Lithium in NaBe-cordierites from El Peñón,Sierra de Córdoba,Argentina

NaBe-cordierites (Fe/(Fe+Mn+Mg)=0.49–0.57) with BeO contents up to 1.16 weight % and additional Li₂O contents up to 0.21 wt.% occur in cordierite-apatite-uraninite-muscovite-biotite-chlorite-feldspar-quartz nodules within pegmatites penetrating gneissic roof pendants of a lower Palaeozoic granite batholith. Occasional small crystals of beryl are interpreted to coexist stably with unaltered cordierite. Be and Li are incorporated in cordieriteaccording to the substitutions Na^[Channel] + Be^[4] Al^[4] and Na^[Ch]+Li_[6]R^2+[6], respectively. The coexisting phyllosilicates do apparently not contain appreciable amounts of Li. According to powder IR-data, the analyzed water contents of the cordierites are dominantly of type II, and there is also little CO₂. Their distortion indices are rather low (0.121–0.145) and so are their optic angles (2V=50-51_°). Considering all eleven NaBeLi-cordierites known thus far there is a strong positive correlation between Na and (Be+Li) with a slope close to 1.0. However, there is virtually no correlation between Be and Li, their incorporation into cordierite depending on the local geochemical environment. A strong negative correlation exists between the distortion indices of the NaBeLi-cordierites and their Be contents. Li has a disturbing influence on this relationship, and the versus Na correlation is also statistically worse than versus Be.Deceased December 20, 1984     

APL computer programs for thermodynamic calculations of equilibria in P-T-X CO 2 space

APL computer programs for the thermodynamic calculation of devolatilization and solid-solid equilibria operate using stored values for the molar volume and entropy of solids, the free energies of H₂O and CO₂, and the free energies of formation for 110 geologically-important phases. P-T-X _{CO 2} calculations of devolatilization equilibria can be made at pressures from 0.2 through 10 kb, and temperatures from 200 through 1,000° C. P-T-X calculations of solid-solid equilibria may be accomplished at pressures to 30 kb and temperatures to 1,000° C. Calculations can be extrapolations from experimental points, or direct calculations from thermochemical data alone. Options are available in these programs to consider effects of: real vs. ideal gas mixing, thermal expansion and compressibility, solid solution, fluid pressure differing from solid pressure, and uncertainties in high-temperature entropies.A collection of thermodynamic data programs accompanies the programs for calculating P-T-X _{CO 2} equilibria. Over a wide range of physical conditions, the data functions report free energies, entropies, fugacities of H₂O and CO₂, high temperature entropies of solids, and activities of components in H₂O-CO₂ mixtures.List of Symbols Activity of H₂O and CO₂ - G_f Free energy of formation of a phase from elements - G_r Free energy change of reaction - G _r ^o Standard state free energy change of a reaction - Free energies of pure H₂O and CO₂ - H _r ^o Standard state enthalpy change for a reaction - K Equilibrium constant - R Gas constant - S _r ^o Standard state entropy change of reaction - S _s ^o Standard state entropy change of solids in a reaction - V_s ^o Standard state volume change of a reaction - V_s ^o Standard state volume change of solids in a reaction - Mole fraction of H₂O and CO₂ - Activity coefficient of H₂O and CO₂     

Rooseveltit von San Francisco de los Andes und Cerro Negro de la Aguadita,San Juan,Argentinien

Zusammenfassung Rooseveltit findet sich in der Oxidationszone der Lagerstätten San Francisco de los Andes und Cerro Negro de la Aguadita, in der Provinz San Juan, Argentinien, auf 30°22 S und 69°33 W. Er bildet sehr feinkörnige, weiß-graue, nach Bismuthinit pseudomorphe Aggregate. Die Brechungsindizes liegen zwischenn=2,10 und 2,30. Die Vickershärte beträgt 513 (4–5 der Mohs'schen Härteskala). Mittels Elektronenmikrosonde wurde folgende chemische Zusammensetzung bestimmt: As=21,5±1%, Bi=60,9±2%. Rooseveltit ist monoklin mita ₀=6,878(1)Å, b₀=7,163(1) Å, c₀=6,735(1) Å, =104° 46±1, Z=4, _calc.=6,94 g·cm^–3, RaumgruppeP 2₁/n.Rooseveltit wurde nach drei verschiedenen Methoden synthetisiert. Die Pulverdiagramme der synthetischen Produkte stimmen mit dem des Minerals überein. Die Brechungsindizes wurden mitn =2,13(2) bzw. n=2,25(2) und die Dichte mit _obs.=7,01 g·cm^–3 bestimmt. Zellparameter: a₀-6,882(1) Å, b₀=7,164(1) Å, c₀=6,734(1) Å, =104° 50,5±0,7, _calc.=6,94 g·cm^–3. Das synthetische Material schmilzt um 950°C. Selbst nach mehrstündigem Erhitzen auf 920°C läßt sich keine Veränderung im Pulverdiagramm des Minerals festellen.Es wird versucht, die natürliche Bildung des Rooseveltits zu erklären.

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Rooseveltite from San Francisco de los Andes and Cerro Negro de la Aguadita, San Juan, Argentina

Summary Rooseveltite occurs in the weathering zone of the San Francisco de los Andes and Cerro Negro de la Aguadita mines, located in the San Juan Province, Argentina, at 30° 22S and 69° 33W. It appears in grey, finegrained aggregates pseudomorph after bismuthinite. Refraction index ranges fromn=2.10 to 2.30. The Vickers microhardness is 513 (4–5 of Mohs' scale). Chemical composition from electron micro probe measurements is As 21.5±1% and Bi 60.9±2%. Rooseveltite is monoclinic, with a₀=6.878(1) Å, b₀=7.163(1) Å, c₀=6.735(1) Å, =104° 46±1, Z=4, _calc.=6,94 g·cm^–3, space groupP 2₁/n.The synthetic compound was prepared by three different methods. The powder pattern are the same as that of the mineral. Refraction index n=2.13(2) and n=2.25(2). The measured specific gravity is _pobs.=7,01 g·cm^–3. Cell parameters: a₀=6.882(1) Å, b₀=7.164(1) Å,c ₀=6.734(1) Å, =104° 50.5±0.7, _calc.=6,94 g·cm^–3. The synthetic material melts at about 950°C. After heating to 920°C no variations were observed in the powder diagram of the mineral.It is tried to explain the formation of rooseveltite in natural environment.

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The upper stability limit of the assemblage paragonite + quartz and its natural occurrences

The assemblage paragonite + quartz is encountered frequently in low- to medium-grade metamorphic rocks. With rising grade of metamorphism they react mutually to yield the condensed assemblage albite + Al₂SiO₅.The univariant curve pertaining to the equilibrium paragonite + quartz=albite + andalusite + H₂O has been located experimentally. The reversed P _{H 2 O}-T data are: 1 kb: 470–490° C 2 kb: 510–530° C 3 kb: 540–560° C 4 kb: 560–580° C 5 kb: 590–600° C The univariant curve pertaining to the equilibrium paragonite + quartz=albite + kyanite + H₂O runs through the following P _{H 2 O}-T-intervals: 5 kb: 570–625° C 6 kb: 600–630° C 7 kb: 620–640° C Thermodynamic calculations of S ₂₉₈ ⁰ , H _f,298 ⁰ and G _f,298 ⁰ of the phase paragonite from the experimental data presented above and those obtained from the equilibria of the reaction paragonite=albite + corundum + H₂O (Chatterjee, 1970), agree within the limits of uncertainty. This prompts the idea that Zen's (1969) suggestion of a possible error of approximately 7 kcal in G _f,298 ⁰ of the Al₂SiO₅ polymorphs may in fact be due to an error of similar magnitude in G _f,298 ⁰ of corundum.A best estimate of S ₂₉₈ ⁰ , H _f,298 ⁰ and G _f,298 ⁰ of paragonite based on these considerations yield: S ₂₉₈ ⁰ : 67.61±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ : –1411.4±2.7 kcal gfw^–1 G _f,298 ⁰ : –1320.9±4.0 kcal gfw^–1 These numbers will be subject to change when better thermochemical data on corundum and albite are available.In medium-grade metamorphic rocks the assemblage paragonite + quartz is commonly found in stable coexistence with such other phases as muscovite, staurolite, andalusite, kyanite, but not with cordierite or sillimanite. However, the assemblage paragonite-sillimanite has been reported to be stable in the absence of quartz. All these petrologic observations can be explained on the basis of the stability data of the phases and phase assemblages concerned.     

Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units

The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, K_x, for the disproportionation reaction among the structural units coexisting in the melts, Si₂O₅(2Q³)SiO₃(Q²)+SiO₂(Q⁴), was calculated (K_x is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln K_x vs l/T relationships the enthalpy (H_x) for the disproportionation reaction is in the range of-30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the H_x increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li₂O·2SiO₂ (Li/Si=1), the H_x is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M=K, Na, Li), there is a positive, near linear correlation between the H_x and the Z/r² of the metal cation. The slope of the H_x vs Z/r² regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, _i, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form _i=a lnX _i+b, where a is between 0.6 and 0.87, and X _i is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.     

Synthetic bayleyite,Mg2[UO2(CO3)3]·18H2O: Thermochemistry,crystallography and crystal structure

Summary Thermochemistry, morphology, optical properties and crystal structure of synthetic bayleyite, Mg₂[UO₂(CO₃)₃]·18H₂O, monoclinic, have been studied. Incongruent melting at 55°, three steps of dehydration and two steps of decarboxylation have been found by thermochemic investigations. Morphology: Prisms along [001] with {100}, {110}, {210}, {001}, {401}, {021}, {211}, {111} and as the most important forms. Optical data:n =1.453,n =1.498,n =1.499, 2V _x =16°,Y=b,X c=11°. Crystal structure: Space groupP2₁/a,a=26.560(3),b=15.256(2),c=6.505(1) Å, =92.90(1)°,Z=4,R=0.029 for 5126 independent reflections measured with MoK -radiation. The structure is built up from isolated Mg(H₂O)₆ octahedra, UO₂(CO₃)₃ units and lattice water molecules, all held together by hydrogen bonds only.

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Synthetischer Bayleyit, Mg₂[UO₂(CO₃)₃]·18H₂O: Thermochemie, Kristallographie und Kristallstruktur

Zuseammenfasung Thermochemie, Morphologie, optische Eigenschaften und Kristallstruktur von Bayleyit, Mg₂[UO₂(CO₃)₃]·18H₂O, monoklin, wurden anhand künstlich hergestellter Kristalle untersucht. Durch thermochemische Untersuchung wurden inkongruentes Schmelzen bei 55°, eine dreistufige Wasserabgabe sowie eine zweistufige CO₂-Abgabe festgestellt. Morphologie: parallel zu [001] gestreckte Prismen mit {100}, {110}, {210}, {001}, {401}, {021}, {211}, {111}, und {311} als wichtigste Formen. Optische Daten:n =1.453,n =1.498,n =1.499, 2V _x =16°,Y=b,X c=11°. Kristallstruktur: RaumgruppeP2₁/a,a=26.560(3),b=15.256(2),c=6.505(1) Å, =92.90(1)°,Z=4;R=0.029 für 5126 unabhängige, mit MoK -Strahlung gemessene Reflexe. Die Struktur enthält isolierte Mg(H₂O)₆-Oktaeder, UO₂(CO₃)₃-Gruppen und freie Wassermoleküle, die ausschließlich durch Wasserstoffbrücken miteinander verknüpft sind.

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With 4 Figures     

Admontit,ein neues Boratmineral aus der Gipslagerstätte Schildmauer bei Admont in der Steiermark (Österreich)

Zusammenfassung Admontit ist ein neues Magnesiumborat, das in der Gipslagerstätte Schildmauer bei Admont in der Steiermark (Österreich) in Vergesellschaftung mit drei weiteren neuen borhaltigen Mineralien sowie Gips, Anhydrit, Hexahydrit, Löweit, Quarz und Pyrit auftritt.Das Mineral bildet undeutlich ausgebildete farblose Kristalle von monokliner Symmetrie, die zum Teil nachc gestreckt und tafelig nach {100} sind. Keine Spaltbarkeit, Bruch muschelig, Härte wahrscheinlich 2–3,D _gem .=1,82,D _x =1,875g·cm^–3;n =1,442±0,002,n =1,504±0,002, 2V 30°,r. AE(010),n c auf (010) ca. 45°. a ₀=12,68,b ₀=10,07,c ₀=11,32 Å (alle Werte±0,02 Å),=109,68° (±0,1°),Z=2, RaumgruppeP2₁/c. Stärkste Linien des Pulverdiagramms: 12,08(9), 7,60(10), 3,93(8), 2,68(9). Formel: 2 MgO·6 B₂O₃·15 H₂O. In Wasser wird Admontit langsam zersetzt. Erhitzungsversuche zeigten, daß das Gitter zwischen 100 und 200°C zerstört wird. Ein Teil des Wassers entweicht schon unterhalb 100°C, der Rest zwischen 150 und 350°C.

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Admontite, a new borate mineral from the gypsum deposit Schildmauer near Admont in Styria (Austria)

Summary Admontite is a new magnesium borate found in the gypsum deposit of Schildmauer near Admont in Styria (Austria) in association with three other new borium-containing minerals and with gypsum, anhydrite, hexahydrite, löweite, quartz and pyrite.The mineral occurs in poorly developed colourless crystals of monoclinic symmetry, which in part are elongated along thec axis and flattened on {100}. No cleavage, fracture conchoidal, hardness probably 2–3,D _meas .=1.82,D _x =1.875g·cm^–3.n =1.442±0.002,n =1.504±0.002, 2V 30°,r. AE(010),n c on (010) about 45°.a ₀=12.68,b ₀=10.07,c ₀=11.32 Å (all±0.02 Å), =109.68° (±0.1°),Z=2,space groupP2₁/c. Strongest lines of the powder pattern: 12.08(9), 7.60(10), 3.93(8), 2.68(9). Chemical composition: 2 MgO·6 B₂O₃·15 H₂O. Admontite is slowly decomposed in water. Investigations of the thermal behaviour show that the lattice breaks down between 100 and 200°C. Part of the water escapes already under 100°C, the rest between 150 and 350°C.

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Herrn Univ. Prof. Dr.H. Meixner zum 70. Geburtstag gewidmet.     

Basaltic glass with high-temperature equilibrated immiscible sulphide bodies with native iron from disko,central West Greenland

Immiscible sulphide bodies show eutectic quench textures in a basaltic glass rock (mg=66) from a native iron-bearing dyke chilled at T=1,200° C and P=250 bars. The sulphide bodies are composed of troilite (90–91%), iron (9–10%) and very scarce vanadium-rich chromite and approach a ternary cotectic in the Ni-poor part of the system Fe-Ni-S. Transition element partition between olivine (mg=83), silicate glass (mg=59) and sulphide blebs indicate that the phases were equilibrated at 1,200° C. D _{vanadium(olivine/glass)} is close to unity and reflect the reducing nature of the rock, for which estimates of f _O210^{–12 to –13} and f _S210^–5 have been made. D _{nickel, cobalt, copper (sulphide/glass)}=4,300, 230 and 380 respectively, are much higher than reported experimentally determined D's on_{monosulphide/basalt glass} at the same temperature and show increasing positive deviation (Ni>Co>Cu) with the increasingly siderophile character of the elements. K _{Dnickel-iron (sulphide/olivine)}=63 is much higher than an experimentally reported value (33) and comparison with published thermodynamic data on Ni-partition between olivine and iron metal suggests that the positive deviation is roughly proportional to the excess metal component in the sulphide melt. The occurrence of strongly Ni-depleted reduced basalts on Disko shows that fractionation of metal and sulphides was a common and geologically important process.     

Laboratory growth of sector zoned clinopyroxenes in the system CaMgSi2O6-CaTiAl2O6

Sector zoning has been experimentally reproduced in CaMgSi₂O₆-CaTiAl₂O₆ clinopyroxene crystals by isothermal crystallization using seed crystals. Element partitioning in different growth sectors and between the core and rim portions in single crystals was analysed in relation to growth rate R and degree of supercooling T. The TiO₂ and Al₂O₃ contents increase with increase in R and T, but when they are compared between different sectors in a single crystal grown at the same T, they correlate negatively with R. The order of faces in respect of contents of TiO₂ and Al₂O₃ is (100)>(110)(010)(111) at T= 13° C and 18° C but changes to (110)>(100)>(010)>(111) at T= 25° C. The growth mechanism is concluded to be controlled by interface kinetics at T= 13–25° C for all these faces, while at T=45° C this relation holds for (100) and (010) faces, but not for (110) and (111), based on the growth rate versus supercooling relation and surface microtopographic observations. The interface kinetics play the essential role in the formation of sector zoning, when the layer growth mechanism takes place.     

Synthesis and upper stability of paragonite

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ₂₉₈ ⁰ ) as well as the enthalpy ( H _f,298 ⁰ ) and Gibbs free energy ( G _f,298 ⁰ ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ₂₉₈ ⁰ 67.8±3.9 cal deg^–1 gfw^–1 63.7±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ –1417.9±2.7 kcal gfw^–1 –1420.2±2.6 kcal gfw^–1 G _f,298 ⁰ –1327.4±4.0 kcal gfw^–1 –1328.5±4.0 kcal gfw^–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968).     

Fe-Mn partitioning between garnet and ilmenite: experimental calibration and applications

A new mineralogic geothermometer based on the partitioning of Fe and Mn between garnet and ilmenite has been calibrated by reversal experiments in the P-T range 600–900° C, 2 and 5 kbars and for fO₂=QFM. The results constitute a sensitive geothermometer applicable over a broad range of composition and conditions. Garnetilmenite thermometry has advantages relative to existing geothermometers because of its accurate calibration, marked temperature sensitivity and the chemical and structural simplicity of the crystalline solutions involved. Application to natural assemblages reveals that the garnet-ilmenite geothermometer yields temperatures that agree well with other estimates. The reactivity of, and relatively rapid Fe-Mn diffusion in ilmenite may lead to retrograde resetting of high temperature partition values, but these factors may be useful for estimating rock cooling rates. Analysis of the experimental data indicates minor positive deviations from ideality for Fe-Mn garnets and ilmenites. Absolute magnitudes of interaction parameters (W _AB) derived from a regression analysis are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the difference between solution parameters. These differences are well constrained with a magnitude of W _FeMn ^ilm –W _FeMn ^gar 300 cal mol^–1. The accuracy and applicability of garnet-ilmenite thermometry will improve with the availability of better thermodynamic data for garnet crystalline solutions.Abbreviations and symbols used in text R universal gas constant (cal/mol/°K) - T absolute temperature (°K or °C) - P pressure (kbars) - V ⁰ volume change of reaction (1) - H _{1, T} ⁰ standard state enthalpy change of reaction (1) at 1 bar and the T of interest, in cal/mole - S _T ⁰ entropy change of reaction (1) at T of interest, in cal/mole/°K - G _P,T ⁰ standard free energy change of reaction (1) at the T and P of interest, in cal/mole - distribution coefficient for Fe-Mn partitioning between garnet and ilmenite - K apparent equilibrium coefficient for reaction (1) - _i ^j activity of component i in phase j - W _A-B binary A-B interaction (Margules) parameter - gar garnet - ilm ilmenite - biot biotite - ol olivine - opx orthopyroxene     

Possible non-equilibrium oxygen isotope effects in mantle nodules,an alternative to the Kyser-O'Neil-Carmichael18O/16O geothermometer

Kyser, O'Neil, and Carmichael (1981, 1982) measured the ¹⁸O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Using Ca-Mg-Fe element-partition geothermometric data, they proposed an empirical¹⁸O/¹⁶O geothermometer: T(°C)=1,151–173–68 ², where is the per mil pyroxene-olivine fractionation. However, this geothermometer has an unusual crossover at 1,150 °C, and in contrast to what might be expected during closed-system equilibrium exchange, the most abundant mineral in the nodules (olivine) shows a much greater range in ¹⁸O (+4.4 to +7.5) than the much less abundant pyroxene (all 50 pyroxene analyses from spinel peridotites lie within the interval +5.3 to +6.5). On ¹⁸O-olivinevs. ¹⁸O-pyroxene diagrams, the mantle nodules exhibit data arrays that cut across the ¹⁸O=zero line. These arrays strongly resemble the non-equilibrium quartzfeldspar and feldspar-pyroxene ¹⁸O arrays that we now know are diagnostic of hydrothermally altered plutonic igneous rocks. Thus, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena, casting doubt on their empirical geothermometer. The peridotite nodules appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO₂, magma, H₂O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels, in agreement with available exchange-rate and diffusion measurements on these minerals. This accounts for the correlation between ¹⁸O pyroxene-olivine and the whole-rock ¹⁸O of the peridotites, which is a major difficulty with the equilibrium interpretation.Contribution No. 3978, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

Very high CO2 cordierite from Norwegian Lapland: Mineralogy,petrology, and carbon isotopes

The most CO₂-rich cordierite thus far encountered in nature with about 2.2 wt.% CO₂ and 0.3 wt.% H₂O occurs as large poikiloblasts in a strange non-foliated reaction rock that dissects well-foliated granulites being part of the classical Lapland granulite area described by Eskola. The cordierite is optically positive with the highest optic angle 2V _x (106°) and birefringence (– = 0.017) ever measured on natural cordierites, but it is also optically very heterogeneous due to secondary loss of CO₂ along fractures and zones paralleling the fluid-bearing channels. Based on the optical properties of the degassed Lapland cordierite and on literature data a ternary diagram is given, which shows the variations of this cordierite in 2V _x and birefringence as a function of channel-filling with both CO₂ and H₂O.Following Losert (1971) the cordierite coexists with calcite, a thus far unique mineral assemblage that is probably only stable at very high CO₂ pressures. In the present case, the of the cordierite (0.75) indicates, on the basis of literature data, a coexisting fluid with >0.95.The carbon isotope composition ¹³C of CO₂ in cordierite lies near –7, that of the calcite is slightly lighter than about –9. Thus, at least for the CO₂ in cordierite, a deep-seated origin may be possible.Based on the geologic occurrence it is speculated that the cordierite-bearing reaction rock could perhaps represent an annealed channel of late degassing in the granulitic lower crust.     

1Tc-Strontiohilgardit (Ca,Sr)2[B5O8(OH)2Cl] und seine Stellung in der Hilgarditgruppe X2/II[B5O8(OH)2Cl]

Zusammenfassung 1Tc-Strontiohilgardit (Ca, Sr)₂ [B₅O₈(OH)₂,Cl] mit Ca : Sr etwa 1 : 1 ist ein neues Mineral der Hilgarditgruppe. Fundpunkt: Reyersbausen (9° 59,7 E, 51° 36,6 N), Grube Königshall-Hindenburg, Flöz Staßfurt in sylvinitischer Ausbildung.Konstanten : triklin-pedial,a ₀=6,38 Å,b ₀=6,480 Å,c ₀=6,608 Å, =75,4°,=61,2°, =60,5°; tafelige-gestreckte Links- und Re chtskristalle, farblos, wasserunlöslich, piezoelektrisch. Härte 5–7, Dichte 2,99 g cm^–3;n =1,638,n =1,639,n =1,670; 2V =19°.Neue Daten für die Hilgarditgruppe : 2 M (Cc)-Calciumhilgardit (=Hilgardit) =4 Ca₂[B₅O₃(OH)₂Cl], Raumgruppe Cc.3Tc-Calciumhilgardit (=Parahilgardit) = 3 Ca₂[B₅O₃(OH)₂Cl]; trinklin-pedial, ₀=6,31 Å,b =6,484 Å,c ₀=17,50 Å; =84,0°,=79,6°, =60,9°.Die Polymorphiebeziehungen sind geometrisch deutbar durch eine spezielle Art der Polytropie (Stapelung von Links- und Rechtskristallen im Elementarbereich).     

Synthesis,lattice constants and OH-valence vibrations of an orthorhombic amphibole with excess OH in the system Li2O-MgO-SiO2-H2O

Orthorhombic amphiboles with excess OH, which can be schematically deduced from anthophyllite by the combined substitutions Mg²⁺ + O^2–Li⁺+OH^– and Mg²⁺2 Li⁺, were synthesized at 750–875° C/1 kbar in the system Li₂O-MgO-SiO₂-H₂O. Their phase relations are presented for 800° C/1 kbar . An amphibole with the analytical composition 2.70 wt% Li₂O, 31.1 wt% MgO, 63.0 wt% SiO₂, and 3.29 wt% H₂O has lattice constants a ₀ 18.588 (11), b ₀ 17.966 (10), c ₀ 5.262 (3) Å, V ₀ 1,757.2 (1.5) ų (referred to Space Group Pnma). The OH-valence vibrational spectrum of this amphibole showed v _OH bands at 3,667, 3,708, and 3,725 (shoulder) cm^–1, which are ascribed to OH in the configurations (MgMgMg)-OH, (MgMgMg)-OH-Li (Li in the A-site) of the pseudotrigonal (M1M1M3)-OH arrangement in the amphibole structure, and to Si-OH, respectively. No explanation can at present be offered for an additional shoulder at 3,695 cm^–1. The proposed structural formula is (Li_{0.27 0.73})(Li_1.11 Mg_0.89)· (Mg₅)(Si_8.01O_21.20(OH)_0.80)(OH)_2.00.     

Carbon isotopic study on coexisting calcite and graphite in the Ryoke metamorphic rocks,northern Kiso district,central Japan

Carbon isotope fractionation between coexisting calcite and grpahite ( ¹³C_cc-gr) has been determined in metamorphosed limestones and calc-silicate rocks from the Ryoke metamorphic belt in the northern Kiso district. In this district, the Ryoke metamorphic rocks, ranging from the lower greenschist facies to the upper amphibolite facies, are widely distributed. The fractionation of ¹³C/¹²C between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute ¹³C values of calcite. This evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification, possibly caused by retrogressive metamorphism, is not distinguished. For T=270–650° C, the fractionation is expressed by the following equation: ¹³C_cc-gr=8.9×10⁶T^–2–7.1 (T in °K).This equation has a slope steeper than the current results on the ¹³C_cc-gr versus 10⁶T^–2 diagram. It can be used as a potential geothermometer for almost the entire temperature range of metamorphism. ¹³C values of carbonaceous matter in unmetamorphosed limestones in this district are approximately –22, due to its biogenic origin. Graphite from metamorphosed limestones is also considered to be of biogenic origin but shows enrichment of ¹³C due to isotopic exchange with calcite. ¹³C values of graphite as well as ¹³C_cc-gr confirm that zone II represents the lowest grade zone of Ryoke metamorphism. The maximum equilibrium fractionation of ¹³C between calcite and graphite is considered to be approximately 23%, which corresponds to 270° C. Below this temperature, it seems that carbon isotopic exchange between the minerals does not occur.Calcite in marble from the higher grade zones has relatively lower ¹³C and ¹⁸O values. The depletion of heavy isotopes is considered to be caused by the loss of ¹³C and ¹⁸O enriched carbon dioxide during decarbonation reactions. For oxygen, it is considered that isotopic exchange with metamorphic fluids plays an important role in lowering the ¹⁸O value of calcite in some higher grade marbles.     

First occurrence of the rare mineral Barylite in Greenland

Summary Barylite has been identified for the first time in Greenland in several specimens from the nepheline syenite pegmatite pockets at Narssârssuk. The crystals are orthorhombic, showing the forms {100}, {210} and {201}, and form thin plates parallel to {100}. Electron microprobe analysis and emission spectrography show a composition close to the ideal formula. The refined unit cell paramters are:a=9.835(2) Å,b=11.654(3) Å andc=4.673(1) Å. The barylite is biaxial negative, 2V =66^o±2^o,n =1.694,n = 1.697 andn _calc.=1.698.

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Das erste grönländische Vorkommen des seltenen Minerales Barylith

Zusammenfassung Der seltene Barylith wurde zum ersten Mal in Grönland in verschiedenen Proben aus Nephelin-Syenit-Pegmatit-Drusen bei Narssârssuk identifiziert. Die Kristalle sind orthorhombisch mit den Formen {100}, {210} und {201}, sie bilden dünne Tafeln nach {100}. Elektronensonden-Analyse und Emissions-Spektrographie ergeben eine chemische Zussammensetzung, die der Idealform nahekommt. Die verfeinerten Zellparameter sind folgende:a=9,835(2) Å,b=11,654(3) Å undc=4,673(1) Å. Barylith ist zweiachsig negativ, 2V =66^o±2^o,n =1,694,n =1.697 undn _calc.= 1,698.

     

Gibbs energy of aluminous minerals

The discrepancy between the tabulated Gibbs Energies of Formation for Al₂SiO₅ and corundum relative to muscovite and kaolinite is considered to lie principally with the latter two minerals. New values for heat of formation of gibbsite [Gbs] will affect the tabulated H _f ⁰ , G _f(298,1) ⁰ for the other aluminous minerals which are referred to gibbsite as calorimetric aluminum reference. Gibbs Energy Difference Functions, calculated from phase equilibria in the system CaO-Al₂O₃-SiO₂-(H₂O-CO₂), can be used to estimate consistent H _f ⁰ , G _f(298,1) ⁰ values for aluminous minerals. A self consistent data set is presented referred to G _f(298,1) ⁰ [Corundum]=–378.08 kcal mol^–1. Two independent values for G _f(298,1) ⁰ [Anorthite]=–961.52 and –960.29 kcal, from a recalculation of the H _f ⁰ [Anor] based upon the revised H _f(298,1) ⁰ [Gbs]=–309.325 kcal mol^–1 and from measurement of silica activity on the anorthite-saturated part of the CaO-Al₂O₃-SiO₂ liquidus, respectively, are considered to show the magnitude of the discrepancy and are used in the calculations.     

Stability of titanian clinohumite: Experiments and thermodynamic analysis

Reversed hydrothermal experiments on a natural titanoclinohumite [Ti-Cl; approximate formula Mg_7.5FeTi_0.5O₁₆(OH)] show that it breaks down at 475°±11° C (3.5 kbar), 620°±11° C (14 kbar) and 675°±8° C (21 kbar) to the assemblage olivine +ilmenite+vapor. An internal-consistency analysis of the data yields _r G _s ^/0 (298 K, 1 bar)=36,760±3,326 cal (mole Ti-Cl)^–1. _r S _s ^/0 (298 K, 1 bar)=34.14±5.91 cal deg^–1(mole Ti-Cl)^–1. Linear correlation coefficient r _G–S 1.0. A solution model that accounts for TiO₂-M(OH)₂ and F-OH substitution shows that the results for our nearly F-free Ti-Cl are in reasonable agreement with the unreversed breakdown experiments of Mer-rill et al. (1972) on a F-bearing Ti-Cl.Because fluorine is necessary to stabilize Ti-Cl under mantle conditions, we suggest that Ti-Cl is much more likely to be a storage device for fluorine than for water in the mantle.     

The olivine — clinopyroxene geobarometer: experimental results in the CaO-FeO-MgO-SiO2 system

The exchange of Ca and Mg between olivine and clinopyroxene has been studied in the CFMS system. Experiments were conducted in a piston-cylinder apparatus in the temperature range of 1,100–1,300° C and the pressure range 10–30 kbar. Results confirm the previous suggestion (Adams and Bishop 1982) that this reaction has a sufficiently large V° to be used as a geobarometer in several basic and ultrabasic systems. Experimental results were corrected for compositional effects using recent activity-composition models for quadrilateral pyroxenes and olivines. The corrected results indicate that the exchange reaction has aH _{1 bar} of 34,900 J, a S° of -7.25J/deg, and a V° of -0.489 J/bar. Corrected results agree well with calculations based on the thermodynamic properties of the endmembers.Application of the olivine-clinopyroxene geobarometer to many systems will require additional calibration of non CFMS components. Preliminary pressure estimates based on simple assumptions about the activity relations of these components have been made for spinel lherzolites from southwestern United States and coarse and porphyroclastic garnet lherzolites from southern Africa. A geotherm calculated from spinel lherzolites near the Rio Grande rift is consistent with a geophysical geotherm based on near-surface heat-flow measurements of 100 mW/m² or greater. Results on garnet lherzolites yield a southern African geotherm with no inflection which falls at somewhat higher temperatures than pyroxene geotherms calculated for the same area.