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1.
Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites. The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite. The garnet forms distinctive coronas around clinopyroxene. The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work.It is thought that under conditions leading up to 750 °C, 8 kb load pressure and 5 kb H2O pressure, partial melting occured in feldspathic hornblende granulites. The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins. The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H2O and Na2O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile.  相似文献   

2.
Crystalline primary inclusions in diamonds from the Argyle and Ellendale lamproites have been analyzed for Mn, Ni, Cu, Zn, Ga, Pb, Rb, Sr, Y, Zr, Nb, Ta, Ba and Mo by proton microprobe. Eclogite-suite inclusions dominate at Argyle and occur in equal proportions with peridotite-suite inclusions at Ellendale. Eclogitic phases present include garnet, omphacitic clinopyroxene, coesite, rutile, kyanite and sulfide. Eclogitic clinopyroxenes are commonly rich in K and contain 300–1060 ppm Sr and 3–70 ppm Zr: K/Rb increases with K content up to 1400 at 0.7–1.1% K. Rutiles have high Zr and Nb contents with Zr/Nb=1.5–4 and Nb/Ta 16. Of the peridotite-suite inclusions, olivine commonly contains > 10 ppm Sr and Mo; Cr-pyropes are depleted in Sr, Y and Zr, and enriched in Ni, relative to eclogitic garnets.Eclogite-suite diamonds grew in host rocks that were depleted in Mn, Ni and Cr, and enriched in Sr, Zn, Cu, Ga and Ti, relative to Type I eclogite xenoliths from the Roberts Victor Mine. Crystallization temperatures of the eclogite-suite diamonds, as determined by coexisting garnet and clinopyroxene from single diamonds, range from 1085 to 1575° C. Log K D (C i cpx /C i gnt ) varies linearly with 1/T for Zr, Sr and Ga in most of the same samples. This supports the validity of the temperature estimates; Argyle eclogite-suite diamonds have grown over a T range 400° C. Comparison with data from eclogite xenoliths in kimberlites suggests that K D Sr and K D Zr are mainly T-dependent, while K D Ga may be both temperature-and pressuredependent. K D Ni , K D Cu and K D Zn show no T dependence in these samples.In several cases, significant major-and/or trace-element disequilibrium is observed between different grains of the same mineral, or between pyroxene and garnet, within single diamonds. This implies that these diamonds grew in an open system; inclusions trapped at different stages of growth record changes in major and trace-element composition occurring in the host rock. Diamond growth may have been controlled by a fluid flux which introduced or liberated carbon and modified the composition of the rock. The wide range of equilibration temperatures and the range of composition recorded in the inclusions of single diamonds suggest that a significant time interval was involved in diamond growth.  相似文献   

3.
Greenstone, blueschist and eclogite metabasaltic blocks from the Franciscan complex of California preserve extensive petrographic and chemical evidence for interaction with hydrous fluids at high-P, low-T metamorphic conditions. The Nd and Sr isotope variations within and among the blocks constrain the origin of the basaltic protoliths, the nature of the fluid metasomatism that occurred within the upper levels (15–45 km) of the paleosubduction zonc, and the character and provenance of the rock that generated the hydrous fluids within the paleosubduction zone. Samples with little or no petrographic evidence of retrograde alteration and unaltered garnet separates have Nd. With increasing degrees of retrograde alteration, Nd isotope compositions are consistently lower, ranging down to Nd(160)=5. Actinolitic alteration rinds which are present on some blocks have the least radiogenic compositions with Nd=1.6 to 6.1. While Nd isotope compositions of unaltered blockes are in the range expected for basalt derived from normal depleted mantle, the Sr isotope compositions are more radiogenic ranging from Sr(160)=–5 to +11. Compositions of unaltered eclogite and blue-schist blocks are consistent with a protolith origin in normal oceanic crust derived from depleted mantle. The Sr isotopy systematics indicate that the protoliths were modified by seawater alteration in an ocean-floor hydrothermal system. Isotopic compositions of samples from parts of blocks that have a retrograde metamorphic overprint show a strong correlation between less radiogenic Nd compositions and the extent of retrograde metamorphism. Maximum Nd isotope ratios of the metasomatizing fluid are provided by analyses of actinolitic rinds, and range from Nd(160)=1.6 to 6.1. A possible source for fluids of this composition is subducted sediment that was derived from a continental craton. Because rind formation occurred while the basaltic blocks were within an ultramafic matrix, the fluids must have migrated from sediments in the accretionary wedge into an overlying wedge of mantle material imbricated with blocks of oceanic crust. This suggests possibly km-scale movement of fluids that carry an amount of the rare-earth elements sufficient to significantly modify the trace-element budget of subducted basalt.  相似文献   

4.
Giester  G. 《Mineralogy and Petrology》1995,53(1-3):165-171
Summary The crystal structure of synthetic KMn[SeO4]2 was determined by single crystal X-ray diffraction methods in space group , a = 4.827(2) Å, b = 4.988(2) Å, c = 7.981(3) Å, = 83.18(1)°, = 85.32(2)°, = 67.92(1)°, V = 176.66 Å3, Z = 1; 1564 unique data, measured up to 2 = 70° (MoK-radiation); R, R(I)w = 0.034, 0.074.KMn[SeO4]2 is closely related to monoclinic yavapaiite, KFe[SO4]2, and isotypic compounds. Jahn-Teller distorted MnO6 octahedra are alternately linked with KO10 polyhedra along [001]. The mean values of the Mn-O and Se-O distances are 2.007 Å and 1.637 Å, respectively.
Die Kristallstruktur vonKMn 3+[SeO4]-einem triklin verzerrten Vertreter der Yavapaiite-Familie
Zusammenfassung Die Kristallstruktur von synthetisch dargestelltem KMn[SeO4]2 wurde mittels Einkristallröntgenmethoden in der Raumgruppe bestimmt: a = 4.827(2) Å, b = 4.988(2) Å, c = 7.981(3) Å, = 83.18(1)°, = 85.32(2)°, = 67.92(1)°, V = 176.66 Å3, Z = 1; 1564 unabhängige Daten bis 2 = 70° (MoK-Strahlung); R, R(I)w = 0.034, 0.074.KMn[SeO4]2 ist eng mit dem monoklinen Mineral Yavapaiit, KFe[SO4]2 und einer Reihe damit isotyper Verbindungen verwandt. Jahn-Teller verzerrte MnO6 Oktaeder sind alternierend mit KO10 Polyedern parallel [001] verbunden. Die Mittelwerte der Mn-O und Se-O Abstände sind 2.007 Å bzw. 1.637 Å.

With 1 Figure  相似文献   

5.
After the discovery of metamorphic coesite in crustal rocks from the Western Alps (Italy) and the Western gneiss region (Norway) in the mid 1980s of the last century, metamorphic diamond was observed only a few years later “in situ” in the Kokchetav Massif (Kazakhstan). Findings of such coesite- and diamond-bearing ultrahigh pressure metamorphic (UHP) rocks with protoliths formed or embedded in crustal levels and subsequently experienced PT-conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes. These occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface. Recent studies even suggest continental subduction to depths exceeding 300 km. These rocks have been extensively studied and many new and important observations have been made. Thus far, more than 350 papers have been published on various aspects of Kokchetav UHP rocks.The Kokchetav Massif of northern Kazakhstan is part of one of the largest suture zones in Central Asia and contains slices of HP and UHP metamorphic rocks. Classical UHP rocks mainly occur in the Kumdy Kol, Barchi Kol and Kulet areas, and include a large variety of lithologies such as calcsilicate rocks, eclogite, gneisses, schists, marbles of various compositions, garnet–pyroxene–quartz rocks, and garnet peridotite. Most of them contain microdiamonds; some of which reach a grain size of 200 μm. Most diamond grains show cuboid shapes but in rare cases, diamonds within clinozoisite gneiss from Barchi Kol occur as octahhedral form. Microdiamonds contain highly potassic fluid inclusions, as well as solid inclusions like carbonates, silicates and metal sulfides, which favour the idea of diamond formation from a C–O–H bearing fluid. Nitrogen isotope data and negative δ13C values of Kokchetav diamonds indicate a metasedimentary origin.PT-estimates of Kokchetav UHP rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950–1000 °C. Some zircon separates show inherited Proterozoic cores and 537–530 Ma UHP metamorphic mantle zones. Several Ar–Ar-ages on micas scatter around 529–528 and 521–517 Ma and reflect different stages of the exhumation history. Migmatization occurred during exhumation at about 526–520 Ma.Isotopic studies on calcsilicate rocks confirm a metasedimentary origin: δ18O values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith. Similar studies on eclogites indicate their basaltic protolith having experienced water–rock interaction prior to UHP metamorphism.A number of unique mineralogical findings have been made on Kokchetav UHP rocks. K-feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx-structure under upper mantle pressures. Other significant observations are coesite exsolutions in titanite, quartz-rods in cpx, the discovery of K-tourmaline as well as new minerals like kokchetavite, a hexagonal polymorph of K-feldspar and kumdykolite, an orthorhombic polymorph of albite.The Kokchetav UHP rocks represent a unique and challenging stomping ground for geoscientists of various disciplines. From crystallography, petrology and geochemistry to geophysics and geodynamics/geotectonics – it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions.  相似文献   

6.
The pseudo-binary system CaMgSi2O6-KAlSi2O6, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi2O6. At higher content of KAlSi2O6 in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi2O6 in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi2O6 content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K2O, i.e. about 22–26 mol% of KAlSi2O6. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi2O6 in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution MgM1C2Al1K2, the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO2-rich alumino-silicate mantle melts (>200 km).Editorial responsibility: J. Hoefs  相似文献   

7.
Thermally metamorphosed and metasomatised fragments of basement actinolite-chlorite-calcite-quartz schists and quartz-bearing marbles are found as inclusions in Quaternary agglomerates and historic (197 B. C.—1950) dacitic lavas of Santorini volcano, Greece.Inclusions in agglomerates preserve the structure of parent schists in the alternation of bands rich in diopside or salite with bands rich in plagioclase. By contrast, inclusions in historic dacites are not banded. Most develop a thin zone of hybrid material at the contact with enclosing lava. The assemblage calcic clinopyroxene-wollastonite-plagioclase is commonly developed. The clinopyroxene is a Fe3+-rich salite or ferrosalite. Andradite-rich garnet and sphene are accessory minerals. Most examples carry interstitial siliceous glass of distinctive chemical composition, and several show minor olivine, augite, hypersthene and calcic plagioclase of magmatic origin.Other inclusions exhibit the assemblage anhydrite-calcic clinopyroxene, the latter mineral ranging widely in Al content. A single example has been observed to develop two distinct assemblages, the first coarsely crystalline melilite-wollastonite-magnetite, the second finely intergrown melilite-wollastonite-andraditic garnet (-xonotlite).Stability data for hedenbergite and andradite as constituents of skarn assemblages suggest that the clinopyroxene-rich assemblages of inclusions in historic dacites formed at temperatures near to or above 800° C and oxygen fugacity (fO2) considerably greater than that which could be imposed upon the inclusions by dacite magma (T 900° C, fO210–13 atm.). Thermal breakdown of original carbonates of the inclusions probably supplied the necessary oxygen. T-fO2 data for the reaction 4 Magnetite+18 Wollastonite 6 Andradite indicate that the assemblage melilite-wollastonite-magnetite of the last inclusion described formed at higher T and/or lower fO2 than the assemblage melilite-wollastonite-garnet. The latter assemblage undoubtedly formed during inclusion of the fragment by dacite magma, while metamorphism by a more basic, high temperature magma may have produced the former. Temperature data for reactions limiting the stability of melilite in the system CaO-Al2O3-SiO2-H2O indicate a minimum temperature of around 800° C for formation of both assemblages.  相似文献   

8.
Summary The crystal structure of papagoite, CaCuAlSi2O6(OH)3, monoclinic,a = 2.926 (3),b = 11.496 (3),c = 4.696 (1) Å, = 100.81 (2)°, V = 685.4 (3) Å3, space groupC 2/m,Z = 4, has been refined to anR index of 3.4% for 913 observed reflections measured with MoK X-radiation. The single unique Cu cation is surrounded by six anions in a pseudo-octahedral arrangement showing strong Jahn-Teller distortion. Edge-sharing rutile-like chains of pseudo-octahedrally coordinated Cu and Al extend in the Y direction, and are cross-linked into an octahedral sheet by Ca2Ø10 (O = unspecified ligand, O or OH) dimers. These sheets are linked by (Si4O12) rings to form a mixed tetrahedral-octahedral framework.
Verfeinerung der Kristallstruktur von Papagoit, CaCuAlSi2O6(OH)3
Zusammenfassung Die Kristallstruktur von Papagoit, CaCuAlSi2O6(OH)3, monoklina = 12,926 (3),b = 11.496 (3),c = 4.696 (1) Å, = 100.81 (2)°,V = 685.4 (3) Å3, RaumgruppeC2/m,Z = 4, wurde für 913 beobachtete, MoK-Röntgenstrahlung gemessene Reflexe aufR = 3,4% verfeinert. Das eine symmetrieunabhängige Kupfer-Kation ist pseudooktaedrisch von sechs Anionen mit einer starken Jahn-Teller-Verzerrung umgeben. Die pseudo-oktaedrisch koordinierten Cu- und Al-Atome werden über Kanten zu parallel zur y-Achse verlaufenden rutil-ähnlichen Ketten verbunden, die zusammen mit den Ca2Ø10-Gruppen (Ø = nicht spezifsierter Ligand, O bzw. OH) Oktaederschichten bilden. Diese Schichten werden durch (Si4O12)-Ringe zu einem aus Tetraedern und Oktaedern bestehenden Netzwerk verknüpft.

With 4 Figures  相似文献   

9.
Multiple regression analysis on an extended dataset has been performed to refine the relationship between temperature, pressure, composition and the Fe–Mg distribution between garnet and clinopyroxene. In addition to a significant dependence between the distribution coefficient KD and X GrtCa and X GrtMg#, as shown by the experimental data, the effect of X GrtMn has also been incorporated using data from natural Mn‐rich garnet–clinopyroxene pairs. Multiple regression of data (n=360) covering a large span in pressure, temperature and composition from 27 experimental datasets, combined with 49 natural high‐Mn granulites from Ruby Range, Montana, USA, and Karnataka, India, yields the P–T –compositional relationship (r2=0.98): where KD=(Fe2+/Mg)Grt/(Fe2+/Mg)Cpx, X GrtCa=Ca/(Ca+Mn+Fe2++Mg) in garnet, X GrtMn= Mn/(Ca+Mn+Fe2++Mg) in garnet, and X GrtMg#=Mg/(Mg+Fe2+) in garnet. The Fe2+–Mg equilibrium between garnet and clinopyroxene does not seem to be affected by variations in the sodic content of the co‐existing clinopyroxene in the range X CpxNa=0–0.51. Comparisons between the new and former calibrations of the garnet–clinopyroxene Fe2+–Mg geothermometer clearly demonstrate how the various parameters in each case affect the calculated temperatures. Application of the new expression gives reasonable results for natural garnet–clinopyroxene pairs from various rock types and settings, and should be preferred to previous formulations. Using the new calibration to the self‐consistent dataset of Pattison & Newton (Contributions to Mineralogy and Petrology, 1989, 101, 87–103) suggests a systematic deviation with regard to both temperature and composition between their dataset and the datasets used in the present calibration.  相似文献   

10.
Post-metamorphic CO2-rich fluid inclusions in granulites   总被引：2，自引：0，他引：2  
In granulite-facies samples from the Adirondack Mountains, NY, estimates of peak-metamorphic CO2 fugacities based on mineral equilibria are not consistent with estimates based on data for high-density, CO2-rich fluid inclusions. Of the 21 Adirondack samples investigated for this study, all contain CO2-rich inclusions. Inclusions occur in quartz, apatite, and garnet. They range in size from 3 to 50 m and are without visible H2O. In a few of the inclusions, freezing point determinations and preliminary Laser Raman spectroscopy show the presence of small amounts (<3%) of other fluids (N2 and H2S). CO2 liquid-vapor homogenization temperatures are between –46 and +31° C, corresponding to densities between 1.14 and 0.5 gm/cc. Some of these densities are consistent with peak-metamorphic entrapment (1.06 to 1.1 gm/cc).Peak metamorphic fluid compositions in these samples are inferred from fluid-buffering equilibria that restrict the fugacity of CO2 (f CO2) directly (i.e., calcite+quartz+wollastonite) or buffer the fugacity of oxygen (f O2). Assemblages that bufferf O2 are important because knowledge off O2 places an upper limit onf CO2. In 13 of the 21 samples, estimates of peak-metamorphic fluid compositions based on these equilibria show that the mole fraction of CO2 (XCO2) in equilibrium with the rock was low, in some cases less than 0.2.The contradiction of mineral equilibria and fluid inclusion data shows that the inclusions record post-metamorphic conditions. At present, there are no criteria to distinguish these primary appearing CO2-rich inclusions from those found in other granulite-facies terranes. Therefore, inferences of pressure-temperature conditions and peakmetamorphic fluid compositions based on fluid inclusions must be viewed with caution.  相似文献   

11.
Summary Based on a X-ray structure analysis it was proved that the mineral schmiederite contains both selenite and selenate groups [a = 9.922(3)Å,b = 5.712(2)Å,c = 9.396(3)Å, = 101.96(3)°, space group P21/m,Z = 2 {Pb2Cu2(OH)4(SeO3)(SeO4)},R w = 0.055 for 1131 reflections up to sin / = 0.65 Å–1]. The crystal structure is closely related to that of linarite [a = 9.701(2) Å,b = 5.650(2) Å,c= 4.690(2)Å, = 102.65(2)°, space group P21/m,Z = 2 {PbCu(OH)2(SO4)},R w = 0.034 for 1991 reflections up to sin / = 1.0 Å–1].The Pb atom in linarite and the Pb(1) atom in schmiederite have each three Pb-O bonds < 2.45 Å with trigonal pyramidal arranged ligands; the Pb(2) atom in schmiederite has only one such near O atom. The Cu atoms are approximately square planar coordinated by hydroxil groups. In addition two further O atoms complete the coordination figure to a strongly distorted octahedron. All the anion groups have the usual geometry.
Kristallstruktur und chemische Formel von Schmiederit, Pb2Cu2(OH)4(SeO3)(SeO4), mit einem Vergleich zu Linarit, PbCu(OH)2(SO4)
Zusammenfassung Basierend auf einer Röntgen-Strukturuntersuchung konnte das Vorliegen von Selenit-und Selenatgruppen im Mineral Schmiederit belegt werden [a=9,922(3) Å,b = 5,712(2) Å,c = 9,396(3) Å, = 101,96(3)°, Raumgruppe P21/m,Z=2 {Pb2Cu2(OH)4(SeO3)(SeO4)},R w = 0,055 für 1131 Reflexe bis sin /, = 0,65 Å–1]. Die Kristallstruktur weist enge Beziehungen zu jener des Linarits auf [a = 9,701(2) Å,b = 5,650(2) Å,c = 4,690(2) Å, = 102,65(2)°, Raumgruppe P21/m,Z=2 {PbCu(OH)2(SO4)},R w = 0,034 für 1991 Reflexe bis sin / = 1,0 Å–1].Das Pb-Atom im Linarit sowie das Pb(1)-Atom im Schmiederit haben jeweils drei Pb-O-Bindungen <,45 Å, wobei die Liganden trigonal pyramidal angeordnet sind; das Pb(2)-Atom im Schmiederit hat hingegen nur ein derart nahes O-Atom. Die Cu-Atome sind etwa quatratisch planar von Hydroxilgruppen koordiniert; zwei weitere O-Atome ergänzen die Koordinationsfigur zur einem stark verzerrten Oktaeder. Die Aniongruppen haben die üblichen Dimensionen.
  相似文献   

12.
Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [1[`1]00 1\bar{1}00 ]Ilm//[0[`1]0 0\bar{1}0 ]Cpx (0001)Ilm//(100)Cpx; and [110]Spl//[0[`1]0 0\bar{1}0 ]Cpx ([`1]11 \bar{1}11 )Spl//(100)Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness <1 μm) and lamellae (with a thickness = 1–10 μm) in host clinopyroxene of sample RZ-11D were probably results of hydration processes, although amphibole plates could otherwise be interpreted as exsolution products. Temporal relations between mineral inclusions in each sample can be established, and a semi-quantitative P–T path for this garnet clinopyroxenite body was derived accordingly. The present results show that the Hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during UHP metamorphism as suggested by previous studies. This study demonstrates that the crystallographic and temporal/spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks.  相似文献   

13.
Single-crystal X-ray and neutron structure refinements carried out on neptunite (KNa2Li(Fe, Mg, Mn)2Ti2Si8O24) from San Benito, California at various temperatures (neutrons: 15 K and 293 K; X-rays: 110 K, 293 K and 493 K) indicate that this mineral crystallizes in the acentric space group Cc (T=293K: a=16.427 Å, b=12.478 Å, c=9.975 Å, = 115.56°, Z=4, V=1844.53 Å3) due to ordering of octahedrally coordinated metals (Ti, Fe, Mn, Mg). In the neptunite structure, Ti and (Fe, Mn, Mg) octahedra share edges to form chains that run along [110] and [110]. These chains are, in turn, linked through shared corners along [001]. The resulting octahedral framework is interwoven by a similar [Si8O22] tetrahedral framework. Li, Na and K occupy 6-, 8- and 10- coordinated sites within the framework. The metal-containing polyhedra show strong distortions at all temperatures. In particular, Ti exhibits a strong off-center displacement (0.25 Å) within its octahedron, leading to four Ti-O distances of 2.0 Å, one of 2.2 Å and one of 1.7 Å. The displaced Ti position is in good agreement with a position that minimizes differences between ionic bond strengths and is interpreted as an energy minimum in an ionic potential model. Mössbauer spectra collected at 77 K, 293 K and 400 K indicate all Fe to be present as octahedral Fe2+. Although two distinct Fe positions were found in the structure, 77 K and 293 K spectra display only one quadrupole doublet. Two Fe sites can only be resolved in the 400 K spectrum. It is suggested that the temperature dependence of octahedral edge distortions is responsible for the separation of the Mössbauer doublets.  相似文献   

14.
Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio   总被引：2，自引：0，他引：2  
The partitioning of Cr and Al between coexisting spinel and clinopyroxene and the dependence of spinel-cpxgarnet equilibria on Cr/Al ratio have been investigated by a combination of phase equilibrium experiments, high temperature solution calorimetry and thermodynamic calculations.The exchange equilibrium: has a measured enthalpy change for pure phases of –2,100±500 cal at 970 K and 1 atm. Experimental reversals of Cr-Al partitioning between the spinel and clinopyroxene phases yield the following partitioning relationship: where X i j refers to atomic fraction of i in the octahedral sites of phase j. The compositional dependence of partitioning implies that Al-Cr mixing in spinel is nonideal with, on the symmetrical model, a W Cr-Al Sp of 2,700±500 cal/gm. atom. In contrast, aluminum-chromium mixing in clinopyroxene is close to ideal.The measured stability field of knorringite (Mg3Cr2Si2O12) and mixing properties of garnet have been used in conjunction with our experimental data to calculate the influence of Cr/Al ratio on the important reaction: orthopyroxene+clinopyroxene+spinel=olivine+garnetThe stability field of spinel lherzolite increases by about 2.8 Kb for every increase of 0.1 in Cr/(Cr+Al) ratio up to Cr/(Cr+Al) of 0.7. The calculated stabilization is in very good agreement with the experimental results of O'Neill (1981). The partitioning relationships are such that, at the low ratios of Cr/Al (0.07) of primitive lherzolite, clinopyroxene buffers spinel composition and sharpens the spinelgarnet reaction interval from 10 Kb (little or no clinopyroxene) down to about 2 Kb in pyroxene-rich pyrolite.  相似文献   

15.
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 Å3. 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.
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 Å3. 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.

Mit 2 Abbildungen  相似文献   

16.
Summary The crystal structure of the Di50CaTs50 clinopyroxene Ca(Mg0.5Al0.5) (Al0,5Si1.5)O6, synthesized at T=1 100°C andP=14 kb, was refined in the C 2/c space group. Cell parameters are:a=9.655,b=8.771,c= 5.278 Å, =106.22°. The finalR value is: 2.1%. Comparison with the Di and CaTs structures shows that the variation of the M 2-0, M 1-0 and T-0 bond lengths is continuous and almost linear in the clinopyroxene solid solutions along the Di-CaTs join. The modifications caused in M 2 and M 1 polyhedra by shifting in tetrahedral chains along z are significant. The different values of shifts with composition account for the anomalous trend for the angle in the DiCaTs solid solution.
La structure cristalline du clinopyroxène synthétique CaMgo0.5AlSil1.5O6 (Di50CaTs50). La cristallochime le long du joint Di-CaTs
Résumé La structure cristalline du clinopyroxáne Ca(Mg0.5Al0.5)(Al0.5Si1.5)O6 (Di50CaTs50), cristallisé á T= 1100° etP=14 kb, a été affinée dans le groupe spatial C 2/c. Les paramétres de la maille sont:a=9.655,b=8.771,c=5.278 Å, =106.22°. La valeur finale deR est 2.1%. La comparaison avec les structures de Di et de CaTs montre que le variation des longueures de liaison M 2-0, M 1-0 et T-0 est continue et presque linéaire pour les clinopyroxénes le long du joint Di-CaTs. Le déplacement des chaines tétraédriques le long de l'axe z cause de remarquables modifications dans les polyèdres M 2 et M 1. Les différentes valeurs du déplacement le long de l'axe z avec la composition expliquent la tendance anomale du paramétre de la maille le long du joint.
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17.
Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (XJd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low XCa bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low XCa bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.  相似文献   

18.
Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO2 fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO2 during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm3. Higher density (=0.90–1.02 g/cm3) IIb inclusions occur only in symplectite phases. Secondary Type III CO2+glass inclusions with =0.47–0.78 g/cm3 occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO2 from the host basalt during its ascent to the surface. These data suggest that CO2 is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO2 from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.  相似文献   

19.
Compositional dependence of apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene from Mt. Higasiakaisi is studied by means of a multicomponent regular solution model. It is shown that garnet and clinopyroxene solid solutions are positively non-ideal, and the non-ideal parameters according to the symmetric regular solution model are 2.58 kcal and 2.39 kcal, respectively, assuming the equilibration temperature of the mass to be 550° C.Notations a i h activity of component i in phase h - ij interaction parameter of component i and j in a solid solution - i activity coefficient of component i - X i mole fraction of component i - K partition coefficient of Fe and Mg between coexisting garnet and clinopyroxene - K apparent partition coefficient of Fe and Mg between coexisting garnet and clinopyroxene - G 0 difference in free energy of the partition reaction - H 0 difference in enthalpy of the partition reaction - S 0 difference in entropy of the partition reaction - R gas constant - G garnet - Alm almandine component - Py pyrope component - Gr grossular component - Sp spessartine component - CPx clinopyroxene - Hd hedenbergite component - Di diopside component - Jd jadeite component - Ts Tschermac's molecule component Deceased on April 17, 1974.  相似文献   

20.
Basic inclusions of two types occur in a kimberlitic diatreme at Kayrunnera in northwestern New South Wales. Type I inclusions comprise assemblages of clinopyroxene+garnet+rutile±plagioclase ±quartz±K-feldspar±scapolite±sphene±apaite. Type II inclusions have assemblages of clinopyroxene +garnet+kyanite+quartz±plagioclase and are lower in Ti, total Fe, and higher in Al and have a higher Mg/Mg+gSFe ratio than the Type I inclusions. Experimental and theoretical data indicate that both inclusion types equilibrated at between 850–900 ° C and 18–23 kb. Due to their low concentrations of incompatible elements, the Type I inclusions are considered to represent a basaltic melt derived from an Fe-rich mantle source rock, and not to be the product of fractionation. The Type II inclusions are believed to represent cumulates which formed from a basaltic magma. The presence of sulphur rich scapolite in the Type I inclusions extends the range of P-T conditions from which this mineral has been reported thus adding further credence to the hypothesis that it may act as a stable repository for S and CO2 in the crust and upper mantle.  相似文献   

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