Volume 106, Issue 7

This study presents a petrogeochemical and boron isotope study on tourmaline from the barren Damai, and the contemporaneous but ore-bearing Dewulu and Meiwu intrusions, to better understand the origins, sources, and fluid evolution of magmatic-hydrothermal ore systems and provide ore formation implications for gold, copper, and iron deposits in the Xiahe-Hezuo polymetallic district in the West Qinling, China. Tourmaline from all three intrusions shows similar compositions and encompasses Na-Fe schorl and Na-Mg dravite. Tourmaline at Dewulu is primarily found in tuffaceous breccias and a quartz diorite porphyry. In the tuffaceous breccia body, tourmaline occurs as fine-grained anhedral masses that fill voids and cement fragments of breccia and sickle quartz. Tourmalines in breccia are texturally similar to those formed in typical breccia pipes, which are attributed to explosion or collapse induced by a transition from magmatic to hydrothermal Si- and B-rich fluids. They display element substitutions controlled by Fe 2+ Mg –1 , indicating a reduced environment. Values of δ 11 B are –6.6 to –4.0‰, representing the primary magmatic-hydrothermal fluids. Tourmaline from the Dewulu quartz diorite porphyry is coarse-grained, euhedral, and found in quartz-sulfide veins. The tourmaline displays oscillatory zoning textures but lacks correlative variations of major elements. The Fe 2+ Mg –1 and Fe 3+ Al –1 substitution mechanisms are dominant, demonstrating more oxidized fluids. The δ 11 B values in the cores, ranging from –7.1 to –5.6‰, suggest that the tourmalines in the quartz veins were inherited from magmatic-hydrothermal fluids that precipitated the fine-grained tourmaline in the tuffaceous breccia body. A large δ 11 B isotopic fractionation that decreases from cores (–5.6‰) to rims (–10.7‰) indicates significant fractionation during degassing occurred, increasing oxygen fugacity of the residual liquid. The Meiwu locality hosts fine-grained euhedral tourmalines coexisting with magnetite. Their composition is controlled by substitution between Al 3+ by Fe 3+ and has the lightest δ 11 B values ranging from –11.4 to –10.0‰. They are interpreted to result from skarn formation under oxidized conditions. In contrast, X ◻Al(NaMg) –1 is the dominant substitution mechanism for Damai tourmalines and attributed to (geochemically) reduced fluids with a low salinity. We conclude that tourmalines with low Fe values, substitution mechanisms dominated by Fe 3+ Al –1 , and large shifts of B isotopic composition are potentially an ore-forming indicator in the Xiahe-Hezuo polymetallic district.

Garnet is an important mineral phase in the upper mantle as it is both a key component in bulk mantle rocks, and a primary phase at high pressure within subducted basalt. Here, we focus on the strength of garnet and the texture that develops within garnet during accommodation of differential deformational strain. We use X‑ray diffraction in a radial geometry to analyze texture development in situ in three garnet compositions under pressure at 300 K: a natural garnet (Prp 60 Alm 37 ) to 30 GPa, and two synthetic majorite-bearing compositions (Prp 59 Maj 41 and Prp 42 Maj 58 ) to 44 GPa. All three garnets develop a modest (100) texture at elevated pressure under axial compression. Elasto-visco-plastic self-consistent (EVPSC) modeling suggests that two slip systems are active in the three garnet compositions at all pressures studied: {110}<111> and {001}<110>. We determine a flow strength of ~5 GPa at pressures between 10 to 15 GPa for all three garnets; these values are higher than previously reported yield strengths measured on natural and majoritic garnets. Strengths calculated using the experimental lattice strain differ from the strength generated from those calculated using EVPSC. Prp 67 Alm 33 , Prp 59 Maj 41 , and Prp 42 Maj 58 are of comparable strength to each other at room temperature, which indicates that majorite substitution does not greatly affect the strength of garnets. Additionally, all three garnets are of similar strength as lower mantle phases such as bridgmanite and ferropericlase, suggesting that garnet may not be notably stronger than the surrounding lower mantle/deep upper mantle phases at the base of the upper mantle.

Several recent papers have purported to find ultra-reduced minerals—as natural examples—within ophiolitic mantle sections, including SiC moissanite, Fe-Si alloys, various metal carbides, nitrides, and borides. All those phases were interpreted to be mantle derived. The phases are recovered from mineral concentrates and are assigned to the deep mantle because microdiamonds and other ultrahigh-pressure (UHP) minerals are also found. Based on these findings, it is claimed that the mantle rocks of ophiolite complexes are rooted in the transition zone (TZ) or even in the lower mantle, at redox states so reduced that phases like SiC moissanite are stable. We challenge this view. We report high-temperature experiments carried out to define the conditions under which SiC can be stable in Earth’s mantle. Mineral separates from a fertile lherzolite xenolith of the Eifel and chromite from the LG-1 seam of the Bushveld complex were reacted with SiC at 1600 K and 0.7 GPa. At high temperature, a redox gradient is quickly established between the silicate/oxide assemblage and SiC, of ~12 log-bar units in f O2 . Reactions taking place in this redox gradient allow us to derive a model composition of an ultra-reduced mantle by extrapolating phase compositions to 8 log units below the iron-wüstite equilibrium (IW-8) where SiC should be stable. At IW-8 silicate and oxide phases would be pure MgO end-members. Mantle lithologies at IW-8 would be Fe° metal saturated, would be significantly enriched in SiO 2 , and all transition elements with the slightest siderophile affinities would be dissolved in a metal phase. Except for the redox-insensitive MgAl 2 O 4 end-member, spinel would be unstable. Relative to an oxidized mantle at the fayalite-magnetite-quartz (FMQ) buffer, an ultra-reduced mantle would be enriched in enstatite by factor 1.5 since the reduction of the fayalite and ferrosilite components releases SiO 2 . That mantle composition is unlike any natural mantle lithology ever reported in the literature. Phases as reduced as SiC or Fe-Si alloys are unstable in an FeO-bearing, hot, convecting mantle. Based on our results, we advise against questioning existing models of ophiolite genesis because of accessory diamonds and ultra-reduced phases of doubtful origin.

Aillikites are carbonate-rich ultramafic lamprophyres, and although they are volumetrically minor components of large igneous province (LIP), these rocks provide important clues to melting and metasomatism in the deep mantle domain during the initial stages of LIPs. In this study, we investigate the Wajilitag “kimberlites” in the northwestern part of the Tarim LIP that we redefine as hypabyssal aillikites based on the following features: (1) micro-phenocrystic clinopyroxene and Ti-rich andradite garnet occurring in abundance in the carbonate-rich matrix; (2) Cr-spinel exhibiting typical Fe-Ti enrichment trend also known as titanomagnetite trend; and (3) olivine showing dominantly low Mg values (Fo < 90). To constrain the magma source and evolution, the major, minor, and trace element abundance in olivine grains from these rocks were analyzed using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. Olivine in the aillikites occurs as two textural types: (1) groundmass olivines, as sub-rounded grains in matrix, and (2) macrocrysts, as euhedral-anhedral crystals in nodules. The groundmass olivines show varying Mg (Fo 89–80 ) with high-Ni (1606–3418 ppm) and Mn (1424–2860 ppm) and low-Ca (571–896 ppm) contents. In contrast, the macrocrysts exhibit a restricted Fo range but a wide range in Ni and Mn. The former occurs as phenocrysts, whereas the latter are cognate cumulates that formed from earlier, evolved aillikite melt. The two olivine populations can be further divided into sub-groups, indicating a multi-stage crystallization history of the aillikite melt. The crystallization temperatures of groundmass olivines and macrocrysts in dunite nodules as computed from the spinel-olivine thermometers are 1005–1136 and 906–1041 °C, respectively. The coupled enrichment of Ca and Ti and lack of correlation between Ni and Sc and Co in the olivine grains suggest a carbonate-silicate metasomatized mantle source. Moreover, the high 100·Mn/Fe (average 1.67) at high Ni (up to 3418 ppm), overlapping with OIB olivine, and the 100 ·Ni/Mg (~1) of primitive Mg-Ni-rich groundmass olivines suggest a mixed source that involved phlogopite- and carbonate-rich metasomatic veins within mantle peridotite.

Density and thermal expansion coeficient of metals are fundamental characteristics to describe the equation of state. Especially for liquid metals, the reported data for density and thermal expansion coeficient vary in the literature, even at ambient pressure. To determine the density of solid and liquid metals precisely at high temperatures and ambient pressure, we have developed a high-temperature furnace. The densities of solid Sn, Ni, and Fe were determined from the sample image with an uncertainty of 0.11–0.7% in the temperature range of 285–1803 K with increments of 1–10 K. The density of solid Sn decreased linearly with increasing temperature up to 493 K, and then the decrease became drastic until the melting temperature ( T m ) was reached. By contrast, for solid Ni and Fe, the densities decreased linearly with increasing temperature up to the T m (1728 and 1813 K) without any drastic density drop near T m . This suggests that Ni and Fe do not exhibit the “premelting effect.” The density of liquid Fe was determined with an uncertainty of 0.4–0.7% in the range of 1818–1998 K with temperature increments of 5 K. The obtained thermal expansion coefficient (α) of liquid Fe was well approximated as either a constant value of α = 2.42(1) × 10 –4 K –1 or a linear function of temperature ( T ); α = 1.37(10) × 10 –3 – [6.0(6) × 10 –7 ] T [K –1 ] up to at least 2000 K.

The timescales of magmatic processes within a volcanic system may be variable over a volcano’s geological history. Crystals reflect environmental perturbations under which they grew, and compositional gradients quenched inside crystals on eruption can be exploited to extract timescales of magmatic processes. Here, we use multi-element diffusion chronometry in olivine macrocrysts to recover their residence time in a melt that ultimately erupted at the surface. The macrocrysts were mobilized by the carrier melt from mushy layers in the magma reservoir, and diffusion timescales likely reflect the time interval between mush disaggregation, ascent, and eruption. To unravel the evolution of mush disaggregation timescales with time, we target early-Holocene, middle-Holocene, and historical magmatic units erupted in the Bárðarbunga-Veiðivötn volcanic system in Iceland’s Eastern Volcanic Zone. Macrocryst contents vary between samples; early-Holocene samples are highly phyric (10–45 vol% macrocrysts) and contain gabbroic nodules, whereas middle-Holocene (5–15 vol%) and historical units (5–10 vol%) tend to be generally less phyric. Early-Holocene olivine macrocrysts have core compositions in the range Fo 84–87 , while middle-Holocene and historical samples record a wider range in core compositions from Fo 80 to Fo 86.5 . Olivine rims are in chemical equilibrium with their carrier liquid and are slightly more evolved in early-Holocene units (Fo 76–81 ) compared to middle-Holocene (Fo 78–80 ) and historical (Fo 81–83 ) units. Diffusion chronometry reveals that the timescale between mush disaggregation and eruption has changed over time, with timescales getting shorter approaching recent times. Early-Holocene olivine macrocrysts dominantly record Fe-Mg diffusion timescales between 200–400 days, while middle-Holocene and historical units typically record timescales of about 70 and 60 days, respectively. Barometric studies suggest that melts and crystals are likely stored and gradually transferred throughout an interconnected multi-tiered system that ultimately culminates in a mid-crustal reservoir(s) at about 6.8–7.5 ± 2.5 km depth, where final disaggregation by the carrier liquid took place. We argue that, as a result of extensional processes enhanced by rifting events, well-mixed melts got drawn into mid-crustal reservoir(s), causing crystal mush loosening and mobilization. In addition, we propose that more energy in the form of heat and/or melt supply was required in the early-Holocene to break up the dense mush fabric and convert it into an eruptible magma. Conversely, as evidenced by the diverse macrocryst content of the historical units and by the lack of gabbroic nodules, the system has become characterized by a less compact mush fabric since at least the middle-Holocene, such that fresh injection of melt would easily loosen and mobilize the mush, resulting in an eruption within a couple of months. This study provides evidence that along axial rift settings, rifting-related processes can help to “pull the mush apart” with no requirement for primitive magma injection as an eruption trigger. Furthermore, we provide evidence that in the Bárðarbunga-Veiðivötn volcanic system specifically, the time between mush disaggregation and eruption has decreased considerably with time, indicating shorter warning times before imminent eruptions.

Hydrogen-induced decomposition of fayalite (Fe 2 SiO 4 ) at high pressure is of considerable interest for a better understanding of the chemical processes occurring in the cores and mantles of icy satellites. At pressures up to 10 GPa and temperatures 250–300 °C (typical of the cores and mantles of Jupiter’s and Saturn’s satellites), a variable amount of hydrogen can react with fayalite contained in their rocks. Volatile compounds that can form via these reactions are usually identified by mass spectroscopy. In our experiments, we used compressed deuterium gas instead of hydrogen to ensure that the volatiles analyzed by mass spectroscopy could only result from the decomposition of fayalite. To study the effect of the amount of deuterium present in the system, the fayalite (Fa) samples were deuterated at P = 7.5 GPa and T = 280 °C with the preset molar ratios D 2 /Fa = 1, 1.5, 2.2, and 5 in the reaction cell. The deuterated samples were further quenched to the liquid N 2 temperature and, after releasing the pressure, removed from the reaction cell and studied by quadrupole mass-spectroscopy, X‑ray diffraction, and Raman spectroscopy. Our results showed that the high-pressure deuteration invariably led to the chemical decomposition of fayalite. The solid products of the reaction varied from a mixture of ferrosilite (FeSiO 3 ) and iron at D 2 /Fa = 1 to a mixture of silica and iron at D 2 /Fa = 2.2. The decomposition occurred via breaking the Fe-O bonds and was always accompanied by the formation of water. Applying the observed reactions to the natural conditions of, e.g., the center of Titan or Ganymede, one may infer that fayalite can be dissolved in the hydrogen fluid or replaced by iron, ferrosilite, or silica depending on the molar ratio H 2 /Fa.

Three single crystals of CaTi 2 O 4 (CT)-type, CaFe 2 O 4 (CF)-type, and new low-density CaFe 2 O 4 (LD-CF) related MgAl 2 O 4 were synthesized at 27 GPa and 2500 °C and also CT-type MgAl 2 O 4 at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively.The structures of CT-type and LD-CF related MgAl 2 O 4 were analyzed by single-crystal X‑ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were a = 2.7903(4), b = 9.2132(10), and c = 9.3968(12) Å, and a = 2.7982(6), b = 9.2532(15), and c = 9.4461(16) Å, respectively, ( Z = 4, space group: Cmcm ) at ambient conditions. This phase has an AlO 6 octahedral site and an MgO 8 bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group: Pnma ), and lattice parameters of a = 9.207(2), b = 3.0118(6), and c = 9.739(2) Å ( Z = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO 6 and a 4+1 AlO 5 trigonal bipyramid, in which MgO 5 trigonal bipyramids are accommodated. The CF-type MgAl 2 O 4 also has the same space group of Pnma but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm 3 among MgAl 2 O 4 polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm 3 ), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl 2 O 4 is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions.

The MgGeO 3 system is a low-pressure analog for the Earth-forming (Mg,Fe)SiO 3 system and exhibits recoverable orthopyroxene, clinopyroxene, and ilmenite structures below 6 GPa. The pressure-temperature conditions of the clinopyroxene to ilmenite phase transition are reasonably consistent between studies, having a positive Clapeyron slope and occurring between 4 and 7 GPa in the temperature range 900–1600 K. There are, though, significant discrepancies in the Clapeyron slope of the orthopyroxene to clinopyroxene phase transition in existing works that also disagree on the stable phase at ambient conditions. The most significant factor in these differences is the method used; high-pressure experiments and thermophysical property measurements yield apparently contradicting results. Here, we perform both high pressure and temperature experiments as well as thermal expansion measurements to reconcile the measurements. High-pressure and -temperature experiments yield a Clapeyron slope of –1.0 –+ 1.0 -0.7 MPa/K for the MgGeO 3 orthopyroxene-clinopyroxene phase transition, consistent with previous high-pressure and -temperature experiments. The MgGeO 3 orthopyroxene-clinopyroxene-ilmenite triple point is determined to be at 0.98 GPa and 752 K, with the ilmenite phase stable at ambient conditions. The high-temperature (>600 K) thermal expansion of the clinopyroxene phase is greater than that of the other phases. Debye-Grüneisen relationships fitted to the volume-temperature data give Debye temperatures for the orthopyroxene, clinopyroxene, and ilmenite phases of 602(7), 693(10), and 758(13) K and V 0 of 897.299(16), 433.192(10), and 289.156(6) Å 3 , respectively. The Clapeyron slopes calculated directly from the Debye-Grüneisen relationships are consistent with previous thermophysical property measurements. The presence of significant anharmonicity and/or formation of defects in the clinopyroxene phase at high-temperatures, which is not apparent in the other phases, accounts for the previous contradictions between studies. The inferred increased heat capacity of the clinopyroxene corresponds to an increase in entropy and an expanded phase field at high temperatures.

This study, along with our previous studies (Yuguchi et al. 2015, 2019a), reveals the hydrothermal alteration processes in a pluton, with a focus on the mass transfer between minerals and hydrothermal fluid. It also depicts the sequential variations in fluid chemistry as alteration progresses. Hydrothermal alteration of the Toki granite in Tono, Japan—the study area of this research—progressed through the successive processes of chloritization, plagioclase alteration, and precipitation of a carbonate. This paper describes the alteration processes of hornblende chloritization, K-feldspar chloritization, and the formation of fracture-filling chlorite through petrography and mineral chemistry. A set of singular value decomposition analyses was conducted to obtain reaction equations for the chloritization processes, which facilitates the quantitative assessment of mass transfer between the reactant and product minerals, and the inflow and outflow of components through the hydrothermal fluid. Several types of chloritization reactions (including biotite chloritization) can be characterized by their reaction with the inflow of Al 3+ , Fe 2+ , Mn 2+ , and Mg 2+ and the outflow of H 4 SiO 4 , Ca 2+ , K + , and F – . The age and thermal conditions of hornblende chloritization (64–54 Ma and 330–190 °C), K-feldspar chloritization (68–53 Ma and 350–210 °C), and precipitation of fracture-filling chlorite (66 and 63 Ma, 340 and 320 °C) overlap with those of biotite chloritization (68–51 Ma and 350–180 °C). The chloritization reactions (this study and Yuguchi et al. 2015) and plagioclase alteration (Yuguchi et al. 2019a) represent sequential variations in fluid chemistry at temporal conditions from 68 to 51 Ma as the temperature decreased from 350 to 180 °C. As the alteration proceeds, the concentrations of aluminum, iron, manganese, and magnesium ions in the hydrothermal fluid decrease gradually, and those of calcium, hydrogen, and fluorine ions increase gradually. Hornblende chloritization is associated with formation of magnetite and ilmenite. The thermal conditions of the hydrothermal fluid yielding the formation of magnetite and ilmenite can be interpreted by the chemical characteristics of chlorite around their associated minerals. The formation temperature of magnetite was higher than that of ilmenite, implying a decrease in oxygen fugacity in the hydrothermal fluid with the decrease in temperature from 280–310 to 220–250 °C.

Acoustic compressional and shear wave velocities ( V P , V S ) of anhydrous (AHRG) and hydrous rhyolitic glasses (HRG) containing 3.28 wt% (HRG-3) and 5.90 wt% (HRG-6) total water concentration (H 2 O t ) have been measured using Brillouin light scattering (BLS) spectroscopy up to 3 GPa in a diamond-anvil cell at ambient temperature. In addition, Fourier-transform infrared (FTIR) spectroscopy was used to measure the speciation of H 2 O in the glasses up to 3 GPa. At ambient pressure, HRG-3 contains 1.58 (6) wt% hydroxyl groups (OH – ) and 1.70 (7) wt% molecular water (H 2 O m ) while HRG-6 contains 1.67 (10) wt% OH – and 4.23 (17) wt% H 2 O m where the numbers in parentheses are ±1σ. With increasing pressure, very little H 2 O m , if any, converts to OH – within uncertainties in hydrous rhyolitic glasses such that HRG-6 contains much more H 2 O m than HRG-3 at all experimental pressures. We observe a nonlinear relationship between high-pressure sound velocities and H 2 O t , which is attributed to the distinct effects of each water species on acoustic velocities and elastic moduli of hydrous glasses. Near ambient pressure, depolymerization due to OH – reduces V S and G more than V P and K S . V P and K S in both anhydrous and hydrous glasses decrease with increasing pressure up to ~1–2 GPa before increasing with pressure. Above ~1–2 GPa, V P and K S in both hydrous glasses converge with those in AHRG. In particular, V P in HRG-6 crosses over and becomes higher than V P in AHRG. HRG-6 displays lower V S and G than HRG-3 near ambient pressure, but V S and G in these glasses converge above ~2 GPa. Our results show that hydrous rhyolitic glasses with ~2–4 wt% H 2 O m can be as incompressible as their anhydrous counterpart above ~1.5 GPa. The nonlinear effects of hydration on high-pressure acoustic velocities and elastic moduli of rhyolitic glasses observed here may provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth.

The humite-group minerals along the brucite-olivine join may be important dense hydrous magnesium silicate (DHMS) phases in the subducting slab. Fluorine and titanium can be incorporated into their structures through the substitutions (OH) – = F – and Mg 2+ + 2(OH) – = Ti 4+ + 2O 2– . These substitutions have significant effects on the hydrogen bonding behavior in the structures. Structure refinements and in situ high-temperature Raman and Fourier transform infrared (FTIR) measurements were conducted on natural humite and norbergite crystals. Both minerals crystallize in space group Pbnm , and their isobaric Grüneisen parameters for the lattice and SiO 4 internal vibrations are compared with those of chondrodite, clinohumite, brucite, and forsterite. For the humite-group minerals, the OH-stretching modes above 3450 cm –1 are affected by local H-H repulsion, whereas the behavior of those below 3450 cm –1 can be explained by F – and Ti 4+ substitutions, either of which may relieve the H-H repulsion effect. The Raman-active OH bands below 3450 cm –1 are affected by Ti 4+ substitution, while the IR-active bands can be affected by either F – or Ti 4+ substitutions. Based on an analysis of the high- T Raman and FTIR spectra, the OH vibrations above and below 3450 cm –1 behave differently as a function of temperature, and similar behavior has also been observed for other dense hydrous silicate phases in the hydrous peridotite system. Hence, the lengths of the oxygen-oxygen edges in MgO 6 octahedra where protonation can occur become similar to each other at elevated temperatures. This may provide an atomistic explanation for the electrical conductivity properties of DHMS phases at high temperatures.

Naturally occurring single crystals of bixbyite, (Fe,Mn) 2 O 3 , from the Thomas Mountain Range in Utah, U.S.A., were studied via (scanning) transmission electron microscopy (S)TEM. With up to 5 cm edge length, these mineral specimens are the largest bixbyite crystals found worldwide. Their hexahedral shapes are often modified by {211} facets at the corners and small {211} truncations along their cube edges. Characteristic lamellar defects, running parallel to the {100} planes, can be observed via TEM imaging. The defects are, according to EDS analyses, attributed to the tetragonal manganese silicate braunite, Mn 7 [SiO 12 ]. In the present study, electron nano-diffraction and atomic resolution (S)TEM were employed to verify the presence of braunite lamellae and to investigate their orientation relationship with bixbyite. The analysis confirmed an epitaxial intergrowth of both phases, with their main-axes being parallel and the unique c -axis of braunite always aligned perpendicularly to the lamellar plane. Moreover, small rectangular-shaped precipitates, which had been, due to their almost identical chemical composition, previously interpreted as small bixbyite inclusions within the host crystal, were often observed in contact with the braunite lamellae. Electron nano-diffraction and atomic resolution (S)TEM imaging revealed these crystallites not to be bixbyite but jacobsite, a cubic iron-manganese spinel with the stoichiometric formula MnFe 2 O 4 , whose occurrence in this unique context had not been reported before. Moreover, due to the higher temperatures needed for spinel crystallization, the occurrence of jacobsite may serve as a geo-thermometer. (S)TEM in conjunction with automated crystal orientation mapping (ACOM)-TEM showed that no orientation relationship exists between the jacobsite inclusions and the bixbyite/braunite matrix. Nevertheless, their characteristic rectangular shape is typically aligned concordantly with the (001) plane of the braunite lamellae. The resulting crystal shape of jacobsite is determined by the presence of the braunite lamellae, while the respective crystallites maintain their freedom of rotation. To the authors’ knowledge, this is a novel observation of exomorphosis of jacobsite, i.e., the change in the habit of the spinel crystallites due to external conditions. Note that the term “exomorphosis” is used here in the mineralogical sense in contrast to the often-used petrological aspect. Based on the TEM results, the formation of the jacobsite precipitates is discussed and a growth model suggested.

Quartz segregations in paragneisses from the Paleozoic basement of the North Patagonian Andes contain highly saline multiphase fluid inclusions with the rare daughter mineral ferropyrosmalite detected by Raman analysis, besides halite, sylvite, hematite, and/ or magnetite. During heating experiments, L-V homogenization occurs (256–515 °C), followed by halite dissolution (287–556 °C) and the dissolution of ferropyrosmalite at 550–581 °C. The latter phase transition triggers the growth of clinoamphibole crystals according to the following idealized reactions, written for potential end-members: 4Fe8Si6O15(OH)6Cl4+6Ca2+(aq)↔ 3Ca2Fe5Si8O22(OH)2+17Fe2+(aq)+16Cl− (aq)+12OH− +3H2 Ferropyrosmalite↔  Ferro-actinoliteFe8Si6O15(OH)6Cl4+2Ca2+(aq)+Fe3+(aq)+2Al3+(aq)+Na+(aq)+H2O↔ NaCa2Fe42+Fe3+Al2Si6O22Cl2+4Fe2+(aq)+2Cl− (aq)+4H2Ferropyrosmalite ↔  Chloro-hastingsite $$\begin{align} & 4\text{F}{{\text{e}}_{8}}\text{S}{{\text{i}}_{6}}{{\text{O}}_{15}}\left[ {{(\text{OH})}_{6}}\text{C}{{\text{l}}_{4}} \right]+6\text{C}{{\text{a}}^{2+}}(\text{aq})\leftrightarrow 3\text{C}{{\text{a}}_{2}}\text{F}{{\text{e}}_{5}}\text{S}{{\text{i}}_{8}}{{\text{O}}_{22}}{{(\text{OH})}_{2}}+17\text{F}{{\text{e}}^{2+}}(\text{aq})+16\text{Cl}-(\text{aq})+12\text{O}{{\text{H}}^{-}}+3{{\text{H}}_{2}} \\ & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{ Ferropyrosmalite}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\leftrightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{ Ferro-actinolite} \\ & \text{F}{{\text{e}}_{8}}\text{S}{{\text{i}}_{6}}{{\text{O}}_{15}}\left[ {{(\text{OH})}_{6}}\text{C}{{\text{l}}_{4}} \right]+2\text{C}{{\text{a}}^{2+}}(\text{aq})+\text{F}{{\text{e}}^{3+}}(\text{aq})+2\text{A}{{\text{l}}^{3+}}(\text{aq})+\text{N}{{\text{a}}^{+}}(\text{aq})+{{\text{H}}_{2}}\text{O}\leftrightarrow \\ & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{NaC}{{\text{a}}_{2}}\left( \text{Fe}_{4}^{2+}\text{F}{{\text{e}}^{3+}} \right)\left( \text{A}{{\text{l}}_{2}}\text{S}{{\text{i}}_{6}} \right){{\text{O}}_{22}}\text{C}{{\text{l}}_{2}}+4\text{F}{{\text{e}}^{2+}}(\text{aq})+2\text{C}{{\text{l}}^{-}}(\text{aq})+4{{\text{H}}_{2}} \\ & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{Ferropyrosmalite }\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\leftrightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{ Chloro-hastingsite } \\ \end{align}$$ The amphibole resembles the composition of ferro-actinolite but also has striking similarities with chloro-hastingsite, as indicated by Raman spectroscopy. During the heating experiment, hematite (when present) transforms to magnetite by the uptake of H 2 , whereas inclusions without Fe-oxides contain traces of H 2 after the reaction. This mineral transformation shows that ferropyrosmalite might result from the retrograde re-equilibration of amphibole with the brine, implying the uptake of Fe 2+ , Cl – , and H 2 O and the enrichment of Ca 2+ in the brine. Pervasive fluid flow and fluid-assisted diffusion are recorded by channel way microstructures, healed microfractures, and dissolution-reprecipitation phenomena, as demonstrated by cathodoluminescence microscopy. These alkali- and FeCl 2 -rich brines, derived from magmatic sources and of possible Mesozoic age, were related to regional metasomatism, coeval with widespread granitoid activity.

In this Issue This New Mineral Names has entries for 10 new species, including huenite, laverovite, pandoraite-Ba, pandoraite-Ca, and six new species of pyrochlore supergroup: cesiokenomicrolite, hydrokenopyrochlore, hydroxyplumbopyrochlore, kenoplumbomicrolite, oxybismutomicrolite, and oxycalciomicrolite.