Stilbite-laumontite equilibrium

The reaction stilbite=laumontite+3 quartz+3 H₂O was experimentally studied using conventional hydrothermal techniques employing mineral mixtures consisting of reactants and products in 91 and 19 ratios. Equilibrium was demonstrated; the univariant curve passes through about 170° C and 2000 bars, 185°±10° C and 3000 bars, about 185° C and 4000 bars, and 183°±10° C at 5000 bars P _fluid. These results combined with published equilibria for analcime, laumontite, wairakite and prehnite permit delineation of the P-T conditions for the zeolite and prehnite-pumpellyite facies metamorphism in the Tanzawa Mountains, Japan.     

The upper thermal stability of clinochlore,Mg5Al[AlSi3O10](OH)8, at 10–35 kb P_{{\text{H}}_{\text{2}} {\text{O}}}

Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at = P _tot to the assemblage enstatite+forsterite+spinel+H₂O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H₂O. The resulting univariant curve has a steep, negative dP/dT slope of –930 bar/ °C at least up to 35 kb.Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.     

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).     

Pyroclastic flows and lavas of the Mogan and Fataga formations,Tejeda Volcano,Gran Canaria,Canary Islands: mineral chemistry,intensive parameters,and magma chamber evolution

The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km³ dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to silicic rocks of the Mogan and Fataga formations follows the roughly 4 Ma duration of basaltic shield volcanism. The most common assemblage in the evolved (Mogan and Fataga) rocks is anorthoclase+ edenitic amphibole+ilmenite+magnetite±augite±hypersthene +apatite+pyrrhotite. A few flows also contain plagioclase, biotite, or sphene. Coexisting Fe-Ti oxides yield equilibrium temperatures between 835 and 930° C and log between –11.2 and –12.6. The lowermost pyroclastic flow of the Mogan formation is zoned from a rhyolitic base (848° C) to a basaltic top (931° C). Unit P1 has an oxygen isotope feldspar-magnetite temperature (850° C) very close to its Fe-Ti oxide temperature. One of the youngest Mogan flows is zoned from a comendite (836° C) at the base to a comenditic trachyte (899° C) at the top. The Fataga formation pyroclastic flows show only slight compositional zonation, and one flow has the same Fe-Ti oxide compositions at top and base.Calculations using the reaction 1/3 magnetite+SiO₂ (melt)=ferrosilite+1/6 O₂ indicate total pressures of 1–4 (±3) kb for six of the Mogan flows and one of the Fataga flows. For four of the pyroclastic flows, equilibria involving tremolite-SiO₂-diopside-enstatite-H₂O and phlogopite-SiO₂-sanidine-enstatite-H₂O imply water contents of 0.9 to 2.6 (±0.5) wt% and between 80 and 610 bars, which indicates that magma within the Tejeda reservoir was H₂O-undersaturated throughout the entire history of Mogan to Fataga volcanism. The fluorine contents of amphibole, biotite, and apatite, and chlorine contents of apatite reveal thatf _HF/ andf _HCl/ high compared to most igneous rocks and are consistent with the peralkaline nature of most of the volcanics. Thef _HCl estimate for one flow is 10^–2 to 10^–1 bars andf _HF for six of the flows ranges from about 10^–1 to 6 bars. Pyrrhotite compositions yield estimates for log between –1 and –3, log between –2 and 1.5, and log between 0.5 and 3, which fall in the range of most intermediate to silicic systems. The lack of a systematic trend with time for magma composition, Fe-Ti oxide temperatures, water contents, phenocryst abundances, and ferromagnesian phase composition indicate that the Tejeda magmatic system was open and kept at nearly the same conditions by the periodic addition of more primitive melts.The intensive thermodynamic parameters estimated from coexisting phenocryst equilibria are used to constrain the eruption dynamics based on solution of the conservation equations for a vapor plus pyroclast mixture. The estimates of magma reservoir temperature, pressure, and water concentration, when combined with a one-dimensional fluid dynamical model of a pyroclastic eruption, imply that the velocities of the ash flows at the vent exit were on the order of 100 to 200 m s^–1, and the mass flow rates were about 10⁷ kg s^–1 for an assumed vent radius of 10 m.     

An experimental study of the diffusion of oxygen in quartz and albite using an overgrowth technique

Diffusion rates of¹⁸O tracer in quartz ( c, 1 Kb H₂O) and Amelia albite ( 001, 2 Kb H₂O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:-quartz ( c), ,Q=98±7 KJ mol^–1 (600–825° C, 1 Kb); Amelia albite ( 001), ,Q=85±7 KJ mol^–1, (400–600° C, 2 Kb). Measured quartz¹⁸O diffusivities decrease discontinuously at the- transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, but-quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted.     

A re-examination of the role of hydrogen in Al−Si interdiffusion in feldspars

Recent experimental studies have shown that the rates of Al–Si order-disorder and interdiffusion in alkali feldspars at high pressures under dry conditions increase dramatically in the approximate pressure range 7–14 kb, depending on temperature and feldspar composition (Goldsmith 1987, 1988). Enhancement of Al–Si interdiffusion rates is ascribed to the involvement of hydrogen, but the species of hydrogen involved is undetermined.A simple kinetic analysis of the data of Goldsmith (1987) on disordering of dry albite at 800°–950° C and 6–24 kb in the solid media press is consistent with the NaCl pressure cell acting as a proton donor by enhancing dissociation of water in the pressure medium, generating a high in the experimental environment. The rate constant for disordering of albite is found to increase linearly with the estimated experimental and with the density of aqueous salt solution, implicating H⁺ as the rate-enhancing species.Further experimental studies confirm the importance of . At 16 kb and 850° C, dry albite in sealed Pt capsules in a NaCl cell containing tantalum powder (which reduces H₂O to H₂) remains highly ordered over the same time that complete disordering would occur in the absence of Ta. H₂ cannot therefore be the rate-enhancing species. At 1 kb and 850° C, the extent of Al–Si disorder in albite in direct contact with various NaCl–H₂O solutions increases from partially disordered for pure H₂O to completely disordered for saturated aqueous NaCl solution, giving strong support to the proton model. SIMS scanning ion imaging of albite run products demonstrates conclusively that solution-reprecipitation is not responsible for enhanced disordering rates.Results of disordering experiments in the solid media apparatus cannot be duplicated in Ar gas media internally-heated pressure vessels, even with the same experimental configuration around the albite-bearing capsules, due to the different proton-buffering capacities of the solid and gas media apparatus.     

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).     

Synthesis and stability relations of wairakite,CaAl2 Si4 O12·2H2O

Hydrothermal investigation of the bulk composition CaO·Al₂O₃·4SiO₂ + excess H₂O has been conducted using conventional techniques over the temperature range 200–500° C and 500–5,000 bars P _fluid. The fully ordered wairakite was synthesized unequivocally in the laboratory, probably for the first time.The gradual, sluggish and continuous transformation from disordered to ordered wairakite evidently accounts for failure by previous investigators to synthesize ordered wairakite in runs of week-long duration. The dehydration of metastable disordered wairakite to metastable hexagonal anorthite, quartz and H₂O has been determined; this reaction takes place at temperatures exceeding 400° C, even at fluid pressures of 500 bars or less. The upper P _fluid-T boundary of the disordered phase is equivalent to the maximum temperature curve of synthetic wairakite presented by previous investigators. The hydrothermal breakdown of natural wairakite above its stability limit appears to be a very slow process.The equilibrium dehydration of wairakite to anorthite, quartz and H₂O occurs at 330±5° C at 500 bars, 348±5° C at 1,000 bars, 372±5° C at 2,000 bars and 385±5° C at 3,000 bars. Where fluid pressure equals total pressure, the thermal stability range of wairakite is about 100° C wide. At lower temperatures wairakite reacts with H₂O to form laumontite. Reconnaissance experiments dealing with the effect of CO₂ on stabilities of calcium zeolites suggest that wairakite or laumontite may be replaced by the assemblage calcite + montmorillonite in the presence of a CO₂-bearing fluid phase.The determined P _fluid -T field of wairakite is compatible with field observations in some metamorphic terrains where it is related to the shallow emplacement of granitic magma and with direct pressure-temperature measurements in certain active geothermal areas. Under inferred conditions of higher _CO2/_H2O ratios, essentially unmetamorphosed rocks grade directly into those characteristic of the greenschist facies; moderately high values of _CO2 in carbonate-bearing rocks result in the downgrade extension of the greenschist facies at the expense of zeolite-bearing assemblages.     

Anthophyllit und Hornblende in einigen metamorphen Reaktionen

Reactions which occur at the lower boundary of the hornblende-hornfels facies and in the so-called pyroxene-hornfels facies were experimentally investigated for an ultrabasic rock at 500, 1000 and 2000 bars H₂O pressure.The starting material used was a mixture of natural chlorite, talc, tremolite and quartz such that its composition, except for surplus quartz, corresponded to that of an ultrabasic rock. The atomic ratio Fe²⁺+Fe²⁺/Mg+Fe³⁺+Fe³⁺ in the system was 0.16.The lower boundary of the hornblende-hornfels facies was defined by the formation of the orthorhombic amphibole anthophyllite and hornblende according to the following idealized reaction: chlorite+talc+tremolite+quartz hornblende+anthophyllite+H₂O In effect, this reaction consists of the two bivariant reactions: chlorite+tremolite+quartz hornblende+anthophyllite+H₂O talc+chlorite anthophyllite+quartz+H₂OThe equilibrium temperatures obtained for the two reactions in the given system are practically the same and are as follows: 535±10°C at 500 bars H₂O pressure 550±20°C at 1000 bars H₂O pressure 560±10°C at 2000 bars H₂O pressure 580±10°C at 4000 bars H₂O pressureAt 2000 bars and higher temperatures within the hornblende-hornfels facies, anorthite is formed in addition to hornblende and anthophyllite, probably according to the following reaction: hornblende₁+quartz hornblende₂+anthophyllite+anorthite+H₂O; because of the formation of anorthite it is to be expected that the hornblende in this case is poorer in aluminium than the hornblende at 500 and 1000 bars. Winkler (1967) suggests renaming the pyroxene-hornfels facies as K-feldspar-cordierite-hornfels facies which, in turn, is subdivided into a lower-temperature orthoamphibole subfacies without orthopyroxene and a higher-temperature orthopyroxene subfacies without orthoamphibole. The orthopyroxene subfacies itself may in its lower temperature part still carry hornblende which finally disappears in the higher temperature part.The appearance of orthopyroxene characterizes the transition from the orthoamphibole to the orthopyroxene subfacies of the K-feldspar-cordierite hornfels facies. The following reaction takes place at pressures lower than 2000 bars: hornblende₁+anthophyllite hornblende₂+enstatite+anorthite+H₂OSince at 2000 bars an Al-poor hornblende already exists in the hornblende-hornfels facies, it is very likely that here only anthophyllite breaks down to give enstatite+quartz+H₂O.The equilibrium temperatures for these reactions which give rise to enstatite are: 650±10°C at 250 bars H₂O pressure 690±10°C at 500 bars H₂O pressure 715±10°C at 1000 bars H₂O pressure 770±10°C at 2000 bars H₂O pressureOnly after an increase in temperature to about 710°C at 500 bars and about 770°C at 1000 bars does hornblende in the system investigated here break down completely according to the reaction: hornblende = enstatite+anorthite+diopside+H₂OExcept at very small H₂O-pressures (see Fig. 3), there exists, therefore, a region within the orthopyroxene subfacies where hornblende, enstatite and anorthite coexist. As a result we have, as mentioned above, a lower-temperature and a higher-temperature part of the orthopyroxene subfacies, and it is only in the latter part that the parageneses correspond to the pyroxene-hornfels facies as stated by Eskola (1939).Summing up, the starting material consisting of chlorite, talc, tremolite plus quartz remains unchanged in the albite-epidote-hornfels facies; this gives rise in the hornblende-hornfels facies to the paragenesis hornblende+anthophyllite, or — at higher pressures — to hornblende+anthophyllite+anorthite. For the particular composition of the starting material, however, no reactions take place at the transition of the hornblende-hornfels facies to the orthoamphibole subfacies of the K-feldspar-cordierite-hornfels facies as this transition is typified by the breakdown of muscovite in the presence of quartz. However, at the end of the orthoamphibole subfacies the breakdown of anthophyllite, by which orthopyroxene is formed, heralds the onset of the orthopyroxene subfacies. In this subfacies — at greater than about 300 bars — hornblende is still present and coexists with enstatite and anorthite, but with rising temperature hornblende breaks down to give way to the paragenesis enstatite+anorthite+diopside. The experimentally determined parageneses confirm known petrographic occurrences.

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Für die Förderung dieser Arbeit danken wir der Deutschen Forschungsgemeinschaft vielmals. Der Dank von Choudhuri gilt dem Akademischen Auslandsamt der Universität Göttingen für ein Stipendium, das ihm den Abschluß seiner Studien an der Universität Göttingen ermöglichte.     

The assemblage orthopyroxene+cummingtonite+quartz. The low-temperature stability limit

The monovariant reaction Opx+H₂O Cum+Ol+Q and the Cum+Opx+Q stability field were studied under hydrothermal conditions at P _total=2940, 4900 bar and the oxygen fugacity of the QFM buffer. Under these conditions, the Opx lower stability brackets were 730°±10° and 740°±5° C, respectively. The kinetics of the reactions in the Cum+ Opx+Q mixture showed that there were only minor differences in the equilibrium compositions of the coexisting Opx and Cum over the 740°–780° C range. At T=780°, 760° and 740° C, the FeO/FeO+MgO ratio, in mol% was: Opx_52.5–Cum_49.5, Opx₆₂–Cum₅₇, Opx₇₂–Cum₆₆ (P=2940 bar) and Opx₆₂–Cum_58.5 Opx_71.5–Cum_66.5, Opx₈₀–Cum₇₅ (P= 4900 bar). The results are in good agreement with earlier studies in the Opx+Ol+Q and Cum+Ol+Q assemblages.Abbreviations Opx Orthopyroxene - Ol olivine - Cum cummingtonite - Mt magnetite - Q quartz - tk talc     

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.     

Paragonite stability at 700°C in the presence of H<Subscript>2</Subscript>O–NaCl fluids: constraints on H<Subscript>2</Subscript>O activity and implications for high pressure metamorphism

We carried out reversed piston-cylinder experiments on the equilibrium paragonite = jadeite + kyanite + H₂O at 700°C, 1.5–2.5 GPa, in the presence of H₂O-NaCl fluids. Synthetic paragonite and jadeite and natural kyanite were used as starting materials. The experiments were performed on four different nominal starting compositions: X(H₂O)=1.0, 0.90, 0.75 and 0.62. Reaction direction and extent were determined from the weight change in H₂O in the capsule, as well as by optical and scanning electron microscopy (SEM). At X(H₂O)=1.0, the equilibrium lies between 2.25 and 2.30 GPa, in good agreement with the 2.30–2.45 GPa reversal of Holland (Contrib Miner Petrol 68:293–301, 1979). Lowering X(H₂O) decreases the pressure of paragonite breakdown to 2.10–2.20 GPa at X(H₂O)=0.90 and 1.85–1.90 GPa at X(H₂O)=0.75. The experiments at X(H₂O) = 0.62 yielded the assemblage albite + corundum at 1.60 GPa, and jadeite + kyanite at 1.70 GPa. This constrains the position of the isothermal paragonite–jadeite–kyanite–albite–corundum–H₂O invariant point in the system Na₂O–Al₂O₃–SiO₂–H₂O to be at 1.6–1.7 GPa and X(H₂O)~0.65±0.05. The data indicate that H₂O activity, a(H₂O), is 0.75–0.86, 0.55–0.58, and <0.42 at X(H₂O)=0.90, 0.75, and 0.62, respectively. These values approach X(H₂O)², and agree well with the a(H₂O) model of Aranovich and Newton (Contrib Miner Petrol 125:200–212, 1996). Our results demonstrate that the presence or absence of paragonite can be used to place limits on a(H₂O) in high-pressure metamorphic environments. For example, nearly pure jadeite and kyanite from a metapelite from the Sesia Lanzo Zone formed during the Eo-Alpine metamorphic event at 1.7–2.0 GPa, 550–650°C. The absence of paragonite requires a fluid with low a(H₂O) of 0.3–0.6, which could be due to the presence of saline brines.     

An ion microprobe study of anhydrous oxygen diffusion in anorthite: a comparison with hydrothermal data and some geological implications

The diffusion rate of ¹⁸O tracer atoms in anorthite (An₉₇Ab₀₃) under anhydrous conditions has been measured using SIMS techniques. The tracer source was ¹⁸O₂ 98.4% gas at 1 bar, in the temperature range 1300° C–850° C. The measured diffusion constants are D ₀=1 _–0.6 ⁺¹ ×10^–9 m²s^–1 Q=236±8 kJ mol^–1 Comparison of these values with published data for ¹⁸O diffusion in anorthite under hydrothermal conditions shows that dry oxygen diffusivities are orders of magnitude lower than equivalent wet values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.     

Machatschkiit,ein neues Arsenatmineral aus der Grube Anton im Heubachtal bei Schiltach (Schwarzwald,Bundesrepublik Deutschland)

Zusammenfassung Machatschkiit ist ein neues wasserhaltiges Calciumarsenat, das in der Grube Anton bei Schiltach im mittleren Schwarzwald (BRD) 1960 gefunden wurde. Es kommt zusammen mit Gips, Pharmakolith, Pikropharmakolith und Sainfeldit als Sekundärbildung krustenförmig auf Granit vor.Als charakteristische Form tritt das Rhomboeder auf. Keine Spaltbarkeit, Bruch muschelig, Härte 2–3,G=2,5 bis 2,6 g·cm^–3,D _x =2,50 g·cm^–3, farblos, durchsichtig bis durchscheinend. Optische Daten:n =1,585±0,002,n =1,593±0,002, einachsig oder schwach zweiachsig, negativ.Gitterkonstanten:a ₀=15,10,c ₀=22,59 Å,a ₀c ₀=11,4960,Z=12;a _rh =11,52 Å, =81°52,Z=4. Mögliche Raumgruppen:R32,R3m, . Stärkste Linien des Pulverdiagramms: 8,59 (10) (110), 5,34 (8) (210), 3,59 (8) . Eine chemische Analyse ergab nach Umrechnung folgende Zusammensetzung: CaO 29,5%, As₂O₅ 40,8%, SO₃ 0,5%, H₂O 29,2%, Summe 100,0%. Dies kommt der Formel Ca₃(AsO₄)₂ · 9 H₂O sehr nahe. In verdünnter HCl und HNO₃ ist der Machatschkiit leicht löslich.

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Machatschkiite, a new arsenate mineral from the Anton Mine in the Heubach valley near Schiltach (Black Forest, Federal Republic of Germany)

Summary Machatschkiite is a new hydrous calcium arsenate which was found 1960 in the Anton Mine near Schiltach in the central Black Forest. It is a secondary mineral which occurs in crusts on granite in association with gypsum, pharmacolite, picropharmacolite and sainfeldite.The characteristic form is the rhombohedron . No cleavage, fracture conchoidal, hardness 2–3,G=2.5–2.6 g·cm^–3,D _x =2.50 g·cm^–3, colourless, transparent to translucent. Optical data:n =1.585±0.002,n =1.593±0.002, uniaxial or weakly biaxial, negative.Unit-cell dimensions:a ₀=15.10,c ₀=22.59 Å,a ₀c ₀=11.4960,Z=12;a _rh =11.52 Å, =81°52,Z=4. Possible space groups:R32,R3m, . Strongest lines of the powder pattern: 8.59 (10) (110), 5.34 (8) (210), 3.59 (8) . A chemical analysis gave the following composition after recalculation: CaO 29.5%, As₂O₅ 40.8%, SO₃ 0.5%, H₂O 29.2%, total 100.0%. This agrees well with the formula Ca₃(AsO₄)₂ · 9 H₂O. Machatschkiite is easily soluble in diluted HCl and HNO₃.

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Mit 3 Abbildungen     

Mid-Pleistocene lavas from the Seguam volcanic center,central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO^*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at 1160°C between about 3–5 kb (±1–2% H₂O), andesites crystallized Plag+Cpx+Opx±Mt at 1000°C between 3–4 kb with 3–5% H₂O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H₂O. All lavas crystallized at f _{o 2} close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al₂O₃, Cr, Ni, and Sr abundances decrease and K₂O, Na₂O, Rb, Ba, Zr, and Pb increase with increasing SiO₂ (50–71%). LREE enrichment [(Ce/Yb)_n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO₂. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K₂O, Na₂O, TiO₂, Rb, Ba, Sr, Zr, Y, and LREE abundances. ⁸⁷Sr/⁸⁶Sr ratios (0.70361–0.70375) and ratios of ²⁰⁶Pb/²⁰⁴Pb (18.88–18.97), ²⁰⁷Pb/²⁰⁴Pb (15.58–15.62), ²⁰⁸Pb/²⁰⁴Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, Nd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of 80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.     

Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pinciná village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An_20–55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6–7% of total iron as Fe³⁺ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8–10 mol% Fe₂O₃ and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm³) primary CO₂ inclusions with minor H₂, CH₄, H₂S, CO and N₂ (<2 mol% total) are present in Qtz and Plg. Equilibrium PT conditions inferred from the intergranular Opx–Ilm–Glass assemblage and fluid density correspond to 1,170±50°C, 5.6±0.4 kbar, respectively. Metaluminous pincinite is more oxidised (25–27% of total iron as Fe³⁺ in diopside-normative intergranular glass of rhyolite–trachyte–dacite composition) and contains Fe₂O₃-rich ilmenite (17–29 mol%) associated with enstatite. Fluid inclusions are composed of CO₂–H₂O mixtures with up to 38 mol% H₂O. Raman spectroscopy revealed H₂S along with dominant CO₂ in the carbonic phase. Equilibrium PT parameters for the intergranular Opx–Ilm–Glass assemblage correspond to 740±15°C, 2.8±0.1 kbar, respectively. Reducing gas species (<2 mol% total) in the CO₂-inclusions of the peraluminous pincinite resulted from hydrogen diffusion due to fH₂ gradient imposed during decrease of redox potential from the log fO₂ values near QFM during Qtz + Plg growth, to QFM-2 incidental to the superimposed Opx + Ilm assemblage in the intergranular melt. The decrease in oxygen fugacity was recorded also in the metaluminous pincinite, where log fO₂ values changed from ~QFM + 2.6 to QFM + 0.4, but hydrogen diffusion did not occur. Absence of OH-bearing minerals, major and trace element abundances (e.g. REE 300–320, Nb 55–57, Th 4–31, Zr 240–300 ppm, FeO_tot/MgO up to 11), and Sr–O isotope ratios in the pincinites are diagnostic of high-temperature anorogenic magmas originated by dehydration melting of biotite in quartz-feldspathoid crust (⁸⁷Sr/⁸⁶Sr>0.705–0.706, ¹⁸O>9 V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10–11 km. Low water activity in the pincinite parental melt was caused by CO₂-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.     

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.     

Feldspar and oxide thermometry of granulites in the Adirondack Highlands

Thermometry of regionally metamorphosed granulites of the Adirondack Highlands has been undertaken using feldspar and iron-titanium-oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An_18–30) and microcline perthite (Or_57–87) pairs from charnockites and granitic gneisses give K_D[Na/(Na+Ca+K]_plag/[Na/(Na+Ca+K)]_or = 2–3 yielding temperatures of 650 ° to 750 ° C in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pressures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magnetite (ulvöspinel_ss 16–45) and ilmenite (hematite_ss 4.7–6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ° to 800 ° C in comparison to Buddington and Lindsley's (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10^–20 and 10^–16 and always lie within 10⁺¹ of the f buffered by fayalite-magnetite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulvöspinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically reasonable results. The inferred 750 ° and 700 ° C isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790 ± 50 ° C) occur between Saranac Lake and Tupper Lake, New York.Contribution No. 336 from the Mineralogical Laboratory, Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan, 48109. U.S.A.     

Die Reaktion Phlogopit + Calcit + Quarz = Tremolit + Kalifeldspat + H2O + C2O

Hydrothermal experiments with mixtures of synthetic minerals have shown the reversibility of the reaction 5 phlogopite + 6 calcite + 24 quartz = 3 tremolite + 5 K-feldspar + 2 H₂O + 6 CO₂. In an isobaric T – diagram the equilibrium curve reaches a maximum at = 0,75. The P, T-values for this maximum are: 2 kb-523°; 4 kb-585°; 6 kb-625°; P±5%, T±10° C. These results give a first approximation of the P, T conditions responsible for a similar mineral reaction which has been recorded from natural metamorphic assemblages.

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Herrn Prof. H. G. F. Winkler danke ich für anregende Diskussionen, desgleichen Herrn Dr. D. Puhan für wichtige Hinweise und Mitteilung seiner exp. Daten. Herrn Prof. V. Trommsdorff und Herrn P. H. Thompson bin ich für petrographische Angaben zu Dank verpflichtet. Der Aufbau der Hydrothermalanlage und die Finanzierung der laufenden Untersuchungen wurde aus den Mitteln des Fonds zur Förderung der wissenschaftlichen Forschung ermöglicht. Für diese Unterstützung gilt daher mein besonderer Dank.     

The crystal structures of CuSO4 · H2O and CuSeO4 · H2O,and their relationships to kieserite

Summary The crystal structures of hydrothermally grown CuSO₄ · H₂O and CuSeO₄ · H₂O were determined by single crystal X-ray methods [Space group ,a = 5.037 (1), 5.129 (1) Å,b = 5.170(1), 5.527(1)Å,c = 7.578(2), 7.469(2)Å, = 108.62(1), 103.98(1)°, = 108.39(1), 106.52(1)°, = 90.93(1), 97.19(1)°; Z = 2; R_w = 0.026, 0.030 for 2065, 2235 reflections with sin / 0.90 Å^–1]. The Cu atoms are [4 + 2]-coordinated to O atoms. These elongated octahedra are corner connected via the H₂O molecule to form chains. The formal units ¹ [Cu₂O₈(H₂O)2]^12- are interconnected by [XO₄]^2- groups (X=S,Se) and hydrogen bonds (bond lengths 2.72–2.83 Å). The crystal structures show pseudomonoclinic symmetry and are strongly related to the structure type of kieserite.[/p]

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Die Kristallstrukturen von CuSO₄ · H₂O und CuSeO₄ · H₂O und ihre Beziehungen zum Kieserit

Zusammenfassung Die Kristallstrukturen von hydrothermal gezüchtetem CUSO₄ · H₂O und CuSeO₄. H₂O wurden an Einkristallen mittels Röntgenbeugung bestimmt [Raumgruppe ;a = 5.037(1), 5.129(1)Å,b = 5.170(1), 5.527(1)Å,c = 7.578(2), 7.469 (2) Å, = 108.62(1), 103.98(1)°, = 108.39(1), 106.52(1)°, = 90.93(1), 97.19(1)°; Z = 2; R_W = 0.026, 0.030 für 2065, 2235 Reflexe mit sin / 0.90)Å^–1]. Die Cu-Atome werden durch O-Atome [4+2]-koordiniert. Diese gestreckten /lOktaeder sind miteinander über Ecken durch die H₂O-Moleküle zu Ketten verknüpft. Die formalen Einheiten ¹ [CU₂O₈(H₂O)₂]^12– werden durch [XO₄]^2–-Gruppen (X = S, Se) und Wasserstoffbrücken (Bindungslängen 2.72–2.83Å) miteinander verbunden. Die Kristallstrukturen zeigen pseudomonokline Symmetrie und sind sehr nahe mit dem Strukturtyp des Kieserits verwandt.

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With 1 Figure