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 American Mineralogist   [SJR: 1.185]   [H-I: 104]   [12 followers]  Follow        Subscription journal    ISSN (Print) 0003-004X    Published by GeoScienceWorld  [16 journals]
• Defining minerals in the age of humans
• Authors: Heaney; P. J.
Pages: 925 - 926
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-6045
Issue No: Vol. 102, No. 5 (2017)

• Bottled samples of Earths lower mantle
• Authors: Bassett; W. A.
Pages: 927 - 927
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-6047
Issue No: Vol. 102, No. 5 (2017)

• Two ways of looking at chemical bonding
• Authors: Brown; I. D.
Pages: 928 - 928
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-6037
Issue No: Vol. 102, No. 5 (2017)

• Diamonds from the lower mantle?
• Authors: Thomson; A. R.
Pages: 929 - 930
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-6061
Issue No: Vol. 102, No. 5 (2017)

• A review and update of mantle thermobarometry for primitive arc magmas
• Authors: Till; C. B.
Pages: 931 - 947
Abstract: Erupted lavas and tephras remain among the best tools we have to ascertain the mantle processes that give rise to the compositional diversity and spatial distribution of near-primary magmas at volcanic arcs. A compilation of mantle-melt thermobarometry for natural, primitive arc magmas to date reveals published estimates vary between ~1000–1600 °C at ~6–50 kbar. In addition to the variability of mantle melting processes within and between different arcs, this range of conditions is the result of different methodology, such as the nature of reverse fractional crystallization calculations, the choice of thermobarometer, how magmatic H2O was quantified and its calculated effect on pressure and temperature, and choices about mantle lithology and oxygen fugacity. New and internally consistent reverse fractionation calculations and thermobarometry for a representative subset of the primitive arc samples with adequate published petrography, measured mineral and melt compositions, and constraints on pre-eruptive H2O content suggest a smaller range of global mantle-melt equilibration conditions (~1075–1450 °C at ~8–19 kbar) than the literature compilation. The new pressure and temperature estimates and major element modeling are consistent with a model whereby several types of primitive arc magmas, specifically hydrous calc-alkaline basalt, primitive andesite and hydrous high-MgO liquid such as boninite, first form at the location of the water-saturated mantle solidus at pressures of ~20–35 kbar and rise into the hot core of the mantle wedge reacting with the mantle en route. Due to their re-equilibration during ascent, these hydrous magmas ultimately record the conditions in the hot, shallow nose of the mantle wedge at the end of their mantle ascent path rather than the conditions at their point of origin as often interpreted. When the mantle residue for this process is lherzolite, calc-alkaline basalt is generated. When the mantle residue is harzburgite to dunite, either high-Mg primitive andesite or high-MgO liquid is generated, depending on the H2O content. A different type of primitive arc magma, specifically nominally anhydrous arc tholeiite, is generated by near-fractional decompression melting at or near the anhydrous lherzolite solidus in the upwelling back limb of corner flow at ~25–10 kbar and is focused into the same region of the shallow mantle wedge as the hydrous melts. The similarity in the terminus of the mantle ascent paths for both wet and dry primitive arc magmas likely explains their eruption in close spatial and temporal proximity at many arcs. The conditions of last mantle equilibration for primitive arc tholeiites generated by decompression melting also imply that the convecting mantle extends to 10 kbar (~30 km) or less below most arcs. The range of mantle-melt equilibration conditions calculated here agrees well with the range of temperatures predicted for the shallow mantle wedge beneath arcs by geodynamic models, although it suggests some subduction zones may have higher maximum temperatures at shallower depths in the wedge than originally predicted. Primitive hydrous arc magmas also constrain natural variation on the order of 200–250 °C in the maximum temperature in the hot shallow nose of the mantle wedge between arcs. Thus the new primitive magma thermobarometry presented here is useful for understanding melt migration processes and the temperature structure in the uppermost part of the mantle wedge, as well as the origin of different primitive magma types at arcs.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5783
Issue No: Vol. 102, No. 5 (2017)

• Using mineral geochemistry to decipher slab, mantle, and crustal input in
the generation of high-Mg andesites and basaltic andesites from the
• Authors: Sas, M; Debari, S. M, Clynne, M. A, Rusk, B. G.
Pages: 948 - 965
Abstract: To better understand the role of slab melt in the petrogenesis of North Cascades magmas, this study focuses on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington. High-Mg andesites (HMA) and basaltic andesites (HMBA) in the Cascade Arc have high Mg# [molar Mg/(Mg+Fe2+)] relative to their SiO2 contents, elevated Nd/Yb, and are Ni- and Cr-enriched. The rock units examined here include the Tarn Plateau HMBA (51.8–54.0 wt% SiO2, Mg# 68–70) and Glacier Creek HMA (58.3–58.7 wt% SiO2, Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek HMBA (54.8–54.6 SiO2, Mg# 69–73) from Glacier Peak. This study combines major and trace element compositions of minerals and whole rocks to test several petrogenetic hypotheses and to determine which, if any, are applicable to North Cascades HMA and HMBA. In the Tarn Plateau HMBA, rare earth element (REE) equilibrium liquids calculated from clinopyroxene compositions have high Nd/Yb that positively correlates with Mg#. This correlation suggests an origin similar to that proposed for Aleutian adakites, where intermediate, high Nd/Yb slab-derived melts interact with the overlying mantle to become Mg-rich, and subsequently mix with low Nd/Yb, mantle-derived mafic magmas with lower Mg#. In the Glacier Creek HMA, elevated whole-rock MgO and SiO2 contents resulted from accumulation of xenocrystic olivine and differentiation processes, respectively, but the cause of high Nd/Yb is less clear. However, high whole-rock Sr/P (fluid mobile/fluid immobile) values indicate a mantle source that was fluxed by an enriched, hydrous slab component, likely producing the observed high Nd/Yb REE signature. The Lightning Creek HMBA is a hybridized rock unit with at least three identifiable magmatic components, but only one of which has HMA characteristics. Cr and Mg contents in Cr-spinel and olivine pairs in this HMA component suggest that its source is a strongly depleted mantle, and high whole-rock Sr/P values indicate mantle melting that was induced through hydration, likely adding the component responsible for the observed high Nd/Yb REE pattern. The elevated SiO2 contents (54.6 wt%) of the HMA component resulted from differentiation or high degrees of partial melting of ultramafic material through the addition of H2O. Therefore the Lightning Creek HMBA is interpreted to have originated from a refractory mantle source that underwent melting through interaction with an enriched slab component. Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA and HMBA in the north Cascade Arc.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5756
Issue No: Vol. 102, No. 5 (2017)

• Sperrylite saturation in magmatic sulfide melts: Implications for
formation of PGE-bearing arsenides and sulfarsenides
• Authors: Bai, L; Barnes, S.-J, Baker, D. R.
Pages: 966 - 974
Abstract: Sperrylite (PtAs2) is one of most common Pt minerals, but the processes whereby it forms are not clearly established. Most commonly it is associated with the major-component base metal sulfide minerals (pyrrhotite, pentlandite, and chalcopyrite), which are believed to have crystallized from magmatic sulfide melts. Hence, sperrylite is thought to have formed by crystallization from a sulfide melt or by exsolution from sulfide minerals. However, sperrylite is also found associated with silicate and oxide minerals where it is thought to have formed by crystallization from the silicate magma. To investigate the conditions under which sperrylite could crystallize from a magmatic sulfide melt we investigated sperrylite saturation in Fe-Ni-Cu-S sulfide melts under controlled $${f}_{{\mathrm{O}}_{2}}$$ and $${f}_{{\mathrm{S}}_{2}}$$ at 910–1060 °C and 1 bar. The As and Pt concentrations in the sulfide melt at sperrylite saturation increase from 0.23–0.41 to 2.2–4.4 wt% and from 0.36–0.65 to 1.9–2.8 wt%, respectively, as the iron concentration in the sulfide melt decreases from 50 to 36 wt% at 910–1060 °C. We show that transitional metal concentrations, particular iron and nickel, as well as sulfur and oxygen fugacities influence As and Pt concentrations in the sulfide melt at sperrylite saturation. These intensive variables appear to effect sperrylite solubility by influencing the oxidation state of As in the sulfide melt. The measured concentrations of As and Pt in sperrylite-saturated sulfide melts produced in our experiments are much higher than that in most natural sulfides, implying that arsenides and sulfarsenides will not reach saturation in natural magmatic sulfide melts at high temperatures unless the magma has been contaminated with an exceptionally As-rich rock. This suggests that the observed arsenides and sulfarsenides in natural sulfide ores were not formed by crystallization from unfractionated sulfide melts at high temperatures above 900 °C, but might form at low temperatures below 900 °C.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5631
Issue No: Vol. 102, No. 5 (2017)

• Water transport by subduction: Clues from garnet of Erzgebirge UHP
eclogite
• Authors: Schmadicke, E; Gose, J.
Pages: 975 - 986
Abstract: A key question concerning the water budget of Earth’s mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust. Hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet. Here we explore if garnet can be used as a proxy to assess the amount of this water. Available data on the water contents of garnet in coesite eclogite vary over orders of magnitude, from a few up to ca. 2000 ppm. By implication, the maximum bulk-rock water contents are unrealistically high (wt% level). New data from the Erzgebirge indicate moderate amounts of structural H2O stored in garnet (43–84 ppm), omphacite (400–820 ppm), and in the bulk coesite eclogite (ca. 280–460 ppm). Higher garnet water contents occur, but these are not primary features. They are related to molecular water in fluid inclusions that can be attributed to eclogite-facies fluid influx postdating the metamorphic peak. Fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions. Such secondary H2O incorporation is only possible in the case of primary water-deficiency indicating that garnet hosted less water than it was able to store. This is insofar astonishing as comparably high H2O amounts are liberated by the breakdown of prograde eclogite-facies hydrous minerals as a result of ultrahigh-pressure (UHP) metamorphism. Judging from Erzgebirge quartz eclogite, dehydration of 5–10% hydrous minerals (±equal portions of zoisite+calcic amphibole) produces 1500–3000 ppm water. We infer that the largest part of the liberated water escaped, probably due to kinetic reasons, and hydrated exhuming UHP slices in the hanging-wall. Depending on the physical conditions, water influx in eclogite during exhumation (1) produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals—as in the Erzgebirge—and/or (2) it may give rise to retrograde hydrous minerals. We conclude that eclogite transports moderate quantities of water (several hundred parts per million) to mantle depths beyond 100 km, an amount equivalent to that in ca. 1% calcic amphibole.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5920
Issue No: Vol. 102, No. 5 (2017)

• Single-track length measurements of step-etched fission tracks in Durango
apatite: "Vorsprung durch Technik"
• Authors: Jonckheere, R; Tamer, M. T, Wauschkuhn, B, Wauschkuhn, F, Ratschbacher, L.
Pages: 987 - 996
Abstract: Fossil and induced confined fission-tracks in the Durango apatite do not etch to their full etchable lengths with the current protocols. Their mean lengths continue to increase at a diminished rate past the break in slope in a length vs. etch-time plot. The mean length of the fossil tracks increases from 14.5(1) to 16.2(1) μm and that of the induced tracks from 15.7(1) to 17.9(1) μm between 20 and 60 s etching (5.5 M HNO3; 21 °C); both are projected to converge toward ~18 μm after ~180 s. This increase is due to track etching, not bulk etching. The irregular length increments of individual tracks reveal a discontinuous track structure in the investigated length intervals. The mean lengths of the fossil and induced tracks for the standard etch time (20 s) for the (5.5 M HNO3; 21 °C) etch are thus not the result of a shortening of the latent fission tracks but instead of a lowering of the effective track-etch rate vT. The rate of length increase of individual fossil confined tracks correlates with their length: older tracks are shorter because they etch slower. Step etching thus makes it possible to some extent to distinguish between older and younger fossil fission tracks. Along-track vT measurements could reveal further useful paleo-temperature information. Because the etched length of a track at standard etch conditions is not its full etchable length, geometrical statistics based on continuous line segments of fixed length are less secure than hitherto held.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5988
Issue No: Vol. 102, No. 5 (2017)

• Transformation of halloysite and kaolinite into beidellite under
hydrothermal condition
• Authors: He, H; Ji, S, Tao, Q, Zhu, J, Chen, T, Liang, X, Li, Z, Dong, H.
Pages: 997 - 1005
Abstract: Understanding clay mineral transformation is of fundamental importance to unraveling geological and environmental processes and to better understanding the unique structure and property of phyllosilicates. To date, two pathways have been identified, i.e., the transformation among 2:1 type clay minerals (e.g., illitization of smectite) and from 2:1 type to 1:1 type (e.g., kaolinization of smectite). However, the transformation of 1:1 to 2:1 type is less commonly observed. In this study, hydrothermal experiments were conducted to investigate the possibility of the transformation of 1:1 type clay minerals (i.e., halloysite and kaolinite) into 2:1 ones (i.e., beidellite). The obtained products were characterized by XRD, TG, FTIR, 27Al and 29Si MAS NMR, and HRTEM. XRD patterns of the hydrothermal products display characteristic basal spacing of smectite group minerals at 1.2–1.3 nm with dramatic decrease/disappearance of the (001) reflection of halloysite and kaolinite. This is consistent with HRTEM observations, in which clay layers with a thickness of 1.2–1.4 nm are observed in all hydrothermal products and the Si/Al ratio determined by EDS analysis is close to that of beidellite. The basal spacing increases to ~1.70 nm upon ethylene glycolation, displaying swelling ability of the resultant minerals. The consumption of surface OH in precursor minerals during the transformation leads to a dramatic decrease of mass loss of dehydroxylation and merging of the well resolved OH stretching vibrations in precursor minerals into one at ca. 3667 cm–1, which is indicative of beidellite. These results demonstrate that both halloysite and kaolinite can be converted to 2:1 beidellite under hydrothermal condition, and the transformation of halloysite is easier than that of kaolinite. Such transformation of 1:1 clay minerals to 2:1 ones could be a new pathway for the transformation of clay minerals in nature. Meanwhile, the substitution of Al3+ for Si4+ is found in all newly formed beidellite, suggesting the chemical composition of the newly formed Si-O tetrahedral sheet is different from the one inherited from the precursor clay minerals. This can well explain the formation of "polar layer" in mixed-layer phyllosilicates. These findings are of high importance for better understanding the transformation among clay minerals and unique structure of mixed-layer phyllosilicates.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5935
Issue No: Vol. 102, No. 5 (2017)

• Controls on trace-element partitioning among co-crystallizing minerals:
Evidence from the Panzhihua layered intrusion, SW China
• Authors: Chen, L.-M; Song, X.-Y, Hu, R.-Z, Yu, S.-Y, He, H.-L, Dai, Z.-H, She, Y.-W, Xie, W.
Pages: 1006 - 1020
Abstract: The factors and processes that control trace-element partitioning among co-crystallizing cumulus minerals in layered intrusions have long been controversial. Here we address this issue using new laser ablation ICP-MS trace element data for magnetite, ilmenite, and clinopyroxene from the Panzhihua layered intrusion in the Emeishan large igneous province, SW China. The cumulus minerals display strong Ni, Co, and Cr depletions, indicative of parental magmas low in concentration of these elements probably due to prior sulfide removal and the fractionation of chromite or Cr-magnetite in a staging magma chamber at depth. Both magnetite and clinopyroxene show cyclical variations in some transition elements (e.g., Cr, V, and Ni) along the stratigraphic section. The average concentrations of these transition elements in magnetite are positively correlated with those in clinopyroxene, likely resulting from co-crystallization of magnetite and clinopyroxene. The incompatible element (e.g., Zr, Hf, and Nb) concentrations of the cumulus minerals from the Lower Zone are highly variable compared to those of the Middle and Upper Zones. These large variations in trace element compositions are attributed to a "trapped liquid shift" in the Lower Zone. Ilmenite crystals from the Panzhihua intrusion may have undergone extensive modification of transition elements during subsolidus re-equilibration with magnetite, leading to the decoupled variations of transition elements in ilmenite across the Lower Zone stratigraphy. Our study indicates that systematic trace element variations of the main cumulus mineral assemblage, rather than a single mineral, need to be considered to better constrain the magmatic differentiation and elemental fractionation of layered intrusions.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5804
Issue No: Vol. 102, No. 5 (2017)

• Using mineral equilibria to estimate H2O activities in peridotites from
the Western Gneiss Region of Norway
• Authors: Kang, P; Lamb, W. M, Drury, M.
Pages: 1021 - 1036
Abstract: The Earth’s mantle is an important reservoir of H2O, and even a small amount of H2O has a significant influence on the physical properties of mantle rocks. Estimating the amount of H2O in rocks from the Earth’s mantle would, therefore, provide some insights into the physical properties of this volumetrically dominant portion of the Earth. The goal of this study is to use mineral equilibria to determine the activities of H2O $$({a}_{{\mathrm{H}}_{2}\mathrm{O}})$$ in orogenic mantle peridotites from the Western Gneiss Region of Norway. An amphibole dehydration reaction yielded values of $${a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ ranging from 0.1 to 0.4 for these samples. Values of $${f}_{{\mathrm{O}}_{2}}$$ of approximately 1 to 2 log units below the FMQ oxygen buffer were estimated from a $${f}_{{\mathrm{O}}_{2}}$$-buffering reaction between olivine, orthopyroxene, and spinel for these same samples. These results demonstrate that the presence of amphibole in the mantle does not require elevated values of $${a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ (i.e., $${a}_{{\mathrm{H}}_{2}\mathrm{O}}\approx 1$$) nor relatively oxidizing values of $${f}_{{\mathrm{O}}_{2}}$$ (i.e., >FMQ).It is possible to estimate a minimum value of $${a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ by characterizing fluid speciation in C-O-H system for a given value of oxygen fugacity $$({f}_{{\mathrm{O}}_{2}})$$. Our results show that the estimates of $${a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ obtained from the amphibole dehydration equilibrium are significantly lower than values of $${a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ estimated from this combination of $${f}_{{\mathrm{O}}_{2}}$$ and C-O-H calculations. This suggests that fluid pressure (Pfluid) is less than lithostatic pressure (Plith) and, for metamorphic rocks, implies the absence of a free fluid phase.Fluid absent condition could be generated by amphibole growth during exhumation. If small amounts of H2O were added to these rocks, the formation of amphibole could yield low values of $${a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ by consuming all available H2O. On the other hand, if the nominally anhydrous minerals (NAMs) contained significant H2O at conditions outside of the stability field of amphibole they might have served as a reservoir of H2O. In this case, NAMs could supply the OH necessary for amphibole growth once retrograde P-T conditions were consistent with amphibole stability. Thus, amphibole growth may effectively dehydrate coexisting NAMs and enhance the strength of rocks as long as the NAMs controlled the rheology of the rock.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5915
Issue No: Vol. 102, No. 5 (2017)

• Rowleyite, Formula, a new mineral with a microporous framework structure
• Authors: Kampf, A. R; Cooper, M. A, Nash, B. P, Cerling, T. E, Marty, J, Hummer, D. R, Celestian, A. J, Rose, T. P, Trebisky, T. J.
Pages: 1037 - 1044
Abstract: Rowleyite, $$[\mathrm{Na}{({\mathrm{NH}}_{4},\mathrm{K})}_{9}{\mathrm{Cl}}_{4}]{[{\mathrm{V}}_{2}^{5+,4+}(\mathrm{P},\mathrm{As}){\mathrm{O}}_{8}]}_{6}\cdot n[{\mathrm{H}}_{2}\mathrm{O},\mathrm{Na},{\mathrm{NH}}_{4},\mathrm{K},\mathrm{Cl}]$$, is a new mineral species from the Rowley mine, Maricopa County, Arizona, U.S.A. It was found in an unusual low-temperature, apparently post-mining suite of phases that include various vanadates, phosphates, oxalates, and chlorides, some containing $${\mathrm{NH}}_{4}^{+}$$. Other secondary minerals found in association with rowleyite are antipinite, fluorite, mimetite, mottramite, quartz, salammoniac, struvite, vanadinite, willemite, wulfenite, and several other potentially new minerals. Analyzed 13C values for the antipinite in association with rowleyite are consistent with a bat guano source. Crystals of rowleyite are very dark brownish green (appearing black) truncated octahedra up to about 50 μm in diameter. The streak is brownish green, the luster is vitreous, very thin fragments are transparent. The Mohs hardness is about 2, the tenacity is brittle, fracture is irregular, there is no cleavage, and the measured density is 2.23(2) g/cm3. Rowleyite is optically isotropic with n = 1.715(5). Electron microprobe analyses yielded the empirical formula $${[{({\mathrm{NH}}_{4})}_{8.81}{\mathrm{Na}}_{3.54}{\mathrm{K}}_{2.58})}_{\Sigma 14.93}{\mathrm{Cl}}_{6.29}{({\mathrm{H}}_{2}\mathrm{O})}_{16}][{({\mathrm{V}}_{9.36}^{5+}{\mathrm{V}}_{2.64}^{4+})}_{\Sigma 12}{({\mathrm{P}}_{5.28}{\mathrm{As}}_{0.72}^{5+})}_{\Sigma 6}{\mathrm{O}}_{48}]$$. Raman and infrared spectroscopy confirmed the presence of NH4 and H2O. Rowleyite is cubic, $$Fd\overline{3}m$$, with a = 31.704(14) Å, V = 31867(42) Å3, and Z = 16. The crystal structure of rowleyite (R1 = 0.040 for 1218 Fo > 4F reflections) contains [V4O16]12+ polyoxovanadate units that link to one another via shared vertices with [(P,As)O4]3– tetrahedra to form a 3D framework possessing large interconnected channels. The channels contain a 3D ordered [Na(NH4,K)9Cl4]6+ salt net, which apparently served as a template for the formation of the framework. In that respect, rowleyite can be considered a salt-inclusion solid (SIS). The rowleyite framework is among the most porous known.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5977
Issue No: Vol. 102, No. 5 (2017)

• Textures and high field strength elements in hydrothermal magnetite from a
skarn system: Implications for coupled dissolution-reprecipitation
reactions
• Authors: Yin, S; Ma, C, Robinson, P. T.
Pages: 1045 - 1056
Abstract: There is increasing evidence to suggest that high field strength elements (HFSE) could be mobile to some extent in hydrothermal fluid due to the influence of halogens (e.g., fluorine and chlorine). However, in natural hydrothermal (fluid) systems, "coupled dissolution reprecipitation" (CDR) reactions at fluid-mineral interfaces that have been emphasized in the past decade may play a key role in controlling the final textures and mineral assemblages. The influences of the CDR reactions in hydrothermal systems on HFSE enrichment or depletion at the mineral scale are enigmatic. In this study, we show that enrichment of Nb and Zr can occur in magnetite on the mineral scale formed by hydrothermal fluids at medium-to-lower temperature in a skarn system. Four stages of mineralization and alteration of magnetite have been identified in the Baishiya iron skarn deposit of the East Kunlun Mountains of China. Magnetite formed in stage 1 (S1) developed obvious oscillatory zonation, whereas that formed in stages 2 (S2) and 3 (S3) shows hydrothermal alteration and metasomatic textures, and that in stage 4 (S4) developed euhedral crystals with simple zoning. Systematic variations in the trace element compositions of different magnetite grains analyzed by EMPA and LA-ICP-MS suggest that the magnetite from S1 to S3 may have formed in a metasomatic process at relatively constant temperature, whereas the magnetite from S4 formed by re-equilibrium processes at lower temperature. The magnetite from each stage can be divided into light and dark domains based on backscattered electron images. The dark domains in the magnetite from S1 and S2 have higher Nb/Ta (8.52–27.00) and Zr/Hf (18.22–52.64) ratios and silicon contents than the light domains (0.55–5.66 and 2.54–16.43, respectively). Compared with other magnetite ores, the ores from S1 and S2 are depleted of V and Ni. This depletion may be induced by increased oxygen and co-crystallized sulfide. However, these variations are unlikely to be responsible for the enrichment of Nb and Zr in magnetite at equilibrium conditions. Conversely, the dark domains of the magnetite from S1 and S2 are porous, irregular, and/or oscillatory with quartz inclusions, indicating nonequilibrium conditions. These textural features could be attributed to the CDR reactions that are ubiquitous in skarn systems. The increased silicon concentrations in magnetite due to the CDR reactions could affect the lattice parameters of the magnetite structure, leading to an overall change in the volume of magnetite ores. The reduplicative processes of volume change, dissolution, and porosity formation within magnetite are further imporved due to an incresed oxygen fugacity and co-crystallized sulfide (e.g., decreased temperature or increased sulfur fugacity) at far-from-equilibrium or local equilibrium conditions, resulting in oscillatory magnetite dark domains of S1. Ripening of the transient porosity can trap nanoscale precipitates of columbite and zircon within pores of Si-magnetite, and this precipitation could be attributed to the co-crystallized phlogopite that would incorporate fluorine from the hydrothermal fluid, and subsequently decrease the solubility of Nb and Zr in the skarn system. This scenario highlights that Nb and Zr could be scavenged and enriched into in the reaction fronts (porosity) by controlling the reaction pathway at a local scale that does not reflect the overall fluid-rock interaction history of the mineral assemblage.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5913
Issue No: Vol. 102, No. 5 (2017)

• X-ray spectroscopy study of the chemical state of "invisible" Au in
synthetic minerals in the Fe-As-S system
• Authors: Trigub, A. L; Tagirov, B. R, Kvashnina, K. O, Chareev, D. A, Nickolsky, M. S, Shiryaev, A. A, Baranova, N. N, Kovalchuk, E. V, Mokhov, A. V.
Pages: 1057 - 1065
Abstract: Minerals of the Fe-As-S system are the main components of Au ores in many hydrothermal deposits, including Carlin-type Au deposits, volcanogenic massive sulfide deposits, epithermal, mesothermal, sedimentary-hosted systems, and Archean Au lodes. The "invisible" (or refractory) form of Au is present in all types of hydrothermal ores and often predominates. Knowledge of the chemical state of "invisible" Au (local atomic environment/structural position, electronic structure, and oxidation state) is crucial for understanding the conditions of ore formation and necessary for the physical-chemical modeling of hydrothermal Au mineralization. In addition, it will help to improve the technologies of ore processing and Au extraction. Here we report an investigation of the chemical state of "invisible" Au in synthetic analogs of natural minerals (As-free pyrite FeS2, arsenopyrite FeAsS, and löllingite FeAs2). The compounds were synthesized by means of hydrothermal (pyrite) and salt flux techniques (in each case) and studied by X-ray absorption fine structure (XAFS) spectroscopy in a high-energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The content of "invisible" Au in the synthesized löllingite (800 ± 300 ppm) was much higher than that in arsenopyrite (23 ± 14 ppm). The lowest Au content was observed in zonal pyrite crystals synthesized in a salt flux. High "invisible" Au contents were observed in hydrothermal pyrite (40–90 ppm), which implies that this mineral can efficiently scavenge Au even in As-free systems. The Au content of the hydrothermal pyrite is independent of sulfur fugacity and probably corresponds to the maximum Au solubility at the experimental P-T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in the structures of löllingite, arsenopyrite, and hydrothermal pyrite. The Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 Å (As), 0.13 Å (S) (arsenopyrite) relative to the Fe-ligand distance in pure compounds. Distortions of the atomic structures are localized around Au atoms and disappear at R > ~4 Å. Chemically bound Au occurs only in hydrothermal pyrite, whereas pyrite synthesized without hydrothermal fluid contains only Au°. The heating (metamorphism) of hydrothermal pyrite results in the decomposition of chemically bound Au and formation of Au° nuggets, which coarsen with increasing temperature. Depending on the chemical composition of the host mineral, Au can play a role of either a cation or an anion: the Bader atomic partial charge of Au decreases in the order pyrite (+0.4 e) > arsenopyrite (0) > löllingite (–0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form a chemically bound refractory admixture in base metal sulfides/chalcogenides. The content of chemically bound noble metals can vary depending on the composition of the host mineral and ore history.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5832
Issue No: Vol. 102, No. 5 (2017)

• Dry annealing of metamict zircon: A differential scanning calorimetry
study
• Authors: Pidgeon, R. T; Chapman, P. G, Danišik, M, Nemchin, A. A.
Pages: 1066 - 1072
Abstract: We report the results of a differential scanning calorimeter (DSC) study of the annealing of a metamict Sri Lankan zircon. Raman measurements on most chips of the powdered zircon starting material, Sri Lankan zircon (WZ19), showed no evidence of a crystalline structure, whereas a few chips retained residual Raman bands typical of highly radiation damaged zircon. DSC runs on aliquots of the powdered sample were heated to 850 and 1000 °C at rates of 2 and 10 °C/min and to 1500 °C at a rate of 10 °C/min. Raman spectroscopy was used to investigate the crystallinity of grains at selected temperature stages. Exothermal peaks were observed at about 910 and 1260 °C during the DSC run to 1500 °C. The 910 °C peak was demonstrated by Raman spectroscopy to mark the crystallization of tetragonal zirconia and the exothermic peak at about 1260 °C was demonstrated to represent the reaction of zirconia and amorphous silica to form crystalline zircon. The degree of crystallinity of these grains was almost identical to that of highly crystalline zircons from recent gem gravels from New South Wales. A small number of experimental chips from DSC analyses under 1000 °C were found to have zircon Raman bands that indicated they had undergone partial annealing. The present experimental results suggest that reconstitution of amorphous zircon to the crystalline state by dry annealing will rarely occur in terrestrial geological settings, even under extreme metamorphic conditions.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5901
Issue No: Vol. 102, No. 5 (2017)

• Tightly bound water in smectites
• Authors: Kuligiewicz, A; Derkowski, A.
Pages: 1073 - 1090
Abstract: Smectites are able to retain molecular tightly bound water (TBW) at temperatures above 100 °C, even after prolonged drying. The presence of TBW affects the stable isotope ratios, the dehydroxylation behavior of smectites and smectite-rich samples and also has implications in measuring various properties of clay-rich rocks. Five reference smectites, in Mg-, Ca-, Na-, and Cs-exchanged forms were subjected to different drying protocols followed by the determination of TBW contents using precise thermogravimetric (TG) analysis. Activation energies (Ea) of the removal of different water fractions at temperatures up to 1000 °C were determined in non-isothermal TG experiments using model-independent methods. Additionally, 4A and 13X zeolites were examined in both cases as apparent OH-free references.After drying at 110 °C, all smectites still contained up to 3 water molecules per interlayer cation. The TBW contents in smectites were found to be primarily dependent on the isothermal drying temperature. For a given temperature, TBW contents decreased with respect to the type of interlayer cation in the following order: Mg > Ca > Na > Cs. The influence of the time of drying and the smectite layer charge were found to be negligible. The Ea of dehydration below 100 °C, as determined by the Friedman method, was quite constant within the 45–60 kJ/mol range. The Ea of TBW removal increased along with the degree of reaction from 90 to 180 kJ/mol, while the Ea of dehydroxylation was found in the 159–249 kJ/mol range, highly depending on the sample’s octahedral sheet structure and the interlayer cation. The Mg2+ cation can hold H2O molecules even beyond 550 °C, making it available during dehydroxylation or—for geologic-scale reactions—pass H2O to metamorphic conditions.High similarities between the TBW contents and the Ea of dehydration for smectites and cationic (low Si/Al-) zeolites lead to the conclusion that TBW in smectites is remarkably similar to zeolitic water in terms of cation bonding and diffusion characteristics. The optimal drying protocol for smectites is to substitute interlayer cations with cations of a low-hydration enthalpy, such as Cs, and to dry a sample at 300 °C, provided that the sample is Fe-poor. Fe-rich smectites should be dried at 200 °C to avoid dehydroxylation that occurs below 300 °C.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5918
Issue No: Vol. 102, No. 5 (2017)

• Mineralogical controls on antimony and arsenic mobility during
tetrahedrite-tennantite weathering at historic mine sites Špania
Dolina-Piesky and Ľubietova-Svatodušna, Slovakia
• Authors: Radkova, A. B; Jamieson, H, Lalinska-Volekova, B, Majzlan, J, Števko, M, Chovan, M.
Pages: 1091 - 1100
Abstract: The legacy of copper (Cu) mining at Špania Dolina-Piesky and Ľubietová-Svätodušná (central Slovakia) is waste rock and soil, surface waters, and groundwaters contaminated with antimony (Sb), arsenic (As), Cu, and other metals. Copper ore is hosted in chalcopyrite (CuFeS2) and sulfosalt solid-solution tetrahedrite-tennantite {Cu6[Cu4(Fe,Zn)2]Sb4S13–Cu6[Cu4(Fe,Zn)2]As4S13} that show wide-spread oxidation characteristic by olive-green color secondary minerals. Tetrahedrite-tennantite can be a significant source of As and Sb contamination. Synchrotron-based μ-XRD, μ-XRF, and μ-XANES combined with electron microprobe analyses have been used to determine the mineralogy, chemical composition, element distribution, and Sb speciation in tetrahedrite-tennantite oxidation products in waste rock. Our results show that the mobility of Sb is limited by the formation of oxidation products such as tripuhyite and roméite group mineral containing 36.54 wt% Sb for samples where the primary mineral chemical composition is close to tetrahedrite end-member. Antimony K-edge μ-XANES spectra of these oxidation products indicate that the predominant Sb oxidation state is 5+. Arsenic and Cu are also hosted by amorphous phases containing 6.23 wt% Sb on average and these are intergrown with tripuhyite and roméite. Antimony in this environment is not very mobile, meaning it is not easily released from solid phases to water, especially compared to As, Cu, and S. For samples where the primary sulfosalt is close to tennantite composition, the oxidation products associated with tennantite relicts contain 2.43 wt% Sb and are amorphous. The variable solubility of the secondary minerals that have been identified is expected to influence mobility of Sb and As in near-surface environment.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5616
Issue No: Vol. 102, No. 5 (2017)

• Deep mantle origin and ultra-reducing conditions in podiform chromitite:
Diamond, moissanite, and other unusual minerals in podiform chromitites
from the Pozanti-Karsanti ophiolite, southern Turkey
• Authors: Lian, D; Yang, J, Dilek, Y, Wu, W, Zhang, Z, Xiong, F, Liu, F, Zhou, W.
Pages: 1101 - 1113
Abstract: The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5850
Issue No: Vol. 102, No. 5 (2017)

• Trace elements and Sr-Nd isotopes of scheelite: Implications for the
W-Cu-Mo polymetallic mineralization of the Shimensi deposit, South China
• Authors: Sun, K; Chen, B.
Pages: 1114 - 1128
Abstract: The Shimensi deposit (South China) is a newly discovered W-Cu-Mo polymetallic deposit with a reserve of 0.76 million tones WO3, one of the largest tungsten deposits in the world. We report elemental and Sr-Nd isotopic data for scheelites from the giant deposit, to determine the source region and genesis of the deposit. Scheelite is the most important ore mineral in the Shimensi deposit. Trace elements (including REEs) and Nd-Sr isotopic compositions of scheelites were used to constrain the origin of the mineralizing fluids and metals. Our data reveal that the REEs of scheelite are mainly controlled by the substitution mechanism 3Ca2+ = 2REE3++ Ca, where Ca is a Ca-site vacancy. Scheelites from the Shimensi deposit show negative Eu anomalies in some samples, but positive Eu anomalies in others in the chondrite-normalized REE patterns. The variation of Eu anomalies recorded the ore-forming processes. Considering the close spatial and temporal relationship between the mineralization and porphyritic granite, we think the negative Eu anomalies were inherited from the porphyritic granite and the positive ones from destruction of plagioclase of country rock during fluid-rock interaction. The variation of cathodeluminescence (CL) color of a single scheelite from red to blue and to yellow was likely associated with the increase of REE contents. The scheelites hosted in the Mesozoic porphyritic granite with negative Eu anomalies formed in a primitive ore-forming fluid, whereas the scheelites hosted in Neoproterozoic granite with positive Eu anomalies precipitated in an evolved ore-forming fluid. The high Nb, Ta, LREE contents, and LREE-enriched REE patterns of scheelites from the Shimensi deposit reveal a close relationship with magmatic hydrothermal fluids.The scheelites from the Shimensi deposit are characterized by low Nd(t) values (–6.1 –8.1) and unusually high and varied initial 87Sr/86Sr ratios (0.7230~0.7657). The Nd(t) values of scheelites are consistent with those of the Mesozoic porphyritic granite, but the Sr isotopic ratios are significantly higher than those of the granites, and importantly, beyond the Sr isotopic range of normal granites. This suggests that the ore-forming fluids and metals cannot be attributed to the Mesozoic porphyritic granites alone, the local Neoproterozoic Shuangqiaoshan Group schists/gneisses with high Rb/Sr ratios and thus radiogenic Sr isotopic compositions should have contributed to the ore-forming fluids and metals, particularly, in a later stage of ore-forming process, by intense fluid-rock interaction. This is different from a commonly accepted model that the ore-forming fluids and metals were exsolved exclusively from the granite plutons.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5654
Issue No: Vol. 102, No. 5 (2017)

• Crystal structure of abelsonite, the only known crystalline geoporphyrin
• Authors: Hummer, D. R; Noll, B. C, Hazen, R. M, Downs, R. T.
Pages: 1129 - 1132
Abstract: The crystal structure of the unique nickel porphyrin mineral abelsonite, NiC31H32N4, has been solved using direct methods with 2195 independent reflections to a final R1 = 0.0406. Abelsonite crystallizes in the triclinic space group $$P\overline{1}$$, with Z = 1 and unit-cell parameters a = 8.4416(5) Å, b = 10.8919(7) Å, c = 7.2749(4) Å, α = 90.465(2)°, β = 113.158(2)°, and = 78.080(2)° at the measurement condition of 100 K, in very good agreement with previous unit-cell parameters reported from powder diffraction. The structure consists of nearly planar, covalently bonded porphyrin molecules stacked approximately parallel to ( $$1\overline{1}1$$), and held together by weak intermolecular Van der Waals forces. The molecules within a layer are slightly tilted such that molecular planes do not overlap, and an up-turned ethyl group on one molecule sits adjacent to a down-turned ethyl group on a neighboring molecule of the same layer. Layers are stacked along a vector normal to ( $$1\overline{1}1$$) such that an aromatic ring at one corner of the molecule lies directly above the opposite aromatic ring of the molecule below. Although a single molecule does not quite possess $$\overline{1}$$ symmetry, matching ethyl groups at roughly opposite ends of the molecule enable orientational disorder, in which molecules can randomly adopt one of two different orientations while still stacking in the same manner. The aggregate of these two random orientations produces an overall symmetry of $$P\overline{1}$$.
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-5927
Issue No: Vol. 102, No. 5 (2017)

• Presentation of the 2016 Roebling Medal of the Mineralogical Society of
America to Robert M. Hazen
• Authors: Hemley; R. J.
Pages: 1133 - 1133
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-AP10251
Issue No: Vol. 102, No. 5 (2017)

• Acceptance of the 2016 Roebling Medal of the Mineralogical Society of
America
• Authors: Hazen; R. M.
Pages: 1134 - 1135
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-AP10252
Issue No: Vol. 102, No. 5 (2017)

• Presentation of the Mineralogical Society of America Award for 2016 to
Anat Shahar
• Authors: Young; E. D.
Pages: 1136 - 1136
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-AP10253
Issue No: Vol. 102, No. 5 (2017)

• Acceptance of the Mineralogical Society of America Award for 2016
• Authors: Shahar; A.
Pages: 1137 - 1137
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-AP10254
Issue No: Vol. 102, No. 5 (2017)

• Presentation of the Dana Medal of the Mineralogical Society of America for
2016 to Sumit Chakraborty
• Authors: Ganguly; J.
Pages: 1138 - 1139
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-AP10255
Issue No: Vol. 102, No. 5 (2017)

• Acceptance of the Dana Medal of the Mineralogical Society of America for
2016
• Authors: Chakraborty; S.
Pages: 1140 - 1142
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-AP10256
Issue No: Vol. 102, No. 5 (2017)

• New Mineral Names,
• Authors: Camara, F; Gagne, O. C, Belakovskiy, D. I.
Pages: 1143 - 1148
PubDate: 2017-05-01T12:00:35-07:00
DOI: 10.2138/am-2017-NMN10258
Issue No: Vol. 102, No. 5 (2017)

• Erratum
• Pages: 1148 - 1148
PubDate: 2017-05-01T12:00:35-07:00
Issue No: Vol. 102, No. 5 (2017)

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