Volume 106, Issue 2

To clarify the effect of water on carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments in the system KAlSi 3 O 8 -CaMgSi 2 O 6 -NaAlSi 2 O 6 -CaMg(CO 3 ) 2 under nominally dry and hydrous conditions by adding 1.5 wt% H 2 O at a pressure of 6 GPa and temperatures of 1000 to 1500 °C. Both systems start to melt at 1050–1100 °C. Under anhydrous condition the melting occurs via the following reaction: 6KAlSi 3 O 8 (K-feldspar) + 6CaMg(CO 3 ) 2 (dolomite) = 2(Ca n ,Mg 1-n ) 3 Al 2 Si 3 O 12 (garnet) + Al 2 SiO 5 (kyanite) + 11SiO 2 (coesite) + 3 K 2 (Ca 1-n , Mg n ) 2 (CO 3 ) 3 (carbonatitic melt) + 3CO 2 (fluid and/or liquid), where n ~ 0.3–0.4. The carbonatitic melt has the following composition 38(K 0.92 Na 0.08 ) 2 CO 3 ⋅62Ca 0.62 Mg 0.38 CO 3 . A second immiscible silicic melt containing (in wt%, volatile free) SiO 2 = 68.8, Al 2 O 3 = 12.6, CaO = 3.7, MgO = 2.4, Na 2 O = 1.1, and K 2 O = 11.3 appears at 1250 °C. Both melts remain stable up to 1500 °C and coexist with the clinopyroxene ± garnet ± coesite residue. In the presence of water stored away in phengite, the melting begins with silicic melt, which contains (in wt%, volatile free) SiO 2 = 61.4, Al 2 O 3 = 15.3, CaO = 4.8, MgO = 3.0, Na 2 O = 2.2, and K 2 O = 13.3, and coexists with phengite, dolomite, clinopyroxene, and coesite. The phengite + dolomite assemblage remains to 1100 °C and disappears at 1200 °C producing two immiscible melts carbonatitic with approximate composition, 19(K 0.89 Na 0.11 ) 2 CO 3 ⋅81Ca 0.57 Mg 0.43 CO 3 , and silicic containing (in wt%, volatile free) SiO 2 = 63.3, Al 2 O 3 = 15.6, CaO = 4.5, MgO = 3.0, Na 2 O = 2.0, K 2 O = 11.6. The present results imply that partial melting of continental material subducted to a depth of 200 km can yield simultaneous formation of two immiscible melts, K-dolomitic and K-aluminosilicate. Under dry conditions, carbonatitic melt appears earlier (at a lower temperature). Given the low density and high mobility of this melt, it must rapidly percolate upward, leaving a refractory eclogite-like residue and leaving no chance for the formation of a second aluminosilicate melt. However, under hydrous conditions silicate melt appears earlier than carbonatitic melt, leaving a phengite- and dolomite-bearing residue, which finally yields the formation of two immiscible silicic and carbonatitic melts. The compositions of these melts fall in the compositional range of carbonatitic and silicic high-density fluids (HDFs) in diamonds worldwide. Thus, we suggest that the presence of water is a necessary requirement for the formation of immiscible HDFs inclusions in diamonds, and this suggestion is strongly supported by natural data from HDFs.

Jasonsmithite (IMA2019-121), M n 4 2 + \begin{equation} \mathrm{Mn}_{4}^{2+} \end{equation}ZnAl(PO 4 ) 4 (OH)(H 2 O) 7 ·3.5H 2 O, is a pegmatite-phosphate mineral from the Foote Lithium Company mine, Kings Mountain district, Cleveland County, North Carolina, U.S.A. It is interpreted as having formed by late-stage, low-temperature hydrothermal alteration. Crystals are colorless to light brown, slightly flattened prisms to about 1 mm in length with wedge-shaped terminations. The mineral is transparent with vitreous luster, white streak, Mohs hardness 2, brittle tenacity, irregular fracture, and perfect {001} cleavage. The density is 2.63(2) g/cm 3 . Jasonsmithite is biaxial (–), with α = 1.561(2), β = 1.580(2), γ = 1.581(2), measured in white light. The 2 V is 25(5)° and dispersion is r < v moderate. The optical orientation is Y = b , X ^ c = 18° in obtuse β. The Raman spectrum is dominated by vibrational modes of PO 4 and ZnO 4 tetrahedra, AlO 6 and MnO 6 octahedra, and OH groups. Electron microprobe analyses gave the empirical formula (Mn 3.09 Fe 0.87 ) Σ3.96 Zn 1.05 Al 0.98 (PO 4 ) 4 (OH)(H 2 O) 7 ·3.5H 2 O. The mineral is monoclinic, P 21/ c , a = 8.5822(3), b = 13.1770(6), c = 20.3040(14) Å, β = 98.485(7)°, V = 2271.0(2) Å 3 , and Z = 4. The structure ( R 1 = 0.0443 for 3685 I >2σ I reflections) contains zigzag chains of edge-sharing MnO 6 octahedra that corner-link to adjacent chains and to PO 4 tetrahedra to form sheets, which are decorated by ZnO 4 tetrahedra. The sheets are linked to one another via dimers of AlO 6 octahedra, forming a framework with large channels containing H 2 O groups. With H 2 O groups removed, the framework has a void space of 70.2% per unit cell, and a framework density of 14.5 polyhedral atoms/1000 Å 3 , which would place jasonsmithite among the most porous minerals.

Thermometry of high-grade metamorphic rocks is difficult due to fast cationic diffusion during slow cooling, but the titanium content of calcium amphibole (Ti-Amp) can retrieve amphibole-forming temperature as high as ~1000 °C. Based on pseudosection modeling and past research survey, we find that Ti-Amp is controlled by temperature under the conditions of oxygen fugacity <2 log units above the Ni-NiO oxygen buffer (ΔNNO+2), in subalkaline systems, and with the presence of Ti-phases (rutile, ilmenite, or titanite). We apply available experimental data to calibrate a new Ti-Amp thermometer for such conditions: T ∘ C = 2400 1.52 − log T i A m p − 273 $$ \begin{equation} T\left({ }^{\circ} \mathrm{C}\right)=\frac{2400}{1.52-\log \mathrm{Ti}^{\mathrm{A} \mathrm{m} \mathrm{p}}}-273 \end{equation}$$ where T is temperature and log Ti Amp is the Ti content of amphibole in atom per formula unit (apfu) expressed in the logarithm to base 10. The standard error of the calibration experiments is ±35 °C. This thermometer can be applied only if the aforementioned conditions are fulfilled. Besides, caution should be taken when applying the thermometer to rocks under subsolidus water-unsaturated conditions, which can be observed as non-equilibrium textures or low bulk-rock water content (<1–1.5 wt%). The results of this thermometer may be underestimated if applied to rocks that are equilibrated above 850 °C and contain rutile and a significant amount of water (>3.5–4 wt%). The new thermometer can be successfully applied to amphibole-bearing natural igneous and metamorphic rocks. In addition, we propose the Si vs. Ti in amphibole diagram that roughly demarcates the boundaries among the high amphibolite, high-temperature granulite, and ultrahigh-temperature granulite facies. Although pseudosection modeling provides details of the P - T evolution of studied rocks, the Ti-Amp thermometer is quick and easy to apply. However, further research is needed to improve our knowledge of the behavior and stability of amphibole and to improve the accuracy of both pseudosection modeling and conventional thermometry.

Micrometer- to submicrometer-scale indium-rich domains are preserved within sphalerite included in hornfels-hosted pyrrhotite from the Dulong polymetallic skarn, Yunnan, China. The nano-mineralogy of the ZnS-bearing blebs was investigated using scanning transmission electron microscopy on thinned foils extracted in situ from pyrrhotite. Indium incorporation in sphalerite occurs via the coupled substitution 2Zn 2+ ↔ Cu + + In 3+ ; the results thus allow insights into phase relationships in the system ZnS-CuInS 2 in which solubility limits are debated with respect to a cubic to tetragonal phase transition. The highest concentrations of In are measured in basket-weave domains from the smallest ZnS blebs or from un-patterned areas in coarser, irregular ZnS inclusions in pyrrhotite. Indium-rich domains contain 17–49 mol% CuInS 2 , whereas In-poor sphalerite contains <5 mol% CuInS 2 . Atomic-scale metal ordering observed in In-(Cu)-rich ZnS domains was modeled as mixed sites in a cubic structure with P 4̅3 m symmetry and empirical formula [(Cu,In,Zn) 3 (Zn 0.5 Fe 0.5 )] 4 S 4 . This sphalerite modification is distinct from the cubic-tetragonal phase transition reported elsewhere for analogous, synthetic phases with abundant planar defects. The Zn 1.5 Fe 0.5 CuInS 4 nanophase described here potentially represents a Fe-bearing polymorph of Zn 2 CuInS 4 , considered as an end-member in the sakuraiite solid-solution series. At ≤50 mol% CuInS 2 in the ZnS-CuInS 2 system, incorporation of In via coupled In+Cu substitution is promoted within a cubic ZnS phase with lower symmetry than sphalerite rather than into the spatially coexisting chalcopyrite of tetragonal symmetry. Solid-state diffusion accounts for phase re-equilibration resulting in the basket-weave textures typical of In-(Cu)-rich domains in the smallest blebs, whereas fluid percolation assists grain coarsening in the irregular inclusions. The results show evidence for the existence of a more complex phase transition than previously recognized from experimental studies, and, intriguingly, also to a potential eutectic in the system ZnS-CuInS 2 . Pyrrhotite-bearing hornfels in skarns may concentrate In and other critical metals hosted in sphalerite and related sulfides due to the efficient scavenging from fluid by these minerals and the subsequent preservation of those included phases by sealing within the pyrrhotite matrix.

Olivine is a major rock-forming mineral in various magmatic and metamorphic rocks and the upper mantle. In this paper, we present the first high-precision analyses of olivine from 15 samples of magnesian skarns and silicate marbles (MSSM) from the collection of the Fersman Mineralogical Museum (Moscow, Russia). Mg# [Mg/(Mg+Fe 2+ )·100, mol%] of olivine from the samples studied varies from 86 to nearly 100. The main distinctive features of the olivine are anomalously low contents of Co (<51 μg/g), Cr (<5 μg/g), and Ni (<44 μg/g) and high content of B (23–856 μg/g), which correlate with host-rock compositions. Phosphorus (5–377 μg/g) incorporation in olivine is charge balanced by the incorporation of Li (0.15–61 μg/g) and Na (<14.3 μg/g). Y and REE contents exhibit positive correlations with Na, which suggests that REE incorporation into MSSM olivine could occur via charge-balanced coupled substitution with Na at low temperature and low a SiO2 conditions during MSSM formation. The documented compositional features of olivine from magnesian skarns and silicate marbles can help reconstruct the genesis of the host-rocks and identify xenocrysts of MSSM olivine in magmatic rocks.

We performed decompression experiments on sulfur-bearing hydrous rhyolite magma at a temperature of 800 °C to investigate redox evolution during magma decompression. The magma was continuously decompressed from 100 MPa to 10–50 MPa at rates of 10 and 100 MPa h –1 . The evolution of the ferric to total iron ratio (Fe 3+ /Fe total ) during decompression was investigated using XANES, and redox evolution was determined based on a thermodynamic calculation and measured Fe 3+ /Fe total . Before decompression, the sample was buffered from NNO to NNO+1, and the pre-exsolved fluid phase and sulfide crystal coexisted. Sulfide crystals were found in all decompressed samples, and Fe 3+ /Fe total showed a slight decrease with decompression. It was confirmed that the sample in a gold capsule was not influenced by the change in redox conditions outside of the capsule for the timescale of the decompression experiments; thus, the decompressed sample reflected the redox evolution in magma during decompression. Our experiments indicated that magma decompression causes a slight reduction when it includes water and sulfur. This evolution is qualitatively explained by sulfur degassing and fluid-melt redox equilibria. During the fluid-melt redox equilibria, magma is reduced if the existence of a pre-exsolved fluid phase is assumed, while the model calculation shows that magma is oxidized when it contains only water or no pre-exsolved fluid phases. This is because sulfur buffers the oxidation of magma through a reaction with oxygen in the fluid phase. Therefore, we inferred that the redox condition of magma is not oxidized during explosive volcanism with a pre-exsolved fluid phase and closed-system degassing. In contrast, if magma experiences open-system degassing, it may be oxidized, resulting in the breakdown of sulfide crystals as observed in some pyroclasts and lavas.

Dark blue lazurite from the Malo-Bystrinskoe lazurite deposit, Baikal Lake area, Eastern Siberian region, Russia, was analyzed by electron microprobe and revealed an unusually high content of total sulfur corresponding to 8.3 wt% S. The relative content of sulfur in sulfate and sulfur in sulfide form was determined by wet chemical analysis. The H 2 O content was measured by means of differential thermal analysis in combination with mass spectrometry and infrared (IR) spectroscopy. The charge-balanced empirical formula of lazurite calculated on the basis of 12 (Al+Si) atoms per formula unit was N a 6.97 C a 0.88   K 0.10 Σ 7.96 A l 5.96 S i 6.04 Σ 12 O 24 S O 4 1.09 2 − S 3 − 0.55   S 0.05 2 − C l 0.04 ⋅ 0.72 H 2 O . $\left(\mathrm{Na}_{6.97} \mathrm{Ca}_{0.88} \mathrm{~K}_{0.10}\right)_{\Sigma 7.96}\left[\left(\mathrm{Al}_{5.96} \mathrm{Si}_{6.04}\right)_{\Sigma 12} \mathrm{O}_{24}\right]\left(\mathrm{SO}_{4}\right)_{1.09}^{2-}\left(\mathrm{S}_{3}^{-}\right)_{0.55} \mathrm{~S}_{0.05}^{2-} \mathrm{Cl}_{0.04} \cdot 0.72 \mathrm{H}_{2} \mathrm{O}.$The presence of H 2 O molecules and (S 3 ) – and (SO 4 ) 2– groups was confirmed by the combination of IR, Raman, electron paramagnetic resonance (EPR), and X‑ray photoelectron spectroscopy (XPS) methods. The idealized formula of lazurite is Na 7 Ca[Al 6 Si 6 O 24 ](SO 4 ) 2– (S 3 ) – ·H 2 O, and it is believed that extra-framework cations and anions are grouped into clusters of [Na 3 Ca·SO 4 ] 3+ and [Na 4 (S 3 ) – ] 3+ . The types of isomorphous substitutions in nosean and haüyne are discussed. Lazurite is a clathrate-type mineral, which may be an effective (S 3 ) – sensor due to the stability of the trisulfur radical anion in isolated cages of the crystal structure. This specific feature makes it possible to study the behavior of this ubiquitous radical anion over larger T and P ranges as compared to free species. This kind of lazurite, with oxidized and reduced sulfur species, seems to be appropriate for the estimation of the fugacity of SO 2 and O 2 in metasomatic systems forming lazurite-containing rocks. The systematic presence of incommensurate modulations is a unique structural feature of Baikal lazurite and may be an important marker indicating provenance of the mineral.

An olivine-melt thermometer based on the partitioning of Ni D N i O 1 / liq $\left(D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq}}\right)$was hypothesized by Pu et al. (2017) to have a negligible dependence on dissolved water in the melt (and pressure variations from 0–1 GPa), in marked contrast to thermometers based on D M g O l / l i q . $D_{\mathrm{Mg}}^{\mathrm{Ol} / \mathrm{liq}}.$In this study, 15 olivine-melt equilibrium experiments were conducted on a basaltic glass starting material (9.6 wt% MgO; 353 ppm Ni) to test this hypothesis by comparing the effect of dissolved H 2 O in the melt on D M g O l / l i q and D N i O 1 / liq $D_{\mathrm{Mg}}^{\mathrm{Ol} / \mathrm{liq}} \,\text{and}\, D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq}}$on the same set of experiments. Results are presented for six anhydrous experiments at 1 bar, two anhydrous experiments at 0.5 GPa, and seven hydrous experiments at 0.5 GPa. Analyzed olivine and glass compositions in the quenched run products were used to calculate D M g O l / l i q and D N i O 1 / liq $D_{\mathrm{Mg}}^{\mathrm{Ol} / \mathrm{liq}} \,\text{and}\, D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq}}$values for each experiment, which in turn permit temperature to be calculated with the Mg- and Ni-thermometers calibrated in Pu et al. (2017) on anhydrous, 1-bar experiments from the literature. The Ni-thermometer recovers the temperatures of all 15 experiments from this study with an average deviation of –3 °C, including those with up to 4.3 wt% H 2 O dissolved in the melt. In contrast, the Mg-thermometer recovers the anhydrous, 1-bar experimental temperatures within +14 °C on average, but overestimates the hydrous experimental temperatures by +49 to +127 °C, with an average of +83 °C. When the Mg-thermometer of Putirka et al. (2007) is applied, which includes a correction for analyzed H 2 O (≤4.3 wt%) in the quenched melts of the run products, all experimental temperatures are recovered with an average (±1σ) deviation of +7 °C. The combined results show that D N i O 1 / liq $D_{\mathrm{Ni}}^{\text{O}1 / \mathrm{liq}}$has a negligible dependence on dissolved water in the melt (≤4.3 wt% H 2 O), which is in marked contrast to the strong dependence of D M g O 1 / liq $D_{\mathrm{Mg}}^{\text{O}1 / \mathrm{liq}}$on water in the melt. An understanding of why D N i O 1 / liq $D_{\mathrm{Ni}}^{\text{O}1 / \mathrm{liq}}$is insensitive to dissolved water, unlike D M g O 1 / liq , $D_{\mathrm{Mg}}^{\text{O}1 / \mathrm{liq}},$is obtained from spectroscopic evidence in the literature, which shows that Ni 2+ (transition metal) and Mg 2+ (alkaline earth metal) have distinctly different average coordination numbers (predominantly fourfold and sixfold, respectively) in silicate melts and that fourfold-coordinated Ni 2+ is unaffected by the presence of dissolved water in the melt. This difference in coordination number explains why D N i O 1 / l i q and D M g O 1 / l i q $D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq} \mathrm{}} \,\text{and}\, D_{\mathrm{Mg}}^{\mathrm{O} 1 / \mathrm{liq}}$each have a different dependence on pressure, anhydrous melt composition, and melt water content. Application of the Ni-thermometer of Pu et al. (2017) to five natural samples from the Mexican arc, for which H 2 O contents (3.6–6.7 wt%) in olivine-hosted melt inclusions are reported in the literature, leads to temperatures that match those obtained from the Putirka et al. (2007) Mg-thermometer that corrects for analyzed H 2 O contents. This study demonstrates that a thermometer based on D N i O l / l i q $D_{\mathrm{Ni}}^{\mathrm{Ol} / \mathrm{liq}}$can be applied to hydrous basalts at crustal depths without the need to correct for dissolved water content or pressure.

Two Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na 2 O + K 2 O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

Two generations of garnet are recognized in a granite and a pegmatite from the Gangdese orogen in southeastern Tibet on the basis of a combined study of petrography, major and trace element profiles, and garnet O isotopes. Zircon U-Pb dating and Hf-O isotope compositions also help constrain the origin of both granite and pegmatite. The first generation of garnet (Grt-I) occurs as residues in the center of garnet grains, and it represents an early stage of nucleation related to magmatic-hydrothermal fluids. Grt-I is dark in backscattered electron (BSE) images, rich in spessartine, and poor in almandine and grossular. Its chondrite-normalized rare earth element (REE) patterns show obvious negative Eu anomalies and depletion in heavy REE (HREE) relative to middle REE (MREE). The second generation of pegmatite garnet (Grt-II) occurs as rims of euhedral garnets or as patches in Grt-I domains of the pegmatite, and it crystallized after dissolution of the preexisting pegmatite garnet (Grt-I domains) in the presence of the granitic magma. Compared with Grt-I, Grt-II is bright in BSE images, poor in spessartine, and rich in almandine and grossular contents. Its chondrite-normalized REE patterns exhibit obvious negative Eu anomalies but enrichment in HREE relative to MREE. The elevation of grossular and HREE contents for Grt-II relative to Grt-I domains indicate that the granitic magma had higher contents of Ca than the magmatic-hydrothermal fluids. The garnets in the granite, from core to rim, display homogenous profiles in their spessartine, almandine, and pyrope contents but increasing grossular and decreasing REE contents. They are typical of magmatic garnets that crystallized from the granitic magma. Ti-in-zircon temperatures demonstrate that the granite and pegmatite may share the similar temperatures for their crystallization. Grt-II domains in the pegmatite garnet have the same major and trace element compositions as the granite garnet, suggesting that the pegmatite Grt-II domains crystallized from the same granitic magma. Therefore, the pegmatite crystallized at first from early magmatic-hydrothermal fluids, producing small amounts of Grt-I, and the fluids then mixed with the surrounding granitic magma. The U-Pb dating and Hf-O isotope analyses of zircons from the granite and pegmatite yield almost the same U-Pb ages of 77–79 Ma, positive ε Hf (t) values of 5.6 to 11.9, and δ 18 O values of 5.2 to 7.1‰. These data indicate that the granite and pegmatite were both derived from reworking of the juvenile crust in the newly accreted continental margin prior to the continental collision in the Cenozoic.

Tourmaline is a common mineral in granites and metamorphic rocks in collisional orogens. This paper describes graphite-bearing, metasomatic tourmalinites in sillimanite-zone schists of the Proterozoic Black Hills Orogen, South Dakota. The tourmalinites bound quartz veins and beyond about 1 m grade into schists with disseminated tourmaline, and ultimately tourmaline becomes only a trace, intrinsic phase in the schists. Next to the quartz veins, tourmaline has almost completely replaced schist minerals, including biotite, muscovite, and plagioclase. The tourmaline is generally anhedral and follows the original foliation direction of the schist. However, tourmaline is euhedral in quartz veinlets cutting through the tourmalinites. Tourmaline is compositionally zoned from having about 22 to 2% of apparent Al occupancy on the Y sites. There are very good negative correlations of Y (Fe 2+ +Mg 2+ ), X Ca 2+ , and Y Ti 4+ with Y Al 3+ , and a very good positive correlation of X-site vacancies with Y Al 3+ . Mg# [molar Mg 2+ /(Mg 2+ +Fe 2+ )] is fairly invariant at approximately 0.5, which is somewhat higher than that in the precursor biotite. This is in contrast to tourmaline in the neighboring peraluminous Harney Peak leucogranite where the range of Y site occupancy of Al is small at about 20%, but the Mg# ranges from 0.12 to 0.5. The compositional trends in the metasomatic tourmaline are dominated by the exchange X o + 4 Y Al 3+ = X Ca 2+ + 3 Y (Fe 2+ +Mg 2+ ) + Y Ti 4+ . Mass-balance calculations suggest the metasomatizing fluid brought in H + and B(OH) 3 and removed K + , SiO 2 , and some Fe 2+ during tourmalinization. Other elements in the tourmaline largely reflect the bulk composition of the replaced schist. The calculations show that silica in the quartz veins was locally derived, not brought in by the metasomatizing fluid. Interstitial graphite in the tourmalinites shows precipitation of carbon from the methane-bearing fluid. The study demonstrates an important effect of boron transfer by fluids during metamorphism and magmatism in the Earth’s crust.

A multi-methodical characterization of a sauconite (Zn-bearing trioctahedral smectite) specimen from the Skorpion ore deposit (Namibia) was performed by combining X‑ray powder diffraction (XRPD), cation exchange capacity (CEC) analysis, differential thermal analysis (DTA), thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM-HRTEM-AEM). The X‑ray diffraction pattern exhibits the typical features of turbostratic stacking disorder with symmetrical basal 00 l reflections and long-tailed hk bands, as confirmed by TEM observations. Besides sauconite, the sample contains minor amounts of kaolinite, dioctahedral smectite, and quartz. CEC analysis provides a total of Ca (~69%), Mg (~26%), Na (~4%), and K (0.7%) exchangeable cations. Therefore, Zn is located exclusively within the octahedral site of sauconite. TG analysis of the sample yields a total mass loss of about 17%. Three endothermic peaks can be observed in the DTA curve, associated with dehydration and dehydroxylation of the material. An exothermic peak at 820 °C is also present as a consequence of decomposition and recrystallization. The infrared spectrum shows the typical Zn 3 OH stretching signature at 3648 cm –1 , whereas, in the OH/H 2 O stretching region two bands at 3585 and 3440 cm –1 can be attributed to stretching vibrations of the inner hydration sphere of the interlayer cations and to absorbed H 2 O stretching vibration, respectively. Diagnostic bands of kaolinite impurity at ~3698 and 3620 cm –1 are also found, whereas 2:1 dioctahedral layer silicates may contribute to the 3585 and 3620 cm –1 bands. Finally, using the one-layer supercell approach implemented in the BGMN software, a satisfactory XRPD profile fitting model for the Skorpion sauconite was obtained. These findings have implications not only for economic geology/recovery of critical metals but also, more generally, in the field of environmental sciences.

Crowningshieldite is the natural analog of the synthetic compound α-NiS. It has a NiAs-type structure and is the high-temperature polymorph relative to millerite (β-NiS), with an inversion temperature of 379 °C. Crowningshieldite is hexagonal, space group P 6 3 / mmc , with a = 3.44(1) Å, c = 5.36(1) Å, V = 55.0(2) Å 3 , and Z = 2. It has an empirical formula (Ni 0.90 Fe 0.10 )S and d calc = 5.47(1) g/cm 3 . The five strongest lines in the powder X‑ray diffraction data are [ d meas in angstroms ( I ) ( hkl )]: 1.992 (100) (102), 1.718 (55) (110), 2.978 (53) (100), 2.608 (35) (101), and 1.304 (17) (202). Crowningshieldite was found as part of a multiphase inclusion in a gem-quality, colorless, type IIa (containing less than ~5 ppm N) diamond from the Letseng mine, Lesotho. The inclusion contains crowningshieldite along with magnetite-magnesioferrite, hematite, and graphite. A fracture was observed that extended from the inclusion to the diamond exterior, meaning that fluids, possibly kimberlite-related, could have penetrated into this fracture and altered the inclusion. Originally, the inclusion might have been a more reduced, metallic Fe-Ni-C-S mixture made up of cohenite, Fe-Ni alloy, and pyrrhotite, akin to the other fracture-free, pristine inclusions within the same diamond. Such metallic Fe-Ni-C-S primary inclusions are a notable recurring feature of similar type IIa diamonds from Letseng and elsewhere that have been shown to originate from the sublithospheric mantle. The discovery of crowningshieldite confirms that the α-NiS polymorph occurs in nature. In this case, the reason for its preservation is unclear, but the relatively iron-rich composition [Fe/(Fe+Ni) = 0.1] or the confining pressure of the diamond host are potential factors impeding its transformation to millerite. The new mineral name honors G. Robert Crowningshield (1919–2006) (IMA2018-072).

For four decades fairbankite was reported to have the formula Pb 2+ (Te 4+ O 3 ), but repeated attempts to isolate fairbankite crystals for structural determination found only the visually similar cerussite and, more rarely, anglesite. The crystal-structure determination of fairbankite using single-crystal X‑ray diffraction, supported by electron microprobe analysis and X‑ray powder diffraction on the type specimen, has shown that fairbankite contains essential S, along with Pb, Te, and O. The chemical formula of fairbankite has been revised to P b 12 2 + T e 4 + O 3 11 S O 4 . $\mathrm{Pb}_{12}^{2+}\left(\mathrm{Te}^{4+} \mathrm{O}_{3}\right)_{11}\left(\mathrm{SO}_{4}\right).$This change has been accepted by the IMA–CNMNC, Proposal 19-I. The crystal structure of fairbankite [space group P 1 (no. 1); revised cell: a = 7.0205(3) Å, b = 10.6828(6) Å, c = 14.4916(8) Å, a = 75.161(5)°, b = 81.571(4)°, g = 83.744(4)°, V = 1036.35(9) Å 3 , and Z = 1] is the first atomic arrangement known to contain a T e 3 4 + O 9 6 − $\mathrm{Te}_{3}^{4+} \mathrm{O}_{9}^{6-}$non-cyclic, finite building unit. Fairbankite has an average structure, formed from a 3D framework of Pb 2+ O n polyhedra, Te 4+ O n polyhedra, and SO 4 tetrahedra in a 12:11:1 ratio. The stereoactive lone pairs of the Pb 2+ and Te 4+ cations are oriented into void space within the structure. Fairbankite contains two mixed sites statistically occupied by Te 4+ and S 6+ in approximately 4:1 and 1:4 ratios. These two sites possess Te 4+ in trigonal-pyramidal environment and S 6+ in tetrahedral environment (with an additional O site to create tetrahedral SO 4 shape for the S-dominant site). Six of the 10 fully occupied Te 4+ sites have Te 4+ in trigonal-pyramidal environment, while four have Te 4+ at the center of highly distorted Te 4+ O 4 disphenoids. The disphenoids allow for the creation of two dimeric T e 2 4 + O 6 4 − $\mathrm{Te}_{2}^{4+} \mathrm{O}_{6}^{4-}$units in addition to the T e 3 4 + O 9 6 − $\mathrm{Te}_{3}^{4+} \mathrm{O}_{9}^{6-}$trimeric unit, which contains two disphenoids. All linkage between disphenoids and trigonal pyramids is via corner-linking. Secondary connectivity is via long Te–O and Pb–O bonds.

It is not known if the thermodynamic behavior of some minerals and their synthetic analogues are quantitatively the same. Olivine is an important rock-forming substitutional solid solution consisting of the two end-members forsterite, Mg 2 SiO 4 , and fayalite, Fe 2 SiO 4 . We undertook the first heat capacity, C P , measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo 0.904 Fa 0.096 . Their C P ( T ) behavior is compared to that of synthetic crystals of similar composition, as found in the literature. The two natural olivines are characterized by X‑ray powder diffraction and 57 Fe Mössbauer spectroscopy. The X‑ray results show that the crystals are well crystalline. The Mössbauer hyperfine parameters, obtained from a fit with two Fe 2+ quadrupole split doublets, are similar to published values measured on synthetic olivines. There are slight differences in the absorption line widths (i.e., FWHM) between the natural and synthetic crystals. C P (2 to 300 K) is measured by relaxation calorimetry. The C P results of the natural nearly end-member fayalite and published values for two different synthetic Fa 100 samples are in excellent agreement. Even C P resulting from a Schottky anomaly and a paramagnetic-antiferromagnetic phase transition with both arising from Fe 2+ are similar. There are slight differences in the Néel temperature between the natural 63 K and synthetic ~65 K fayalites. This is probably related to the presence of certain minor elements (e.g., Mn 2+ ) in the natural crystal. The third-law entropy, S °, value is 151.6 ± 1.1 J/(mol·K). C P behavior of the natural forsterite, Fo 0.904 Fa 0.096 , and a synthetic olivine, Fo 90 Fa 10 , are in excellent agreement between about 7 and 300 K. The only difference lies at T < 7 K, as the former does not show Debye T 3 behavior, but, instead, a plateauing of C P values. The S ° value for the natural forsterite is 99.1 ± 0.7 J/(mol·K).