Carbon in silicate liquids: the systems NaAlSi3O8-CO2, CaAl2Si2O8-CO2, and KAlSi3O8-CO2

To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO₂ vapor. The depression of the temperature of the solidus of albite by CO₂ decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO₂ in NaAlSi₃O₈ liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO₂ lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si₃O₈ composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO₂ and CaO and depleted in Al₂O₃ relative to CaAl₂Si₂O₈.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO₂ component in the liquid ( ). We predict that this increases the ratio of molecular CO₂/CO ₃ ^2– in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO₂ in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi₃O₈ liquids, which is in contrast to that in CaAl₂Si₂O₈ liquid. Also, reaction of CO₂ with NaAlSi₃O₈ liquid to form CO ₃ ^2– that is complexed with Na⁺ must be accompanied by a change in Al³⁺ from network-former to network-modifier, as Na⁺ is no longer abailable to charge-balance Al³⁺ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO ₃ ^2– with the excess Ca²⁺ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO₂ increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO₂-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant relative to those of albite and anorthite, and CO₂ produces a nearly constant freezing-point depression of about 170° C. Because of the low at pressures above the singular point, we infer that most of the carbon dissolves as CO ₃ ^2– , resulting in a low CO₂/ CO ₃ ^2– , but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744     

Die Kristallstruktur des Voltaits,K2Fe 5 2+ Fe 3 3+ Al[SO4]12·18H2O

Zusammenfassung Die Kristallstruktur von künstlichem Voltait, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· ·18 H₂O, kubisch hexakisoktaedrisch,Fd3c–O _h ⁸,a ₀=27,254 ,Z-16, wurde mittels photographischer Röntgendaten bestimmt. Die Aufklärung der Struktur erfolgte mit Patterson- und Fouriermethoden unter Zuhilfenahme des multiplen isomorphen Ersatzes. Die Verfeinerung nach der Methode der kleinsten Quadrate ergab mit anisotropen Temperaturfaktoren für 726 beobachteteF _hkl R=0,033. Das Hauptmerkmal der Struktur ist ein 3dimensionales Gerüst aus [Fe³⁺O₆]-Oktaedern, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-Oktaedern und [K⁺O₁₂]-Polyedern, die durch SO₄-Tetraeder verknüpft werden. Hohlräume dieses Gerüstes werden von ungeordnet orientierten [Al(H₂O)₆]-Oktaedern eingenommen. Es wird gezeigt, daß Al als wesentlicher Bestandteil dieses Voltaits angesehen werden muß.

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The crystal structure of voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂·18H₂O

Summary The crystal structure of synthetic voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· · 18 H₂O, cubic hexakis-octahedral, space groupFd3c–O _h ⁸,a ₀=27.254 ,Z=16, was determined from photographic X-ray data. The structure was solved by Patterson and Fourier-methods with the aid of multiple isomorphic substitution. Least squares refinement with anisotropic temperature factors resulted inR=0.033 for 726 observedF _hkl . The dominant structural feature is a continous framework composed of [Fe³⁺O₆]-octahedra, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-octahedra and [K⁺O₁₂]-polyhedra linked by SO₄-tetrahedra. The arrangement gives rise to cages occupied by disordered [Al(H₂O)₆]-octahedra. It is shown that Al must be considered to be a essential constituent of such voltaites.

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The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

Solubility and solution mechanisms of H₂O in depolymerized melts in the system Na₂O-Al₂O₃-SiO₂ were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na₂Si₄O₉-Na₂(NaAl)₄O₉] with Al/(Al+Si) = 0.00-0.25. The H₂O solubility was fitted to the expression, X_H2O=0.20+0.0020f_H2O-0.7X_Al+0.9(X_Al)², where X_H2O is the mole fraction of H₂O (calculated with O = 1), f_H2O the fugacity of H₂O, and X_Al = Al/(Al+Si). Partial molar volume of H₂O in the melts, , calculated from the H₂O-solulbility data assuming ideal mixing of melt-H₂O solutions, is 12.5 cm³/mol for Al-free melts and decreases linearly to 8.9 cm³/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H₂O, , increases with Al/(Al+Si).Solution mechanisms of H₂O were obtained by combining Raman and ²⁹Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H₂O content. The rate of NBO/T-change with H₂O is negatively correlated with H₂O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q⁴-species to form Q² species. Steric hindrance appears to restrict bonding of H⁺ with nonbridging oxygen in Q³ species. The presence of Al³⁺ does not affect the water solution mechanisms significantly.     

Sigioite : The oxidation mechanism in [M 2 3 + (PO4)2(OH)2(H2O)2]2 - structures

Summary The crystal structure of sigloite, Fe³ [(H₂O)₃OH] [Al₂(PO₄)₂(OH)₂(H₂O)₂]- 2 H₂O, triclinic, a 5.190 (2), b 10.419 (4), c 7.033 (3) Å, 105.00 (3), 111.31(3), 70.87 (3)°, V 330.5 (2) Å3, Z = 1, space group P , has been refined to anR index of 5.3% using 1713 observed (I > 2.5 1) reflections collected with graphite-monochromated MoK X-rays. Sigloite is isostructural with the laueite-group minerals. Corner-linked [A1₅] chains (: unspecified ligand) are cross-linked by (PO₄) tetrahedra to form a mixed corner-linked tetrahedral-octahedral sheet of composition [A1₂(PO₄)₂(OH)₂(H₂O)₂]²-. These sheets are linked by (Fe³⁺O₂(OH, H₂O)₄) octahedra and two (H₂O) groups that participate in a hydrogen-bonding network. Sigloite is the oxidized equivalent of paravauxite, Fe²⁺(H₂O)4[Al₂(PO₄)₂(OH)₂(H₂O)₂]-² H₂O, and detailed comparison of the two structures shows that the oxidation mechanism involves loss of hydrogen from one of the (H₂O) groups coordinating the Fe³⁺, and positional disorder of both the Fe³⁺ and (OH) and (H₂O) ligands.

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Siggloit: Der Oxidationsmechanismus in (M ₂ ³ ₊ (PO₄)₂(OH)₂(H2O)₂]²- Strukturen

Zusammenfassung Die Kristallstruktur von Sigloit, Fe³⁺ [(H₂O)₃OH] [Al₂(PO₄)₂(OH)₂(H₂O)₂].2 H₂O, triklin, a 5,190 (2), b 10,419 (4), c 7,033 (3) Å, 105,00 (3), 111,31 (3), 70,87 (3)°, V 330,5 (2) ų,Z = 1, Raumgruppe P , wurdefür 1713 beobachtete Reflexe (I > 2,5 I), die mit MoKa-Röntgenstrahlung (Graphit-Monochromator) gesammelt wurden, auf einen R-Wert von 5,3% verfeinert. Sigloit ist isotyp mit den Mineralen deer Laueit-Gruppe. Über Ecken verknüpfte [A1₅]-Ketten (: nicht spezifizierter Ligand) werden über (P0₄)-Tetraeder zu ebenfalls über Ecken verknüpfte Tetraeder-OktaederSchichten der Zusammensetzung [A1₂(PO₄)₂(OH)₂(H₂O)₂]²- verbunden. Diese Schichten werden über (Fe³⁺O₂(OH, H2O)₄)-Oktaeder und zwei (H₂O)-Gruppen, die amWasserstoffbrücken-Netzwerk beteiligt sind, verbunden. Sigloit ist das oxidierte Analogon zu Paravauxit, Fe²⁺(H₂O)₄[A1₂(PO₄)₂(OH)₂(H₂O)₂] - 2 H₂O; ein detaillierter Vergleich dieser beiden Strukturen zeigt, daß der Oxidationsmechanismus sowohl den Verlust eines Wasserstoffatoms (H₂O)-Gruppe, welche ein Fe³⁺-Atom koordiniert, als auch eine Fehlordnung der Punktlagen von Fe³⁺ und von den (OH) und (H₂O) Liganden bedingt.

     

Hydrothermalsynthese und Strukturtyp des Berylliumborates ∞ 2 Be2BO3OH.H2O

Zusammenfassung Hydrothermalsynthese von Be₂BO₃OH.H₂O und Strukturbestimmung der Verbindung nach der trial and error. Methode aus Pulveraufnahmen wurden durchgeführt. Be₂BO₃OH.H₂O krístallisiert in der Raumgruppe P321-D ₃ ² a₀=4,43 Å, c₀=5,34 Å, und repräsentiert einen neuen Strukturtyp mit ausgeprägtem Schichtcharakter. Bor ist von drei Sauerstoffen koordiniert, Beryllium nahezu tetraedrisch von drei Sauerstoffen und einem (OH, H₂O). Zwischen letzteren sind Wasserstoffbrücken von 2,72Å Länge ausgebildet; diese bewirken die Bindung zwischen den Schichten.

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Summary Be₂BO₃OH.H₂O was synthesized under hydrothermal conditions, and its crystal structure was determined from powder data by the trial and error-method. The compound crystallizes in space group P 321-D ₃ ² a₀=4,43 Å, c₀=5,34 Å, and represents a new structure type with pronounced layers. Boron is coordinated by three oxygens, beryllium almost tetrahedrally by three oxygens and one (OH, H₂O). Between the latters, there are hydrogen bridges of 2,72 Å length which cause the chemical bonds between the layers.

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A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.     

Garnet-pyroxene equilibria in the system CaO-MgO-Al2O3-SiO2 (CMAS): prospects for simplified (‘T-independent’) lherzolite barometry and an eclogite-barometer

New experimental data on compositions of garnets in two-pyroxene — garnet assemblages in the system CaO –MgO –Al₂O₃ –SiO₂ (CMAS) are presented for conditions between 1,100 and 1,570° C and 30 to 50 kb. Garnets in these assemblages become less calcic with increasing pressure. Garnet-orthopyroxene barometry (Al-solubility-barometry) pertinent to geobarometry for garnet lherzolites has been evaluated with a set of experimental data covering the range 900 to 1,570° C and 15 to 100 kb. Various formulations of this barometer work well to 75 kb. Phase equilibria are not sufficient to positively verify the thermodynamic validity of any of such models. Empirical garnet-orthopyroxene barometry at least in the system CMAS can be formulated to obtain a pressure estimate without previous temperature estimation (P(kb)=34.4-19.175 1n X _Al ^M1 +17.702 1n X _Ca ^M2 ). The potential application of an analogous garnetclinopyroxene equilibrium is limited because the amount of Ca-Tschermaks in natural clinopyroxenes is usually quite small in garnet lherzolites and many eclogites. The Ca-Mg exchange between garnet and clinopyroxene appears however sufficiently sensitive to pressure to allow calibration of a CMAS barometer. The reaction 3CaMgSi₂O₆+Mg₃Al₂Si₃O₁₂=3Mg₂Si₂O₆+Ca₃Al₂Si₃O₁₂ has a V ^o of 3.5 cm³. The total pressure dependency of this reaction is however closer to a theoretical V ^o of about 5 cm³ when excess volume properties of the phases involved are taken into account. We have calibrated such a barometer (mean error of estimate 2.8 kb) for assemblages with pyrope-rich (py>80) garnets and orthopyroxenes. This may provide the basis for a geobarometer for eclogites from kimberlites.Abbreviations Used in the Text CaTs Ca-tschermak's molecule, CaAl₂SiO₆ - cpx clinopyroxene - di diopside, CaMgSi₂O₆ - en enstatite, Mg₂Si₂O₆ - gr grossular, Ca₃Al₂Si₃O₁₂ - gt garnet - MgTs Mg-Tschermak's molecule, MgAl₂SiO₆ - opx orthopyroxene - px pyroxene - py pyrope, Mg₃Al₂Si₃O₁₂ - a _i ^j activity of component i in phase j - activity coefficient - G(I) molar Gibbs free energy difference of reaction (I) at standard state unless specified otherwise - H(I), (H _I) molar enthalpy (difference) of phase (reaction) (I) at standard state unless specified otherwise - S (I), (S _I) molar entropy (difference) of phase (reaction) (I) at standard state unless specified otherwise - V ^o, (V _I ^o) molar volume (difference) of phase (reaction) (I) at standard state - X _i ^j mole fraction of component i in phase j     

High-pressure,high-temperature stability of surinamite in the system MgO-BeO-Al2O3-SiO2-H2O

The MgAl surinamite end member, (Mg₃Al₃)^[6]O[AlBeSi₃O₁₅], was synthesized in the requisite system with and without water. The new phase is monoclinic, space group P2/n, with a=9.881(1)Å; b=11.311(1) Å; c=9.593(1) Å; =109.52(2)°. Refractive indices are n _x=1.7015(20); n _y=1.7035(20); n _z=1.7055(20). The infrared spectrum shows characteristic differences against the structurally related and optically extremely similar phase sapphirine.Using the seeding technique, the preliminary stability field for MgAl surinamite was found to lie at high temperatures (650 °C) and high pressures (4 kbar). At lower temperatures breakdown takes place to hydrous assemblages of chlorite, talc, and chrysoberyl with kyanite or yoderite; at lower pressures chrysoberyl forms parageneses with sapphirine and cordierite. In crystal chemical terms the underlying principle for the stability of surinamite versus that of the low-pressure assemblages is the higher proportion of octahedrally coordinated Al in surinamite (75%). Following the same principle surinamite itself decomposes at still higher pressures to a paragenesis, in which all Al enters octahedral coordination (pyrope+a chrysoberyl-type phase and some unidentified X-ray peaks).The stability field of synthetic MgAl surinamite is in good agreement with P, T-estimates of some 8–12 kbar, 800°–950° C as taken from the literature for the few occurrences of natural, Fe-bearing surinamite in granulite and upper amphibolite facies environments. The incorporation of iron in surinamite must be limited, because this mineral is known to coexist with its more iron-rich breakdown assemblage almandine-rich garnet+chrysoberyl. As the minimum melting curve of granite under hydrous conditions lies outside the surinamite field up to a water pressure of about 20 kbar, the absence of surinamite in normal granitic pegmatites can already be explained by physical constraints. However, there are probably also chemical constraints in the generally high Fe/Mg bulk chemistry of the pegmatite environments.Now at Institut für Kristallographie, Technische Hochschule, Templergraben 55, D-5100 Aachen, FRG     

Potassic cordierites: characteristic minerals for high-temperature,very low-pressure environments

The mineral chemistry of cordierites from three different sanidinite facies localities-1) volcanic xenoliths from the Eifel, Germany; 1) buchites of the Blaue Kuppe, Germany; 3) paralavas from the Bokaro coalfield, India-is characterized by unusually high potassium contents up to 1.71 wt%, equivalent to 0.22 K atoms per formula unit (p.f.u.) based on 18 oxygens. Significantly, these cordierites are either hexagonal highcordierites (indialites) with =0 or exhibit intermediate -values 0<<0.20 relative to well Al,Si-ordered orthorhombic low-cordierite. Based on microprobe analyses, the predominant substitutional mechanism for alkali incorporation is Alk^[Channel]+Al^[4] for +Si^[4], thus leading to Al/Si-ratios deviating considerably from the value 4:5 in ideal cordierite M₂[Al₄Si₅O₁₈]. The most highly substituted cordierite from Blaue Kuppe is about (K_0.22Na_0.07)^[Ch](Mg_1.33Fe _0.66 ²⁺ )^[6][Al_4.16Si_4.79O₁₈]. Bokaro cordierites are further characterized by obvious (Al+Si)-deficiencies against the ideal value of 9.0 p.f.u., a tendency of which is apparent in most Blaue Kuppe analyses as well. As the tetrahedral deficiencies are often equivalent to excess cations in the octahedra, we assume that ferric iron fills up the remaining tetrahedral sites, again linked with the introduction of potassium according to K+Fe³⁺ for +Si. In comparison with the available experimental data, these natural potassic cordierites are considered stable high-temperature phases regarding their compositions, but not their structural states. Although the substitution KAl for Si in Mg-cordierite is known to lower the maximum -value to be attained, the hexagonal nature of the cordierites must be due to very rapid crystallization and subsequent quenching. The higher -values of the Blaue Kuppe cordierites might be caused by their topotactic origin from preexisting biotite. The complicated twin and domain patterns of the hexagonal Eifel and Bokaro cordierites as observed in thin section could perhaps be attributed to structural modulations as postulated recently for hexagonal cordierite shortly after its growth.     

Synthesis and stability of deerite,Fe 12 2+ Fe 6 3+ [Si12O40](OH)10, and Fe3+⇋Al3+ substitutions at 15–28 kb

Deerite, Fe ₁₂ ²⁺ Fe ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀, thus far known from ten localities in glaucophane schist terranes, was synthesized at water pressures of 20–25 kb and temperatures of 550–600 °C under the of the Ni/NiO buffer. The X-ray powder diagram, lattice constants and infrared spectrum of the synthetic phase are closely similar to those of the natural mineral. A solid solution series extends from this ferri-deerite end member to some 20 mole % of a hypothetical alumino-deerite, Fe ₁₂ ²⁺ Al ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀. The upper temperature breakdown of ferri-deerite to the assemblage ferrosilite +magnetite+quartz+water occurs at about 490 °C at 15 kb, and 610 °C at 25 kb fluid pressure for the of the Ni/NiO buffer. Extrapolation of these data to lower water pressures indicates that deerite can be a stable mineral only in very low-temperature, high-pressure environments.     

The crystal structure of curite, [Pb6.56(H2O,OH)4] [(UO2)8O8(OH)6]2

Summary The crystal structure of curite, of which until now only the arrangement of the U and Pb atoms was known, has been redetermined with a synthetic crystal using three-dimensional X-ray techniques.R=0.043 for 1270 observed reflections. Curite is orthorhombic, space groupPnam-D _2h ¹⁶ ,a=12.513,b=13.002,c=8.373 ,Z=6.56 PbO·16UO₃·9.44H₂O. The structure consists of a novel type of washboard like puckered layers ² [(UO₂)₈O₈ (OH)₆]^6– formed by tetragonal bipyramidal [(UO₂)O₃(OH)] and pentagonal bipyramidal [(UO₂)O₃ (OH)₂] polyhedra. The layers are parallel to {100} and are directly connected by hydrogen bonds. Lead atoms and oxygen atoms (H₂O+OH) are located in folds between the layers, helping to connect them. The interlayer atomic positions are slightly disordered and one of them is partially occupied. The variable concentrations of the interlayer atoms are responsible for the changes in chemical composition.The structural formula [Pb_8–x (OH)_4–2x (H₂O)_2x ] [(UO₂)₈(OH)₆]₂ is suggested for curite;x=1.44 for the investigated synthetic curite. Within the three different U–O polyhedra the axial U–O distances are 1.81–1.88 , the equatorial 2.14–2.57 . The two different Pb atoms have ionic coordinations, each by ten oxygens with Pb–O distances of 2.46–3.32 , on the average 2.82 .

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Die Kristallstruktur von Curit, [Pb _6,56 (H ₂ O, OH) ₄ ] [(UO ₂)₈ O ₈(OH)₆]₂

Zusammenfassung Die Kristallstruktur von Curit, von der bisher nur die Lagen der Uran- und Bleiatome bekannt waren, wurde anhand eines künstlichen Kristalls mit dreidimensionalen Röntgendaten neu bearbeitet und für 1270 Reflexe aufR=0,043 verfeinert. Curit kristallisiert rhombisch, RaumgruppePnam-D _2h ¹⁶ ,a=12,513,b=13,002,c=8,373 ,Z=6,56 PbO·16 UO₃·9,44 H₂O. Die Struktur enthält gewellte Schichten eines neuen Typs, ² [(UO₂)₈O₈(OH)₆]^6–, die sich aus tetragonal bipyramidalen [(UO₂)O₃(OH)]- und pentagonal-bipyramidalen [(UO₂)O₃(OH)₂]-Polyedern zusammensetzen. Die Schichten verlaufen parallel {100} und sind über Wasserstoffbrücken miteinander unmittelbar verknüpft. Zwischen den Schichten befinden sich Bleiatome und zusätzliche Sauerstoffatome (H₂O+OH). Diese Atome weisen zum Teil Fehlordnung auf; eine der beiden Pb-Lagen ist nur partiell besetzt. Für Schwankungen in der chemischen Zusammensetzung von Curit ist der unterschiedliche Gehalt an Zwischenschichtatomen verantwortlich. Aufgrund dieser Untersuchung wird die Strukturformel [Pb_8–x (OH)_4–2x (H₂O)_2x ] [(UO₂)₈O₈(OH)₆]₂ vorgeschlagen; für den untersuchten Curit istx=1,44. Die drei verschiedenen U–O-Polyeder der Struktur besitzen axiale bzw. äquatoriale U–O-Abstände von 1,81–1,88 bzw. 2,14–2,57 . Die zwei Arten von Bleiatomen besitzen eine ionische Koordination; beide sind von 10 Sauerstoffatomen in Abständen von 2,46–3,32 (Mittelwert 2,82 ) umgeben.

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With 3 Figures     

The restricted stability of osumilite under hydrous conditions in the system K2O-MgO–Al2O3–SiO2–H2O

Osumilite_ss was synthesized as a single phase product in the model system K₂O-MgO-Al₂O₃-SiO₂ at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with 0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg₂(Al_3-xMg_x) (Al_2–xSi_10+x)O₃₀ with 0x0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0x0.6. Reversed stability experiments for the synthetic osumilite KMg₂(Al_2.75Mg_0.25)(Al_1.75Si_10.25)O₃₀ determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H₂O vapor break down to cordierite+K feldspar+phlogopite_ss+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for dry conditions in rocks formed at total pressures higher than about 0.8 Kbar.     

Ellenbergerite,a new high-pressure Mg-Al-(Ti,Zr)-silicate with a novel structure based on face-sharing octahedra

Ellenbergerite occurs as purple millimetre-size grains associated with talc, kyanite, clinochlore, rutile, and zircon in composite inclusions within decimetre-large pyrope crystals (90–98 mole percent end-member) in the quartzite layer of the Dora Maira massif, Western Alps, from which coesite has been recently reported (Chopin 1984). It is hexagonal, a=12.255(8), c=4.932(4) Å, Z=1, space group P6₃. Mohs hardness 6.5; D_mes 3.15, D_cal 3.10; no cleavage. Uniaxial negative and vividly pleochroic, colourless, colourless to deep lilac with colour zoning. The intensely coloured variety has 1.6789(5), 1.670(1); microprobe analysis yields SiO₂ 39.1, P₂O₅ 0.45, Al₂O₃ 25.1, TiO₂ 4.0, MgO 22.2, FeO 0.20, sum 99.05 wt.% including H₂O 8.0 (coulometrically). The formula calculated on a O₂₈(OH)₁₀ basis (implying 7.5 wt.% H₂O) is Mg_6.71 Fe_0.03 Ti_0.61 Al_6.00 Si_7.92 P_0.08 O₂₈(OH)₁₀ The colour zoning is due to nearly complete TiZr substitution. In addition ellenbergerite may contain more than 8 wt.% P₂O₅ with strictly correlated changes of Si, Mg, Al and Ti+Zr contents, over 80% of which represent the SiAlPMg substitution.The structure has been determined from 1049 observed independent reflections and refined to R=0.034, R_w=0.031, including six of ten protons. It consists of single chains of face-sharing octahedra with one third vacancies extending along the six-fold screw axes, and of pairs of fully occupied face-sharing octahedra linked by edge-sharing to form octahedral double chains parallel to the twofold screw axes, all interconnected by SiO₄ tetrahedra. It may be compared with the dumortierite polymorph with space group P6₃mc derived hypothetically by Moore and Araki (1978). The structural formula is (Mg,Ti,Zr,)₂ Mg₆(Al,Mg)₆ (Si,P)₂ Si₆ O₂₈(OH)₁₀ Face-sharing octahedra are an unusual feature in silicates which results in a dense structure and reflects, considering the common bulk composition, the uncommon high-pressure formation conditions (about 25–30 kbar, 700–800° C). Ti⁴⁺-Fe²⁺ charge transfer between face-sharing octahedra on the six-fold screw axes most likely accounts for the absorption scheme.     

Succession de subfacies métamorphiques en Vanoise méridionale (Savoie)

The Southern Vanoise is localized in the internal part of the Western Alps, in the Briançonnais zone. In Vanoise the following units can be distinguished (Fig. 1): a pre-hercynian basement (micaschists, glaucophanites, basic rocks), a permian cover (micaschists) and a mesozoic-paleocene cover (carbonate rocks). This area has been affected by the alpine metamorphic event characterized here by high and intermediate pressure facies. The rocks paragenesis are often unbalanced.The paleozoic rocks (Table 1) contain mainly: quartz, albite, paragonite, phengite, blue amphibole, chlorite, green biotite, garnet (Table 2). These minerals were analysed by an electron microprobe (Tables 3, 4 and 5). Mineral composition is highly variable: glaucophane is zoned (Table 5), white micas are more or less substituted with phengite (3.22O₃/FeO + MgO)<0.53] whereas the Al rich chlorites [(Al₂O₃/FeO + MgO)>0.6] are associated with the less substituted white micas (Si=3.2) (Tables 3 and 4). The phengites with a Si content 3.2 occur in rocks where the retromorphic evolution is the most pronounced and penetrative. A metamorphic evolution is characterized by the disappearance of glaucophane which corresponds to the appearance of Al rich chlorite and to the decrease of phengitic substitution.The samples analysis are plotted in the tetraedric diagram: K₂O-Al₂O₃-Na₂O, Al₂O₃-FeO, MgO, on which a special mathematical treatment was applied. This method calculates the location of rocks composition in the four minerals space. This location is internal when the per cent amounts of all four relevant minerals are positive, if any of them is negative, the point is external (Tables 6–9).In Southern Vanoise micaschists, 2 subfacies are successively present (Fig. 3):Subfacies I: glaucophane-chlorite-phengite (Si⁴⁺ 3.5)-paragonite. Then subfacies II: chlorite-albite-phengite (Si⁴⁺ 3.2)-paragonite.In basic rocks is found essentially: Subfacies III: glaucophane-garnet-phengite-paragonite or IV: glaucophane-garnet-phengite-albite. Then subfacies V: green biotite-chlorite-albite-paragonite.The assemblages I and II proceed through reaction: 2 glaucophane +1 paragonite+2 H₂O4.2 albite + 1 chlorite.The assemblage V appears with reactions: 1.8 glaucophane +2 phengite0.4 chlorite+2 green biotite + 3.6 albite +0.4 H₂O or 2 glaucophane +2 phengite +0.5 garnet+ 6 H₂O2 green biotite +1 chlorite+4 albiteThese reactions are controlled by hydratation: the composition variation of phengite and associated chlorite during the metamorphic evolution determines the stability of some minerals (particularly the glaucophane in Na₂O poor rocks).In same rocks the results of mathematical treatment is not consistent with the data (Tables 2, 6–9). This discrepancy corresponds to a desequilibrium between chlorite and phengite.These results imply a continuous metamorphic evolution between two stages (Fig. 6): a first stage (1) at 8 kb, 350 ° C; a second stage (2) at 2 to 3 kb, 400–450 ° C.     

Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer

The Al-in-hornblende barometer, which correlates Al^tot content of magmatic hornblende linearly with crystallization pressure of intrusion (Hammarstrom and Zen 1986), has been calibrated experimentally under water-saturated conditions at pressures of 2.5–13 kbar and temperatures of 700–655°C. Equilibration of the assemblage hornlende-biotite-plagioclase-orthoclasequartz-sphene-Fe-Ti-oxide-melt-vapor from a natural tonalite 15–20° above its wet solidus results in hornblende compositions which can be fit by the equation: P(±0.6 kbar) = –3.01 + 4.76 Al _hbl ^tot r ²=0.99, where Al^tot is the total Al content of hornblende in atoms per formula unit (apfu). Al^tot increase with pressure can be ascribed mainly to a tschermak-exchange ( ) accompanied by minor plagioclase-substitution ( ). This experimental calibration agrees well with empirical field calibrations, wherein pressures are estimated by contact-aureole barometry, confirming that contact-aureole pressures and pressures calculated by the Al-in-hornblende barometer are essentially identical. This calibration is also consistent with the previous experimental calibration by Johnson and Rutherford (1989b) which was accomplished at higher temperatures, stabilizing the required buffer assemblage by use of mixed H₂O-CO₂ fluids. The latter calibration yields higher Al^tot content in hornblendes at corresponding pressures, this can be ascribed to increased edenite-exchange ( ) at elevated temperatures. The comparison of both experimental calibrations shows the important influence of the fluid composition, which affects the solidus temperature, on equilibration of hornblende in the buffering phase assemblage.     

Die Kristallstruktur des Johannits,Cu(UO2)2(OH)2(SO4)2·8H2O

Zusammenfassung Die Kristallstruktur des Johannits wurde anhand eines verzwillingten Kristalls von Joachimsthal, Böhmen, mit dreidimensionalen Röntgendaten bestimmt und für 2005 unabhängige Reflexe aufR=0,039 verfeinert. Johannit kristallisiert triklin, RaumgruppeP1, mita=8,903 (2),b=9,499 (2),c=6,812 (2) Å, =109,87 (1) =112,01 (1), =100,40 (1)° undV=469,9 ų. Chemische Formel und Zellinhalt lauten Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O, das ist um zwei H₂O-Moleküle mehr als bisher angenommen. In der Struktur sind pentagonal dipyramidale (UO₂)(OH)₂O₃-Polyeder paarweise über eine von zwei OH-Gruppen gebildete Kante zu Doppelpolyedern und diese wiederum durch SO₄-Gruppen zu (UO₂)₂(OH)₂(SO₄)₂-Schichten parallel (100) verknüpft. Die Schichten sind parallel über gestreckte Cu(H₂O)₄O₂-Oktaeder und Wassermoleküle miteinander verbunden. Folgende Bindungslängen wurden gefunden: U–O=1,78 Å (2x) und 2,34–2,39 Å (5x); Cu–O=1,97 Å (4x) und 2,40 Å (2x); =1,47 Å; O–O in Wasserstoffbrücken 2,71–2,91 Å (8x) und 3,30 Å.

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The crystal structure of johannite, Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O

Summary The crystal structure of johannite has been determined from threedimensional X-ray data measured on a twinned crystal from Joachimsthal, Böhmen, and has been refined toR=0.039 for 2005 independent reflections. Johannite crystallizes triclinic, space groupP1, witha=8.903 (2),b=9.499 (2),c=6.812 (2) Å, =109.87(1), =112.01(1), =100.40 (1)° andV=469.9 ų. Chemical formula and cell content are Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O, by two H₂O molecules more than previously assumed. Pairs of pentagonal dipyramidal (UO₂) (OH)₂O₃ polyhedra form double polyhedra by edgesharing via two OH groups. The double polyhedra are linked by the SO₄ tetrahedra to form layers (UO₂)₂(OH)₂(SO₄)₂ parallel zu (100). These layers are interconnected parallel toa by elongated Cu(H₂O)₄O₂ octahedra and water molecules. Following bond lengths have been observed: U–O=1.78 Å (2x) and 2.34–2.39 Å (5x); Cu–O=1.97 Å (4x) and 2.40 Å (2x); =1.47 Å; O–O for hydrogen bonds 2.71–2.91 Å (8x) and 3.30 Å.

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Mit 2 Abbildungen     

Structure and properties of fluorine-bearing aluminosilicate melts: the system Na2O-Al2O3-SiO2-F at 1 atm

The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO₂-SiO₂] has been studied with Raman spectroscopy. Fluorine is dissolved to form F^– ions that are electrically neutralized with Na⁺ or Al³⁺. There is no evidence for association of fluorine with either Si⁴⁺ or Al³⁺ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si⁴⁺ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na⁺) reacting to form fluoride complexes. In this process an equivalent amount of Na⁺ (orginally required for Al-³⁺charge-balance) or Al³⁺ (originally required Na⁺ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.     

Relationships among Gibbs free energies and enthalpies of formation of phosphates,oxides and aqueous ions

Two parameters _GO^2– and _HO^2– are defined as the differences between respectively the Gibbs free energies and the enthalpies of formation of an oxide and its corresponding aqueous cation. The Gibbs free energies and enthalpies of formation of phosphates from their consituent oxides are shown to be linear functions of respectively _GO^2– and _HO^2– of their constituent cations.     

Diamondiferous peridotite xenoliths from the Argyle (AK1) lamproite pipe,Western Australia

This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al₂O₃ and CaO < 1%, Na₂O0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ^91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr₂O₃ (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ^92–94 ) have comparatively high Cr₂O₃ (0.2%–0.45%) and Na₂O (up to 0.18%) but very low Al₂O₃ contents (0.5%–0.7%). Diopsides (Mg ^92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr₂O₃) and have low Al₂O₃ (0.7%–2.2%) and Na₂O (0.5%–1.6%) contents. Many have high K₂O contents, typically 0.1%–0.4% but up to 1.3% K₂O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe²⁺)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr₂O₃, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr₂O₃) titanate resembling armalcolite but containing significant K₂O (1%–2.5%), CaO (0.6%–2.2%), ZrO₂ (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f _O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly ¹³C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (2 Ga) enrichment in incompatible elements.     

Phase relations in the system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-SiO2 at 1 kilobar water vapour pressure

Crystal-melt relations at a water vapour pressure of 1 kilobar have been determined for planes at 3, 5, 7.5, and 10 weight per cent anorthite in the system NaAlSi₃O₈KAlSi₃O₈-CaAl₂Si₂O₈-SiO₂. The ratio of the silicate components in the liquids which are in univariant equilibrium with plagioclase, alkali feldspar, quartz and gas are Ab₃₁Or₂₈Q₃₈An₃ (weight per cent) at 730°±5–10° C, Ab₂₁Or₃₄Q₄₀An₅ at 745°±5–10° C and Ab₁₀Or₃₉ Q_43.5An_7.5 at 780°±10° C. The univariant curve on which the above compositions lieoriginates on the H₂O-saturated Or-An-Q plane at a composition containing less than 10 weight per cent An and terminates within 1.5 weight per cent An of the H₂O-saturated Or-Ab-Q plane. Experimental data for the synthetic system have been used to illustrate a discussion on the partial melting of metasediments and the possible significance of such a process with respect to the genesis of granitic rocks. Data taken from the literature (Winkler and v. Platen, 1960, 1961a) have been used to illustrate that the normative salic composition of a sediment has a strong influence on the composition of any melt which form when such a rock is subjected to high temperatures and pressures.