Published by De Gruyter

Volume 94 Issue 4

Issue of American Mineralogist

Contents
Journal Overview

April 1, 2015

Growth controls in colloform pyrite

Craig D. Barrie, Adrian J. Boyce, Alan P. Boyle, Patrick J. Williams, Kevin Blake, Takahiko Ogawara, Junji Akai, David J. Prior

Page range: 415-429

Abstract

Primary colloform textures preserved in ore deposits can be a useful tool in understanding changing conditions of ore formation due to the sequential development of the colloform layers. However, the growth controls that influence formation of these textures are poorly understood. To try to address this problem, samples from two ore deposits, Greens Creek in Alaska and Ezuri in Japan, have been systematically analyzed for grain size and shape, crystal preferred orientation (CPO), sulfur isotope composition, and trace element content. Grain size and shape varies between layers of equant, ~20 μm crystals to acicular and elongate crystals up to several millimeters in length. Electron backscatter diffraction (EBSD) reveals that both samples have an initial random orientation of crystals with CPO in subsequent layers developed either about <100>, <110>, or <111> crystallographic axes. Despite similarity in texture, the sulfur isotope results from Greens Creek colloforms have a very negative, open-system bacteriogenic δ 34 S between -40 and -32‰, whereas the Ezuri colloform has a positive δ 34 S of ~+5‰, typical of hydrothermal sulfur in Kuroko ores. Trace element results indicate variability in As, Sb, and Cu distribution. Whereas trace element variability at Greens Creek appears to be related to changes in δ 34 S, with a heavier signature correlating with sequestration of Sb in outer layers, overall the detailed analyses reveal that in both Greens Creek and Ezuri, there is no systematic correlation between sulfur source or trace element sequestration and CPO. This suggests that the abrupt changes in CPO recorded appear most likely to be influenced by changes in degree of supersaturation.

April 1, 2015

Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barroviantype metapelites, Imjingang belt, central Korea

Yoonsup Kim, Keewook Yi, Moonsup Cho

Page range: 430-438

Abstract

The paragenetic relationships and Th-U distributions among allanite, monazite, and xenotime were investigated in a progressive sequence of garnet- to kyanite-zone metapelites of the Imjingang belt, Korea. Allanite is predominant in the garnet and staurolite zones, whereas monazite and xenotime predominate in the kyanite zone. Epidote grains in the lower garnet zone are commonly zoned, from allanite (core) to relatively Y-rich, rare-earth-element (REE)-epidote to clinozoisite (rim), although both REE-epidote and clinozoisite disappear in higher-grade metapelites. Moreover, allanite and REEepidote often contain minute inclusions of thorium silicate. The isogradic distributions and similarity of REE patterns between allanite and monazite suggest that the latter has grown at the expense of the former. In addition, the discontinuous Th zoning in monazite is apparently inherited from heterogeneous Th distribution and thorium silicate inclusions in allanite. Thus, thorium silicate possibly provided the additional Th and U necessary for the monazite formation. Paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite-facies conditions allanite is stable at higher pressures than monazite. Xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and REE-epidote, whereas those in the kyanite zone are grown primarily at the expense of garnet. Incorporation of Th and U into monazite and xenotime is governed mainly by the brabantite and thorite substitutions, respectively. Taken together, our results suggest that the allanite-to-monazite transformation is primarily responsible for the distributions of REEs, Th, and U among metapelitic phases, and that the xenotime formation was facilitated by the contribution from major silicates, particularly garnet.

April 1, 2015

Magmatic vs. hydrothermal origins for zircon associated with tantalum mineralization in the Tanco pegmatite, Manitoba, Canada

Marieke Van Lichtervelde, Frank Melcher, Richard Wirth

Page range: 439-450

Abstract

Complex textures in zircon associated with Ta oxides have been used to assess the processes at the origin of zircon crystallization and associated Ta mineralization in pegmatites, in particular the role of magmatic vs. hydrothermal processes. The Tanco pegmatite is used as an example because its zircon is devoid of post-magmatic alteration and its petrogenesis is well constrained. Zircon in primary units is metamict with high U-Pb-Th contents. By contrast, in secondary assemblages affected by latemagmatic micaceous alteration, zircon is devoid of radiogenic elements, but it may contain abundant Ta 2 O 5 (up to 4.7 wt%). The incorporation of Ta into zircon may occur through coupled substitution mechanisms involving other minor elements such as P, Al, Mn, or Li. The presence of Ta accounts for the distorted structures in the Ta-rich zircon, as revealed in high-resolution TEM images. Four zircon types occur sequentially in single zircon crystals, which permits a new model for zircon growth and evolution in rare element pegmatites to be advanced: (1) zircon (Z1) is Ta-rich and crystallized possibly before discrete Ta phases; (2) Z1 was overgrown by regularly zoned zircon Z2 and Z3, which show lower Ta and extreme Zr/Hf fractionation (HfO 2 up to 38.9 wt%), suggesting crystallization from a highly fractionated melt, possibly at the onset of Ta mineralization; and (3) close to the solidus, the aqueous fluid at the origin of micaceous alteration corroded the distorted structure of Z1, and a low- Ta zircon (Z4) replaced Z1 by dissolution-reprecipitation, whereas Z2 and Z3 resisted this alteration. Tantalum was no longer stable in the zircon structure and crystallized as Ta-oxide inclusions in the reequilibrated zircon (Z4).

April 1, 2015

Molecular conformation and anion configuration variations for As₄S₄ and As₄Se₄ in an anion-substituted solid solution

Atsushi Kyono

Page range: 451-460

Abstract

Molecular crystals of an As 4 S 4-x Se x (0 ≤ x ≤ 4) solid solution were grown at 400 °C using vacuumsublimation and were characterized using single-crystal X-ray diffraction and electron microprobe analyses. The solid solution crystallizes in the monoclinic space group P2 1 /n with lattice parameters of a = 9.33-9.57 Å, b = 13.57-13.82 Å, c = 6.60-6.74 Å, β = 106.4-106.7°, V = 801-854 Å 3 , and Z = 4. Substitution of Se for S in the As4X4 molecule causes nearly isotropic unit-cell expansion retaining identical molecular packing to the low-temperature form α-As 4 S 4 (natural realgar). Crystal structure refinements show that Se is distributed with a strong preference for the X2 site. The As-X3 bond is slightly elongated in the solid-solution series. The bond length difference between As-X3 and the other As-X bonds increases with the Se content, thereby inducing atomic shift of the As4 bonds to X3 and X4 toward the side of X3. The atomic position of As4 returns to the place on the bisector between X3 and X4 at the As 4 Se 4 end-member composition. Although As-As bonds shorten with Se content, the volume of the As 4 X 4 molecule and the intercentroid distance between the nearest As4X4 molecules increase continuously, resulting in isotropic expansion of the unit cell in this solid-solution series. The strong site preference of Se for the X2 site suggests that S may be detached from that site more easily than from the remaining ones. On the contrary, very slight substitution of Se for S in the X3 site indicates that S in the X3 site is strongly bonded to two As atoms. Therefore, S is more stably incorporated into the X3 site than into the X2 site; these molecular features of the Se substitution in the As 4 X 4 molecule are consistent with the proposed photoinduced transformation dynamics in realgar. Continuous illumination of the Se-containing realgar engenders alteration to pararealgar under identical transformation mechanisms to those of the realgar.

April 1, 2015

Low-pressure clino- to high-pressure clinoenstatite phase transition: A phonon-related mechanism

Yonggang G. Yu, M. Wentzcovitch

Page range: 461-466

Abstract

We have investigated by first principles the compressional behavior of low-pressure (LP) and highpressure (HP) MgSiO 3 clinoenstatite. We have carefully examined unit-cell shapes, chain angles, and polyhedral volume responses, such as angle variances and quasi-elongations, under pressure at room temperature. We have observed opposite behavior of the tetrahedra in the S-rotated and O-rotated chains with pressure in the LP phase, with a slight increase (decrease) in angle variance and quasi-elongation in the former (latter). Inspection of zone center modes of both phases under pressure reveals a transition path that converts the S-rotated chain in the LP phase into the O-rotated chain in the HP phase. This conversion is related to a slight softening of an A g “metastable” Raman mode under pressure.

April 1, 2015

Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part I. Influence of a multi-electron excitation on EXAFS analyses of La, Pr, Nd, and Sm

Atsuyuki Ohta, Hiroyuki Kagi, Masaharu Nomura, Hiroshi Tsuno, Iwao Kawabe

Page range: 467-475

Abstract

Coordination states of rare earth elements (REEs) adsorbed by iron oxyhydroxide (FeOOH) and manganese dioxide (δ-MnO 2 ) (REE = La, Pr, Nd, and Sm) were determined using extended X-ray absorption fine structure (EXAFS) spectroscopy. Multi-electron excitation, resulting in double-electron excitation (2p, 4d → 5d, 5d) for REE-LIII edge EXAFS spectra, possibly causes a considerable error in EXAFS analyses for light REEs. To obtain reliable structural parameters this study elucidates the effects of double-electron excitation on the local structure determination of light REEs on poorly crystallized FeOOH and δ-MnO 2 . For this study, anomalous features attributable to excitation are superimposed on EXAFS signals of La, Pr, Nd, and Sm samples in the 5.0-7.0 Å -1 k range. The relative intensity of the double-electron excitation to the L III adsorption edge is <1.1%. Consequently, the double-electron excitation engenders a smaller error than those estimated in fitting for Pr, Nd, and Sm samples. However, significant correction is necessary for the determination of local coordination states of La samples: interatomic distances are 0.007-0.036 Å shorter after correction. The EXAFS analyses of REE-sorbed Fe samples show that adsorbed La, Pr, Nd, and Sm have a mixture of eightfold and ninefold coordination structures and form inner sphere complexes at the FeOOH surface. The determined structural parameters of light REE-sorbed δ-MnO 2 suggest that adsorbed light REEs have distorted tenfold-coordination spheres consisting of six short and four long REE-O bonds and form inner sphere complexes. Their coordination structures are more disordered than those of aquo ions and FeOOH samples.

April 1, 2015

Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part II. Correspondence of structural change to irregular variations of partitioning coefficients and tetrad effect variations appearing in interatomic distances

Atsuyuki Ohta, Hiroyuki Kagi, Masaharu Nomura, Hiroshi Tsuno, Iwao Kawabe

Page range: 476-486

Abstract

Experimental distribution coefficients of rare earth elements (REEs) between Fe oxyhydroxides (FeOOH) and Mn dioxide (δ-MnO 2 ) and solutions [K D (REE)] exhibit anomalous variations: preferential uptake of light REEs by Mn dioxide, a step-like trend in K D (REE) in the Er-Tm-Yb-Lu region, and fractionation of K D (Y) from K D (Ho). Extended X-ray absorption fine structure (EXAFS) spectroscopy was applied to determine coordination states of Er, Tm, Yb, Lu, and Y adsorbed onto FeOOH and δ-MnO 2 to assess structural changes around the REE site. The structures obtained, combined with previously determined structures of light REEs-sorbed Fe and Mn samples, corresponded to variations found in K D (REE). The structural parameters in the first coordination sphere suggest that La, Pr, and Nd adsorbed onto δ-MnO 2 have a distorted tenfold-coordination sphere and differ greatly from La-, Pr-, Nd-, and Sm-sorbed FeOOH, which have a mixture of eightfold- and ninefold-coordination spheres. In contrast, heavy REEs including Y adsorbed onto Fe and Mn samples’ local structures have an eightfold-coordination sphere. The preferential uptakes of light REEs by δ-MnO 2 are explained by the structural change. The irregular variations of heavy REEs and Y fractionation from Ho in K D (REE) do not, however, correspond to any change found in the coordination sphere. During characterization of the first coordination sphere, the W-type tetrad effect appears in the series variation of interatomic distances of REE 3+ aq and REE-sorbed FeOOH and δ-MnO 2 . The occurrence of a tetrad effect indicates that the interatomic distances relate not only to the electrostatic field but also to a quantum field.

April 1, 2015

Aluminocerite-Ce: A new species from Baveno, Italy: Description and crystal-structure determination

Fabrizio Nestola, Alessandro Guastoni, Fernando Cámara, Luciano Secco, Alberto Dal Negro, Danilo Pedron, Anton Beran

Page range: 487-493

Abstract

Aluminocerite-(Ce), ideally (Ce,Ca) 9 Al(SiO 4 ) 3 [SiO 3 (OH)] 4 (OH) 3 , is isostructural with cerite-(Ce) and cerite-(La). The holotype was found at the Ratti quarry, near Baveno, Italy, in millimeter-sized secondary cavities hosted in aplite-pegmatite veins and pods within pink granite. Aluminocerite-(Ce) forms aggregates of pseudo-octahedral to rhombohedral crystals flattened on the c axis. The cotype of aluminocerite-(Ce) was discovered at the Locatelly quarry, also near Baveno, where it occurs in centimeter-sized miarolitic cavities in pink granite. The mineral is pale pink to pink-reddish, with a white streak, and is translucent with a vitreous luster. Aluminocerite-(Ce) is non-fluorescent. The hardness based on the Mohs scale is 5, and the tenacity is brittle. Neither cleavage, fracture, or twinning were observed. Calculated density is 4.675 g/cm 3 . It is uniaxial, optically positive, with n o = 1.810-1.816 and n e = 1.812-1.822 (λ = 589 nm) and non-pleochroic. The average of 15 electron microprobe analyses for the holotype gave (wt%): Ce 2 O 3 23.37; Nd 2 O 3 15.59; La 2 O 3 7.43; Sm 2 O 3 4.38; Pr 2 O 3 3.54; Gd 2 O 3 3.12; Y 2 O 3 1.68; Dy 2 O 3 0.46; Yb 2 O 3 0.07; CaO 8.31; Fe 2 O 3 0.47; Al 2 O 3 2.47; SiO 2 24.01; and H 2 O 3.63 (calculated from crystal-chemical constraints), total 98.53 wt%, corresponding to the empirical formula (Ca 2.60 Ce 2.49 Nd 1.62 La 0.80 Sm 0.44 Pr 0.38 Gd 0.30 Y 0.26 Dy 0.04 Yb 0.01 ) Σ8.94 (Al 3+ 0.85 Fe 3+ 0.10 ) Σ0.95 (SiO 4 ) 3 [SiO 3 (OH)] 4 (OH) 3.06 , calculated on the basis of Si = 7. Aluminocerite-(Ce) is trigonal, space group R3c, with a =10.645(1), c = 38.019(5) Å, V = 3731 Å 3 . The strongest eight lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 3.405(27)(122), 3.250(26)(124), 2.914(100)(02,10), 2.647(58)(220), 2.198(40)(03,12), 1.923(34) (238), 1.826(24)(051), and 1.732(46)(03,18). The crystal structure has been refined to R1 = 0.056 for 745 observed reflections. The name is an allusion to the fact that it is the Al-dominant analog of cerite-(Ce).

April 1, 2015

A thermodynamic model for the plagioclase-liquid hygrometer/thermometer

Rebecca A. Lange, Holli M. Frey, Jacob Hector

Page range: 494-506

Abstract

A new thermodynamic model for the plagioclase-liquid exchange reaction between the albite (NaAlSi 3 O 8 ) and anorthite (CaAl 2 Si 2 O 8 ) components is presented, which can be used as a plagioclaseliquid hygrometer or thermometer. The model incorporates calorimetric and volumetric data for the pure liquid and crystalline components, which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition. This allows a more accurate assessment of the effect of melt composition (including dissolved water concentration) on the exchange reaction from plagioclase-liquid equilibrium experiments. Activity-composition relations for the plagioclase solid solution are taken from Holland and Powell (1992). The new hygrometer is calibrated on 71 plagioclase-liquid experiments, of which 45 are hydrous and 26 are anhydrous. Three filters were applied to the phase-equilibrium data: (1) crystallanities <30%; (2) pure H 2 O fluidsaturated; and (3) compositional totals (including H 2 O component) of 97-101% for hydrous quenched glasses. The final data set spans a wide range of liquid compositions (46-74 wt% SiO 2 ), plagioclase compositions (An 93 -An 37 ), temperatures (825-1230 °C), pressures (0-300 MPa), and dissolved melt water concentrations (0-7 wt% H 2 O). The standard error of estimate (SEE) for the model is ±0.32 wt% H 2 O, and all liquid compositions are fitted equally well. When the model is used as a thermometer, all measured temperatures are recovered equally well within ±14 °C on average. The model is only recommended for applications that fall within the compositional bounds of the calibration data set (i.e., metaluminous basalts through rhyolites in equilibrium with An 95 -An 35 ). It is not yet calibrated for rhyolites crystallizing plagioclase more sodic than An 30 , owing to an absence of phase-equilibrium experiments on rhyolites that pass the required filters. The new plagioclase-liquid hygrometer/thermometer is available as a Visual Basic program that runs on Excel 2004.

April 1, 2015

Solid-state NMR and IR spectroscopic investigation of the role of structural water and F in carbonate-rich fluorapatite

Harris E. Mason, Francis M. McCubbin, Alexander Smirnov, Brian L. Phillips

Page range: 507-516

Abstract

We have studied the substitutions in a natural well-crystallized carbonate-containing apatite-(CaF) (var. staffelite) using infrared (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopic techniques. Our results show the presence of both A- and B-type carbonate plus a large amount of structural water (0.44 pfu). This sample also contains 0.21 pfu excess F, but only weak C-F dipolar coupling is observed indicating that a tetrahedral CO 3 F 3- complex does not occur. 19 F NMR results indicate the presence of a second F environment in the apatite structure at a concentration similar to that of B-type carbonate but which does not differ from channel F in terms of coupling to 31 P or 1 H.

April 1, 2015

Chemical-structural modularity in the tetradymite group: A HRTEM study

Cristiana Liana Ciobanu, Allan Pring, Nigel John Cook, Peter Self, David Jefferson, Gabriel Ionel Dima, Volodymir Melnikov

Page range: 517-534

Abstract

Mixed-layer compounds from the tetradymite group, in the range Bi 2 Te 3 -Bi 8 Te 3 , were studied by HRTEM. The formula S′(Bi 2k X 3 )·L′[Bi 2(k+1) X 3 ] (X = chalcogen; S′, L′ = number of short and long modules, respectively) was introduced as a working model. Diffraction patterns show that all phases are N-fold (N = layers in the stacking sequence) superstructures of a rhombohedral subcell with c/3 = d1 ~ 0.2 nm. The patterns, with two brightest reflections about the middle of d 1 *, are described by monotonic decrease of two modulations with increase in Bi: (1) q = γc sub * (q ~ homoatomic interval; γ = 1.8-1.64 for analytical range; c sub ~ 3d 1 ), based on displacive modulation between chalcogen and Bi atoms; and (2) q F = γ F c sub *; q F = (i/N)d 1 * = id N *, i = S′ + L′, relating changes in module size and number to displacements in a basic substructure. The q F model, besides underpinning the stacking sequences, was adapted to incorporate the homology for S′, L′ modules related by k. The displacements are quantifiable by fractional shifts between reflections in the derived and basic structures. The condition for “the brightest two reflections about the middle of d 1 * to be separated by id N *” is fulfilled only if the shift at this position is minimal (equal to 1/N b ; N b = layers in the basic structure). This model and accompanying program compiled to find suitable N b and simulate intensity pattern(s) can be used to (1) constrain stacking sequences estimated from observation; (2) predict polysomes as larger building blocks; and (3) discriminate single-phases from random polysomes. The formula nBi 2 ·mBi 2 X 3 describing the configuration for Bi 2k X 3 modules by n/m = k - 1 is proven by lattice fringes, but is not underpinned by q F and does not constrain assumed homology.

April 1, 2015

Stability of pargasite during ultrahigh-temperature metamorphism: A consequence of titanium and REE partitioning?

K. Sajeev, Y. Osanai, Y. Kon, T. Itaya

Page range: 535-545

Abstract

Orthopyroxene-clinopyroxene-plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine-bearing titanian-ferroan pargasite from the Highland Complex, Sri Lanka, are interpreted as evidence for dehydration melting at ultrahigh-temperature conditions. High Ti (up to 0.4 pfu) and F (X F up to 0.56) content in pargasite extends its stability to higher temperatures, and the composition indicates the dehydration melting reaction may take place at ultrahigh-temperatures (~950 °C) at a pressure around 10 kbar, close to peak metamorphic conditions. The increase of Ti content close to the grain boundaries and cleavage planes in pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh-temperature conditions. The REE content in the pargasite shows a similar behavior to that of titanium. The cores with no breakdown assemblage consist of low and flat REE concentrations with respect to the high and Eu-depleted rim. Clinopyroxene in symplectite and needle-shaped lamellae within the pargasite porphyroblasts have similar REE patterns with slightly low-concentrations relative to that of pargasite. In the breakdown assemblage, LREEs are partitioned mainly into plagioclase while the HREEs are partitioned into orthopyroxene. The REE enrichment in the pargasite rims signals their relative partitioning between pargasite rims and melt. Modeling of the partitioning of Ti and REEs associated with pargasite breakdown demonstrates that its stability is greatly enhanced at UHT conditions. This investigation implies that the stability of hydrous minerals such as amphibole can be extended to UHT conditions, and expands our knowledge of metamorphism in the lower crust.

April 1, 2015

Raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh-pressure metamorphic rocks from diamonds in sample preparation materials

Maria Perraki, Andrey V. Korsakov, David C. Smith, Evripidis Mposkos

Page range: 546-556

Abstract

Natural diamond from three ultrahigh-pressure metamorphic (UHPM) terranes (Erzgebirge Massif, Germany; Kokchetav Massif, Northern Kazakhstan; Rhodope Metamorphic Province, Greece) and synthetic diamond from cutting and polishing materials (paste, spray, saw blade) were studied by means of optical microscopy and Raman microspectroscopy, to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond. Several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones [i.e., diamond as residual particles (contaminants) from the cutting and polishing material] such as the diamond size, the presence of inclusions, coatings, or coexistent phases and two diamond Raman band parameters, i.e., the Raman shift and the full-width at half maximum height (FWHM).

April 1, 2015

Lattice compression and structural behavior of NaVSi₂O₆ clinopyroxene to 11 GPa

Angela Ullrich, Ronald Miletich, Fabrizio Nestola, Christian Weikusat, Haruo Ohashi

Page range: 557-564

Abstract

High-pressure behavior of a synthetic single crystal of the C2/c clinopyroxene NaVSi 2 O 6 was investigated using X-ray diffraction and Raman spectroscopy in combination with diamond-anvil cell techniques. Lattice-parameters, single-crystal intensity data, and Raman spectra were collected up to pressures of about 11 GPa, revealing no discontinuity in the evolution of the lattice with increasing pressure. The fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields a bulk modulus of K 0 = 114.7 (1.8) GPa with its pressure derivative K′ = 5.4 (4). Strain analysis reveals compressional anisotropy with unit strain axial ratios of 1:2.9:3.1. The direction of largest compression was found to be perpendicular to the (3̅02) plane, which matches closely the direction of largest interlayer spacing between oxygen monolayers forming the structure of clinopyroxenes. The relatively large pressure dependency of the bulk modulus, compared to that of isostructural transition-metal compounds, was related to an increase in stiffness of the M2 polyhedron occurring above 4.6 GPa.

April 1, 2015

A predictive model for the enthalpies of hydration of zeolites

Philippe Vieillard, Romain Mathieu

Page range: 565-577

Abstract

A compilation of the average hydration enthalpies per mole of water of 145 diversely originating zeolites measured using different technical methods [76 data from transposed-temperature drop calorimetry (TTDC), 57 data from immersion calorimetry (IC), 6 data from phase equilibria (PE), 5 data from gas-adsorption calorimetry (GAC), and 3 data from hydrofluoric acid solution calorimetry (HF)] was generated. Statistical regressions between three parameters involving the average hydration enthalpy per mole of water {ΔH hyd-W , ΔH hyd-W /(Al/Si), ln[-ΔH hyd-W /(Al/Si)]}and six parameters namely: (1) the charge defined by the Al/Si ratio; (2) the ratio of the framework charge to the number of H 2 O molecules (Al/H 2 O); (3) the framework density (FD) calculated from the molecular volume of the anhydrous zeolite, FD anh , and hydrated zeolite, FD hyd ; (4) the average cation electronegativity in the exchange site characterized by parameter Δ H O=(site A) aq ; and (5) the intracrystalline water porosity (WP) determined from the volume of liquid water that can be recovered upon thorough outgassing of the hydrated zeolite. The regressions were performed by taking into account either the nature of the measurement technique, or the nature of the zeolite family. Within the zeolites from the TTDC and IC populations (133 data), the best results were obtained with ln[-ΔH hyd-W /(Al/Si)] and Al/(Al + Si). Whatever the measurement technique, considering the nature of the zeolite family having a constant framework density of the anhydrous form (129 data), the Al/(Al + Si) ratio remains the best parameter and the enthalpy of hydration can be expressed as follows: ΔH hyd-w = -(Al/Si)*e {5.491 - 4.674*[Al/(Al + Si)]} This general relationship can be improved by considering the following parameters: FD anh , Δ H O=(site A) aq , WP and a new parameter that is the product of three parameters Al/Si, Δ H O=(site A) aq , and WP weighing the variation of the water porosity related to the nature of the cation and to the total charge of the exchange site. Therefore, an understanding of the chemical formulae and unit-cell volumes of anhydrous and hydrated zeolites is required to evaluate the enthalpy of hydration with an accuracy of ±3.25 kJ/mol H 2 O.

April 1, 2015

Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Carlos Rodriguez-Navarro, Encarnacion Ruiz-Agudo, Ana Luque, Alejandro B. Rodriguez-Navarro, Miguel Ortega-Huertas

Page range: 578-593

Abstract

Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2DXRD), and transmission electron microscopy coupled with selected area electron diffraction (TEMSAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO 3(s) → CaO (s) + CO 2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (101̅4) calcite display the following topotactic relationships: (1) (12̅10) calcite //(110) CaO ; (2) (1̅104) calcite ┴ (110) CaO ; (3) (1̅018) calcite //(110) CaO ; and (4) (01̅14) calcite ┴(110) CaO ; with [841] calcite //[11̅0] CaO in all four cases. At this stage, the reaction mechanism is independent of PCO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy E α = 176 ± 9 kJ/ mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

April 1, 2015

Synchrotron Mössbauer spectroscopic study of ferropericlase at high pressures and temperatures

Jung-Fu Lin, Alexander G. Gavriliuk, Wolfgang Sturhahn, Steven D. Jacobsen, Jiyong Zhao, Michael Lerche, Michael Hu

Page range: 594-599

Abstract

The electronic spin state of Fe 2+ in ferropericlase, (Mg 0.75 Fe 0.25) O, transitions from a high-spin (spin unpaired) to low-spin (spin paired) state within the Earth’s mid-lower mantle region. To better understand the local electronic environment of high-spin Fe 2+ ions in ferropericlase near the transition, we obtained synchrotron Mössbauer spectra (SMS) of (Mg 0.75 ,Fe 0.25 )O in externally heated and laser-heated diamond anvil cells at relevant high pressures and temperatures. Results show that the quadrupole splitting (QS) of the dominant high-spin Fe 2+ site decreases with increasing temperature at static high pressure. The QS values at constant pressure are fitted to a temperature-dependent Boltzmann distribution model, which permits estimation of the crystal-field splitting energy (Δ 3 ) between the d xy and d xz or d zy orbitals of the t 2g states in a distorted octahedral Fe 2+ site. The derived Δ 3 increases from approximately 36 meV at 1 GPa to 95 meV at 40 GPa, revealing that both high pressure and high temperature have significant effects on the 3d electronic shells of Fe 2+ in ferropericlase. The SMS spectra collected from the laser-heated diamond cells within the time window of 146 ns also indicate that QS significantly decreases at very high temperatures. A larger splitting of the energy levels at high temperatures and pressures should broaden the spin crossover in ferropericlase because the degeneracy of energy levels is partially lifted. Our results provide information on the hyperfine parameters and crystal-field splitting energy of high-spin Fe 2+ in ferropericlase at high pressures and temperatures, relevant to the electronic structure of iron in oxides in the deep lower mantle.

April 1, 2015

Effective radii of noble gas atoms in silicates from first-principles molecular simulation

Liqun Zhang, James A. Van Orman, Daniel J. Lacks

Page range: 600-608

Abstract

An understanding of how noble gas atoms are dissolved in mantle minerals and melts is necessary to infer geological information from the constraints provided by noble gas geochemistry. For this purpose, first-principles molecular simulations are carried out on liquid and crystalline (stishovite) silica systems with dissolved noble gas atoms (He, Ne, Ar, Kr, and Xe). The first principles nature of the simulations, which do not involve empirical force field parameters, enables the determination of the effective radii and structural environments of the noble gas atoms. The noble gas atoms are shown to be highly compressible, so that their effective radii depend strongly on the molar volume of the system (which in turn depends on pressure). Due to the continuous nature of interatomic forces, the effective radii also depend on the extent to which the surrounding atoms can relax in response to the presence of the noble gas atom. In this regard, different definitions of effective radii are relevant in different situations: “equilibrium radii” that correspond to the optimal interatomic distances at the molar volume of the system, and “repulsive wall” radii that correspond to the interatomic distances where the interatomic potentials of mean force change from attractive to repulsive at that molar volume. The equilibrium radii determine the interatomic distances in a melt, and the repulsive wall radii determine the interatomic distances for interstitial sites in a crystal. Based on these effective radii, the structural environment surrounding the noble gas atoms at high pressure is shown to correspond to a close packing of O atoms around the central noble gas atom. Compression of the noble gas atoms is shown to correspond closely to the compression of the porosity within the silicate melt structure.

April 1, 2015

Fettelite, [Ag₆As₂S_{7][Ag₁₀HgAs₂S₈] from Chañarcillo, Chile: Crystal structure, pseudosymmetry, twinning, and revised chemical formula}

Luca Bindi, Frank N. Keutsch, Carl A. Francis, Silvio Menchetti

Page range: 609-615

Abstract

The crystal structure of the rare mineral fettelite was solved using intensity data collected from a twinned crystal from Chañarcillo, Copiapó Province, Chile. This study revealed that, in spite of the strong hexagonal pseudosymmetry, the structure is monoclinic (space group C2) with a = 26.0388(10), b = 15.0651(8), c = 15.5361(8) Å, β = 90.48(1)°, and V = 6094.2(5) Å 3 . The refinement of an anisotropic model led to an R index of 0.0656 for 7143 observed reflections [I > 2σ(I)] and 0.0759 for all 17 447 independent reflections. Fettelite is intimately twinned with six twin domains. The structure consists of the stacking of two module layers along [001]: an A module layer with composition [Ag 6 As 2 S 7 ] 2- and a B module layer with composition [Ag 10 HgAs 2 S 8 ] 2+ . The As atoms form isolated AsS 3 pyramids typical of sulfosalts, Hg links two sulfur atoms in linear coordination, and Ag occupies sites with coordination ranging from quasi linear to almost tetrahedral. The A module layer found for fettelite is identical to that described for the minerals belonging to the pearceite-polybasite group. On the basis of information gained from this characterization the crystal chemical formula was revised according to the structural results, yielding [Ag 6 As 2 S 7 ][Ag 10 HgAs 2 S 8 ] (Z = 8).

April 1, 2015

High-pressure phase transformation in LiFeGe₂O₆ pyroxene

Fabrizio Nestola, Günther J. Redhammer, Martha G. Pamato, Luciano Secco, Alberto Dal Negro

Page range: 616-621

Abstract

A synthetic pyroxene with composition LiFeGe 2 O 6 and space group P2 1 /c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, β, and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P2 1 /c phase is estimated to be 110 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P2 1 /c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.

April 1, 2015

Letter. Determination of the crystal structure of sanderite, MgSO₄·2H₂O, by X-ray powder diffraction and the charge flipping method

Hongwei Ma, David L. Bish, Hsiu-Wen Wang, Steve J. Chipera

Page range: 622-625

Abstract

The crystal structure of sanderite, MgSO 4 ∙2H 2 O, was determined from laboratory X-ray powder diffraction data measured from 2-140 °2θ using CuKα radiation. Sanderite is orthorhombic, space group P2 1 2 1 2 1 , with unit-cell parameters a = 8.8932(1) Å, b = 8.4881(1) Å, c = 12.4401(2) Å, V = 939.16(3) Å 3 , Z = 8. The crystal structure model was determined using the charge flipping method and was refined using fundamental parameters Rietveld refinement method to R wp = 6.52%, R exp = 1.89%, and χ 2 = 3.43. Bond-valence calculations for the refined model show that the structure is chemically reasonable. In the refined structure, Mg 2+ cations are coordinated by four O atoms from [SO 4 ] 2- groups and by two H 2 O molecules, forming distorted octahedra. By sharing vertex O atoms, [SO 4 ] tetrahedra and [MgO 4 (H 2 O) 2 ] octahedra build up a 3-D framework.

April 1, 2015

Letter. Forsterite, wadsleyite, and ringwoodite (Mg₂SiO₄): ²⁹Si NMR constraints on structural disorder and effects of paramagnetic impurity ions

Jonathan F. Stebbins, Wendy R. Panero, Joseph R. Smyth, Daniel J. Frost

Page range: 626-629

Abstract

We present here high-resolution 29 Si MAS NMR data for synthetic samples of forsterite (α-Mg 2 SiO 4 ), wadsleyite (β), and ringwoodite (γ). Enrichment to >99% 29 Si provides greatly enhanced signal-tonoise ratios and thus great sensitivity to small features in the spectra. At a detection limit of 0.1 to 0.5%, no six-coordinated Si ( VI Si) is observed in any of the polymorphs, although these results could be consistent with theoretical predications of 1 to 2% Mg-Si site disorder in ringwoodite if re-ordering occurs rapidly during cooling. Several small IV Si peaks in ringwoodite samples may be related to residual defects from this process. In forsterite and wadsleyite, several very small “extra” peaks are observed, many of which are at positions far outside the known range of chemical shifts for 29 Si in silicates. These may be caused by “pseudo-contact” shifts from dipolar interactions with unpaired electron spins on trace impurities of paramagnetic transition metal cations.

April 1, 2015

Letter Crystal structure and Raman spectrum of a high-pressure Li-rich majoritic garnet, (Li₂Mg)Si₂(SiO₄)₃

Hexiong Yang, Jürgen Konzett, Robert T. Downs, Daniel J. Frost

Page range: 630-633

Abstract

A Li-rich majoritic garnet (LiMGt), (Li 2 Mg)Si 2 (SiO 4 ) 3 , was synthesized at 15 GPa and 1500 °C and its structure studied with single-crystal X-ray diffraction and Raman spectroscopy. It is cubic with space group Ia3̄̅d and unit-cell parameters a = 11.2660(2) Å and V = 1429.91(1) Å 3 . The 8-, 6-, and 4-coordinated cation sites in LiMGt are occupied by (Li + + Mg 2+ ), Si 4+ , and Si 4+ , respectively. Whereas the SiO 6 octahedron is nearly regular, the XO 8 dodecahedron is the most distorted of all known silicate garnets in terms of the bond-length distortion index. All Raman peaks of LiMGt are broader than those of pyrope, due to the substitution of Li + for Mg 2+ at the dodecahedral site. Furthermore, both Si-O symmetric stretching (A 1g -ν 1 ) and O-Si-O symmetric bending (A 1g -ν 2 ) modes of LiMGt shift significantly to higher frequencies relative to the corresponding ones of pyrope. In contrast, the A 1g -(SiO 4 ) rotational mode of LiMGt displays a much lower frequency than that of pyrope. This study represents the first structural report on a garnet with an all-silicate framework and suggests that, like Na incorporation in garnets, the pressure-dependent coupled substitution of (Li + + Si 4+ ) for (Mg 2+ + Al 3+ ) is likely one of the primary mechanisms for Li enrichment in garnets in the mantle and the transition zone.

April 1, 2015

Letter. Quasi-ice-like C_P behavior of molecular H₂O in hemimorphite Zn₄Si₂O₇(OH)₂·H₂O: C_P and entropy of confined H₂O in microporous silicates

Charles A. Geiger, Edgar Dachs

Page range: 634-637

Abstract

Hemimorphite, Zn 4 Si 2 O 7 (OH) 2 ·H 2 O, and its dehydrated analog Zn 4 Si 2 O 7 (OH) 2 were studied by lowtemperature relaxation microcalorimetry and their heat capacity determined to analyze the behavior of the confined H 2 O between 5 and 300 K. An analysis of the data, which are corrected for the presence of a phase transition, shows that the C P of H 2 O in hemimorphite behaves more similar to the C P of ice than to liquid water or steam. The H 2 O molecule, with its four planar hydrogen bonds in hemimorphite, as well as its tetrahedral coordination in ice, is more rigidly hydrogen bonded in both than in liquid water. This is reflected in their respective C P behavior. The heat capacity and entropy for the dehydration reaction at 298 K are ΔC P rxn = -2.1 ± 3.6 J/(mol·K) and ΔS rxn = 134.7 ± 4.0 J/(mol·K). C P behavior at 0 < T < 300 K and entropy values at 298 K for confined H 2 O in hemimorphite and hydrous Mg cordierite are compared to those in several zeolites. The entropy for confined H 2 O in hemimorphite, analcime, and mordenite is around 54 J/(mol·K) at 298 K. The strength of the interactions (e.g., H bonding) between an H 2 O molecule and its surroundings increases approximately from steam > cordierite > analcime > hemimorphite ≥ mordenite > heulandite > natrolite ≈ scolecite > liquid H 2 O > ice and, in the case of microporous silicates, is inversely proportional to the S of the confined H 2 O.

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American Mineralogist: Journal of Earth and Planetary Materials (Am Min), is the flagship journal of the Mineralogical Society of America (MSA), continuously published since 1916. Am Min is home to some of the most important advances in the Earth Sciences. Our mission is a continuance of this heritage: to provide readers with reports on original scientific research, both fundamental and applied, with far reaching implications and far ranging appeal. Topics of interest cover all aspects of planetary evolution, and biological and atmospheric processes mediated by solid-state phenomena. These include, but are not limited to, mineralogy and crystallography, high- and low-temperature geochemistry, petrology, geofluids, bio-geochemistry, bio-mineralogy, synthetic materials of relevance to the Earth and planetary sciences, and breakthroughs in analytical methods of any of the aforementioned.

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Volume 94 Issue 4

Issue of American Mineralogist

Growth controls in colloform pyrite

Abstract

Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barroviantype metapelites, Imjingang belt, central Korea

Abstract

Magmatic vs. hydrothermal origins for zircon associated with tantalum mineralization in the Tanco pegmatite, Manitoba, Canada

Abstract

Molecular conformation and anion configuration variations for As4S4 and As4Se4 in an anion-substituted solid solution

Abstract

Low-pressure clino- to high-pressure clinoenstatite phase transition: A phonon-related mechanism

Abstract

Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part I. Influence of a multi-electron excitation on EXAFS analyses of La, Pr, Nd, and Sm

Abstract

Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part II. Correspondence of structural change to irregular variations of partitioning coefficients and tetrad effect variations appearing in interatomic distances

Abstract

Aluminocerite-Ce: A new species from Baveno, Italy: Description and crystal-structure determination

Abstract

A thermodynamic model for the plagioclase-liquid hygrometer/thermometer

Abstract

Solid-state NMR and IR spectroscopic investigation of the role of structural water and F in carbonate-rich fluorapatite

Abstract

Chemical-structural modularity in the tetradymite group: A HRTEM study

Abstract

Stability of pargasite during ultrahigh-temperature metamorphism: A consequence of titanium and REE partitioning?

Abstract

Raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh-pressure metamorphic rocks from diamonds in sample preparation materials

Abstract

Lattice compression and structural behavior of NaVSi2O6 clinopyroxene to 11 GPa

Abstract

A predictive model for the enthalpies of hydration of zeolites

Abstract

Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Abstract

Synchrotron Mössbauer spectroscopic study of ferropericlase at high pressures and temperatures

Abstract

Effective radii of noble gas atoms in silicates from first-principles molecular simulation

Abstract

Fettelite, [Ag6As2S7][Ag10HgAs2S8] from Chañarcillo, Chile: Crystal structure, pseudosymmetry, twinning, and revised chemical formula

Abstract

High-pressure phase transformation in LiFeGe2O6 pyroxene

Abstract

Letter. Determination of the crystal structure of sanderite, MgSO4·2H2O, by X-ray powder diffraction and the charge flipping method

Abstract

Letter. Forsterite, wadsleyite, and ringwoodite (Mg2SiO4): 29Si NMR constraints on structural disorder and effects of paramagnetic impurity ions

Abstract

Letter Crystal structure and Raman spectrum of a high-pressure Li-rich majoritic garnet, (Li2Mg)Si2(SiO4)3

Abstract

Letter. Quasi-ice-like CP behavior of molecular H2O in hemimorphite Zn4Si2O7(OH)2·H2O: CP and entropy of confined H2O in microporous silicates

Abstract

Molecular conformation and anion configuration variations for As₄S₄ and As₄Se₄ in an anion-substituted solid solution

Lattice compression and structural behavior of NaVSi₂O₆ clinopyroxene to 11 GPa

Fettelite, [Ag₆As₂S_{7][Ag₁₀HgAs₂S₈] from Chañarcillo, Chile: Crystal structure, pseudosymmetry, twinning, and revised chemical formula}

High-pressure phase transformation in LiFeGe₂O₆ pyroxene

Letter. Determination of the crystal structure of sanderite, MgSO₄·2H₂O, by X-ray powder diffraction and the charge flipping method

Letter. Forsterite, wadsleyite, and ringwoodite (Mg₂SiO₄): ²⁹Si NMR constraints on structural disorder and effects of paramagnetic impurity ions

Letter Crystal structure and Raman spectrum of a high-pressure Li-rich majoritic garnet, (Li₂Mg)Si₂(SiO₄)₃

Letter. Quasi-ice-like C_P behavior of molecular H₂O in hemimorphite Zn₄Si₂O₇(OH)₂·H₂O: C_P and entropy of confined H₂O in microporous silicates