Volume 105, Issue 1

The founding of the Mineralogical Society of America (MSA) in 1919 followed so closely on the heels of the discovery of X‑ray diffraction (XRD) in 1912 that one might hypothesize a causal link. Was MSA born out of this scientific revolution? The formation of our Society conventionally is attributed to the desire for a professional journal and the need to emerge from the shadow of the Geological Society of America, but these issues were not new in 1919. This review argues that MSA’s birth can be understood by an exploration of two historical strains: (1) Although modern notions of atomism traditionally are associated with the emergence of the kinetic theory of gases in the late 19th century, mineralogists had invoked ordered atomic spheres as the fundamental metaphor for crystalline structures over a century earlier, leading directly to W.L. Bragg’s discovery of the atomic design in halite; and (2) in contrast to the broader chemistry community, mineralogists were uniquely poised to embrace X‑ray diffraction and the revolution in crystallography that attended it. This revelation gave rise to a sense of distinct identity. An examination of MSA’s early records unambiguously reveals that U. S. mineralogists were closely attuned to the crystallographic insurgency as it took place across the Atlantic. In particular, Edgar T. Wherry, one of the organizers of the American Mineralogist and of the Society, actively disseminated information about the new discoveries to his colleagues in the United States. Other founders of MSA similarly championed the transformative character of the new crystallography, thereby warranting the establishment of a specialized professional society. The continuing force of the revolution is revealed in a counter-reaction a century later, as mineralogists have renewed their focus on phases at the boundary between crystals and glasses, leading to reconsiderations of the meaning of crystallinity.

Reactions involving carbon in the deep Earth have limited manifestations on Earth’s surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth’s accretion and may have sequestered substantial carbon in Earth’s core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth’s inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies.

Much of Earth’s carbon resides in the “deep” realms of our planet: sediments, crust, mantle, and core. The interaction of these deep reservoirs of carbon with the surface reservoir (atmosphere and oceans) leads to a habitable surface environment, with an equitable atmospheric composition and comfortable range in temperature that together have allowed life to proliferate. The Earth in Five Reactions project (part of the Deep Carbon Observatory program) identified the most important carbon-bearing reactions of our planet, defined as those which perhaps make our planet unique among those in our Solar System, to highlight and review how the deep and surface carbon cycles connect. Here we review the important reactions that control the concentration of carbon dioxide in our atmosphere: outgassing from magmas during volcanic eruptions and during magmatic activity; and uptake of CO 2 by alkaline surface waters. We describe the state of our knowledge about these reactions and their controls, the extent to which we understand the mass budgets of carbon that are mediated by these reactions, and finally, the implications of these reactions for understanding present-day climate change that is driven by anthropogenic emission of CO 2 .

Unraveling the evolution of Mississippi Valley-type (MVT) hydrothermal system is crucial for understanding ore genesis and exploration. In this paper, we take the Wusihe Pb-Zn deposit in the western Yangtze Block (South China) as a case study, using detailed ore deposit geology, quartz in situ trace elements, and sulfides in situ S-Pb isotopes, to propose a new integrated model for the evolution of MVT hydrothermal system. Four hydrothermal stages were identified in the Wusihe ore district: (I) lamellar pyrite-sphalerite; (II) disseminated, stock-work, and brecciated sphalerite-galena; (III) massive galena, and (IV) veined calcite-bitumen. Within the most representative stage (stage II), Al concentrations in quartz (Q) increase from 8.46–354 ppm (mean 134 ppm) of Q1 to 171–3049 ppm (mean 1062 ppm) of Q2, and then decrease to 3.18–149 ppm (mean 25.4 ppm) of Q3. This trend indicates the role of acid-producing processes that resulted from sulfide precipitation and acid consumption by carbonate buffering. The occurrence of authigenic non-altered K-feldspar provides further evidence that the ore-forming fluids were weakly acidic with pH values of > ~5.5. Moreover, new bulk δ 34 S values of sulfides (+1.8 to +14.3‰) are overall lower than those previously reported (+7.1 to +20.9‰), implying that in addition to thermochemical sulfate reduction (TSR), bacterial sulfate reduction (BSR) may play an important role in the formation of S 2– . In situ δ 34 S values show a larger range (–4.3 to +26.6‰), and significantly, varies within single grains (up to +12.3‰), suggesting mixing of two isotopically distinct S 2– end-members produced by TSR and BSR. The diagenetic and hydrothermal early phase (stage I) sulfides were formed within a nearly closed system of BSR, whereas the formation of late phase (stage II and stage III) sulfides was caused by the input of hydrothermal fluids that promoted TSR. New galena in situ Pb isotopic ratios ( 206 Pb/ 204 Pb = 18.02–18.19, 207 Pb/ 204 Pb = 15.66–15.69, and 208 Pb/ 204 Pb = 38.14–38.39) suggest that the sources of mineralizing metals in the Wusihe deposit are mainly Proterozoic basement rocks. Hence, a multi-process model (i.e., basin-mountain coupling, fluid mixing, local sulfate reduction, in situ acid-producing and involvement of black shales and carbonate sequences) was responsible for the formation of the Wusihe deposit, while S 2– was produced by both TSR and BSR, providing new insights into the evolution of MVT hydrothermal system.

We report here the first occurrence of celestine (SrSO 4 ) in recent oceanic basalts. Celestine was found in moderately altered accidental volcanic blocks from Ka‘ula Island, a rejuvenated tuff cone in the northern Hawaiian Islands. This occurrence is novel not only for the presence of celestine but also for the absence of barite, the sulfate mineral most commonly found in oceanic hydrothermal deposits. Celestine was found lining vesicles and partially fillings voids within the matrix of several high Sr (2200–6400 ppm) Ka‘ula basalts. High-quality wavelength-dispersive microprobe analyses of celestine are reported here for near end-member celestine (>90%). The Ka‘ula celestine deposits are compositionally heterogeneous with large variations in Ba content (0.9–7.5 wt%) within single mineral aggregates. The most likely source of the Sr for celestine in the Ka‘ula basalts was the host basalt, which contains ~1200 ppm. This is about 10 times higher than normally found in mid-ocean ridge basalts and 4 times greater than commonly observed in Hawaiian basalts. Hydrothermal alteration by S-bearing fluids related to the eruption that transported these accidentally fragments probably mobilized Sr in the blocks. These S-rich solutions later precipitated celestine during or following the eruption. We were unable to confirm the origin for the Sr via Sr isotope measures because the Ka‘ula celestine was too fine grained, friable, and widely dispersed to be concentrated for Sr isotope analyses. Future studies of basalts from active volcanoes on oceanic islands, especially for basalts with elevated Sr contents (>1000 ppm), should be aware of the possible presence of celestine in moderately altered lavas.

Quantitative X‑ray element maps of cassiterite crystals from four localities show that Ti, Fe, Nb, Ta, and W define oscillatory zonation patterns and that the cathodoluminescent response is due to a complex interplay between Ti activated emission paired with quenching effects from Fe, Nb, Ta, and W. Sector zonation is commonly highlighted by domains of high Fe, incorporated via a substitution mechanism independent of Nb and Ta. A second form of sector zonation is highlighted by distributions of W separate to the Fe-dominant sector zone. Both sector zones show quenched cathodoluminescence and are indistinguishable under routine SEM CL imaging. For cassiterite already high in Fe (and Nb or Ta), such as in pegmatitic or granitic samples, the internal structure of the grain may remain obscured when imaged by cathodoluminescence techniques, regardless of the presence of sector zonation. Care‑ ful petrogenetic assessments using a combination of panchromatic and hyperspectral CL, aided by quantitative elemental X‑ray mapping, is a prerequisite step to elucidate cassiterite petrogenetic history and properly characterize these grains for in situ microanalysis. The absence of a clear petrogenetic framework may lead to unknowingly poor spot selection during in situ analyses for geochronology and trace element geochemistry, and/or erroneous interpretations of U-Pb and O isotopic data.

Bulk-rock compositions are commonly used as proxies for melt compositions, particularly in silicic plutonic systems. However, crystal accumulation and/or melt loss may play an important role in bulk-rock compositional variability (McCarthy and Hasty 1976; McCarthy and Groves 1979; Wiebe 1993; Wiebe et al. 2002; Collins et al. 2006; Deering and Bachmann 2010; Miller et al. 2011; Vernon and Collins 2011; Lee and Morton 2015; Lee et al. 2015; Barnes et al. 2016a; Schaen et al. 2018). Recognizing and quantifying the effects of crystal accumulation and melt loss in these silicic systems is challenging. Hornblende-melt Fe/Mg partitioning relationships and hornblende (Hbl) chemometry are used here to test for equilibrium with encompassing bulk-rock and/or glass compositions from several plutonic and volcanic systems. Furthermore, we assess the extent to which these tests can be appropriately applied to Hbl from plutonic systems by investigating whether Hbl from the long-lived (~10 Ma) Tuolumne Intrusive Complex preserves magmatic crystallization histories. On the basis of regular zoning patterns, co-variation of both fast- and slow-diffusing trace elements, Hbl thermometry, and compositional overlap with volcanic Hbl we conclude that Hbl from plutons largely preserve records supporting the preservation of a magmatic crystallization history, although many compositional analyses yield calculated temperatures <750 °C, which is unusual in volcanic Hbl. Hornblende is only rarely in equilibrium with host plutonic bulk-rock compositions over a wide range of SiO 2 contents (42–78 wt%). Hornblende chemometry indicates that the majority of Hbl from the plutonic systems investigated here is in equilibrium with melts that are typically more silicic (dacitic to rhyolitic in composition) than bulk-rock compositions. These results are consistent with crystal accumulation and/or loss of silicic melts within middle- to upper-crustal plutons. Although the processes by which melts are removed from these plutonic systems is uncertain, it is evident that these melts are either redistributed in the crust (e.g., leucogranite dikes, plutonic roofs, etc.) or are instead erupted. In contrast, Hbl from volcanic rocks is more commonly in equilibrium with bulk-rock and glass compositions. In most cases, where Hbl is out of equilibrium with its host glass, the glasses are more evolved than the calculated melts indicating crystallization from a less fractionated melt and/or mixed crystal populations. Where Hbl is not in equilibrium with volcanic bulk-rocks, the bulk-rock compositions are typically more mafic than the calculated melts. In some intermediate volcanic samples, the occurrence of wide-ranges of calculated melt compositions is indicative of magma mixing. The general absence of Hbl with temperatures <750 °C from volcanic systems suggests that magmatic mushes below this temperature are unlikely to erupt. Our results indicate that bulk-rock compositions of granitic plutonic rocks only rarely approximate melt compositions and that the possibility of crystal accumulation and/ or melt loss cannot be ignored. We suggest that detailed assessments of crystal accumulation and melt loss processes in magmatic systems are crucial to evaluating magma differentiation processes and discerning petrogenetic links between plutonic and volcanic systems.

Rare earth elements (REEs) have become increasingly important to our modern society due to their strategical significance and numerous high technological applications. Regolith-hosted heavy rare earth element (HREE) deposits in South China are currently the main source of the HREEs, but the ore-forming processes are poorly understood. In these deposits, the REEs are postulated to accumulate in regolith through adsorption on clay minerals. In the Zudong deposit, the world’s largest regolith-hosted HREE deposit, clay minerals are dominated by short, stubby, nanometer-scale halloysite tubes (either 10 or 7 Å) and microcrystalline kaolinite in the saprolite and lower pedolith and micrometer-sized vermicular kaolinite in the humic layer and upper pedolith. A critical transformation of the clay minerals in the upper pedolith is coalescence and unrolling of halloysite to form vermicular kaolinite. Microcrystalline kaolinite also transformed to large, well-crystalline vermicular kaolinite. This transformation could result in significant changes in different physicochemical properties of the clay assemblages. Halloysite-abundant clay assemblages in the deep regolith have specific surface area and porosity significantly higher than the kaolinite-dominant clay assemblages in the shallow soils. The crystallinity of clay minerals also increased, exemplified by decrease in Fe contents of the kaolinite group minerals (from ~1.2 wt% in the lower saprolite to ~0.35 wt% in the upper pedolith), thereby indicative of less availability of various types of adsorption sites. Hence, halloysite-abundant clay minerals of high adsorption capacity in deep regolith could efficiently retain the REEs released from weathering of the parent granite. Reduction in adsorption capacity during the clay transformation in shallow depth partially leads to REE desorption, and the released REEs would be subsequently transported to and adsorbed at deeper part of the soil profile. Hence, the clay-adsorbed REE concentration in the lower pedolith and saprolite (~2500 ppm on average) is much higher than the uppermost soils (~400 ppm on average). Therefore, weathering environments that favor the release of the REEs in the shallow soils but preservation of halloysite in the deep regolith can continuously adsorb REEs in the clay minerals to form economically valuable deposits.

The classification of the tetrahedrite group minerals in keeping with the current IMA-accepted nomenclature rules is discussed. Tetrahedrite isotypes are cubic, with space group symmetry I4¯3m.$I\overline{4}3m.$The general structural formula of minerals belonging to this group can be written as M (2) A 6 M (1) (B 4 C 2 ) X (3) D 4 S(1) Y 12 S(2) Z, where A = Cu + , Ag + , o (vacancy), and (Ag 6 ) 4+ clusters; B = Cu + , and Ag + ; C = Zn 2+ , Fe 2+ , Hg 2+ , Cd 2+ , Mn 2+ , Cu 2+ , Cu + , and Fe 3+ ; D = Sb 3+ , As 3+ , Bi 3+ , and Te 4+ ; Y = S 2– and Se 2– ; and Z = S 2– , Se 2– , and o. The occurrence of both Me + and Me 2+ cations at the M (1) site, in a 4:2 atomic ratio, is a case of valency-imposed double site-occupancy. Consequently, different combinations of B and C constituents should be regarded as separate mineral species. The tetrahedrite group is divided into five different series on the basis of the A, B, D, and Y constituents, i.e., the tetrahedrite, tennantite, freibergite, hakite, and giraudite series. The nature of the dominant C constituent (the so-called “charge-compensating constituent”) is made explicit using a hyphenated suffix between parentheses. Rozhdestvenskayaite, arsenofreibergite, and goldfieldite could be the names of three other series. Eleven minerals belonging to the tetrahedrite group are considered as valid species: argentotennantite-(Zn), argentotetrahedrite-(Fe), kenoargentotetrahedrite-(Fe), giraudite-(Zn), goldfieldite, hakite-(Hg), rozhdestvenskayaite-(Zn), tennantite-(Fe), tennantite-(Zn), tetrahedrite-(Fe), and tetrahedrite-(Zn). Furthermore, annivite is formally discredited. Minerals corresponding to different end-member compositions should be approved as new mineral species by the IMA-CNMNC following the submission of regular proposals. The nomenclature and classification system of the tetrahedrite group, approved by the IMA-CNMNC, allows the full description of the chemical variability of the tetrahedrite minerals and it is able to convey important chemical information not only to mineralogists but also to ore geologists and industry professionals.

Caseyite, [(V 5+ O 2 )Al 10– x (OH) 20–2 x (H 2 O) 18–2 x ] 2 [H 2 V 4+ V 9 5+ O 28 ][V 5+ 10 O 28 ] 2 [(Na,K,Ca) 2– y (SO 4 ) 2– z ⋅(60+8x+ y +4 z ) H 2 O], where x = 0–2.5, y = 0–2, z = 0–2, is a new mineral (IMA 2019-002) occurring in low-temperature, post-mining, secondary mineral assemblages at the Burro, Packrat, and West Sunday mines in the Uravan Mineral Belt of Colorado, U.S.A. Crystals of caseyite are yellow tapering needles or blades, with a pale yellow streak, vitreous luster, brittle tenacity, curved fracture, no cleavage, Mohs hardness between 2 and 3, and 2.151 g/cm 3 calculated density. Caseyite is optically biaxial (+) with a = 1.659(3), b = 1.670(3), g = 1.720(3) (white light), 2 V = 52.6(5)°, has strong r < v dispersion, optical orientation Z ≈ a (elongation of needles), and no pleochroism. Electron-probe microanalysis provided the empirical formula [(V 5+ O 2 )Al 8.94 (OH) 17.88 (H 2 O) 15.88 ] 2 [H 2 V 4+ V 9 5+ O 28 ][V 5 10 + O 28 ] 2 [(Na 0.82 Ca 0.35 K 0.27 ) S1.44 (SO 4 ) 1.33 ⋅70.24H 2 O] (+0.94 H). Caseyite is monoclinic, P 21/ n , a = 14.123(8), b = 30.998(15), c = 21.949(11) Å, b = 97.961(8)°, V = 9516(9) Å 3 , and Z = 2. The crystal structure ( R 1 = 0.0654 for 9162 I o >2s I reflections) contains both normal [V 10 O 28 ] 6– and doubly protonated mixed-valence [H 2 V 1 4+ V 9 5+ O 28 ] 5– decavanadate isopolyanions, and a novel vanadoaluminate heteropolycation (“flat-Al 10 V☐2”), ideally [(V 5+ O 2 )Al 10 (OH) 20 (H 2 O) 18 ] 11+ , closely related to the technologically important flat-Al 13 polyoxocation.

The crystal structure and chemical composition of a crystal of Mg-bearing phase Egg with a general formula M1−x3+Mx2+SiO4H1−x(M3+=Al,  Cr;M2+=Mg, Fe),$M_{1-x}^{3+}M_{x}^{2+}\text{Si}{{\text{O}}_{4}}{{\text{H}}_{1-x}}\left( {{M}^{3+}}=\text{Al,}\,\,\text{Cr;}{{M}^{2+}}=\text{Mg,}\,\text{Fe} \right),$where x = 0.35, produced by subsolidus reaction at 24 GPa and 1400 °C of components of subducted oceanic slabs (peridotite, basalt, and sediment), was analyzed by electron microprobe and single-crystal X‑ray diffraction. Neglecting the enlarged unit cell and the consequent expansion of the coordination polyhedra (as expected for Mg substitution for Al), the compound was found to be topologically identical to phase Egg, AlSiO 3 OH, space group P 21/ n , with lattice parameters a = 7.2681(8), b = 4.3723(5), c = 7.1229(7) Å, b = 99.123(8)°, V = 223.49(4) Å 3 , and Z = 4. Bond-valence considerations lead to hypothesize the presence of hydroxyl groups only, thereby excluding the presence of the molecular water that would be present in the hypothetical end-member MgSiO 3 ·H 2 O. We thus demonstrate that phase Egg, considered as one of the main players in the water cycle of the mantle, can incorporate large amounts of Mg in its structure and that there exists a solid solution with a new hypothetical MgSiH 2 O 4 end-member, according to the substitution Al 3+ ↔ Mg 2+ + H + . The new hypothetical MgSiH 2 O 4 end-member would be a polymorph of phase H, a leading candidate for delivering significant water into the deepest part of the lower mantle.

Trace-element zoning in igneous phenocrysts and cumulus phases is an informative record of magmatic evolution. The advent of microbeam X‑ray fluorescence (XRF) mapping has allowed rapid chemical imaging of samples at thin section to decimeter scale, revealing such zoning patterns. Mapping with synchrotron radiation using multidetector arrays has proved especially effective, allowing entire thin sections to be imaged at micrometer-scale resolution in a matter of hours. The resolution of subtle minor element zoning, particularly in first-row transition metals, is greatly enhanced in synchrotron X‑ray fluorescence microscopy (XFM) images by scanning with input beam energy below the Fe K α line. In the examples shown here, from a phenocryst rich trachybasalt from Mt Etna (Italy) and from a Ni-Cu-PGE ore-bearing intrusion at Norilsk (Siberia), the zoning patterns revealed in this way record aspects of the crystallization history that are not readily evident from XFM images collected using higher incident energies and that cannot be obtained at comparable spatial resolutions by any other methods within reasonable scan times. This approach has considerable potential as a geochemical tool for investigating magmatic processes and is also likely to be applicable in a wide variety of other fields.