Volume 85, Issue 11-12

Patterns in the occurrence of minerals in metamorphic rocks suggest additional opportunities for investigating chemical and physical processes during metamorphism. Three such patterns are reviewed. First, trace minerals in metamorphic rocks commonly occur with regular distributions indicating their participation in prograde reactions that can be mapped as isograds. Examples include the distribution of allanite and monazite in pelitic rocks and of zircon and baddeleyite in siliceous dolomites. Recognition of these isograds points to the potential for developing a chronology of specific chemical reactions during metamorphism and for defining the P-T conditions of those reactions. Second, the mineralogical products of retrograde metamorphism in many cases occur in distinctive associations that are consistent with partial mineral-fluid equilibrium. Examples include the distribution of retrograde calcite, quartz, and tremolite in siliceous limestones and of retrograde tremolite, dolomite, brucite, and serpentine in siliceous dolomites from contact aureoles. Among other things, application of partial equilibrium to retrograde metamorphic rocks leads to constraints on the amount and direction of fluid flow in contact aureoles as they cool. Third, pseudomorphs are typically absent from prograde metamorphic rocks but are common in retrograde metamorphic rocks. The distribution may be explained by the effect of “force of crystallization.” The pattern of occurrence of pseudomorphs thus suggests novel phenomena during metamorphism that develop from an interplay between chemical and mechanical processes

Gigantic silicate magma chambers have been postulated to account for seismically inferred ultra-low-velocity zones at the base of the mantle, acting to modulate heat transport out of the core and into plumes. In the core, rising plumes of liquid metal and light elements pump the latent heat outward from the inner core boundary by compositional convection. The outer core thus pumps heat to the core-mantle boundary (CMB), and the giant magma chamber pumps heat into the mantle to act as a plume source. Magmatic action at the CMB depends critically on the balance between melt composition, compressibility, and temperature. A stably resident magma above the CMB must be denser than the mantle crystals, mostly silicate perovskite (pv) and magnesiowüstite (mw). Dense melts should form at the CMB, where they must be saturated with Fe and FeO from the core. Given normal Mg-Fe partitioning, mw will be denser than pv, and melts from pv + mw + iron = liq should also be intrinsically dense relative to the bulk crystal assemblage. Such melts may become superheated to overcome the compositional density, and rise upward in the magma chamber, assisted by turbulent mixing. Cooled melt sheds dense metal and metal oxide crystals, and growth of these crystals releases light solute that quickly transports heat upward toward the chamber roof, where it melts the mantle. Near the roof, dense melt grows from the heat transported upward by compositional convection, and sinks. Excess heat and occasionally some melt escapes to feed plumes in the mantle. CMB magmas must be among the most Fe-rich in the Earth

Calc-silicate xenoliths occur as part of a xenolith and autolith swarm in the Lower Zone of the Kiglapait Intrusion, Labrador. All xenoliths are lighter-colored than the local troctolite host rock, and are surrounded by a 1-2 cm thick reaction zone of clinopyroxenite. The typical mineral assemblage in calc-silicate xenoliths is diopside + forsterite + monticellite + spinel, and one also contains åkermanite. Reaction zones are dominated by diopside and spinel. All clinopyroxene compositions are “fassaitic,” with high concentrations of Al 2 O 3 (4-12 wt%), TiO 2 (1-4 wt%), and calculated Fe 2 O 3 (Fe 3+ > Fe 2+ ). In addition, clinopyroxene grains locally display extreme zoning in Si, Al, Ti, and X Mg . Åkermanite contains up to 1.5 wt% Na 2 O. Textures in the xenoliths are complex, with most containing highly irregular intergrowths of diopside, forsterite, and monticellite. Åkermanite and forsterite are separated locally by a coarse symplectic intergrowth of diopside and monticellite, which resulted from a retrograde reaction. The dominant assemblage probably represents the production of monticellite from calcite + clinopyroxene + forsterite, until calcite was consumed. These reactions require metamorphic temperatures of ~900 °C and pressures ≤ 0.4 kbar, such that the xenoliths were probably derived from the roof of the magma chamber. Whole-rock compositions reflect either substantial chemical exchange with the Kiglapait magma, or a very impure dolomitic protolith. The clinopyroxenite reaction zones were produced primarily by assimilation of carbonate by the Kiglapait magma, in combination with the effects of magma undercooling near the xenoliths

This paper presents studies of samples from different localities of the Garnet-Amphibole-Magnetite (GAM)-Quartzite Member of the Ammonoosuc Volcanics from southern New England, USA. Based on stratigraphic and geochemical evidence, earlier workers interpreted the Ammonoosuc Volcanics as part of a volcanic island-arc complex. As such, these rocks have a highly varied stratigraphy with a wide spectrum of chemical compositions. During subsequent metamorphism, these rocks reached amphibolite-facies conditions (6 kbar and 580-640 °C). Within the Ammonoosuc Volcanics, a finegrained, garnet-, magnetite-, and amphibole-bearing quartzite forms a distinct stratigraphic horizon. The quartzite commonly shows delicate millimeter- to centimeter-thick layering, which is interpreted as an original depositional feature that could have formed by precipitation from solutions enriched in silica, iron, and manganese. Volcanic detritus that was rich in Al, Ca, Mg, and Na could account for the variation in concentration of these elements, which are present along with Fe, Mn, and Si in some laminae of these quartzites. The mineralogy of layers within these quartzites can vary greatly on the scale of several centimeter. The most mineralogically complex layers contain, in addition to quartz, garnet, and magnetite, and one or more of the amphiboles cummingtonite, anthophyllite, or hornblende. In the most recent investigations of these rocks, both orthopyroxene and clinopyroxene have been identified in a few samples. A comparison of Mn contents of minerals that are common to different assemblages indicates that the pyroxenes are present in bulk compositions rich in Mn. The Mn content of the bulk composition is the most important factor controlling the mineralogy, and increasing the Mn content causes changes in the mineralogy that mimic the same progression of assemblages that is seen in mafic rocks undergoing the amphibolite- to granulite-facies transition

Phyllites from the Charlotte belt (Central Piedmont) of South Carolina contain porphyroblasts of black corundum in a matrix of margarite and minor muscovite. The margarite-corundum phyllites formed during Ordovician (?) amphibolite facies metamorphism of an aluminous protolith with a probable mineralogy of pyrophyllite, diaspore, and calcite through reactions such as 6Dia + Prl + 2Cal = 2Mrg + 2CO 2 + 2H 2 O and 2Dia = Crn + H 2 O. The protolith was probably a hydrothermally altered felsic tuff of Late Proterozoic to Cambrian age. Based on a whole-rock chemical analysis of margarite-corundum phyllite, the protolith is believed to have consisted of about 7 mol% calcite, 23 mol% pyrophyllite, and 70 mol% diaspore. A T-XCO 2 plot of pertinent reactions in the system CaOAl 2 O 3 - SiO 2 -H 2 O-CO 2 shows that in the presence of an H 2 O-rich fluid (X CO2 ~0.08) margarite began forming by the reaction 6Dia + Prl + 2Cal = 2Mrg + 2CO 2 + 2H 2 O at about 350 °C at 5 kbar. This reaction continued along a univariant path until pyrophyllite and calcite were consumed. The remaining diaspore was converted to corundum by the reaction 2Dia = Crn + H 2 O at about 430 °C for this H 2 O-rich fluid at 5 kbar. The upper temperature limit of this assemblage is constrained by the reaction Mrg = Crn + An + H 2 O, which should occur at about 575 °C under this pressure. The absence of plagioclase in these rocks suggests that the conditions of this reaction were never achieved. Fluids associated with subsequent Alleghanian greenschist facies metamorphism reacted with corundum to form fine-grained margarite at corundum margins

The Chrystalls Beach-Brighton coastal section, southwest of Dunedin, New Zealand, exposes Otago Schist, including the weakly metamorphosed Triassic accretionary melange of the Chrystalls Beach Complex and higher-grade rocks of a largely felsic, arc-derived provenance. Melange zones contain pelagic sediments, some of which are manganiferous, and metabasites. The grade of metamorphism increases progressively from southwest to northeast. Four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks: pumpellyite-chlorite, pumpellyite-actinolite, epidote-actinolite, and biotite. d 002 data for carbonaceous material, b 0 values for phengite, and metamorphic phengite K-Ar ages are reported for 32 pelitic and semipelitic samples. Remarkably tight correlations are revealed between d 002 for progressively graphitized carbonaceous material, K-Ar ages that range from 184.6 to 138.7 Ma, and linear distance along the section. Both the d 002 values and the K-Ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes. All rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements. A convex trend for phengite b 0 values plotted against advancing graphitization suggests P/T conditions during metamorphism as in the high-pres sure intermediate facies series. The age of the metamorphic peak is inferred to be 175-155 Ma (Middle to Late Jurassic), with cooling to the mica closure temperature at 155-135 Ma (Late Jurassic to Early Cretaceous). Unloading, denudation, and cooling were accompanied by localized hydrothermal events. The oldest covering strata were deposited at ca. 100 Ma and denudation was completed by ca. 75-80 Ma. Middle Triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 Ma. Associated turbidites contain suspected late Middle to early Late Triassic tube fossils

Quantitative estimates of metamorphic conditions are combined with previously reported structural analysis to develop the thermotectonic evolution of two separate lithotectonic units metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. A structurally higher plate, regionally correlated with the Blåhø Nappe, contains kyanite eclogites associated with microdiamond-bearing kyanite-garnet-graphite gneiss on the north coast of Fjørtoft and correlated with kyanite eclogites at Nogva, Flemsøya. The kyanite eclogites, containing the assemblage kyanite- garnet-omphacite-coesite (now polycrystalline quartz pseudomorphs) ± zoisite ± phengite, yield conditions of 820 °C and 34-39 kbar at Fjørtoft and of 820 °C and 30-36 kbar at Nogva, best characterized by two recently recalibrated geothermobarometers. The conditions at Fjørtoft overlap the diamond-graphite phase boundary and represent the first quantitative petrologic determination of UHP diamond-forming conditions in crustal rocks outside the Dabie Mountains, China and the Kokchetav Massif, Kazahkstan. In a structurally lower plate, eclogitized mylonite with the assemblage kyanite-garnet-omphacite-quartz-oligoclase, produced from the mid-Proterozoic Haram Gabbro that has intruded diorite country rock, yields 780 °C and 18 kbar. This result agrees with other normal HP estimates by Mørk (1985) from partially to completely eclogitized gabbro in the same unit on Flemsøya. We propose that the UHP plate reached a maximum depth of 125 km and then experienced 65 km of exhumation during top-southeast thrusting that brought it into contact with the HP plate. Following this, both plates were exhumed together until reaching a depth of 37 km where they experienced extensive amphibolite-facies re-equilibration and top-west or left-lateral shearing. Temporal details of these histories were determined by monazite U-Th-Pb geochronology described in a companion paper

U-Th-Pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of UHP and HP plates (820 °C, 39 kbar; 780 °C, 18 kbar) metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. The Upper Plate contains a microdiamond-bearing, kyanitegarnet- graphite gneiss and associated kyanite eclogites, independently indicating probable diamond-forming conditions. In situ dating of monazite in the microdiamond sample, using the SHRIMP II at the Geological Survey of Canada, yielded ages of 415 ± 6.8 Ma for those included in garnet and 398 ± 6 Ma for those in the matrix. These ages compare to 408.0 ± 5.6 and 397.5 ± 4.4 Ma determined using the electron microprobe at the University of Massachusetts. Both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100-950 Ma and scattered grains with ages of 900-500 Ma, but no ages of 1680-1650 Ma equivalent to the local Baltica basement were found. Agreement between the two techniques allowed evaluation of monazite age domains (198 analyses) from the microdiamond rock and a kyanite-garnet-sillimanite mylonite produced from it, using a combination of high-resolution element imaging and trace-element analysis of U, Th, Pb, and Y. This comparison yielded three mean ages of 407.0 ± 2.1 Ma, 394.8 ± 2.3 Ma, and 374.6 ± 2.7 Ma. Combining this geochronology with previous P-T estimates, we propose that the UHP unit reached its maximum depth of 125 km, at a maximum age of 407 Ma when monazite was included in garnet, and experienced 65 km of exhumation at an average rate of 10.9 mm/year during top-southeast thrusting that brought it into contact with the HP unit. Following these events, both units were exhumed together at an average rate of 3.8 mm/year until reaching a depth of 37 km at 395 Ma, where these rocks experienced extensive re-equilibration, and top-west and left-lateral shearing. After 395 Ma, these units continued to be exhumed at an average rate of 0.8 to 1.4 mm/year until 375 Ma, the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite. The exhumation histories of these units record a change in mechanism from syncollisional exhumation through late- to post-orogenic collapse that was a consequence of plate reorganization

The solubilities of noble metals (NM: Ir, Pd, Au, Pt, and Ru) in FeO-free silicate melts are known from recent experimental work. In this paper, calculations are presented that relate solubilities of NM in FeO-free melts to those in FeO-containing melts. The main difference between these two systems is the formation of Fe-NM alloys in FeO-containing melts. At f O₂ conditions of the QFM buffer and 1200 °C, binary alloys of Fe with Au, Ru, Ir, Pd, or Pt containing 1, 2, 10, 17, and 25 at% Fe, respectively, are in thermodynamic equilibrium with a silicate melt with 10 mol% FeO. Thus alloy formation leads to a significant reduction in the solubility of Pt and to a lesser reduction in the solubility of Pd. The effects for Ir and Ru are small, and for Au almost negligible. The reduction in solubilities depends on temperature, oxygen fugacity, and FeO content of the silicate melt. Formation of FePt-alloys would lead to a preferred depletion of Pt in partial melts from the Earth’s mantle, which, however, is not observed. One explanation is that mantle melting occurs under very oxidizing conditions (QFM+2). The calculations presented here should be considered a first step toward gaining a better understanding of the behavior of NM during igneous processes

The distribution of dissolved water in a haplogranitic glass between molecular water and reacted OH groups, as measured by Sowerby and Keppler (1999) at temperature and pressure, is compared to a Langmuir model. In this model the number of reactive sites in the glass is limited, so as the total water content increases, the concentration of reacted OH groups saturates. The comparison between the model and experimental measurements is good with two fitting parameters. The total water solubility in NaAlSi 3 O 8 melts as measured by Holtz et al. (1995) is the sum of molecular water dissolved from the gas phase and reacted OH groups; the concentration of OH groups calculated from these solubility measurements also fits the Langmuir model. The partial molar volumes for water in NaAlSi 3 O 8 melts are constant above about 4 wt% total water, but decrease at lower water contents; this dependence can be understood from the speciation measurements

The energetics of different ordering schemes for NaAlSi 3 O 8 (albite) were investigated by both empirical potential and ab initio methods. These computations indicate that the ordered structure of natural low albite, in which aluminum atoms reside on the T 10 site, is favored by 30 meV (2.9 kJ/ mol) over the corresponding structure in which aluminum atoms are ordered onto the T 20 site. Permissible lattice relaxation of the T 10 structure, with an associated substantial decrease in the γ lattice angle, is unique to the T 10 structure and appears to be responsible for its substantially lower enthalpy

The heat capacity of synthetic eskolaite, Cr 2 O 3 , and of the synthetic spinels magnesiochromite, MgCr 2 O 4 , and chromite, FeCr 2 O 4 were measured from 1.5 K to 340 K. For MgCr 2 O 4 , a substantial magnetic contribution to the entropy is revealed by a sharp peak in the heat capacity curve at 12.55 ± 0.05 K, which indicates the transition to antiferromagnetic long-range order. Integration of the heat capacity curve yields a value of 118.3 ± 1.2 J/(mol·K) for the standard entropy at 298.15 K, which is in excellent agreement with that calculated from phase equilibria studies on the reaction MgCr 2 O 4 + SiO 2 = Cr 2 O 3 + MgSiO 3 . The new calorimetric results for Cr 2 O 3 indicate a standard entropy at 298.15 K of 82.8 ± 0.8 J/(mol·K). The measurements for FeCr 2 O 4 show three distinct heat capacity anomalies, one of which (peaking at 36.5 ± 0.2 K) was missed by previous low temperature heat capacity measurements, which only extend down to 53 K. Integration of the heat capacity curve yields a value for the standard entropy at 298.15 K of 152.2 ± 3.0 J/(mol·K) for FeCrO 4 , some 6 J/ (mol·K) greater than the previous calorimetric value. These low-temperature heat capacity data were combined with high-temperature heat content measurements from the literature to derive heat capacity equations for all three phases to 1800 K. The resulting heat capacity equations were then used to extract revised recommended values of the standard enthalpies of formation and entropies of MgCr 2 O 4 and Cr 2 O 3 from phase equilibrium data. For FeCr 2 O 4 , the phase equilibrium data are of dubious accuracy, the enthalpy of formation is only approximate

The thermodynamics of cation ordering in the ilmenite-hematite solid solution (FeTiO 3 )x(Fe 2 O 3 )1-x were re-examined in view of a recent in-situ neutron diffraction study of the R3̅ to R3̅ phase transition. A macroscopic thermodynamic model is developed, in which the excess enthalpy of ordering is described by a fourth-order polynomial function of the long-range order parameter, Q, and the excess entropy is described by the configurational point entropy. The enthalpy coefficients are determined as a function of composition by fitting to the neutron diffraction data. The fourth-order energy coefficient increases smoothly with increasing Ti-content, indicating a change in the character of the transition from second-order (x < 0.87) to first-order (x > 0.87). Monte Carlo simulations confirm that the excess enthalpy is well described by a fourth-order polynomial, and demonstrate that the reduction in configurational entropy due to short-range ordering can be described by simply scaling the point entropy. This allows the enthalpy coefficients in the macroscopic model to be corrected for the effects of short-range ordering, yielding a revised estimate of x = 0.92 for the position of the tricritical point. Monte Carlo simulations are used to study both ordering and exsolution in the solid solution, allowing the paramagnetic portion of the equilibrium phase diagram to be derived. The changing character of the phase transition is reproduced successfully by treating Fe 2+ -Ti and Fe 3+ -Ti interactions separately in the simulations. Intralayer Fe 2+ -Ti interactions are negative (i.e., ordering of Fe 2+ and Ti within the 001 layers is favored) whereas Fe 3+ -Ti intralayer interactions are positive (i.e., separation of Fe 3+ and Ti is favored). The λ-line for the R3̅ - to R3̅ - c phase transition intersects the lowtemperature miscibility gap at a tricritical point near x = 0.6 and T = 800 °C, in reasonable agreement with previous thermodynamic models of the solid solution

The structural properties of katoite, Ca 3 Al 2 (O 4 H 4 ) 3 , are investigated as a function of applied pressure. The calculated structure at ambient pressure, the bulk modulus and its pressure derivative are in good agreement with the available experimental data. The strength of the hydrogen bond increases under pressure, accompanied by a lengthening of the O-H bonds. This conclusion agrees with experimental spectroscopic data and casts serious doubts on the reliability of neutron diffraction results that predict strong compression of the O-H bonds. Calculations for Mg 3 Al 2 (O 4 H 4 ) 3 have been used to predict the structure and relative stability of the hypothetical hydropyrope

Fourier-transform infrared spectra (FTIR) were recorded on a series of synthetic amphiboles along the join Ca 1.8 Mg 5.2 Si 8 O 22 (OH) 2 -Ca 1.8 (Mg 4.2 Al)(Si 7 Al)O 22 (OH) 2 . The spectra were fitted by up to six component bands by optimization and non-linear least-squares techniques. 27 Al MAS NMR and triple-quantum (3Q) MAS NMR spectra were recorded for the synthetic amphibole Ca 1.8 (Mg 4.8 Al 0.4 )(Si 7.6 Al 0.4 )O 22 (OH) 2 . The fitted FTIR and NMR spectra show that VIAl occurs at the M2 site and at the M1 or M3 sites; by analogy with previous crystal-structure refinement results on natural amphiboles, VIAl occupancy of M2 and M3 is presumed. The fine structure present in the FTIR spectra indicates that they are also affected by NNN (next-nearest-neighbor) interactions. There are two types of NNN arrangements: (1) SiSi or SiAl at adjacent T1T1 dimers; (2) permutation of Mg/Al over M2M2M3 sites. Discounting those arrangements unlikely on bond-valence grounds, there are two arrangements that give rise to five distinct bands in the infrared spectra. There are two principal conclusions: (1) infrared spectra of amphiboles in the principal OH-stretching region can be affected significantly by NNN effects; (2) the small number of bands due to NNN effects indicates that amphiboles show strong short-range order

Changes in frequency and linewidth of the 206 and 464 cm -1 A 1 Raman modes of quartz were determined over temperatures from 23 to 800 °C and simultaneously at pressures ranging between 0.1 MPa and 2.1 GPa using a hydrothermal diamond-anvil cell (HDAC). The frequency shift of the 464 cm -1 peak can be used as a secondary pressure standard for SiO 2 -saturated systems in HDAC experiments at temperatures up to 560 °C. The frequency of this peak depends quasilinearly on pressure in the studied pressure range. The global slope (∂ν464/∂P)T is 9 ± 0.5 cm -1 /GPa. A significant variation of this slope with temperature was not observed. Including literature data, the temperature induced frequency shift of the 464 cm -1 mode is described by (ΔV T ) 464, P = 0.1 MPa (cm -1 ) = 2.50136·10 -11 ·T 4 + 1.46454·10 -8 ·T 3 - 1.801·10 -5 ·T 2 - 0.01216·T + 0.29 where -196 ≤ T (°C) ≤ 560. The pressure dependence of the linewidth of the 464 cm -1 line increases with temperature. The frequency shifts and linewidths for the 206 cm-1 mode indicate that this line can be used as an alternative to the ruby fluorescence technique as a pressure sensor to about 5 GPa for experiments at room temperature. Both the frequency and linewidth of this mode show significant cross-derivatives (∂ 2 V 206 /∂P∂T) and (∂ 2 FWHM 206 /∂P∂T)

Changes in the hydroxyl surfaces of potassium acetate-intercalated halloysite were studied over ambient to predehydroxylation temperature range using a combination of X-ray diffraction and Raman spectroscopy. XRD shows that the halloysite is completely expanded to 13.80 Å. Upon heating the intercalation complex to 50 °C under nitrogen, two expanded phases are observed with d-spacings of 11.47 and 8.95 Å. Upon thermal treatment, the 11.47 Å phase is stable in both the heating and cooling cycles. The ~9.0 Å phase undergoes an expansion at 100 °C to 9.2 Å. Upon exposure to air, the intercalated kaolinite returns to a 13.80 Å phase. The completely intercalated halloysite showed a band at 3602 cm -1 attributed to the inner surface hydroxyl hydrogen bonded to the acetate ion together with bands at 3620 and 3695 cm -1 assigned to the inner hydroxyl and the inner surface hydroxyls, which do not react with the acetate. Mild heating of the intercalation complex to 50 °C caused a rearrangement of the surface structure with Raman bands being observed at 3606 and 3597 cm -1 . Further thermal treatment at 100 °C caused these bands to shift to 3615 and 3601 cm -1 . At the predehydroxylation temperature for potassium acetate intercalated halloysite (300 °C) two bands were observed at 3602 and 3612 cm -1 . Above this temperature no hydroxyls are spectroscopically evident. Upon cooling to room temperature, the Raman spectra of the hydroxyl surfaces are altered with two bands observed at 3604 and 3596 cm -1

In annealing experiments on natural crystals of apatite and titanite, two types of fission track were observed that had not been identified previously: (1) “stable” tracks, and (2) tracks through fluid inclusions. Stable tracks do not shorten significantly even when heated to temperatures well above those normally sufficient for complete annealing of fission tracks. Tracks through fluid inclusions owe their excessive length to the fact that one fission fragment traveled through a fluid inclusion without appreciable energy loss. If they go undetected, both types of track can distort the temperature-time paths constructed on the basis of confined fission-track-length measurements. Simple measures are proposed to avoid these pitfalls

The reactivity of arsenopyrite (FeAsS) fractured surfaces toward oxygen was studied using synchrotron radiation excited photoelectron spectroscopy (SXPS). The spectra of the pristine surface provide evidence of different As and S surface sites. Signals for As3d with 0.33 eV lower and 0.37 eV higher binding energy than the bulk signal are attributed to arsenic surface sites with filled and unfilled dangling bonds, respectively, caused by the rupture of Fe-As and As-S bonds. Sulfur surface sites with filled dangling bonds, bonded to three iron as well as to two iron and one arsenic atom, give rise to a composite signal in the S2p spectra shifted by 0.79 eV to lower binding energy. Reaction of oxygen with FeAsS surfaces in ultra-high vacuum reveals fast oxidation of As surface sites with filled dangling bonds to As species of increasing oxidation state. The detection of oxidation states As 0 , As 2+ , As 3+ , and As 5+ indicate a consecutive reaction scheme for arsenic oxidation involving elementary one-electron transfer steps. The Fe3p spectra have a corresponding intensity increase of a component with a binding energy 1.1 eV higher than the Fe3p signal emitted from the pristine surface. This signal is assigned to Fe bonded to oxidized arsenic. The very small changes in the S2p spectra together with their decreased intensity indicate the formation of an arsenic and iron containing overlayer of oxidation products on top of the FeAsS mineral surface where the S2p signal arises from. In air oxidation of arsenic continues with As 5+ being the final oxidation product. An additional Fe3p signal at 56.1 eV binding energy is attributed to Fe bonded to O atoms formed during Fe oxidation. Sulfur oxidation leads to numerous intermediate oxidation products with sulfate being the final product. During air oxidation of up to 30 min, the sulfur signal at the low binding energy side of the S2p spectrum is broadened which is probably caused by S 2- formed in layers underneath As and Fe oxidation products. These oxidation products reach the surface by diffusion from the bulk (reaction induced segregation). A model of homogeneous oxidized layers on arsenopyrite indicates that reaction with air has produced a layer containing iron bonded to oxygen on top of the increasingly oxidized arsenic and iron containing layer. The Fe-O overlayer is about 1.8 monolayers thick and is probably formed through interaction of water with iron surface sites

Surface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA) was measured to investigate controls on this parameter for several primary silicate minerals (PSM) used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ olivine ≈ albite < oligoclase ≈ bytownite < hornblende ≈ diopside. Reproducibility of BET SSA values range from ±70% (SSA < 1000 cm 2 /g) to ± 5% (SSA > 4000 cm 2 /g) and values measured with N 2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia albite and San Carlos olivine, SSA can be calculated using log (SSA, cm 2 /g) = b + m log (d), where d = grain diameter (μm), b = 5.2 ± 0.2 and m = -1.0 ± 0.1. A similar equation was previously published for laboratory-ground quartz. Some other samples showed SSA higher than predicted by these equations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA increased with observed hysteresis in the adsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers documented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapolation of interfacelimited rates of dissolution of many silicates if internal surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coarse and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimate precisely. Quantification of the porosity in laboratory- ground and naturally weathered samples may help to alleviate some of the discrepancy between laboratory- and field-based estimates of weathering rate

The environmental scanning electron microscope (ESEM) allows high-resolution, high-magnification imaging of conductivity differences in uncoated geological samples. Under normal ESEM operating conditions, negative charge buildup at the sample surface (from bombardment by the electron beam) is prevented by the presence of a gas (usually water vapor) in the sample chamber. Backscattered and secondary electrons from the sample ionize this chamber gas, and the resultant positively charged gaseous ions migrate toward the negatively charged sample. When chamber gas pressures lower than approximately 250 Pa are used, however, charging of the sample can occur because insufficient charge balancing positively charged gaseous ions are produced. Charge implantation in the sample alters secondary electron emission, and, because intracrystalline conductivity contrasts occur in response to variations in defect density, secondary electron images reflect compositional variations and/or microstructural features. These secondary electron images are referred to as charge contrast images (CCI). To demonstrate potential geological applications of CCI, we present images of growth zones, microfractures, differential diffusion domains, pleochroic haloes, and relict fluid pathways from zircon (strongly luminescent), quartz (weakly luminescent), and biotite and cordierite (non-luminescent). CCI detect defects in a similar way to cathodoluminescence (CL), but have a higher resolution because the CCI signal is composed of secondary electrons that are generated from a much smaller interaction volume than photons utilized in CL. CCI imaging also can be applied to a wider variety of geological samples than CL, because electronic charge trapping is not restricted to wide-band gap electronic configurations. One of the most important potential applications of the CCI technique may lie in the direct imaging of relict fluid pathways in rocks that have experienced metasomatism or alteration

Jadeitic pyroxenes in two Franciscan samples, one, a possible segregation vein in metabasite, and another, a metabasite, both from Ward Creek, Cazadero area, western California, were examined with the electron microprobe. The samples were collected from the area of Type III rocks, the highestgrade portion of the pumpellyite zone. Jadeitic pyroxenes from both samples are very heterogeneous and cannot be ascribed to crystallization under conditions of surface equilibrium with the coexisting phases. Two possibilities for their origin are suggested: one is as tectonic blocks, and the other as the highest-grade rocks from the zone that crystallized close to or within the P-T stability field of pure jadeite + quartz

The crystal structure of CaMg 2 Al 6 O 12 , a new high-pressure phase synthesized at 21.8 GPa and 1200 °C using a multi-anvil apparatus, was solved by a newly developed structure program and refined by Rietveld analysis of the powder X-ray diffraction profile. The structure is hexagonal with cell constants a = 8.7616(2) and c = 2.7850(1) Å, and space group P6 3 /m. The structure of this phase contains double chains of edge shared AlO 6 octahedra running along the c axis. Three double chains share corners to form sixfold positions in which octahedrally coordinated Mg atoms reside. The large Ca atoms are randomly distributed at ninefold sites with half-occupancy in the hexagonal tunnel. Previously reported Al-rich silicate phases could possibly have the same structure. This structure could thus qualify as one of the possible major host phases for aluminum in the lower mantle

X-ray intensities for the “tetragonal almandine-pyrope phase” (TAPP), *(FeMg) VIII (AlMgFe) 4 [ IV (SiAl)O 4 ] 3 where * refers to a capped tetrahedral site, reported by Harris et al. (1997) were recollected using the identical crystal, and the crystal structure was refined from these new data. The basic structure of the previous report is confirmed; however, some of the details regarding cation occupancies are revised. The chemical composition of this phase corresponds to a high-pressure origin; however, significant questions remain regarding the crystallization pressure of this phase

The structure of tetranatrolite from Mont Saint-Hilaire, Québec (U.S. National Museum sample R1830) with a = 13.197(7) Å, c = 6.630(9) Å, and space group I4 - 2d, was refined using single-crystal X-ray data. A representative formula of tetranatrolite determined from electron microprobe analysis is Na 12.50 K 0.01 Ca 2.93 Sr 0.11 Al 19.09 Si 20.91 O 79.74 ·nH 2 O. The structure has the basic natrolite Si-Al-O framework configuration with Na, Ca, Sr, and K residing within inter-framework cages. Aluminum is disordered over the T1 and T2 tetrahedral sites, with T2 > T1. Water molecules O4 and O5 coordinate the intercage atoms and have high displacement parameters, indicating disorder within the cages. The Mont Saint-Hilaire tetranatrolite structure is compared to four previously determined structures, two tetranatrolite samples from Khibiny and Lovozero, Russia and two “gonnardite” samples from Tvedalen, Norway and Gignat, France. Observations are given to indicate that the Norwegian sample deduced to be tetranatrolite rather than gonnardite. Although the crystal structures of tetranatrolite and gonnardite are very similar, it is shown that the tetranatrolite compositions differ significantly from those of gonnardite. The tetranatrolite composition series varies along the join Na 16 Al 16 Si 24 O 80 -Na 12 Ca 4 Al 20 Si 20 O 80 , and is represented by the formula Na 16 - x Ca x Al 16+x Si 24-x O 80 ·nH 2 O, where x extends from approximately 2.4 to 3.9. In contrast, gonnardites from Arkansas and Austria have compositions that vary along the join Na 16 Al 16 Si 24 O 80 -Na 4 Ca 8 Al 20 Si 20 O 80 , which are represented by the formula ⃞ x Na 16-3x Ca 2x Al 16+x Si 24-x O 80 ·nH 2 O and where ⃞ indicates vacant intercage cation sites and x varies from approximately 0.3 to 3.2. Tetranatrolite is a dehydration product of paranatrolite and probably does not have a true stability field

The crystal structure of elyite, Pb 4 Cu(SO 4 )O 2 (OH) 4 ·H 2 O, a = 14.233(2), b = 11.532(1), c = 14.611(2) Å, β = 100.45(1)°, V = 2358.4(5) Å 3 , Z = 8, was solved by direct methods and refined in space group P2 1 /c to R1 = 3.64% and wR2 = 5.10% for the 5861 independent reflections. Data were collected on a tiny untwinned crystal fragment with a four-circle diffractometer (MoKα radiation, CCD area detector). The structure contains eight unique Pb atoms, two isolated Cu atoms in planar fourfold-coordination (<Cu-O> = 1.933, 1.927 Å) and two isolated, almost ideal SO 4 tetrahedra. All anions coordinating Cu are OH groups. Two H 2 O molecules are weakly bound to Pb atoms. The Pb atoms show highly variable coordinations due to variable stereochemical activities of the Pb 2+ lone electron pairs. The connectivity of the structure is based on Pb-O polyhedra which are closely linked by common O ligands to form rod-like structure elements parallel to the b axis. The structure framework is held together by sharing ligands with CuO 4 squares and SO 4 tetrahedra. The CuO 4 squares can be considered as struts connecting the Pb-O rods along the c axis and, intermittently, along the a axis. A complex hydrogen bond system provides additional strengthening. The non-merohedral twinning parallel to {100} reported previously is explained by the presence of a pseudo-mirror plane in the structure. Comparisons are drawn with the structures of the related Pb-Cu-sulfates chenite, Pb 4 Cu(SO 4 ) 2 (OH) 6 , and linarite, PbCu(SO 4 )(OH) 2 . The violet color of elyite and other Cu compounds might be related to the planar fourfold-coordination of Cu.

Dukeite, Bi 3+ 24 Cr 8 6+ O 57 (OH) 6 (H 2 O) 3 , space group P31c, a = 15.067(3), c = 15.293(4) Å, V = 3007(1) Å 3 , Z = 2, is a new mineral found on a museum specimen labeled as originating from the São José Mine, Brejaúba, Minas Gerais, Brazil. The strongest seven lines of the X-ray powder-diffraction pattern [d in Å (I) (hkl)] are: 7.650 (50) (002), 3.812 (40) (004), 3.382 (100) (222), 2.681 (70) (224), 2.175 (40) (600), 2.106 (40) (226), 1.701 (50) (228). It occurs as groupings of tightly bound 1 × 0.3 mm2 sized sheaves that are associated with pucherite, schumacherite, bismutite, and hechtsbergite. Individual acicular crystals do not exceed 100 μm in length by 1-2 μm in width. Crystals are yellow inclining to a dirty yellow-brown, possess a bright yellow streak, are transparent, brittle, resinous, and do not fluoresce under ultraviolet light. The estimated Mohs hardness is between 3 and 4, the calculated density (for the empirical formula) is 7.171 g/cm 3 , and the mineral is slowly soluble in concentrated HCl. Electron-microprobe analyses yielded Bi 2 O 3 85.06, CrO 3 11.65, V 2 O 5 0.59, H 2 O (calc.) [1.67], total [98.97] wt%. The empirical formula, derived from the crystalstructure analysis and electron-microprobe analyses, is Bi 3+ 23.95 (Cr 6+ 7.64 V 5+ 0.43 )Σ 8.07 O 56.84 (OH) 6.16 ⋅3.01 H 2 O, based on O = 66. In reflected plane-polarized light in air it is gray to purplish gray with strong yellow internal reflections. Bireflectance is very weak. Measured reflectance values, in air and in oil, are tabulated: indices of refraction calculated from these at 590 nm are 2.33 and 2.37. The name honors Duke University, Durham, North Carolina, in whose collection the mineral was found and also recognizes the contribution of the Duke family to the advancement of scientific knowledge. The crystal structure of dukeite was solved by direct methods and refined on the basis of F 2 using all unique reflections measured with MoKα X-radiation on a CCD-equipped diffractometer. The final R1 index was 8.7%, calculated using 1033 observed reflections. It contains irregular layers of Biφn polyhedra (φ: O, OH-, H 2 O) parallel to (001), separated and connected by CrO 4 tetrahedra to form a framework structure. One CrO 4 tetrahedron shares all of its vertices with Biφn polyhedra, whereas the other three CrO 4 tetrahedra share only three vertices each with the Biφn polyhedra on either side

Pseudosinhalite, Mg 2 Al 3 [O|OH|(BO 4 ) 2 ] and chondrodite, Mg 5 [(F,OH) 2 |(SiO 4 ) 2 ] are shown to be isostructural. Compositional and crystallographic relationships between members of the forsteriteclinohumite polysomatic series and their borate isotypes are demonstrated

We performed a series of experiments in the system Fe-FeS at a pressure of 21 GPa and temperatures between 950 and 1400 °C, and we found two new iron-excess iron-sulfur compounds, Fe 3 S and Fe 2 S, formed at subsolidus temperatures. Powder X-ray diffraction data revealed that Fe 3 S has a tetragonal cell, isostructural with Fe 3 P (space group I4 - ). The tetragonal unit-cell dimensions for Fe 3 S are a = 9.144(2) Å and c = 4.509(2) Å, with a zero-pressure density of 7.033 g/cm 3 . Static compression experiments on Fe 3 S were carried out in a diamond-anvil cell, using synchrotron X-ray diffraction technique. A least-squares fit to the experimental data at room temperature yielded bulk modulus K 0 = 170 ± 8 GPa with a corresponding pressure derivative K 0 ' = 2.6 ± 0.5 or K0 = 150 ± 2 GPa with fixed K 0 ' = 4

The single-crystal elastic moduli of lawsonite [CaAl 2 (Si 2 O 7 )(OH) 2 ·H 2 O] were measured by Brillouin spectroscopy at ambient conditions. The Voigt-Reuss-Hill averaged aggregate elastic moduli are K S = 125(2) GPa and μ = 52(2) GPa, for the adiabatic bulk modulus and shear modulus, respectively. Our acoustic results resolve discrepancies between the bulk moduli obtained in earlier compression studies. Lawsonite has distinctive acoustic properties, being characterized by extremely high shear elastic anisotropy (74%), a high V P to V S ratio (V P /V S = 1.94), and a large Poisson’s ratio (σ = 0.318)

The viscosity of liquid FeS and Fe-FeS eutectic was measured at pressures between 0.5 and 5.0 GPa using a synchrotron-based falling sphere technique. We obtain viscosities of 2 × 10 -2 to 4 × 10 -3 Pa-s in FeS at 1450 to 1700 °C and 2 × 10 -2 to 8 × 10 -3 Pa-s in Fe-Seut at 1150 to 1380 °C. These results are consistent with recent viscosity measurements in Fe-Seut at 5 to 7 GPa (Urakawa, in preparation), measured diffusivities (Dobson 2000) and ab initio simulated viscosity (Vočadlo et al. 2000). The results are also similar to the values for pure iron at low pressure (Shimoji and Itami 1986). A systematic increase in viscosity and activation energy is seen with increasing sulfur content. Interpolation between the data presented yields a viscosity of 1.4 × 10 -2 Pa-s for an outer core composition with ~10 wt% S at the melting temperature. There is good evidence of homologous behavior for Fe-S liquids which implies that the liquid alloy at the inner core boundary may have a similar viscosity