On local structural changes in lizardite-1T: {Si4+/Al3+}, {Si4+/Fe3+}, [Mg2+/Al3+], [Mg2+/Fe3+] substitutions

Geometrical changes induced by cation substitutions {Si⁴⁺/Al³⁺}[Mg²⁺/Al³⁺], {2Si⁴⁺/2Al³⁺} [2Mg²⁺/2Al³⁺], {Si⁴⁺/Fe³⁺} [Mg²⁺/Al³⁺] or [Mg²⁺/Fe³⁺], where {} and [] indicate tetrahedral and octahedral sheet in lizardite 1T, are studied by ab-initio quantum chemistry calculations. The majority of the models are based on the chemical compositions reported for various lizardite polytypes with the amount of Al in the tetrahedral sheets reported to vary from 3.5% to 8% in the 1T and 2H ₁, up to ~30% in the 2H ₂ polytype. Si⁴⁺ by Fe³⁺ substitution in the tetrahedral sheet with an Al³⁺ (Fe³⁺) in the role of a charge compensating cation in the octahedral sheet is also examined. The cation substitutions result in the geometrical changes in the tetrahedral sheets, while the octahedral sheets remain almost untouched. Substituted tetrahedra are tilted and their basal oxygens pushed down from the plane of basal oxygens. Ditrigonal deformation of tetrahedral sheets depends on the substituting cation and the degree of substitution.     

DFT study of the cation arrangements in the octahedral and tetrahedral sheets of dioctahedral 2:1 phyllosilicates

Quantum mechanical calculations based on the density functional theory (DFT) are used to study the crystal structures of dioctahedral 2:1 phyllosilicates. The isomorphous cation substitution is investigated by exploring different substitutions of octahedral Al³⁺ by Mg²⁺ or Fe³⁺, tetrahedral substitution of Si⁴⁺ by Al³⁺, and different interlayer cations (IC) (Na⁺, K⁺, Ca²⁺, and Mg²⁺). Samples with different kinds of layer charges are studied: only tetrahedrally charged, only octahedrally charged, or mixed octahedral/tetrahedral charged. The effect of the relative arrangements of these substitutions on the lattice parameters and total energy is studied. The experimental observation of segregation tendency of Fe³⁺ and dispersion tendency of Mg²⁺ in the octahedral sheet is reproduced and explained with reference to the relative energies of the octahedral cation arrangements. These energies are higher than those due to the IC/tetrahedral and IC/octahedral relative arrangements. The tetrahedral and octahedral substitutions that generate charged layers also tend to be dispersed. The octahedral cation exchange potentials change with the IC-charge/ionic radius value.     

Variations in composition and abundance of white mica in the hydrothermal alteration system at Hellyer,Tasmania, as revealed by infrared reflectance spectroscopy

Short-wave infrared (SWIR) spectral reflectance of hydrothermally altered volcanic rocks in the footwall of the Hellyer massive sulfide deposit was measured with a portable PIMA-II infrared spectrometer. The Al–OH band was used to derive information on the octahedral Al content and the abundance of white mica (sericite) in the hydrothermal alteration and mineralization system. The range of the Al–OH band wavelength from 2192 nm to 2222 nm corresponds to the number of octahedral Al (Al^vi) in white mica approximately from 3.9 to 3.0 (based on 4 octahedral cations per formula). This Al^vi range represents a significant compositional variation, covering most of the compositional region between muscovite (Al^vi = 4.0) and phengite (Al^vi = 3.0). Furthermore, the spectral reflectance data show that the compositional variation of white mica is spatially related to hydrothermal alteration zoning, such that phengitic white mica tends to occur in 1) main upflow fluid channel, 2) intensely altered volcanic rocks, and 3) Pb–Zn mineralization, whereas muscovitic white mica was formed preferentially distal to massive sulfide mineralization on the margin of the footwall alteration system. The results suggest that the Al–OH band wavelength, and therefore the octahedral Al content, of white mica can be used as vectors to mineralization to map the hydrothermal system at Hellyer.     

Copper Adsorption by Chernozem Soils and Parent Rocks in Southern Russia

Laboratory data in Cu²⁺ adsorption by chernozems and parent rocks in Rostov region show that adsorption isotherms can be approximated by the Langmuir equation, whose parameters (K_l and C_∞) were calculated for all of the samples. The values of C_∞ show a strong negative correlation with the values of cationexchange capacity (CEC) (r =–0.88 at Р = 0.95), and K_l is correlated with the content of physical clay (particles <0.01 mm) (r = 0.78) and with clay (particles <0.001 mm) content in ordinary chernozem and southern chernozems of various particle size distribution (r = 0.80). Even stronger correlations were detected between these parameters in southern chernozems (r = 0.89 for the physical clay (PC) and r = 0.91 for the silt). However, none of the samples displays a significant correlation of C_∞ and K_l with the contents of physical clay and silt. This led us to conclude that the composition of the samples, for example, their organic matter, can affect Cu²⁺ adsorption by the soils and parent rocks. Acidification mechanisms of the equilibrium solutions during the Cu²⁺ adsorption by soils are discussed, as also are the reasons for the absence of balance between Cu²⁺ adsorbed by soils and exchangeable cations transferred into solution. Analysis of the fine structures of the XANES and EXAFS spectra suggests that Cu²⁺ can form coordinated chelate complex compounds with humic acids (HA) of soils and can substitute Al³⁺ at octahedral sites when interacting with clay minerals in soils.     

Cation distribution and structure modelling of spinel solid solutions

 This paper presents an improved generalisation of cation distribution determination based on an accurate fit of all crystal-chemical parameters. Cations are assigned to the tetrahedral and octahedral sites of the structure according to their scattering power and a set of bond distances optimised for spinel structure. A database of 295 spinels was prepared from the literature and unpublished data. Selected compositions include the following cations: Mg²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺ and vacancies. Bond distance optimisation reveals a definite lengthening in tetrahedral distance when large amounts of Fe³⁺ or Ni²⁺ are present in the octahedral site. This means that these cations modify the octahedral angle and hence the shared octahedral edge, causing an increase in the tetrahedral distance with respect to the size of the cations entering it. Some applications to published data are discussed, showing the capacity and limitations of the method for calculating cation distribution, and for identifying inconsistencies and inaccuracies in experimental data. Received: 19 February 2001 / Accepted: 1 June 2001     

Sapphirine I

Microprobe analyses of 26 natural sapphirines from 17 localities indicate that the predominant chemical substitutions in this mineral occur along the solid solution join^VI(Mg,Fe)²⁺+^IVSi⁴⁺=^VI(Al, Fe)³⁺+^IVAl³⁺. Chromium and manganese are minor substituents. Evidence for the substitution SiAl+1/2Mg+1/2 vacancy is absent within the limits of analytical error.A partitioning scheme based on electrostatic charge balance considerations has been devised permitting calculation of Fe²⁺ and Fe³⁺ from total iron content. Results are in good agreement with previous Mössbauer studies which indicate Fe³⁺ is sometimes in octahedral and/or tetrahedral coordination.Distribution coefficients for Fe²⁺-Mg exchange equilibria between sapphirine-spinel and sapphirine-orthopyroxene are similar for most mineral pairs and suggest that most of the assemblages equilibrated at about the same temperature or that the exchange reactions are insensitive to temperature.Compositions of synthetic sapphirines as a function of temperature and pressure are qualitatively predictable from crystal chemical considerations. Changes in sapphirine composition along the MgSi= AlAl solid solution join toward more aluminous compositions stabilize the sapphirine structure at high temperatures and low pressures. The limited extent of MgSi=AlAl solid solution observed in natural sapphirines appears to be related to the requirements of geometrical fit among octahedra and tetrahedra in the almost idealized cubic closest-packed anion framework.     

Effect of the Si/Al ordering on structural parameters and the energetic stabilization of vermiculites – a theoretical study

The effect of the Si/Al distribution in the tetrahedral sheets of the vermiculite mineral has been investigated employing density functional theory. The structures of six models for vermiculite with the structural formula (Mg₄)(Mg₁₂)(Si₈Al₈)O₄₀(OH)₈·24(H₂O) per unit cell were fully optimized. The models differ by the T···Mg²⁺···T coordination of the interlayer Mg²⁺ cations by two central cations from the adjacent tetrahedral sheets of the 2:1 vermiculite layers (T,T=Si,Al). We observed the formation of very strong hydrogen bonds between water molecules solvating the interlayer Mg²⁺ cations and the surface basal oxygen atoms of the 2:1 layers. The directionality of hydrogen bonds is the major factor determining the layer stacking in the vermiculite structure. Results showed that the most stable model is that where only silicon atoms in the tetrahedral sheets coordinate all interlayer Mg²⁺ cations.     

Influence of structural factors on 29Si and 27Al NMR chemical shifts of phyllosilicates 2:1

The ²⁹Si and ²⁷Al nuclear magnetic resonance (NMR) analysis of synthetic trioctahedral phyllosilicates 2:1, with tetrahedral ratios Al _T/(Si + Al _T) ranging from 0 to 0.5, has shown that the ditrigonal distortion of tetrahedral rings (angle ) is the main factor controlling chemical shift values of tetrahedral components in both signals. The increase of ditrigonal rotation angle shifts these components towards more positive values. For each sample, the composition of tetrahedral and octahedral sheets determine the value of , and from this parameter, the mean tetrahedral Tot angle and the chemical shift values of components are deduced. For a given environment, variations on ditrigonal angle are responsibles for the observed evolution of chemical shift values with bulk composition. The comparative analysis of micas and saponite samples has demonstrated that the location of compensating charge (interlayer and octahedral sheet) does not affect chemical shift values.     

ELNES spectroscopy of mixed Si coordination minerals

 Si K- and L-edge ELNES spectroscopy and multiple-scattering (MS) calculations are used to examine mixed Si coordination compounds varying in Si^VI:Si^IV ratio. As in previous studies, the edges are influenced mainly by silicon coordination (tetrahedral vs. octahedral), as supported by the MS calculations. We demonstrate two methods semi-quantitatively to extract the value of Si^VI/(Si^VI+Si^IV): (1) A linear relationship between the L_2,3-L₁ splitting and Si^VI/(Si^VI+Si^IV) is observed, (2) a fitting method based on the coaddition of reference tetrahedral and octahedral Si spectra is applied to both Si K- and L-edge ELNES spectra. Received: February 10, 1997 / Revised, accepted: May 23, 1997     

Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

Microscopic and spectroscopic studies were conducted to assess mineral transformation processes in aqueous suspensions of illite (Il), vermiculite (Vm) and montmorillonite (Mt) that were subjected to weathering in a simulated high-level radioactive tank waste leachate (0.05 m Al_T, 2 m Na⁺, 1 m NO₃⁻, pH ∼14, Cs⁺ and Sr²⁺ present as co-contaminants). Time series (0 to 369 d) experiments were conducted at 298 K, with initial [Cs]₀ and [Sr]₀ concentrations from 10⁻⁵ to 10⁻ mol kg⁻. Incongruent clay dissolution resulted in an accumulation of secondary aluminosilicate precipitates identified as nitrate-sodalite, nitrate-cancrinite and zeolite X, by molecular spectroscopy and electron microscopy (XRD, IR, NMR, SEM-EDS and TEM-EDS). Contaminant fate was dependent on competing uptake to parent clays and weathering products. TEM-EDS results indicated that high Il affinity for Cs was due to adsorption at frayed edge sites. The Il system also comprised Sr-rich aluminous precipitates after 369 d reaction time. In Mt systems, Cs and Sr were co-precipitated into increasingly recalcitrant spheroidal precipitates over the course of the experiment, whereas contaminant association with montmorillonite platelets was less prevalent. In contrast, Cs and Sr were found in association with weathered Vm particles despite the formation of spheroidal aluminosilicate precipitates that were comparable to those formed from Mt dissolution.     

Crystal-chemistry of natural and heated aluminous orthopyroxenes

X-ray single crystal diffraction data of natural and heated Al-rich orthopyroxenes were used to study the cation ordering and the geometrical changes induced by Si+R ²⁺=Al^IV+R ³⁺ substitution. The calculated site populations and the observed bond distances in tetrahedral and octahedral sites suggest a total ordering of Al^IV in the TB tetrahedron and of R ³⁺ cations in the M1 octahedron, even in samples heated at 1000° C. The mismatch between the tetrahedral and octahedral layers along the c axis seems to play a crucial role in determining the limits of Si/Al^IV substitution.     

Crystal chemistry, surface morphology and X-ray photoelectron spectroscopy of Fe-rich osumilite from Mt. Arci, Sardinia (Italy)

Microprobe analysis, single crystal X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray absorption spectroscopy were applied on Fe-rich osumilite from the volcanic massif of Mt. Arci, Sardinia, Italy. Osumilite belongs to the space group P6/mcc with unit cell parameters a = 10.1550(6), c = 14.306(1) Å and chemical formula (K_0.729)_C (Na_0.029)_B (Si_10.498 Al_1.502)_T1 (Al_2.706 Fe _0.294 ²⁺ )_T2 (Mg_0.735 Mn_0.091 Fe _1.184 ²⁺ )_AO₃₀. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe²⁺ content, unlike the c parameter which does not seem to be affected by chemical composition. The determination of the amount of each element on the mineral surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si_2p, Al_2p, Mg_1s and Fe_2p core levels, suggests that Fe presents Fe²⁺ oxidation state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si⁴⁺ and Si¹⁺ components, respectively, both in tetrahedral coordination. The binding energy of Al_2p, at 74.5 eV, indicates that Al is mostly present in the distorted T2 site, whereas the Mg peak at 1,305.2 eV suggests that this cation is located at the octahedral site. X-ray absorption at the Fe L_2,3-edges confirms that iron is present in the mineral structure, prevalently in the divalent state and at the A octahedral site.     

Geochemical gradients and artifact mass densities on the lowermost Bed II eastern lake margin (~ 1.8 Ma), Olduvai Gorge, Tanzania

Bulk geochemistry of ~ 1.8 Ma lacustrine claystone at Olduvai Gorge, Tanzania, is controlled principally by the geochemistry of ultrafine (< 0.1 μm), authigenic clay minerals. Authigenic clays have an average structural formula of (Si_3.83Al_0.17)(Al_0.43Fe_0.49Mg_0.84)(Na_0.99K_0.22Ca_0.16)O₁₀(OH)₂; octahedral composition varies, with Mg/(Al + Fe) ranging from 0.7 to 2.3. These clay minerals have a complex history of interaction with saline, alkaline water, followed by secondary diagenetic reactions that leached Mg in freshwater paleoenvironments. Lateral variations in whole-rock and clay geochemistry show westward enrichment in Mg, from Mg_oct = 0.6–1.6. This is consistent with persistence of saline, alkaline Paleolake Olduvai to the west, and the presence of groundwater wetlands and other freshwater paleoenvironments to the east. Stone artifact mass density also varies systematically across the basal Bed II deposits, ranging from 10^0.0 to 10^4.3 g of artifacts per cubic meter of excavated sediment. Significant correlation is found between clay geochemistry and the density of artifacts excavated from associated archeological trenches (r² = 0.59, p < 0.01). This relationship supports models of hominin land use in which artifact use and discard is concentrated near freshwater paleoenvironments such as wetlands associated with surface and groundwater discharge. Independent paleoenvironmental proxies such as clay geochemistry allow quantitative hypothesis testing to improve our understanding of early hominin behavior and paleoecology.     

海南岛不同地质背景下的土壤类型、质量特征和作物适宜性

文章利用海南岛SOTER数据库及自动土地评价模型ALES评价结果,对海南岛4种主要母岩上发育的土壤类型、土壤性质及香蕉种植适宜性与母质的关系进行了分析。海南岛不同母岩上土壤类型的发育呈现多样性,土壤类型的分布明显受到母岩类型、成土年龄、成土环境等因素的影响。酸性火成岩上主要发育有雏形土和富铁土,基性岩上则主要是发育良好的铁铝土,碎屑岩上主要是雏形土,而海相沉积物由于土壤形成时间的差异出现了多样的土壤类型。在海南岛湿热的气候条件下,原生母岩上发育的土壤,尽管成土母岩成分各不相同,但一些土壤性质已经没有明显差别,如交换性钙、镁、钠等,部分土壤化学性质如交换性钾的含量继承了母岩特性。各种母岩上发育的土壤,在自然条件下,适宜香蕉种植的比例都很低,但基性火成岩和海积物上发育的土壤,更容易受到人为管理措施的影响,如果满足一定的技术和经济投入,海南岛热带作物的种植潜力可以得到进一步的挖掘。     

Mössbauer and infrared spectrometry of lizardite-1T from Monte Fico, Elba

A well crystallized and homogeneous specimen of lizardite from Monte Fico, Elba, Italy, has been studied by Mössbauer and Fourier transform infrared (FTIR) spectrometries. One of the aims was the determination of the oxidation state and the distribution of iron in the structure of this reference sample. Mössbauer data indicate the presence of octahedral ferrous iron, octahedral ferric iron and tetrahedral ferric iron (59.9, 31.3 and 8.8% of total iron, respectively). The existence of only one octahedral site, previously suggested by X-ray structure refinement, is confirmed. The occurrence of tetrahedrally coordinated iron is indicated also by FTIR spectrometry, in particular by the presence of an absorption band at 790 cm^–1. Based also on new electron microprobe data, the improved crystal chemical formula for lizardite from Monte Fico is: (Mg_2.74Fe²⁺ _0.10Fe³⁺ _0.05Al_0.11)_Σ=3.00 ?· (Si_1.94Al_0.05Fe³⁺ _0.01)_Σ=2.00O_5.05(OH)_3.95.     

The formation of high temperature clay minerals from basalt alteration during hydrothermal discharge on the East Pacific Rise axis at 21°N

Aluminous, high-temperature clay minerals form from alteration of tholeiitic basaltic glass and calcic plagioclase during hydrothermal venting on the crest of the East Pacific Rise at 21°N. The clay alteration assemblages are layered crusts (up to 1 mm thick) completely replacing glass and calcic plagioclase adjacent to surfaces exposed to hydrothermal fluids. The interiors of the affected basalt samples have unaltered appearances and oxygen isotopic compositions just slightly heavier than that of MORB. The surficial alteration crusts are mixtures of beidellitic smectite (aluminous, dioctahedral), randomly interstratified mixed-layer Al-rich chlorite/smectite, minor chlorite, an x-ray amorphous aluminosilicate material, and possible minor serpentine (amesite). A δ¹⁸O value of +4.1 ± 0.2%. (SMOW) is determined for the beidellitic smectite. Assuming that this smectite equilibrated with hydrothermal fluid having an oxygen isotope value between that of seawater (0%.) and 350°C hydrothermal fluid from EPR, 21°N vents (+1.6%.), an equilibration temperature between 290°C and 360°C is calculated for the beidellitic smectite. This is substantially higher than any previously reported temperature for an oceanic smectite. The mixed-layer Al-rich chlorite/smectite has a δ¹⁸O value of +3.5%., which corresponds to equilibration at 295°–360°C. The aluminous composition of the alteration assemblage is uncommon for clay minerals produced by submarine hydrothermal basalt alteration. We suggest that this assemblage is largely the product of high-temperature interaction between basalt glass + plagioclase and Mg-poor, acidic hydrothermal fluids, with possibly some contribution of Mg from bottom seawater, and that the aluminous clays either incorporate Al³⁺ remobilized from basalt by lowpH hydrothermal fluids, or are residual phases remaining after intense alteration of basaltic glass + plagioclase.     

Pyroxene mineral chemistry of the Jinchuan intrusion,China

Summary Chemical compositions of orthopyroxene and clinopyroxene from the Jinchuan ultramafic intrusion have been obtained by electron microprobe analysis. The Mg number (MgO/(MgO + FeO)) for both pyroxenes falls within narrow ranges, 82–87 for clinopyroxene and 81–85.5 for orthopyroxene, suggesting limited magma differentiation in regard to the present igneous body. The Al₂O₃ content ranges from 2.44 wt.% to 4.43 wt.% and increases with decreasing Mg of the pyroxenes, i.e., with the more evolved magma. This is attributed to the relatively greater effects of Al₂O₃, TiO₂, Cr₂O₃ and Fe₂O₃ than that of SiO₂ on pyroxene crystallization.Negative linear relationships between Ti⁴⁺ and Si⁴⁺, and Al³⁺ and Si⁴⁺ characterize the pyroxenes. In clinopyroxene, regression of Si⁴⁺ versus Al³⁺ results in a straight line with a slope of –1.012, indicating that the decrease of Si⁴⁺ in the crystal structure is matched by an increase only in tetrahedral Al³⁺; octahedral Al³⁺ has remained relatively constant. The negative linear relationship between Ti⁴⁺ and Si⁴⁺ in clinopyroxene reflects either a greater tendency of Ti⁴⁺ to occupy octahedral sites than Al³⁺, or that replacement of Al³⁺ for Si⁴⁺ demands a more efficient charge balance. The scatter in plots of Ti⁴⁺ versus Si⁴⁺ for orthopyroxene indicates that charge balance is not as critical as structure symmetry.The crystallization temperature of pyroxene is calculated to be 1108–1229°C usingWood andBanno's (1973) two pyroxene thermometer, and is within 40°C of that calculated fromWells's (1977) thermometer. The distribution coefficient (Kd) for Mg²⁺ and Fe²⁺ between clinopyroxene and orthopyroxene is estimated to be 0.86, which is higher than that of the other intrusions and lower than that of mantle nodules, but still falls within their Kd-1/T trend. This suggests that the Kd value of pyroxene is controlled mainly by temperature.

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Mineralchemie der Pyroxene der Jinchuan-Intrusion, China

Zusammenfassung Die chemische Zusammensetzung von Orthopyroxenen und Klinopyroxenen aus der ultramafischen Jinchuan Intrusion wurden mit der Mikrosonde bestimmt. Die Mg-Zahl (MgO/(MgO + FeO)) beider Pyroxene liegt innerhalb enger Grenzen, 82–87 für Klinopyroxen und 81–85.5 für Orthopyroxen. Dies weist auf beschränkte magmatische Differentiation der Intrusion hin. Der Al₂O₃-Gehalt liegt zwischen 2.44 Gew.%. und 4.43 Gew.%. und nimmt mit der abnehmenden Mg-Zahl der Pyroxene ab, d.h. mit dem mehr entwickelten Magma. Dies wird damit erklärt, daß Al₂O₃, TiO₂, Cr₂O₃ und Fe₂O₃ einen größeren Einfluß auf die Kristallisation der Pyroxene ausüben als SiO₂.Die Pyroxene werden durch negative lineare Beziehungen zwischen Ti⁴⁺ und Si⁴⁺, sowie Al³⁺ und Si⁴⁺ charakterisiert. In Klinopyroxenen resultiert die Regression von Si⁴⁺ gegen Al³⁺ in einer geraden Linie mit einer Neigung von –1.012. Dies weist darauf hin, daß die Abnahme der Si⁴⁺ Gehalte in die Kristallstruktur durch Zunahme von ausschliesslich tetraedrischem Al³⁺ kompensiert wird; oktaedrisches Al³⁺ ist relativ konstant geblieben. Die negative lineare Beziehung zwischen Ti⁴⁺ und Si⁴⁺ in Klinopyroxenen geht entweder auf eine stärkere Tendenz des Ti₄O₂, oktaedrische Plätze zu besetzen zurück, oder darauf daß ein Ersatz von Al³⁺ für Si⁴⁺ einen effizienteren Ladungsausgleich verlangt. Die unregelmäßige Verteilung der Plots von Ti⁴⁺ gegen Si⁴⁺ in Orthopyroxenen läßt erkennen, daß Ladungsausgleich hier nicht so kritisch ist wie die Symmetrie der Struktur.Die Kristallisationstemperatur der Pyroxene wurde mit dem Zwei Pyroxenthermometer nachWood undBanno (1973) mit 1108–1229°C bestimmt. Diese Werte liegen innerhalb von 40°C des vonWells (1977) berechneten. Der Verteilungskoeffizient (Kd) für Mg²⁺ und Fe²⁺ zwischen Klinopyroxen und Orthopyroxen wird auf 0.86 berechnet; das ist höher als der aus anderen Intrusionen und niedriger als der von Mantelxenolithen, fällt aber immer noch innerhalb des Kd-1/T Trends derselben. Dies legt den Gedanken nahe, daß der Kd Wert der Pyroxene hauptsächlich durch Temperatur bestimmt wird.

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With 6 Figures     

Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

The equilibrium (Mg, Fe, Zn)₃Al₂Si₃O₁₂+2Al₂SiO₅=3(Mg, Fe, Zn)Al₂O₄+5SiO₂ garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100^*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al₂O₃–SiO₂–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.     

Si+4 Content of natural phengites

The chemical compositions of white micas separated from adjacent rocks of glaucophane and greenschist facies are compared with respect to their Si⁺⁴ content. The micas are predominantly phengitic, i.e. between muscovite, K[Al₂Si₃AlO₁₀(OH)₂] and celadonite, K[(R⁺²R⁺³)Si₄O₁₀(OH)₂] in composition. Constancy of Si content in micas coming from rocks of different bulk chemical composition but closely similar physical conditions indicates that the silica content of a potassic dioctahedral mica can be used to indicate the pressure and temperature conditions of its formation. This conclusion is in part based upon previous experimental data obtained for synthetic phengites.     

Fe-Mg partition and miscibility gap between coexisting calcic amphiboles from the southern Abukuma Plateau,Japan

Lamellar intergrowths of actinolite and hornblende or aluminous actinolite occur in metamorphosed igneous rocks in the Hitachi metamorphic district, southern Abukuma Plateau. Electron microprobe analyses of five pairs are presented. The Fe-Mg partitioning and the miscibility gap are discussed in terms of an Mg-Fe-Al^IV diagram. The Fe-Mg partition coefficients depend on the Al^IV contents in hornblendes in a manner indicating that the pairs are close to equilibrium. Calcic amphibole pairs of high Fe/Mg ratio are richer in Al^IV than those with low ratio. The Al^IV content of the coexisting actinolite first increases with rising temperature, but then decreases as the temperature increases further. On the other hand, the Al^IV content of coexisting hornblende-aluminous actinolite successively decreases with rising temperature.