Published by De Gruyter

Volume 87 Issue 8-9

Issue of American Mineralogist

Contents
Journal Overview

March 28, 2015

Evidence for magmatic vapor deposition of anhydrite prior to the 1991 climactic eruption of Mount Pinatubo, Philippines

Ryan T. Jakubowski, John Fournelle, Sue Welch, R. J. Swope, Patrick Camus

Page range: 1029-1045

Abstract

Anhydrite (CaSO 4 ) phenocrysts from Mount Pinatubo pumices show evidence of having responded dynamically to changing conditions prior to the June 15, 1991 climactic eruption. Micrometer-sized and smaller pyramidal surface growth features and lesser numbers of etch pits on anhydrite surfaces are documented by scanning electron microscopy. Chemical analyses indicate that the pyramids are a CaSO 4 polymorph and electron backscatter diffraction patterns show conclusively that the pyramids are indeed orthorhombic anhydrite and not another Ca-sulfate phase. Unit-cell measurements of volcanic anhydrite are identical with evaporitic anhydrite, as determined from single-crystal X-ray diffraction patterns. The computer program SOLVGAS was used to identify conditions under which the pyramids may have precipitated. Thermodynamic modeling of a cooling magmatic gas (H 2 O-CO 2 -SO 2 ) at 500 bars (maximum model pressure) and NNO +1.7 was performed. Assuming that the gas contained >10 -9 mol% Ca and 4 mol% SO 2 , the program indicates that anhydrite will precipitate homogeneously at approximately 780 °C, whereas an isothermal drop in pressure would likely lead to dissolution. Pyramids located between a phenocryst and adjacent glass provide physical evidence that at least a portion of the pyramids nucleated and grew before the melt quenched. We propose a mechanism to account for these previously unrecognized surface growths, which is that the anhydrite pyramids precipitated from a fluid or vapor phase that had separated from the magma at depth. At least a portion of the Pinatubo anhydrite phenocrysts provided substrates for nucleation and epitaxial growth of anhydrite. Because the anhydrite pyramids resemble products of chemical vapor deposition of metals and ceramics, we propose that this previously unrecognized process be termed magmatic vapor deposition. Evidence of high-temperature, magmatic vapor deposition of anhydrite could be used as an indicator that a pre-eruptive gas phase was accumulating or stored at depth, especially at arc volcanoes where excess amounts of sulfur are vented into the atmosphere

March 28, 2015

Experimental and modeled solubilities of chlorine in aluminosilicate melts, consequences of magma evolution, and implications for exsolution of hydrous chloride melt at Mt. Somma-Vesuvius

James D. Webster, Benedetto De Vivo

Page range: 1046-1061

Abstract

Solubility experiments were conducted with forty-one aluminosilicate rock compositions to determine how extensively Cl dissolves in hydrous chloride melt- ± vapor-saturated silicate melts containing low to moderate water contents at 2000 bars. Chlorine solubility in most silicate melts is dominated by the abundances of Mg ≈ Ca > Fe > Na > K > network-forming Al > Li ≈ Rb ≈ Cs, but Ti, F, and P also have strong influences. The relationship of composition to Cl solubility is more complex in peraluminous and peralkaline felsic melts, because network-modifying Al, Na, and K have a greater influence than their network-forming counterparts. Also, the effects of Ca, Mg, and Al in mafic melts characterized by high (Ca + Mg + Al)/(Na + K + Li) are much greater than their effects in silica-enriched melts. Association coefficients that express the influence of each ion on Cl solubility were determined, and the solubility data and coefficients were employed to develop a model that predicts Cl solubility at 2000 bars for water-undersaturated melts ranging from rhyolite to basalt. The coefficients were also used to investigate the predominant chloride complexes in melt, and the bulk of the solubility data are consistent with the interaction of Cl with alkaline-earth metals that provide charge balance for network-forming Al. The Cl solubility model is applied to Mt. Somma-Vesuvius magmas as they evolved from phonotephrite to phonolitic compositions, via fractional crystallization, to investigate the role of Cl in magmatic degassing. The results clearly demonstrate that Cl solubility was dramatically reduced by subtle changes in melt composition. Decreasing abundances of Ca, Mg, and Fe in the residual melt induced a dramatic reduction in Cl solubility that occurred simultaneously with gradual increases in the abundance of volatiles in melt due to crystallization of volatile-free minerals. The increasing abundance of volatiles and concurrent reduction in Cl solubility may have forced the exsolution of a hydrous chloride melt directly from the Cl-enriched mafic magmas. It is likely that the exsolution of hydrous chloride melt may occur in other Cl-enriched magmas, because Cl solubility depends so strongly on melt composition.

March 28, 2015

Temperature-induced Al -zoning in hornblendes of the Fish Canyon magma, Colorado

Olivier Bachmann, Michael A. Dungan

Page range: 1062-1076

Abstract

An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution (<10 μm steps) core-to-rim traverses across large euhedral phenocrysts, provides: (1) temporal constraints on the immediately pre-eruptive P-T-f H₂O evolution of the magma, and (2) a means of evaluating recent calibrations of the Al-inhornblende barometer (Anderson and Smith 1995; hereafter AS1995) and thermometers (Blundy and Holland 1990; thermometers A and B of Holland and Blundy 1994; hereafter BH1990, HB1994TA, and HB1994TB). Hornblende phenocrysts are variable for most major elements (e.g., 5-9 wt% Al 2 O 3 and 44-50 wt% SiO 2 ). This compositional range is controlled by two major temperature-sensitive coupled substitutions. Approximately 50% of the total Al variation (~0.8 atoms per formula unit = apfu) is due to the edenite exchange [ T Si + A ■ = T Al + A (Na + K)] and another 25-30% is the consequence of a Ti- Tschermak exchange ( T Si + M1-M3 Mn = T Al + M1-M3 Ti). In contrast, the pressure-sensitive Al-Tschermak substitution ( T Si + M1-M3 Mg = T Al + M1-M3 Al) did not play a significant role, as M1-M3 Al does not correlate with T Al and is always <0.2 apfu. In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of BH1990, HB1994TA (requiring silica saturation), HB1994TB (not requiring silica saturation) and AS1995, we have calculated pressures and temperatures for two selected populations of analyses wherein Al 2 O 3 contents are within analytical error (5.95 to 6.05 wt% Al 2 O 3 , N = 78 and 7.7 to 7.8 wt% Al 2 O 3 , N = 40). The barometric formulation of AS1995 gives a mean pressure of 2.24 ± 0.05 for the high-Al population at 760 °C, which is indistinguishable from the 2.4 ± 0.5 kbar estimate of Johnson and Rutherford (1989a). A high sensitivity to temperature at low P is suggested by the geologically implausibly shallow depths calculated for the low-Al population (<1 kbar at 760 °C). The three thermometric formulations give reasonable results between 706 and 814 °C, but the HB1994TA calibration gives a mean temperature higher by ~50 °C and is more sensitive to small analytical differences (~100 °C spread for each population). HB1994TB is considered the most reliable calibration of the Al-in-hornblende thermometer as it most precisely reproduces T estimates determined by independent methods. Nine out of 14 traverses across large phenocrysts from the Fish Canyon magma display rimward increases in T Al, A(Na + K), and M1-M3 Ti, compensated by decreases in T Si, and M1-M3 Mn. Using the HB1994TB algorithm, the low-Al population, typical of near-core compositions, gives a mean temperature of ~715 °C, which is ~35-45 °C above the water-saturated granite solidus at 2-2.5 kbar. The high-Al population, representing the average rim composition, gives a value around 760 °C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mag oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a temperature range of ~40 °C, which is consistent with our model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma (Bachmann et al. 2002). Crystallization of hornblende from a high-SiO 2 , low-MgO melt during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma.

March 28, 2015

Analytical techniques for volatiles: A case study using intermediate (andesitic) glasses

P.L. King, T.W. Vennemann, J.R. Holloway, R.L. Hervig, J.B. Lowenstern, J.F. Forneris

Page range: 1077-1089

Abstract

Small-scale analyses of volatiles in minerals and glasses provide information on how volatiles influence high-temperature geologic processes and low-temperature alteration processes. Four techniques for determining the C-O-H volatile contents of andesitic glasses are compared: manometry, secondary ion mass spectrometry, micro-Fourier transform infrared spectroscopy, and a technique where the H 2 O content is calculated using the difference between electron microprobe analysis totals and 100% sum. We present a method to determine the H content of a wide range of glass and mineral compositions using secondary ion mass spectrometry and a model for calibration factors. The extinction coefficients for H-O volatile contents in intermediate composition synthetic glasses are determined, and it is demonstrated that C-O speciation changes as total H 2 O content increases, with molecular CO 2 decreasing, CO 3 2- increasing, and carbonate peak splitting increasing. For glasses with low H 2 O content and oxy-substituted minerals, the methods of choice for volatile analysis are secondary ion mass spectrometry or micro-Fourier transform infrared spectroscopy.

March 28, 2015

Anomalous optical properties of fibrous tremolite, actinolite, and ferro-actinolite

Jennifer R. Verkouteren, Ann G. Wylie

Page range: 1090-1095

Abstract

The fibrillar growth habit and {100} twinning of fibrous amphiboles tend to produce anomalous optical properties. Commercial amosite and crocidolite always exhibit uniaxial-like optical properties including parallel extinction and two principal indices of refraction. Fibrous members of the actinolite series, however, exhibit a range in optical properties from normal to anomalous. The types of anomalous optical properties that can be displayed by asbestiform members of the actinolite series are described based on a study of twelve samples. One sample displays uniaxial-like properties, nine display partial development of uniaxial-like properties, and two contain fibers with both orthorhombic and monoclinic optical properties. “Byssolitic” samples of the actinolite series, a fibrous non-asbestiform habit, contain fibers that do not go to extinction in sections on or near (010), probably as a result of {100} twinning. Although anomalous optical properties may confound the identification of fibrous amphiboles, in most cases the refractive indices are predictable and can be used for identification. Because of the range in optical properties, especially extinction angle, reliance solely on parallel extinction to distinguish asbestos from non-asbestiform varieties is not recommended. The fibrillar structure, however, remains the hallmark of the asbestiform habit.

March 28, 2015

Protoanthophyllite from three metamorphosed serpentinites

Hiromi Konishi, István Dódony, Peter R. Buseck

Page range: 1096-1103

Abstract

This is the first report of a natural Mg-rich protoanthophyllite. It is common in metamorphosed serpentinites from three Japanese ultramafic complexes, and some crystals contain anthophyllite (Pnma) lamellae. The Mg/(Mg + Fe) ratios of the Hayachine, Tari-Misaka, and Takase protoanthophyllites are 0.90, 0.92, and 0.91, respectively. The samples have identical optical properties: X = a, Y = b, Z = c, and 2V x = 64 ± 5°. Their space group is Pnmn (or Pn2n), as revealed by systematic extinctions in selected-area electron-diffraction patterns. The protoanthophyllite and anthophyllite have similar compositions and orthorhombic symmetry. They are difficult to distinguish using optical, microanalytical, and powder X-ray diffraction measurements. This problem raises the possibility that some of the published data on geological and synthetic anthophyllite samples may be of misidentified materials, potentially leading to errors in the published stability relations of anthophyllite. We provide a method to identify protoanthophyllite and differentiate it from its polymorphs using selected-area electron diffraction and high-resolution transmission electron microscopy methods.

March 28, 2015

Synthesis of beryllian sapphirine in the system MgO-BeO-Al₂O₃-SiO₂-H₂O and comparison with naturally occurring beryllian sapphirine and khmaralite. Part 1: Experiments, TEM, and XRD

A.G. Christy, Y. Tabira, A. Hölscher, E.S. Grew, W. Schreyer

Page range: 1104-1112

Abstract

Beryllian sapphirine Mg 4-x Al 4+x [Al 4+x-2y BeySi 2-x+y O 18 ]O 2 has been synthesized from starting compositions with y ≤ 1 at x = 0 and y ≤ 0.5 at x = 0.5, P = 0.1-1.3 GPa, T = 700-1350 °C. Electron diffraction shows the sapphirines are dominantly the 1A polytype but lamellae of a 2M phase are consistently present. This is the first 2M sapphirine synthesized in the laboratory, and the first known to be devoid of Fe 2+ . No superstructure reflections corresponding to the doubled tetrahedral chain repeat of khmaralite were observed, probably due to insufficient annealing time. Cell parameters of the synthetic sapphirine decreased strongly and linearly with Be content (2.7 vol% decrease from y = 0 to y = 1). In agreement with crystal-chemical considerations, experiments with starting compositions of y > 1.0 resulted in additional crystalline phases either coexisting with the limiting sapphirine (y = 1) or without it. At 900 °C, 1.3-2.0 GPa, the saturating assemblage is surinamite + chrysoberyl + forsterite, which is chemically equivalent to sapphirine with y = 1.5. The current lack of natural khmaralite with Be > 0.78 cations per formula unit (pfu) is likely due to the bulk composition of the host rocks being too rich in SiO 2 and Al 2 O 3 for forsterite to be stable. Addition of BeO to the MgOAl 2 O 3 -SiO 2 system evidently enlarges the stability field of sapphirine + forsterite relative to its restricted range in the BeO-free system.

March 28, 2015

Intersite distribution of Fe²⁺ and Mg in the spinel (sensu stricto)–hercynite series by singlecrystal X-ray diffraction

Giovanni B. Andreozzi, Sergio Lucchesi

Page range: 1113-1120

Abstract

The influence of composition on Fe 2+ -Mg intracrystalline distribution was studied in eleven synthetic crystals belonging to the spinel (sensu stricto)-hercynite series (Mg 1-y Fe 2+ y )Al 2 O 4 , with 0 ≤ y ≤ 1, produced by flux-growth at 800 °C. Samples were analyzed by single-crystal X-ray diffraction and electron microprobe methods, and found to be chemically homogeneous with only minor Fe3+, which substitutes for Al and increases up to 0.09 atoms per formula unit with total Fe. Structural parameters a, u, T-O, and M-O increase with hercynite content and, among bond distances, T-O shows the maximum change, from 1.920 to 1.968 Å. The a variation from 8.0855 to 8.1646 Å is essentially caused by the T-O increase that, in turn, is due to the cooperative effects of (1) Fe 2+ → Mg substitution and (2) decrease of inversion from 0.23 to 0.15 along the series. Intracrystalline cation distribution was obtained by a minimization procedure that takes into account structural and chemical data. The T site is mainly populated by Mg and Fe 2+ but, at a given temperature, Fe 2+ shows a marked preference for tetrahedral coordination with respect to Mg. The influence of composition and temperature on Fe-Mg intracrystalline distribution was modeled within the framework of the general thermodynamic model of O’Neill and Navrotsky for spinel binary solid solutions. The inversion values observed in our samples agree very well with those calculated by the model. Both measured and calculated amounts of octahedral Fe 2+ ( VI Fe 2+ ) show a non-linear increase from spinel s.s. to hercynite. Consequently, the VI Fe 2+ /Fe 2+ tot. ratio is not constant along the join, but increases from zero to 15% toward the hercynite end-member. This behavior explains the very limited Fe 2+ inversion observed in natural spinels, which usually belong to the hercynite-poor part of the join.

March 28, 2015

Structural relationships in (Mn_1–xZn_x)Mn₂O₄ (0 ≤ x ≤ 0.26): The “dragging effect” of the tetrahedron on the octahedron

Ferdinando Bosi, Sergio Lucchesi, Antonio Della Giusta

Page range: 1121-1127

Abstract

Ten hausmannite crystals (from Ilfeld and Friedrichrode, Harz, Germany), belonging to the (Mn 1-x Zn x )Mn 2 O 4 (0 ≤ x ≤ 0.26) system (I4 1 /amd hausmannite structure type), were characterized by chemical (electron microprobe) and structural (single-crystal X-ray diffractometer) analysis. The prevailing trivalent cation is Mn 3+ , with very minor Al (not higher than 0.005 apfu). Among divalent cations, the main substitution involves Zn → Mn 2+ . Cation distribution was obtained by comparing chemical and structural data, and results confirm normal distribution, with Mn 3+ ordered on the octahedral site. A specific bond distance of 2.030 Å was refined for VI Mn 3+ -O. Unit-cell parameters a and c range from 5.752 to 5.763 Å and from 9.408 to 9.461 Å, respectively. The smallest values are characteristic of the sample with the highest hetaerolite content. T-O bond distance (2.027-2.041 Å) shows a strong positive correlation with unit-cell constants, while the O-T-O angle (103.3-103.7°) is related only to the oxygen coordinate, z. The two octahedral bond distances show limited variations: the shorter one, M-O S , ranges from 1.927 to 1.930 Å, and is not significantly correlated with unit-cell parameters. The longer one, M-O L , shows a larger variation, from 2.281 to 2.290 Å, and is positively correlated with c. Regularization of the octahedron with increasing hetaerolite content coincides with an increase in the oxygen coordinate y and a decrease in c and c/a. Of particular interest is the positive linear relation between octahedral elongation and VT. As the octahedral content of all samples is almost constant, given the closeness of Mn 3+ to stoichiometry, all structural distortions are linked to IV Zn → IV Mn 2+ that reduces the T-O bond distance and causes movement of the structure toward cubic symmetry. This interaction is due to the “dragging effect” of the tetrahedron on the octahedron. In hausmannite-type structures, besides the main structural distortion produced by the Jahn-Teller effect, a secondary one, without symmetry modification, is determined by the geometrical effects of the tetrahedron on the octahedron.

March 28, 2015

The grid-work texture of authigenic microcrystalline quartz in siliceous crust-type (SCT) mineralized horizons

Gianfranco Camana, Daniel Chateigner, Michele Zucali, Gilberto Artioli

Page range: 1128-1138

Abstract

Siliceous crust-type (SCT) formations are pervasive silicified horizons associated with fluoritebarite- polymetallic sulfide mineralization in carbonate rocks. They almost invariably show a peculiar grid-work texture formed by microcrystalline quartz. The mineralogical and textural features of several SCT samples from Italy, Spain, France, China, and Brazil were investigated by optical and electron microscopy and X-ray and neutron powder diffraction. The resulting {001} pole figures exhibit two main components of orientation of the crystallographic c axis with respect to the normal to the sample surface, one located at about 35° and the other in the range 60-85°. The observed textural features are seen in all samples and they appear to be independent of the age and geographical location of the SCT formation. The observed grid-work texture is tentatively interpreted as forming by nucleation of quartz seeds on specific faces of surrounding crystals. The observed morphology and the regularly oriented populations of crystals are consistent with simultaneous growth of quartz crystals having well-defined reciprocal orientations. The grid-work texture of quartz in SCT horizons is similar to the type reported for Magadi-type cherts, which are commonly interpreted as formed by magadiite-mediated silica precipitation processes. Since the genesis of SCT deposits is totally unrelated to alkaline-lake genetic processes, it is concluded that cherts showing strongly textured patterns of microcrystalline quartz may have a different origin than Magadi-type deposits.

March 28, 2015

The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si₃B₃O₁₈)^15– polyanion

Paolo Ballirano, Athos Callegari, Franca Caucia, Adriana Maras, Fiorenzo Mazzi, Luciano Ungaretti

Page range: 1139-1143

Abstract

The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th) 15 Fe 3+ (SiO 4 ) 3 (Si 3 B 3 O 18 )(BO 3 )(As 5+ O 4 )(As 3+ O 3 ) x (NaF 3 ) 1-x F 7 ·0.2H 2 O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers. Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO 4 ) 6 group and a threefold B 3 O 9 borate ring. Each tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si 3 B 3 O 18 15- polyanion. Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO 4 tetrahedron and a BO 3 triangle. Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As 3+ O 3 3- and NaF 3 2- polyhedra, the occurrence of which is mutually exclusive and statistically disordered. A 3-dimensional network of M-(O,F) n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.

March 28, 2015

Submicrometer optical characterization of the grain boundary of optically active Cr³⁺ doped polycrystalline Al₂O₃ by near-field spectroscopy

Yoshihito Narita, Hiroshi Murotani

Page range: 1144-1147

Abstract

Near-field fluorescent spectroscopy was used to characterize the grain boundary of Cr-doped polycrystalline alumina. The results show that the peak widths in near-field spectra are narrower than in spectra obtained by conventional microspectrometry, and this is attributed to the difference in spatial resolution of the two methods. The R-line peaks of Cr 3+ fluorescence were observed to shift to a lower wavelength at the grain boundary compared to the bulk crystal, which is attributed to stress relief at the crystal boundary. This peak shift at the boundary decreases as a function of the time a polycrystal is annealed at 1700 °C.

March 28, 2015

High-temperature, high-pressure optical spectroscopic study of ferric-iron-bearing tourmaline

Michael N. Taran, George R. Rossman

Page range: 1148-1153

Abstract

The optical spectra of dravite (tourmaline) with an unusually high ratio of Fe 3+ to Fe 2+ and a synthetic elbaite with high Fe 3+ content were studied at high temperature (297 to 600 K) and high pressure (to 10.51 GPa) conditions. Individual absorption bands derived from paired Fe atoms were identified in the spectra and their temperature and pressure dependence was studied. The most pronounced effects are the intensification of the two Fe 2+ -related bands (~9090 cm -1 and ~14300 cm -1 ) at pressure, their shift to higher energy at pressure, and their pronounced intensity decrease with increasing temperature. Such behavior is assumed to be caused by an electronic exchange interaction in an Fe 2+ -Fe 3+ pair at adjacent Y sites in the structure. Temperature and pressure dependencies of the bands attributed to Fe 3+ -Fe 3+ exchange-coupled pairs are noticeably different from those of Fe2+-Fe 3+ pairs. This shows that the two types of pairs have different exchange interactions, and points to the need for further experimental and theoretical investigation. The intensity of the intense E⊥c-polarized band at ~ 20580 cm -1 originating from the 6 A 1g → ( 4 A 1g , 4 E g ) transition of the Fe 3+ (Y)-Fe 3+ (Y) pair, depends moderately on temperature and pressure. The intensity of a weak shoulder at ~18350 cm-1 (E⊥c), also attributed to an Fe 3+ (Y)-Fe 3+ (Y) pair, decreases and nearly disappears between 5.44 and 9.55 GPa. The intensity of the E⊥c-polarized band at ~18500 cm -1 , attributed to an Fe 3+ (Z)-Fe 3+ (Z) pair, displays a strong inverse temperature dependence, whereas the energy of the band remains nearly constant.

March 28, 2015

Infrared and Mössbauer study of Brazilian tourmalines from different geological environments

Ester Figueiredo De Oliveira, Cristiane Castañeda, Sigrid Griet Eeckhout, Messias Menezes Gilmar, Rogério Ribeiro Kwitko, Eddy De Grave, Nilson Francisquini Botelho

Page range: 1154-1163

Abstract

Infrared (IR) spectra in the OH stretching region and Mössbauer spectra (MS) have been acquired for natural tourmalines from magmatic and hydrothermal deposits in the state of Minas Gerais, Brazil. In the magmatic environment, samples of tourmaline were collected from granitic pegmatites and granitoids. In the former, the elbaite-schorl series is prevalent, while in the latter the schorl species is dominant. Tourmalines from hydrothermal gold deposits are typically intermediate members of the dravite-schorl series, whereas the uvite and Fe-bearing uvite species are present in tourmaline samples from a magnesite deposit. Different behaviors are discussed in terms of local lattice environment, geological history, fluid/rock interaction or magma evolution, and degree of crystallinity.

March 28, 2015

Isothermal compression of staurolite: A single-crystal study

Paola Comodi, Michela Montagnoli, Pier Francesco Zanazzi, Tiziana Boffa Ballaran

Page range: 1164-1171

Abstract

The response of staurolite to pressure was studied by single crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals with composition: (Fe 3.365 Zn 0.025 Li 0.114 Co 0.009 Mn 0.034 )[T2, M4] (Al 2 Mg 0.307 )[M3] (Al 15.491 Fe 3+ 0.104 Mg 0.394 Cr 0.004 Ti 0.07 )[M1, M2] (Si 7.534 Al 0.466 )[T1] O 48 H 3 . Lattice parameters, measured at various pressure up to 7.264(6) GPa, were fitted using a thirdorder Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V 0 = 740.85(7) Å 3 , K 0 = 180(2) GPa and K' = 4.7(6), a 0 = 7.8723 (2) Å, K 0 = 189(2) GPa, and K' a = 4.1 (6), b 0 = 16.62453(1) Å, K 0 = 179(2) GPa, K' b = 6.1(6) and c 0 = 5.6604 (4) Å, K 0 = 179(5) GPa, K' c = 2(1); whereas the angle b remained almost constant with increasing pressure. These data suggest an almost isotropic compressibility. Structural evolution was studied by comparison of structural refinements carried out with data collected at 0.0001, 2.48, 4.15, 5.43, 6.84, and 8.74 GPa. All refinements were made in the Ccmm space group. Polyhedral evolution with P is a function of occupancy: whereas the T1 tetrahedron and the M1 and M2 octahedra, occupied by Si and Al, are practically incompressible, the T2 tetrahedron, and the M4 and M3 octahedra, only partially occupied principally by Fe (the first two) and by Al (the last), show larger changes as a function of pressure. As a consequence, the two kyanite and Fe-Al hydroxide layers, which can be used to describe the staurolite structure, have different compressibilities.

March 28, 2015

Equation of state measurements of chlorite, pyrophyllite, and talc

A.R. Pawley, S.M. Clark, N.J. Chinnery

Page range: 1172-1182

Abstract

The compressibilities of five sheet silicates-three chlorite compositions, pyrophyllite, and talc- have been measured in situ using energy-dispersive powder diffraction in a multi-anvil apparatus at the synchrotron radiation source, Daresbury Laboratory, U.K. Some high-temperature data were also collected. The three chlorite samples were a natural Mg-rich chlorite [Mg 4.49 Fe 0.28 Cr 0.12 Al 2.10 Si 2.95 O 10 (OH) 8 ], a natural Fe-rich chlorite [Mg 0.28 Fe 4.11 Al 2.71 Si 2.77 O 10 (OH) 8 ], and a synthetic clinochlore [Mg 5 Al 2 Si 3 O 10 (OH) 8 ]. The compressibility data for all three samples, measured up to between 3 and 6 GPa, can be fit to a single equation of state with a bulk modulus of 86.9(16) GPa (using K' = 4). The thermal expansivity measured for the natural Mg-chlorite [α 0 = 2.5(6) × 10 -5 /K] is similar to previous measurements on various chlorite compositions, suggesting that the thermal expansivity of chlorite also can be described by a single equation, independent of composition. The bulk modulus of the natural pyrophyllite [Al 2 Si 4 O 10 (OH) 2 ], measured up to 6.3 GPa, is 37(3) GPa, with K' = 10(1). The few high-temperature points agree with previous measurements. The natural talc [Mg 3 Si 4 O 10 (OH) 2 ], measured up to 6.2 GPa, has a bulk modulus of 41(4) GPa, with K' = 6(2). Our bulk modulus of chlorite is much higher than previously determined using X-ray diffraction. This finding explains why calculations of chlorite reactions using the previous data considerably overestimate its stability when compared to experimentally determined reaction positions. The pyrophyllite compressibility data should be useful in calculations of metamorphic reactions involving this mineral. The talc data are essentially identical to previous measurements.

March 28, 2015

Structure change of Ca_1–xSr_xTiO₃ perovskite with composition and pressure

Takamitsu Yamanaka, Noriyuki Hirai, Yutaka Komatsu

Page range: 1183-1189

Abstract

Structure refinements of solid solutions of (Ca 1-x Sr x )TiO 3 (x = 0.0, 0.25, 0.5, 0.6, 0.65, and 1.0) were undertaken using single crystals at ambient conditions. Their lattice constants, c/a axial ratios, and cell volumes indicate continuous changes from orthorhombic to cubic through a tetragonal phase. The orthorhombic structure is continuous between x = 0.0 and x = 0.6, and a phase at x = 0.65 shows a tetragonal structure with space group I4/mcm. With increasing Sr substitution, the symmetry changes to cubic with Pm3̄m space group. A-O and B-O distances in ABO 3 perovskite were determined as a function of the composition of the A cation (Ca and Sr). Tilting and rotation angles of the TiO 6 octahedral linkage with x of (Ca 1-x Sr x )TiO 3 were also evaluated. Single-crystal structure refinements of Ca 0.35 Sr 0.65 TiO 3 perovskite at 3.5, 4.1, and 7.0 GPa at 300 K were carried out using a diamond anvil cell. The tetragonal phase transforms to an orthorhombic structure with space group Pbnm at 3.5 GPa. The polymorphic transition of VIII A 2+VI B 4+ O 3 perovskites under compression is discussed.

March 28, 2015

Mechanism of 2/1- to 3/2-mullite transformation at 1650 °C

Martin Schmücker, Bernd Hildmann, Hartmut Schneider

Page range: 1190-1193

Abstract

The transformation of 2/1-mullite single crystals into stable 3/2-mullite was investigated using reaction couples of 2/1-mullite cut parallel to (001) and silica glass at 1650 °C. A thin layer of 3/2- mullite formed at the surface of the parent 2/1-mullite crystal. Between 3/2- and 2/1-mullite a gradual transition layer of about 1 mm exists. This gradual transition zone and the lack of any interface between the substrate 2/1-mullite and the newly formed 3/2-mullite implies a topotactic transformation mechanism controlled by Al, Si, and O interdiffusion with preservation of the basic crystal structure, especially the AlO 6 octahedral chains. The number of O atom vacancies is reduced going from 2/1- to 3/2-mullite, as shown by a gradual decrease of the corresponding superlattice intensities in electron diffraction patterns. As the transformation requires diffusion of Al out of and of Si and O atoms into the 2/1-mullite crystal, a coexisting silicate melt is required as an Al sink and as a source of Si and O atoms.

March 28, 2015

High-pressure deformation mechanism in scolecite: A combined computationalexperimental study

Pietro Ballone, Simona Quartieri, Alessandra Sani, Giovanna Vezzalini

Page range: 1194-1206

Abstract

Pressure-induced structural modifications in scolecite were studied by means of in situ synchrotron X-ray powder diffraction and density functional computations. The experimental cell parameters were refined up to 8.5 GPa. Discontinuities in the slope of the unit-cell parameters vs. pressure dependence were observed; as a consequence, an increase in the slope of the linear pressure-volume dependence is observed at about 6 GPa, suggesting an enhanced compressibility at higher pressures. Weakening and broadening of the diffraction peaks reveals increasing structural disorder with pressure, preventing refinement of the lattice parameters above 8.5 GPa. Diffraction patterns collected during decompression show that the disorder is irreversible. Atomic coordinates within unit cells of different dimensions were determined by means of Car-Parrinello simulations. The discontinuous rise in compressibility at about 6 GPa is reproduced by the computation, allowing us to attribute it to re-organization of the hydrogen bonding network, with the formation of water dimers. Moreover we found that, with increasing pressure, the tetrahedral chains parallel to c rotate along their elongation axis and display an increasing twisting along a direction perpendicular to c. At the same time, we observed the compression of the channels. We discuss the modification of the Ca polyhedra under pressure, and the increase in coordination number (from 4 to 5) of one of the two Al atoms, resulting from the approach of a water molecule. We speculate that this last transformation triggers the irreversible disordering of the system.

March 28, 2015

Far infrared spectra of K⁺ in dioctahedral and trioctahedral mixed-layer minerals

Magali Diaz, Valérie Laperche, James Harsh, René Prost

Page range: 1207-1214

Abstract

Mixed-layer minerals were analyzed by far infrared (FIR) spectrometry. The FIR spectra show two bands, which occur at 84-87 and 108-114 cm -1 for mixed-layer illite-smectite (I-S) and at 71-79 and 103 cm -1 for mixed-layer phlogopite/vermiculite (P-V). The 103 cm -1 band seen in the P-V series from Palabora is independent of K + content and is attributed to lattice vibrations. The lower wavenumber band observed at 85 cm -1 for I-S mixed-layers and 71-73 cm -1 for P-V mixed-layers corresponds to K + fixed in the two-layer stacking sequences. This low wavenumber band shifts to higher wavenumbers as the thickness of coherent scattering domains increases: shifting toward 97 cm -1 for the illite end-member of the I-S series and to 79 cm -1 for Palabora phlogopite, the endmember of the P-V series.

March 28, 2015

Ab initio quantum mechanical modeling of infrared vibrational frequencies of the OH group in dioctahedral phyllosilicates. Part I: Methods, results and comparison to experimental data

Sara Martínez-Alonso, James R. Rustad, Alexander F.H. Goetz

Page range: 1215-1223

Abstract

The infrared (IR) spectra of small clusters of atoms ([MM'(OH) 2 ] and [MM'(OH) 2 (H 2 O) 6 ], where M, M' = Al 3+ , Mg 2+ , Fe 2+ , Fe 3+ ) mimicking the environment of the OH group in dioctahedral phyllosilicates have been modeled using ab initio quantum mechanical calculations. These modeling results are relevant to establishing the connections between IR spectra of phyllosilicates and their composition, and to investigate the utility of quantum mechanical models for calculating IR frequencies of minerals. This study focused on the OH stretch and in-plane bend fundamentals, because they give rise to a combination band near 4545 cm -1 (2.2 μm) that can be observed in imaging spectrometer or hyperspectral remote sensing data. A comparison among results obtained using both ab initio [Hartree-Fock (HF) and Density Functional Theory (DFT)], and semi-empirical [PM3(tm)] methods, showed that the DFT model approaches IR frequency experimental values most closely. IR spectra of phyllosilicates were modeled using the DFT method. The modeled frequencies were scaled using a mode-dependent linear transformation, and experimental frequencies were reproduced satisfactorily. The modeling results show that most of the variability observed in the OH in-plane bend fundamental of dioctahedral phyllosilicates can be explained by the effects of neighboring octahedral cations alone. Discrepancies between modeling and experimental results in the case of the OH stretch point to the existence of factors other than the nature of the neighboring octahedral cations, such as tetrahedral substitution, affecting this fundamental mode

March 28, 2015

Ab initio quantum mechanical modeling of infrared vibrational frequencies of the OH group in dioctahedral phyllosilicates. Part II: Main physical factors governing the OH vibrations

Sara Martínez-Alonso, James R. Rustad, Alexander F.H. Goetz

Page range: 1224-1234

Abstract

The physical factors responsible for the variability observed in OH infrared (IR) fundamentals in dioctahedral phyllosilicates, due to octahedral substitution of Al 3+ by Mg 2+ , Fe 2+ , and Fe 3+ , are discussed here. The data analyzed consist of experimental frequencies as well as frequencies modeled using Density Functional Theory (DFT) calculations. The charge of the octahedral cations surrounding the OH is one of the main factors affecting both the OH stretch and the in-plane bend; cationic electronegativity and ionic radius play important roles in the stretch and bend modes, respectively. The mass of the octahedral cations does not affect the OH fundamental vibrations. The nature of the octahedral cations alone can explain most of the variability observed in the OH in-plane bend, making this fundamental vibration the most suitable for assessing octahedral composition. Discrepancies between modeled and experimental OH stretch frequencies indicate the existence of other factors governing this fundamental vibration. Further DFT calculations indicate that apical O atoms of the tetrahedral sheet with unsatisfied charges due to octahedral and/or tetrahedral substitutions can explain these discrepancies. The modeling results are utilized to predict the frequency of the OH stretch and in-plane-bend combination band that occurs near 4545 cm -1 (2.2 μm) in phyllosilicates. This band can be observed in imaging spectrometer data, allowing for the detection and analysis of phyllosilicates and other minerals in large natural systems. The modeling results confirm that the variability observed in the combination band of dioctahedral phyllosilicates reflects octahedral and, to a certain degree, tetrahedral composition, but not interlayer composition.

March 28, 2015

On geological interpretations of crystal size distributions: Constant vs. proportionate growth

D.D. Eberl, Daniel E. Kile, V.A. Drits

Page range: 1235-1241

Abstract

Geological interpretations of crystal size distributions (CSDs) depend on understanding the crystal growth laws that generated the distributions. Most descriptions of crystal growth, including a population-balance modeling equation that is widely used in petrology, assume that crystal growth rates at any particular time are identical for all crystals, and, therefore, independent of crystal size. This type of growth under constant conditions can be modeled by adding a constant length to the diameter of each crystal for each time step. This growth equation is unlikely to be correct for most mineral systems because it neither generates nor maintains the shapes of lognormal CSDs, which are among the most common types of CSDs observed in rocks. In an alternative approach, size-dependent (proportionate) growth is modeled approximately by multiplying the size of each crystal by a factor, an operation that maintains CSD shape and variance, and which is in accord with calcite growth experiments. The latter growth law can be obtained during supply controlled growth using a modified version of the Law of Proportionate Effect (LPE), an equation that simulates the reaction path followed by a CSD shape as mean size increases.

March 28, 2015

Allabogdanite, (Fe,Ni)₂P, a new mineral from the Onello meteorite: The occurrence and crystal structure

Sergey N. Britvin, Nikolay S. Rudashevsky, Sergey V. Krivovichev, Peter C. Burns, Yury S. Polekhovsky

Page range: 1245-1249

Abstract

Allabogdanite, (Fe,Ni) 2 P, is a new mineral from the Onello iron meteorite (Ni-rich ataxite). It occurs as thin lamellar crystals disseminated in plessite. Associated minerals are nickelphosphide, schreibersite, awaruite, and graphite. Crystals of the mineral, up to 0.4 × 0.1 × 0.01 mm, are flattened on (001) with dominant {001} faces, and other faces that are probably {110} and {100}. Mirror twinning resembling that of gypsum is common, with possible twin composition plane {110}. Crystals are light straw-yellow with bright metallic luster. Polished (001) sections look silverywhite against an epoxy background. In reflected light in air, the mineral has a creamy color, with distinct anisotropy from light to dark creamy tint. No bireflectance was observed. R 1 /R 2 (λ, nm) in air: 48.4/37.2(440), 46.7/36.8(460), 47.0/37.6(480), 47.5/38.1(500), 47.6/38.8(520), 48.2/39.2(540), 49.0/39.9(560), 49.6/40.7(580), 50.1/41.6(600), 50.5/41.9(620), 51.9/43.0(640), 52.3/44.3(660), 53.3/ 45.0(680), 54.4/46.2(700). No cleavage or parting was observed. Moh’s hardness is 5-6; the mineral is very brittle, and its calculated density 7.10 g/cm 3 . Its chemical composition (determined by microprobe methods, average of nine analyses) is: Fe 57.7, Ni 20.7, Co 1.4, P 20.4, Total 100.2 wt%, corresponding to (Fe 1.51 Ni 0.50 Co 0.03 ) 2.04 P 0.96 (three atoms per formula unit). Crystal structure: R1 = 0.040 for 138 unique observed (|F o |≥ 4σ F ) reflections. Orthorhombic, Pnma, unit-cell parameters refined from powder data: a = 5.748(2), b = 3.548(1), c = 6.661(2) Å, V = 135.8(1), Å 3 , Z = 4; unitcell parameters refined from single-crystal data: a = 5.792(7), b = 3.564(4), c = 6.691(8) Å, and V = 138.1(3) Å 3 . Strongest reflections in the X-ray powder diffraction pattern are [d in Å, (I) (hkl)]: 2.238(100)(112), 2.120(80)(211), 2.073(70)(103), 1.884(50)(013), 1.843(40)(301), 1.788(40)(113), 1.774(40)(020). The mineral is named for Alla Bogdanova, Geological Institute, Kola Science Centre of Russian Academy of Sciences

March 28, 2015

Cristobalite inclusions in the Tatahouine achondrite: Implications for shock conditions

Karim Benzerara, François Guyot, Jean Alix Barrat, Philippe Gillet, Aurice Lesourd

Page range: 1250-1256

Abstract

The mineralogy of the Tatahouine diogenite was investigated by optical microscopy, Raman micro-spectrometry, and scanning and transmission electron microscopies. Inclusions of α-cristobalite in orthopyroxenes, locally in symplectic association with chromites, or associated with metal, have been characterized for the first time in a diogenite. Mosaicism of the orthopyroxenes indicates shock effects in the meteorite. The shock history of the meteorite must be consistent with the presence of vein-like structures containing inclusions of well-crystallized cristobalite, a low-pressure, high-temperature phase. Several possible mechanisms to account for these observations are discussed. The simplest one, consistent with all observations, is that a shock event would have occurred in a hot orthopyroxenite, either before extensive cooling of the asteroid, or in materials heated by previous impacts and maintained hot under an ejecta blanket.

March 28, 2015

Letters. Natural occurrence of Fe₂SiO₄ -spinel in the shocked Umbarger L6 chondrite

Zhidong Xie, Naotaka Tomioka, Thomas G. Sharp

Page range: 1257-1260

Abstract

Here we report the first natural occurrence of Fe 2 SiO 4 -spinel in a shock-induced melt pocket of the Umbarger L6 chondrite. Optical microscopy, scanning electron microscopy, electron microprobe analysis, and analytical transmission electron microscopy were used to examine the sample. Fe 2 SiO 4 - spinel was identified by TEM using selected-area electron diffraction and energy-dispersive X-ray spectroscopy. The symmetry of the diffraction patterns, the ratios of d-spacings, and interplanar angles are consistent with the spinel structure. However, the cell parameter of Fe 2 SiO 4 -spinel (8.52 Å), calculated from d-spacing data, is 3.5% larger than that of synthetic Fe 2 SiO 4 -spinel (8.235 Å). Chemical analyses of the spinel show olivine stoichiometry with Fe/(Fe + Mg) ratios ranging from 0.62 to 0.99. Fe 2 SiO 4 -spinel and stishovite occur within FeO-SiO 2 -rich zones in the melt pocket, surrounded by SiO 2 -rich glass and Fe-rich phyllosilicates. Fe 2 SiO 4 -spinel plus stishovite also occur with other high-pressure minerals in the melt pocket: ringwoodite, akimotoite, augite, and hollanditestructured plagioclase. We infer that the Fe 2 SiO 4 -spinel crystallized from a zone of FeO-SiO 2 -rich melt within the shock-induced melt pocket. Two models for FeO-SiO 2 -rich melt are discussed: it was either a residual melt after crystallization of MgO-rich silicates in a chondritic melt pocket, or it was produced by shock melting of FeO-SiO 2 -rich material.

March 28, 2015

Structural refinements of magnesite at very high pressure

Guillaume Fiquet, François Guyot, Martin Kunz, Jan Matas, Denis Andrault, Michael Hanfland

Page range: 1261-1265

Abstract

Unit-cell parameters of magnesite were measured between ambient pressure and 80 GPa using angle dispersive powder X-ray diffraction. The isothermal bulk modulus determined from a third order Birch Murnaghan equation of state is K T = 108(3) GPa with K' T = 5.0(2), and V 0 = 279.2(2) Å 3 , in agreement with previously reported values. Combining this result with previous measurements, we show that magnesite with R3̄c structure is stable compared to the assemblage periclase + carbon dioxide at pressures and temperatures corresponding to the core-mantle boundary. Crystal structure refinements have also been carried out up to 80 GPa. The main structural change is a strong compression of the MgO 6 octahedra with increasing pressure, largely reflected in the anisotropic compression of the c axis. This compression, however, tends to level off at around 50-60 GPa. On the other hand, the CO 3 groups do not remain invariant since they undergo first a slight expansion and then a compression above the same threshold pressure of 60 GPa above which Mg-O bonds cannot compress further. Thus, in this structure-type, the energy gain due to a drastic volume reduction of the MgO 6 octahedron compensates in a given pressure range for the energy cost of the small expansion of the CO 3 carbonate unit.

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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 87 Issue 8-9

Issue of American Mineralogist

Evidence for magmatic vapor deposition of anhydrite prior to the 1991 climactic eruption of Mount Pinatubo, Philippines

Abstract

Experimental and modeled solubilities of chlorine in aluminosilicate melts, consequences of magma evolution, and implications for exsolution of hydrous chloride melt at Mt. Somma-Vesuvius

Abstract

Temperature-induced Al -zoning in hornblendes of the Fish Canyon magma, Colorado

Abstract

Analytical techniques for volatiles: A case study using intermediate (andesitic) glasses

Abstract

Anomalous optical properties of fibrous tremolite, actinolite, and ferro-actinolite

Abstract

Protoanthophyllite from three metamorphosed serpentinites

Abstract

Synthesis of beryllian sapphirine in the system MgO-BeO-Al2O3-SiO2-H2O and comparison with naturally occurring beryllian sapphirine and khmaralite. Part 1: Experiments, TEM, and XRD

Abstract

Intersite distribution of Fe2+ and Mg in the spinel (sensu stricto)–hercynite series by singlecrystal X-ray diffraction

Abstract

Structural relationships in (Mn1–xZnx)Mn2O4 (0 ≤ x ≤ 0.26): The “dragging effect” of the tetrahedron on the octahedron

Abstract

The grid-work texture of authigenic microcrystalline quartz in siliceous crust-type (SCT) mineralized horizons

Abstract

The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15– polyanion

Abstract

Submicrometer optical characterization of the grain boundary of optically active Cr3+ doped polycrystalline Al2O3 by near-field spectroscopy

Abstract

High-temperature, high-pressure optical spectroscopic study of ferric-iron-bearing tourmaline

Abstract

Infrared and Mössbauer study of Brazilian tourmalines from different geological environments

Abstract

Isothermal compression of staurolite: A single-crystal study

Abstract

Equation of state measurements of chlorite, pyrophyllite, and talc

Abstract

Structure change of Ca1–xSrxTiO3 perovskite with composition and pressure

Abstract

Mechanism of 2/1- to 3/2-mullite transformation at 1650 °C

Abstract

High-pressure deformation mechanism in scolecite: A combined computationalexperimental study

Abstract

Far infrared spectra of K+ in dioctahedral and trioctahedral mixed-layer minerals

Abstract

Ab initio quantum mechanical modeling of infrared vibrational frequencies of the OH group in dioctahedral phyllosilicates. Part I: Methods, results and comparison to experimental data

Abstract

Ab initio quantum mechanical modeling of infrared vibrational frequencies of the OH group in dioctahedral phyllosilicates. Part II: Main physical factors governing the OH vibrations

Abstract

On geological interpretations of crystal size distributions: Constant vs. proportionate growth

Abstract

Allabogdanite, (Fe,Ni)2P, a new mineral from the Onello meteorite: The occurrence and crystal structure

Abstract

Cristobalite inclusions in the Tatahouine achondrite: Implications for shock conditions

Abstract

Letters. Natural occurrence of Fe2SiO4 -spinel in the shocked Umbarger L6 chondrite

Abstract

Structural refinements of magnesite at very high pressure

Abstract

Synthesis of beryllian sapphirine in the system MgO-BeO-Al₂O₃-SiO₂-H₂O and comparison with naturally occurring beryllian sapphirine and khmaralite. Part 1: Experiments, TEM, and XRD

Intersite distribution of Fe²⁺ and Mg in the spinel (sensu stricto)–hercynite series by singlecrystal X-ray diffraction

Structural relationships in (Mn_1–xZn_x)Mn₂O₄ (0 ≤ x ≤ 0.26): The “dragging effect” of the tetrahedron on the octahedron

The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si₃B₃O₁₈)^15– polyanion

Submicrometer optical characterization of the grain boundary of optically active Cr³⁺ doped polycrystalline Al₂O₃ by near-field spectroscopy

Structure change of Ca_1–xSr_xTiO₃ perovskite with composition and pressure

Far infrared spectra of K⁺ in dioctahedral and trioctahedral mixed-layer minerals

Allabogdanite, (Fe,Ni)₂P, a new mineral from the Onello meteorite: The occurrence and crystal structure

Letters. Natural occurrence of Fe₂SiO₄ -spinel in the shocked Umbarger L6 chondrite