moho range from ~4 to ~9 wt%. Magma-mush ponding depth variations fully explain the observed westward increase of average surface heat flux along the TMVB, supporting a new model of mafic arc magma ascent, where rapidly rising, initially aphyric melts pick up their antecrystic crystal cargo from a restricted crustal depth range, in which small unerupted batches of previously risen co-genetic magmas typically stall and solidify. This implies that, globally, mafic arc magmas may be used to constrain the depths of degassing and mush zone formation, as well as the amount of H 2 O in the primary melts.

Wayneburnhamite (IMA2015-124), Pb 9 Ca 6 (Si 2 O 7 ) 3 (SiO 4 ) 3 , is a new mineral from the Commercial quarry, Crestmore, Riverside County, California, where it occurs as a metasomatic mineral on fracture surfaces in vesuvianite/wollastonite rock. Wayneburnhamite crystals are sky-blue hexagonal tablets and prisms up to 0.5 mm in maximum dimension. The streak is white. Crystals are transparent to translucent with vitreous to resinous luster. The Mohs hardness is 3%, the tenacity is brittle, the fracture is conchoidal, and there is no cleavage. The calculated density is 5.271 g/cm 3 . The mineral is optically uniaxial (+), with m = 1.855(5), and e = 1.875(5) (white light). The pleochroism is E sky blue and O lighter sky blue; E > O weak. Raman and infrared spectra are consistent with the crystal structure, but suggest a very minor hydrous component. The empirical formula (based on 9 Si apfu) is (Pb 8.33 Sr 0.04 n 0.63 )Σ 9.00 (Ca 5.40 Cu 2+ 0.27 □ 0.33 )Σ 6.00 Si 9 S 0.21 O 32.64 Cl 0.05 . Wayneburnhamite is hexagonal, P 6 , a = 9.8953(9), c = 10.2054(7) Å, V = 865.40(17) Å 3 , and Z = 1. The eight strongest lines in the X-ray powder diffraction pattern are [ d obs in Å (I)(hkl) ]: 4.95(52)(110); 4.45(64)(111); 3.550(77)(112); 3.232(54)(120); 3.086(100)(121); 2.847(60)(300); 2.798(48)(113); and 2.734(83)(212). The structure determination ( R 1 = 3.01% for 1063 F o > 4σF ) shows wayneburnhamite to be an apatite polysome isostructural with ganomalite, differing only in that the site occupied dominantly by Mn in the structure of ganomalite is occupied dominantly by Ca in the structure of wayneburnhamite. The structure refinement of wayneburnhamite appears to represent a rare case in which the approximate locations of the Pb 2 + 6s 2 lone-electron pairs can be seen as electron density residuals.

Apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only U-bearing mineral available to date mafic rocks because primary zircon and/or baddeleyite are not present. In this paper, U-Pb LA-ICP-MS dating of apatite was applied to seven different dike and sill samples of dolerite from the Variscan belt of Brittany (Armorican Massif, western France). These dolerites, which are characterized by a within-plate tholeiite geochemical signature, are organized in several dense swarms across the belt. Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings. Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40 Ar/ 39 Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon. Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less. Consequently, the U-Pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions. Our results demonstrate that, in all cases, the apatite grains do carry enough radiogenic Pb to be dated by in situ U-Pb analyses and yield a 207 Pb-corrected mean age of 363.4 ± 5.8 Ma. These results reveal the existence of a major and short-lived magmatic event in the Variscan belt of Brittany during the Devonian-Carboniferous transition, a feature further highlighted by field evidence. Beyond the geological implications of these results, U-Pb LA-ICP-MS dating of apatite appears to represent an ideal tool to date small size mafic intrusions.

We have calibrated the infrared (IR) method for determining CO 2 concentrations in apatite with absolute concentrations obtained through nuclear reaction analysis (NRA). IR data were obtained on double-polished apatite wafers using polarized transmission IR spectroscopy. Due to the various sites and orientations of CO 2- 3 in apatite, the IR spectra are complicated and do not have the same shape in different apatite samples. Hence, simple peak heights are not used to characterize CO 2 concentrations in apatite. The total absorbance ( A total ) was derived using the integrated area under the curves in a given polarized spectral region. Then A tot!ll is calculated as A E//c + 2 A E±c . The calibration has been carried out for two wavenumber regions, one with high sensitivity and the other applicable to apatite with high CO 2 concentrations. The first calibration is for the fundamental asymmetric CO32−$\rm CO^{2-}_3$ stretching at wavenumbers of 1600-1300 cm -1 , and the CO 2 concentration in parts per million can be obtained as (0.0756 ± 0.0036) A total /d where d is sample thickness in centimeters. The fundamental stretching bands are strong and hence sensitive for measuring low CO 2 concentrations in apatite, down to parts per million level. The second calibration is for the CO32−$\rm CO^{2-}_3$ bands at wavenumbers of2650-2350 cm -1 , and the CO 2 concentration in parts per million is (9.3 ± 0.6) A total /d where d is sample thickness in centimeters. These bands are weak and hence are useful for measuring high CO 2 concentrations in apatite without preparation of super-thin wafers. The anisotropy is significant. The difference between A E//c and A E±c can reach a factor of 2.73. Hence, for high accuracy, it is best to use polarized IR to determine CO 2 concentrations in apatite. For rough estimation, unpolarized IR spectra may be used by estimating A total = 3 A unpol , where A unpol is the integrated absorbance from unpolarized spectra.

Phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids. We have carried out a petrographic study of the phosphate minerals, merrillite and apatite, in metamorphosed H group ordinary chondrites of petrologic type 4-6, to understand development of phosphate minerals and associated fluid evolution during metamorphism. In unbrecciated chondrites, apatite is Cl rich and shows textural evolution from fine-grained apatite-merrillite assemblages in type 4 toward larger, uniform grains in type 6. The Cl/F ratio in apatite shows a similar degree of heterogeneity in all petrologic types, and no systematic change in compositions with metamorphic grade, which suggests that compositions in each meteorite are dictated by localized conditions, possibly because of a limited fluid/rock ratio. The development of phosphate minerals in H chondrites is similar to that of L and LL chondrites, despite the fact that feldspar equilibration resulting from albitization is complete in H4 chondrites but not in L4 or LL4 chondrites. This suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions, and chemical equilibrium was not achieved throughout the H chondrite parent body or bodies during the late stages of metamorphism. A relict igneous clast in the H5 chondrite, Oro Grande has apatite rims on relict phenocrysts of (possibly) diopside that have equilibrated with the host chondrite. Apatite in the Zag H3-6 regolith breccia records a complex fluid history, which is likely related to the presence of halite in this meteorite. The porous dark H4 matrix of Zag, where halite is observed, has a high apatite/merrillite ratio, and apatite is extremely Cl rich. One light H6 clast contains similarly Cl-rich apatite. In a second light H6 clast, apatite compositions are very heterogeneous and more F-rich. Apatites in both H4 matrix and H6 clasts have very low H2O contents. Heterogeneous apatite compositions in Zag record multiple stages of regolith processing and shock at the surface of the H chondrite parent body, and apatite records either the passage of fluids of variable compositions resulting from different impact-related processes, or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith. Unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies, with implications for physical and mechanical properties of chondritic asteroids. The behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general: if impact-melt materials close to the surface of a chondritic asteroid are readily degassed, the volatile inventories of terrestrial planets could be considerably more depleted than the CI carbonaceous chondrite abundances that are commonly assumed.

Metasomatic alteration of fluorapatite has been reported in several iron-oxide apatite (IOA) deposits, but its effect on elemental and isotopic variations has not been well understood. In this study, we present integrated elemental, U-Pb, Sr, and O isotopic microanalytical data on fresh and altered domains in fluorapatite from the Taocun IOA deposit, Eastern China, to evaluate the timing and nature of the metasomatism and its effects on the ore-forming event. Orebodies of the Taocun deposit are spatially associated with a subvolcanic, intermediate intrusion, which displays zonal alteration patterns with albite in the center and increasing actinolite, chlorite, epidote, and carbonate toward the margin. Both disseminated and vein-type ores are present in the Taocun deposit, and fluorapatite commonly occurs with magnetite and actinolite in most ores. Fluorapatite grains from the both types of ores have been variably metasomatized through a coupled dissolution-reprecipitation mechanism. Many trace elements, including Na, Cl, S, Si, Mg, Sr, U, Th, and (REEs+Y), were variably leached from the fluorapatite grains during this process and the Sr and O isotopic signatures of the grains were also modified. The altered fluorapatite grains/domains have in situ 87 Sr/ 86 Sr ratios (0.70829–0.70971) slightly higher than those of the fresh fluorapatite (0.70777–0.70868), and δ 18 O values (–3.0 to +3.4‰) variably lower than the primary domains (+5.3 to +7.5‰). The Sr and O isotopes of the primary fluorapatite are consistent with or slightly higher than those of the ore-hosting intrusion, implying that the early-stage, ore-forming fluids were magmatic in origin but underwent weak interaction with the country rocks. U-Pb dating of the fresh and altered domains of the fluorapatite yielded indistinguishable ages of ~131 Ma, which are the same as the age of the ore-hosting intrusion. In combination with fluid inclusion data, we propose that the metasomatism of fluorapatite was induced by hydrothermal fluids at a late stage of the ore-forming event. The shifts to higher 87 Sr/ 86 Sr ratios and lower δ 18 O values in the altered fluor-apatite indicate that the alteration was induced by fluids with more radioactive Sr and lighter O isotope signatures. The metasomatic fluids were likely dominated by meteoric waters that were mixed with the earlier magmatic fluids and interacted with sedimentary rocks. Our study highlights that elemental and isotopic compositions of fluorapatite can be significantly modified by hydrothermal fluids during ore-forming events. Thus, instead of traditional bulk-rock analysis, in situ microanalysis is important to provide accurate constraints on the magmatic and/or hydrothermal evolution of complex ore-forming systems.

Acidic geothermal environments such as those in Lassen Volcanic National Park (LVNP) may provide novel organisms and enzymes for conversion of plant lignocellulose into ethanol, a process that typically requires hot and acidic pre-treatment conditions to hydrolyze cell wall polysaccharides to fermentable sugars. We evaluated seven Ascomycete fungi associated with LVNP’s Boiling Springs Lake (BSL) for utilization of lignocellulose material. We screened the fungi for growth pH and temperature optima, and for growth on purified or natural plant cell wall components. We also examined potential lignin degradation using a (per)oxidase assay, and screened for the presence of potential (hemi)cellulose degradation genes with PCR. Growth analysis showed Acidomyces and Ochroconis grew best at 35–45 °C and pH < 4, and grew up to 48–53 °C. In contrast, Aspergillus, Paecilomyces , and Penicillium preferred cooler temperatures for acidic media (25-35 °C), but grew up to 48 °C. Phialophora only grew up to 27 °C under both acidic and neutral conditions, and Cladosporium showed a preference for cool, neutral conditions. The most promising material utilizers, Acidomyces , Ochroconis , and Paecilomyces , used cellobiose and xylan, as well as pine and incense cedar needles, for growth at 40 °C and pH 2. Acidomyces and Ochroconis showed extracellular (per)oxidase activity at 40 °C and pH 2, and PCR screening showed Acidomyces , Paecilomyces , and Ochroconis contain orthologs to known fungal lignocellulose degradation genes, including glucanases and xylanases. We conclude that the BSL-adapted taxa Acidomyces, Ochroconis , and Paecilomyces may be promising sources of enzymes that combine heat- and acid-tolerance, potentially valuable in streamlining the pre-treatment of lignocellulosic biofuels.

Seven clinopyroxene compositions along the join M2 Li M1 Al T Si 2 O 6 (spodumene) to M2 Li M1 Fe 3+T Si 2 O 6 (ferri-spodumene) were synthesized at 2 GPa, 800 °C under highly oxidizing conditions (using H 2 O 2 fluid) in an end-loaded piston cylinder. In addition, the LiFe 3+ Si 2 O 6 composition was also synthesized under the intrinsically reducing conditions in a piston cylinder, to check the effect of f O 2 on iron speciation. The run products were characterized by field emission scanning electron microscope (FE-SEM), Rietveld refinements on XRPD synchrotron data, and space groups were assigned using SAED-TEM patterns. Run products are composed mainly of lithium clinopyroxene (Li-Cpx), plus minor amounts of hematite (magnetite under reducing condition) and corundum, as independently detected by image analysis (area%) and Rietveld refinements (wt%); moreover, Rietveld results were used to derive cell parameters, M1-site occupancy (Al vs. Fe 3+ ), atomic positions, and average bond lengths of all these Li-Cpx indexed in the C 2/ c space groups according to SAED-TEM. Li-Cpx with Al and Fe 3+ amounts close to 50:50 are actually slightly richer in Al apfu than nominal; the LiFe 3+ Si 2 O 6 grown under very oxidized and reducing conditions have very similar cell parameters, indicating that f O 2 is unable to induce a significant incorporation of Fe 2+ in these Li-Cpx. The replacement of Al with Fe 3+ induces a linear (%) increase of the cell edges following b > a > c , whereas β is roughly constant and the cell volume increases linearly. Furthermore, the substitution of Al with Fe 3+ only weakly affects the T-O average length (<1%), whereas M2-O and M1-O bonds increase linearly of 2.3 and 5.0%, respectively. These new experimental data have been compared with other available on Li-, Na-, and Ca-Cpx, i.e., M2 (Li,Na,Ca,Mg,Fe 2+ ) M1 (Mg,Fe 2+ Al,Ni,Cr,Ga,V,Fe 3+ ,Mn,Sc,In) T Si 2 O 6 , to model lattice strain, bond lengths, steric effects, and phase transitions behaviors. The replacement of Al with progressively larger cations in LiM 3+ Si 2 O 6 Cpx (M 3+ : Ni, Cr, Ga, V, Fe 3+ , Ti, Sc, and In) results in a linear increase following V > b > a > c , whereas β is roughly constant except for Ti-end-member and P 2 1 / c compositions. Lattice strains induced by X , T , and P for Li-Cpx in the C 2/ c stability field show that when the M1 site is progressively filled with a large cation, ε 1 axis (ε 1 > ε 2 > ε 3 ) increases along b , whereas ε 2 and ε 3 are nearly parallel to a and at about 30° from c . Conversely, T will provoke a similar enlargement of ε 1 and ε 2 along b and a edges, respectively, whereas ε 3 is again oriented at about 30° from c ; the increasing of P will instead shorten all strain tensor components (ε 1 , ε 2 , and ε 3 ) with a similar percentage amount; notably, high- P is the only stress that induces a strain component to be almost parallel to c edge. Moreover, finite lattice strains and orientation in C 2/ c LiMe 3+ Si 2 O 6 Li-Cpx induced by Me 3+ : Al-Fe 3+ , Fe 3+ -Sc, Sc-In are slightly different, with ε 1 invariably lying along b ; conversely, Li-Na cation substitution is completely different with the highest and lowest deformations on the ac plane and ε 2 along b ; ε 3 vector is negative and oriented at about 30° from T-chains. The ideal replacement of Al with larger cations up to In in Li-Cpx induces the M1-O, M2-O, and T-O average bond lengths to increase by 10.6, 4.3, and <0.5%. Steric effects in Li M1 Me 3+ Si 2 O 6 and Na M1 Me 3+ Si 2 O 6 Cpx are significant and very similar, whereas several other Me 1+ and Me 2+ substitutions in Cpx at both the M1 and M2 site, keeping fixed the other site, display less or even the absence of steric effects. Our new data also better elucidate relationships between Li-Cpx composition, symmetry at room and non-ambient conditions and T c . The aggregate cation radii at the M1 site does not exclusively control the stability of C 2/ c and P 2 1 / c polymorphs; instead valence electrons can profoundly favor the stabilization of a polymorph.

In low-temperature, high-pressure hydrothermal environments coesite transforms into hydrous forms of stishovite. We studied hydrous stishovite produced from hydrothermal treatment of silica glass as initial SiO 2 source at temperatures of 350–550 °C and pressures around 10 GPa. The P-T quenched samples were analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis, and IR and magic-angle spinning (MAS) NMR spectroscopy. The presence of significant amounts of H 2 O (ranging from 0.5 to 3 wt%) is shown from thermogravimetric measurements. PXRD reveals that at temperatures below 400 °C, hydrous stishovite is obtained as two distinct phases that may relate to the solid ice-VII environment present at prevailing P-T conditions. Initially formed hydrous stishovite is metastable and dehydrates over time in the low-temperature, high-pressure hydrothermal environment. The primary mechanism of H incorporation in stishovite is a direct substitution of 4H + for Si 4+ yielding unique octahedral hydrogarnet defects. In IR spectra this defect manifests itself by two broad but distinct bands at 2650 and 2900 cm –1 , indicating strong hydrogen bonding. These bands are shifted in the deuteride to 2029 and 2163 cm –1 , respectively. Protons of the octahedral hydrogarnet defect produce

Magnesian calcites are important components of sediments and biominerals. Although Raman spectra of calcite, dolomite, and magnesite are well known, those of magnesian calcites deserve further investigation. Nineteen syntheses of magnesian calcites covering the range 0–50 mol% MgCO 3 have been carried out at high pressure and temperature (1–1.5 GPa, 1000–1100 ℃). The crystalline run products have been characterized by μ-Raman spectroscopy. For all lattice and internal modes (L, T, ν 1 , ν 4 , 2ν 2 ) but ν 3 , wavenumbers align closer to the calcite– dolomite line than the calcite–magnesite line. The compositional dependence is strong and regression curves with high correlation coefficients have been determined. Full-width at half maximum (FWHM) plot along parabolas that depart from the calcite–dolomite or calcite–magnesite lines. The limited data dispersion of both shifts and FWHM allow using Raman spectral properties of magnesian calcites to determine the Mg content of abiotic calcites. A comparison with Raman data from the literature obtained on synthetic magnesian amorphous calcium carbonate (Mg ACC) shows that the wavenumber position of the ACC ν 1 mode is systematically shifted toward lower values, and that their FWHM are higher than those of their crystalline counterparts. The FWHM parameters of crystalline and amorphous materials do not overlap, which allows a clear-cut distinction between crystalline and amorphous materials. In synthetic magnesian calcites, the shift and FWHM of Raman bands as a function of magnesium can be interpreted in terms of changes of metal-O bond lengths resulting from the replacement of calcium by magnesium. The facts that the wavenumber of magnesian calcites are close to the calcite–dolomite line (not calcite-magnesite), that the FWHM of the T, L, and ν 4 modes reach a maximum around 30 ±5 mol% MgCO 3 , and that a peak specific to dolomite at 880 cm –1 is observed in high-magnesian calcites indicate that dolomite-like ordering is present above ~10 mol% MgCO 3 . Mg atom clustering in cation layers combined with ordering in successive cation basal layers may account for the progressive ordering observed in synthetic magnesian calcites.

A new chain-silicate mineral species, yangite, ideally PbMnSi 3 O 8 ·H 2 O, has been found on a specimen from the Kombat mine, Otavi Valley, Namibia. Associated minerals are melanotekite and rhodochrosite.Yangite is colorless to pale brown in transmitted light, transparent with white streak and vitreous luster.Broken pieces of yangite crystals are bladed or platy, and elongated along [010]. It is sectile with a Mohs hardness of ~5; cleavage is perfect on {101} and no twinning or parting was observed. The measured and calculated densities are 4.14(3) and 4.16 g/cm 3 , respectively. Optically, yangite is biaxial (-), with n α = 1.690(1), n β = 1.699(1), n γ = 1.705(1), Y = b, Z ˄ c = 11°, and 2 V meas = 77(2)°. It is insoluble in water, acetone, and hydrochloric acid. An electron microprobe analysis demonstrated that the sample was relatively pure, yielding the empirical formula (with calculated H 2 O) Pb 1.00 Mn 1.00 2+ Si 3.00 O 8 ·H 2 O.Yangite is triclinic and exhibits space group symmetry P 1¯$\overline{1}$ with unit-cell parameters a = 9.6015(9), b = 7.2712(7), c = 7.9833(8) Å, α = 105.910(4), β = 118.229(4), γ = 109.935(5)°, and V = 392.69(7) Å 3 .Its crystal structure is based on a skeleton of double wollastonite SiO 4 tetrahedral chains oriented parallel to [010] and interlinked with ribbons of Mn- and Pb-polyhedra. Yangite represents the first chain silicate with two-connected double chains and possesses all of the structural features of a hypothetical triclinic Ca 2 Si 3 O 8 ·2H 2 O phase proposed by Merlino and Bonaccorsi (2008)as a derivative of the okenite structure. The difference in the H 2 O component between the hypothetical phase and yangite likely is a consequence of the larger Pb 2+ with its lone-pair electrons in yangite replacing the smaller Ca 2+ in the hypothetical phase.

A hypothetical ideal wollastonite with regular octahedra and T3 tetrahedron is presented and used to compare and contrast the pyroxenes and pyroxenoids. While clinopyroxenes have close-packed arrangements of oxygen anions, several lines of evidence demonstrate that pyroxenoids do not. One such line is the number of tetrahedra in a single tetrahedral chain per octahedra in a single associated octahedral chain (interior to the octahedral band), referred to as the “single-chain T:O ratio,” which is 1:1 in pyroxenes but 3:2 in wollastonite and always greater than 1:1 in other MSiO 3 pyroxenoids. Because the Si-tetrahedron is extremely resistant to distortion, this forces marked distortion in at least one pyroxenoid octahedral site. The octahedral layers in pyroxenes and pyroxenoids are compared by placing them in the context of a fully occupied, closest-packed sheet of idealized octahedra, and it is shown that they are fundamentally different. The new mineral yangite is analyzed from the perspective developed in this study. It is structurally similar to the pyroxenoids, but the structure is a new type because it contains double tetrahedral chains and mixed polyhedral layers containing double chains of tetrahedra and bands of octahedra of width two. The tetrahedral chains are wollastonite-type chains and the wollastonite-type double chain is shown to have important differences from the amphibole double chain. A possible explanation for the existence of this crystal structure based on a hydrogen bond between Pb and O is presented.

Tourmalines, XY 3 Z 6 T 6 O 18 (BO 3 ) 3 V 3 W, are excellent petrogenetic indicators as they capture the signature of the host-rock bulk composition. Raman spectra of tourmalines can be used as fingerprints for species identification and crystal-chemical analysis. While Li-bearing species are directly distinguishable by the shape of the OH-stretching vibrations, the discrimination of Mg- and Fe-dominant species can be hindered by the coexistence of at least two types of octahedrally coordinated R n+ cations. Thirty Li-free tourmaline samples comprising 14 different species were studied by Raman spectroscopy and electron microprobe. All nine Fe 3+ -bearing samples were also analyzed by single-crystal X-ray diffraction and Mössbauer spectroscopy. The Raman scattering analysis shows that Mg-dominant species can be immediately distinguished from Fe-dominant species by the shape of the vibrational modes at ~200–240 cm –1 arising from the YO 6 vibrations. Trivalent Fe can be observed and quantified by shifts of the framework vibrations toward lower wavenumbers. The position of the main ZO 6 vibrational mode (275–375 cm –1 ) can be used to determine the Z Fe 3+ content, while the Y Fe 3+ content can be inferred from the position of the peak at ~315 cm –1 . Fits to the data points indicate that the homovalent substitution of Fe 3+ for Al 3+ leads to a considerably larger downward shift of the ZO 6 vibrational mode than the heterovalent substitution Mg 2+ for Al 3+ . The intensity ratio of the two major YO 6 vibrational modes (200–240 cm –1 ) of the fully characterized Fe 3+ -bearing samples reflects the amount of Y-site Mg and thus can be used to deduce the site-occupancy disorder of Mg over the Y and Z site for tourmaline species with Mg ≤2 apfu. By combining the information from framework and OH-stretching vibrations, Raman spectroscopy alone can be used as a micrometer-scale sensitive non-destructive method for the analysis of tourmaline crystal chemistry including trivalent Fe, which is the major tracer for oxygen fugacity and central for intersite geothermometry.

The 10 Å phase, Mg 3 Si 4 O 10 (OH) 2 ·H 2 O, is a high-pressure hydrous phyllosilicate considered as an important link in the succession of hydrous phases transporting water into the deep mantle. In this study, in situ synchrotron X-ray diffraction combined with external heating diamond-anvil cell was used to determine limits of the 10 Å phase stability at pressures above 7 GPa. A reaction “ 10 Å phase + H 2 O → hydroxide-perovskite (3.65 Å phase) + stishovite ” at about 10 GPa was found to be a high-pressure boundary of the 10 Å phase stability field. A dehydration temperature of the 10 Å phase “10 Å phase → enstatite + stishovite + H 2 O ” decreases with pressure from 690 °C at 7 GPa to 450 °C at 10 GPa; a nonvariant point where 10 Å phase, hydroxide-perovskite and enstatite coexist in the presence of stishovite and hydrous fluid was found near 10 GPa and 450 °C.