Distribution of Al between coexisting micas in metamorphic rocks from the Central Alps

In 61 pairs of coexisting biotites and muscovites from the Central Alps total Al scatters considerably, but in both series a gradual increase is noticed with increasing metamorphic grade. The ratio Al _Mu ^tot /Al _Bi ^tot remains virtually constant (1.61 average for greenschist facies, 1.57 for amphibolite facies). Tetrahedral Al varies little in biotites and increases in muscovites-phengites with rising metamorphic grade; accordingly the ratio Al _Mu ^IV /Al _Bi ^IV increases slightly with grade. Far the best control of metamorphism is evidenced by octahedral Al. In the muscovite series, and still more pronounced in the biotite series, Al^VI increases with increasing metamorphic grade. Consequently 1 $$K_D = \frac{{Al_{Mu}^{VI} }}{{Al_{Bl}^{VI} }}$$ decreases from 14 to 3. A map (Fig. 6) representing the regional distribution of the K_D values locates a 100 km long and 23 km broad central zone with low K_D. The outline of this central core almost coincides with the isograds anorthite-diopside-calcite and labradorite-pyroxene-hornblende of the Tertiary regional metamorphism; with some deviations this core also agrees with the zone in which phenomena of partial anatexis are observed. The K_D values of micas from anateotic pegmatites agree with those of associated gneisses and schists. The study demonstrates that in the course of progressive regional metamorphism equilibrium has been approached to an unexpected extent and that the two micas coexisted in a strict sense.     

Actinolite-hornblende pairs in metamorphosed gabbros,Hidaka mountains,Hokkaido

Actinolite-actinolitic hornblende and actinolitic hornblende-hornblende pairs are described from gabbroic amphibolites and epidote amphibolites formed by dynamic metamorphism during uplift of gabbroic rooks in the Hidaka Metamorphic Belt, Hokkaido. Electron microprobe analyses indicate that coupled substitutions involved in the transition from actinolite to hornblende are essentially those of edenite and tschermakite-ferritschermakite together with smaller amounts of glaucophane-riebeckite, i.e. Al^IV, Al^VI, Fe³⁺, A-site occupancy and Na^M4 increase with replacement of Mg by Fe²⁺+ Mn and Si by Al^IV. During metamorphism the amount of deformation due to shearing has affected the degree of compositional discontinuity in the actinolite-hornblende series and the compositional gap is most pronounced in the epidote amphibolite. The coexisting actinolite-hornblende do not represent an equilibrium pair as textural relations indicate that the actinolitic amphiboles are relics. It is suggested that shearing deformation during uplift has caused an overstepping of the changing physico-chemical conditions of metamorphism so that compositional readjustment of amphiboles was not achieved. Equilibrium-disequilibrium actinolite-hornblende pairs are discussed from other localities where rocks of basaltic composition have been metamorphosed.     

Coarsening kinetics of the exsolution microstructure in alkali feldspar

Actinolite, hornblende and biotite coexisting in greenschist mafic metagreywackes have been analysed with the electron microprobe to obtain information on their chemical relationship during metamorphism. As in some other parts of the world, the two calcic amphiboles coexist in the greenschist facies because of a miscibility gap between them which is observed under conditions of low-pressure regional metamorphism; it is thought that the two amphiboles are in equilibrium, or at least that the actinolite participated in hornblendeforming reactions. Contact metamorphism by granitic intrusives of these metagreywackes has converted them to hornblende hornfelses with the assemblage hornblende, andesine, quartz, biotite±cummingtonite; the hornblendes of the hornfelses are found to have compositions between actinolite and hornblende of the greenschists, and frequently show fine exsolution lamellae of cummingtonite as a result of oversaturation in this component. The distribution of Fe-Mg between hornblende and biotite changes from the greenschist to the hornblende hornfels facies, and the K _D is probably dependent on Al^VI in the hornblende.     

Coexisting pyroxenes — a multivariate statistical approach

A population of 117 coexisting nonalkaline pyroxene pairs has been studied statistically to evaluate compositional and thermal effects on the element distribution. K_DMg^opx-cpx is influenced by the Fe/Mg-ratio, by the Ca content—especially of clinopyroxene—and by the content of tetrahedral Al. Fe and tetrahedral Al are found to be negatively correlated. A principal component analysis based on the variation of Si, Al^IV, Al^VI, Fe, Mg, Mn, Ca is performed. Dropping of highly correlated variables does not affect the result significantly. The first principal component reflects the major chemical variation in Fe and Mg. When using ferrous and ferric iron as separate entries of the analysis, either the second or the third component demonstrates a temperature dependence. It is, however, not possible to obtain pure temperature and chemical components due to the composition not being uncorrelated to temperature of formation. From these components a graph reflecting temperature of formation has been constructed.     

IIb trioctahedral chlorite from the Barberton greenstone belt: crystal structure and rock composition constraints with implications to geothermometry

IIb trioctahedral chlorite in the Barberton greenstone belt (BGB) metavolcanic rocks was formed during pervasive greenschist metamorphism. The chem‐ical composition of the chlorite is highly variable, with the Fe/(Fe+Mg) ratio ranging from 0.12 to 0.8 among 53 samples. The chemical variation of the chlorite results from the chemical diversity of the host rock, especially the MgO content of the rock, but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite. All major cation abundances in the chlorite are strongly correlated with each other. Sil‐icon increases with Mg and decreases with Fe, while Al^IV and Al^VI decrease with Mg and increase with Fe²⁺. A complex exchange vector explains over 90% of the chlorite compositional variation: Mg₄SiFe²⁺ ₋₃Al^VI ₋₁ Al^IV ₋₁, which has 3 parts Fe-Mg substitution coupled with one part tschermakite substitution. This ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets. The subtle change in Al substitution in chlorite implies that Al^VI is preferentially ordered in the M(4) site, and about 84% of the Al^VI present is in the M(4) sites when they are nearly filled with Al^VI. Based on 47 analyzed chlorite-bearing rock samples, chlorite (Chl) composition is strongly correlated with the MgO content of the host rock. Calculated correlation coefficients are +0.91 for SiO_2Chl-MgO_Rock, −0.87 for Al₂O_3Chl-MgO_Rock, +0.89 for MgO_Chl-MgO_Rock, and −0.85 for FeO_Chl-MgO_Rock. Only weak correlations have been found between chlorite oxides and other oxides of rock (between same oxides in chlorite and rock: SiO₂−0.67, Al₂O₃ + 0.59, FeO −0.41). However, MgO_Chl is saturated at about 36 wt% in rocks that have MgO above 22 wt%.The MgO_Chl is about 5 wt% when the host rock approaches 0 wt% of MgO. This implies that Mg substituting into the chlorite is approximately limited to 1.5–9.2 Mg atoms per formula unit and 1.0–3.2 Al^IV. Chlorite geothermometers can not be applied to all BGB samples. However, the empirical chlorite geothermometer based on Al^IV of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study. An estimated temperature of about 320°C for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers. Received: 22 January 1996 / Accepted: 15 August 1996     

The geochemistry of orthoamphiboles and coexisting cordierites and phlogopites from South Norway

A number of orthoamphiboles (18), cordierites (9) and phlogopites (5) in Mg-rich rocks from the Bamble Sector of the Fennoscandian Shield have been separated and analysed for major and 20 trace elements.The ratio of Na to Al^IV in the Bamble orthoamphiboles is approximately 15, which is a smaller ratio than calculated for many other areas. This may result from the low Na content of the majority of parent rocks.The AFM diagram shows that orthoamphiboles termed anthophyllites by one author overlap with gedrites of other authors and vice-versa. In addition there is a wide variation of the AFM components for all orthoamphiboles. This variation can be accounted for by the presence or absence of coexisting cordierite or clinoamphibole. Finally the AFM positions of orthoamphiboles from 3 separate investigations are split into Al-poor and Al-rich fields. This can be explained, at least in part, by the relative abundance of Si and Na in the rock.The partition of Fe²⁺ and Mg between the coexisting minerals is comparable with that established for other metamorphic terrains in spite of the Mg enrichment of the South Norway rocks, indicating probable stability between the minerals.The three coexisting phases show a different preferential partition of trace elements. Orthoamphiboles are enriched in Sc, V, Y, Zr and lanthanides, cordierites in Cu, Ga and Sr, and phlogopites in Cl, Cr, Co, Ni, Rb and Ba. The trace elements concentrations and partitions, for orthoamphibole-phlogopite pairs display strong similarities to those of hornblende-biotite pairs from igneous granites.     

Non-stoichiometry of interlayer cations in micas from low- to middle-grade metamorphic rocks in the Ryoke and the Sanbagawa belts,Japan

Micas in 17 pelitic (K-feldspar-free) and 8 psammitic (K-feldspar-bearing) rocks from the Wazuka and the Asemi-gawa areas in the Ryoke and the Sanbagawa metamorphic terrains, respectively, were analyzed on an electron-probe microanalyzer. The deficiency of alkali cations in the low- to middle-grade metamorphic micas is ascribed to the illite substitution, K^XII+Al^IV=^XII (vacancy)+Si^IV.At the same metamorphic grade, the deficiency of interlayer cations in micas from the pelitic rocks is greater than that from the psammitic rocks. However, it decreases with rising temperature in both rock-types, irrespective of the pressure of metamorphism.K-feldspar and biotite buffer the illite substitution. Two reactions are proposed to explain the decrease of the alkali-cation deficiency in both muscovite and biotite.     

Authigenic layer silicate minerals in borehole Elmore 1, Salton Sea Geothermal Field,California, USA

A combined petrographic/X-ray/electron microprobe and energy dispersive system investigation of sandstone cuttings from borehole Elmore # 1 near the center of the Salton Sea Geothermal Field has revealed numerous regular variations in the composition, texture, mineralogy and proportions of the authigenic layer silicate minerals in the temperature interval 185° C (411.5 m depth) to 361° C (2,169 m). At temperatures near 190° C, dolomite/ankerite+calcite-bearing sandstones contain an illite/mixed layer phase with 10% expandable layers (dolomite/ankerite zone). In shale, the percentage of expandable layers in the mixed layer phase changes from 10–15% at 185° C to 5% at 210° C (494 m). In the interval 250° C (620 m) to 325° C (1,135 m), the calcite+pyrite+epidote-bearing sandstones contain a layer silicate assemblage of chlorite and illite (chlorite-calcite zone). In the shallower portions of this metamorphic zone, the illite contains 0–5% expandable layers, while at depths greater than 725 m (275° C) it is completely free of expandable layers. On increasing temperature, the white mica shows regular decreases in Si^IV, Mg and Fe, and increase in Al^IV, Al^VI, and interlayer occupancy, as it changes gradually from fine-grained illite (=textural sericite) to coarse-grained recrystallized phengitic white mica. In the same interval, chlorite shows decreases in Al^VI and octahedral vacancies and an increase in total Mg+Fe. The sandstones range from relatively unmodified detrital-textured rocks with porosities up to 20% and high contents of illite near 250° C to relatively dense hornfelsic-textured rocks with trace amounts of chlorite and phengite and porosities near 5% at 325° C. Numerous complex reactions among detrital (allogenic) biotite, chlorite, and muscovite, and authigenic illite and chlorite, occur in the chlorite-calcite zone.Biotite appears, and calcite disappears, at a temperature near 325° C and a depth of 1,135m. The biotite zone so produced persists to 360° C in sandstone, at which temperature orthoclase disappears and andradite garnet appears at a depth near 2,155 m. Throughout the biotite zone and into the garnet zone, the biotite undergoes compositional changes that are very similar to those observed in illite/phengite in the chlorite-calcite zone, including increases in interlayer occupancy, Al^IV, Al^VI, and Ti, and decreases in F^–, Si^IV, and Mg/Fe_t+Mg, on increasing temperature. Biotite thus changes from a siliceous, K-deficient biotite at the biotite isograd to a typical low-grade metamorphic biotite at temperatures near 360° C. Minor amounts of talc appear with biotite at the biotite isograd in sandstone, while actinolite appears in both sandstone and shale at temperatures near 340° C (1,325 m). Chlorite completely disappears from sandstone at temperatures of approximately 350° C (1,500 m), and diminishes abruptly in amount in the more chloritic shales at the same depth.     

Corrensite and mixed-layer chlorite/corrensite in metabasalt from northern Taiwan: TEM/AEM,EMPA, XRD,and optical studies

Many chloritic minerals in low-grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties, expand slightly upon glycolation (expandable chlorite) and/or have excess Al^VI relative to Al^IV, as well as significant Ca, K and Na contents. Chloritic minerals with these properties fill vesicles and interstitial void space in low-grade metabasalt from northern Taiwan and have been studied with a combination of TEM/AEM, EMPA, XRD, and optical microscopy. The chloritic minerals include corrensite, which is an ordered 1:1 mixed-layer chlorite/smectite, and expandable chlorite, which is shown to be a mixed-layer chlorite/corrensite. Corrensite and some mixed-layer chlorite/corrensite occur as rims of vesicles and other cavities, while later-formed mixed-layer chlorite/corrensite occupies the vesicle cores. The TEM observations show that the mixed-layer chlorite/corrensite has ca. 20%, and the corrensite has ca. 50% expandable smectite-like layers, consistent with XRD observations and with their abnormal optical properties. The AEM analyses show that high Si and Ca contents, high Al^VI/Al^IV and low Fe^VI/(Fe+Mg)^VI ratios of chlorites are correlated with interstratification of corrensite (or smectite-like) layers in chlorite. The AEM analyses obtained from 200–500 Å thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low Al^IV/Si^IV and low Fe^VI/(Fe+Mg)^VI, while chlorite has high Al^IV/Si^IV and high Fe^VI/(Fe+Mg)^VI. This implies that the trioctahedral smectite-like component of corrensite has significantly lower Al^IV/Si^IV and Fe^VI/(Fe+Mg)^VI. The ratios of Fe^VI/(Fe+Mg)^VI and Al^IV/Si^IV thus decrease in the order chlorite, corrensite, smectite. The proportions of corrensite (or smectite-like) layers relative to chlorite layers in low-grade rocks are inferred to be controlled principally by Fe/Mg ratio in the fluid or the bulk rock and by temperature. Compositional variations of chlorites in low-grade rocks, which appear to correlate with temperature or metamorphic grade, more likely reflect variable proportions of mixed-layered components. The assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases, such as chlorite-corrensite, corrensite-smectite, or chlorite-corrensite-smectite. A model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1:1 ordered mixed-layer chlorite/smectite.     

Distribution of tetrahedral Al and Si in coexisting biotite and Ca-amphibole

The distribution of tetrahedrally coordinated ions between coexisting biotite and Ca-amphibole is found to follow a regular trend of approximately Al _Bi ^IV =1.95+0.35 Al _A ^IV . It is concluded that deviations permit the recognition of disequilibrium mineral associations. The potential influence of crystal-chemical factors is considered.     

Processes of regional metamorphism and ultrametamorphism and some problems of the geology of the deep-lying sections of folded zones

On the basis of 135 pairs of chemical analyses of coexisting hornblendes and biotites, we have established a relationship between the contents of Al^IV, Al^VI, Fe³⁺, Mg, Ti, Na, and K and the overall iron index in the hornblendes and the depth of granitoid formation. This relationship has been emphasized by the R-method of factor analysis. We have examined the strength and nature of the correlations between the elements in the hornblendes and have considered the types of Isomorphism in the amphiboles according to depth, from the viewpoint of crystal chemistry. A regular increase in the amounts of A^IV in hornblende from <0.8 to > 1.6 formula units; of (Al^VI + Fe³⁺ + Ti) from <0.5 to >1. 0 formula units; of (K + Na) from <0.35 to >0. 64 formula units; and of Group A from <0.24 to >0.51 formula units has been recorded from the near-surface granitoids to the ultra-abyssal types. Biotites In this respect display no adequately clear and reliable information.—Authors.     

Amphiboles of a Metagabbro--Amphibolite Sequence, Hidaka Metamorphic Belt, Hokkaido

The western part of the Hidaka Metamorphic Belt, Hokkaido, consistsof primary pyroxene gabbro and lesser amounts of olivine gabbrothat have been dynamically metamorphosed to metagabbro—gabbroicamphibolite-amphibolite-epidote amphibolite during uplift andshearing about 23 m.y. ago. Textures and the presence of relic and recrystallized amphiboleand plagioclase in the same rock indicate incomplete reactionand non attainment of equilibrium during recrystallization. EPMA and bulk analyses of 165 amphiboles indicate a continuousoverall compositional range from actinolite to dark green hornblende(with 100 mg/(Mg+Fe²⁺+Fe³⁺Mn) ratios varying from 89.5 to 32.0)marked by increasing Al, Fe, Ti, and Na. A compositional gapis usually present between relic and recrystallized amphibolesin any one rock which becomes more prominent with increasingshearing. In addition to host rock chemical control, amphibole compositionis largely dependent on the An content of coexisting plagioclase.Locally epidote and sphene exert a strong influence on bothamphibole and plagioclase compositions. Amphibole Ti and Mncontents decrease with shearing and Fe enrichment of the hostrocks largely as a result of the incoming of rutile, sphene,and Fe-Ti oxides. Analysis of host rock oxidation ratio andamphibole compositions indicates that the rocks essentiallybehaved as closed systems to oxygen during metamorphism. Al^1V-Al^IV, Al^IV-Fe³⁺, and Al^IV-(Na, K)^A are the main substitutionsin the amphiboles. Within any one rock the recrystallized amphibolesare enriched in Al, Fe, Ti, and Na relative to the relice amphiboles.Increasing metamorphism results in a progressive change of amphiboles(recrystallized) to more Fe and Si (rather than Al) rich compositionsreflecting the trend towards greenschist where Fe-actinolite(+Mg chlorite) would be stable. Differentiation of the amphiboles is within the limits of SiAlreplacement and the compositional limits of the early stagereaction rim and replacement amphiboles in the host olivineand pyroxene metagabbros.     

Coexisting Amphiboles

Chemical analyses for the following amphibole pairs are presented:anthophyllite—tremolite (or actinolite, or hornblende),cummingtonite (or grunerite)–actinolite (or hornblende),cummingtonite (or grunerite)–anthophyllite (or gedrite),and manganoan cummingtonitemagnesioriebeckite. Nineteen analyses of such pairs are quoted from the literature,and thirty-seven additional pairs have been newly analyzed byelectron probe techniques. Quantitative microprobe determinationsof Si, Al, Fe, Mn, Mg, Ca, and Na were made on polished thin-sections,using naturally occurring, analyzed, homogeneous amphibolesas standards. The literature analyses and the electron probeanalyses for metamorphic, two-amphibole assemblages are givenfor amphiboles in physical contact, which show no textural evidenceof one amphibole being a reaction or alteration product of theother. The chemical data for some of the volcanic, two—amphiboleassemblages were obtained from occurrences that probably donot represent equilibrium pairs. The chemical data are used to determine the extent of the miscibilitygaps between the various amphibole series and the fractionationof the major elements between the two amphiboles of a pair.Anthophyllite and members of the cummingtonite-grunerite seriesgenerally have a larger Fe(total)/Mg ratio than the coexistingcalcic amphibole. The maximum CaO, Al₂O₃and Na₂O contents ofcummingtonite in metamorphic cummingtonite—hornblendepairs are 19 and 32, 02 weight per cent, respectively. Themaximum CaO, A1₂O₃, and Na₂O contents of cummingtonite in metamorphiccummingtonite-hornblende pairs are 19, 32, and 02 weightper cent, respectively. Larger CaO and Al₂O₃ values reportedin the literature were found to be too high because of admixtureof actinolite or hornblende in the analyzed separates. Smallamounts of MnO tend to concentrate preferentially in anthophylliteor cummingtonite of anthophyllite-hornblende and cummingtonite-hornblendepairs. Anthophyllite-cummingtonite pairs may show very similarFe(total)/Mg ratios and differ slightly in Al₂O₃ content only.     

冀东太平寨地区麻粒岩及有关岩石的辉石研究

冀东早太古代迁西群是由火山-沉积岩系变质生成的一套变质杂岩。它的主体是麻粒岩类以及退变质生成的斜长角闪岩类的岩石。该套变质岩系的下部夹有层状和透镜状变质超镁铁质岩(变橄辉岩、变辉橄岩、变辉石岩及变闪辉岩),而上部为变质的含铁岩系。以石渣子山和娄子山为例,它们的含铁岩层分别为辉石磁铁石英岩和英榴易熔岩。 本区的麻粒岩和斜长角闪岩在岩相学上呈渐变关系。绝对不含角闪石和/或富钛黑云母的麻粒岩是很少的。本文将以角闪石和斜长石为主要组成矿物的岩石归为斜长角闪岩类。在斜长角闪岩类岩石中,角闪石含量大大超过辉石,另外常见被角闪石交代的辉石残晶。     

Metamorphic evolution of a subduction complex, South Shetland Islands, Antarctica

A subduction complex composed of ocean floor material mixed with arc-derived metasediments crops out in the Elephant Island group and at Smith Island, South Shetland Islands, Antarctica, with metamorphic ages of 120–80 Ma and 58–47 Ma, respectively. Seven metamorphic zones (I–VII) mapped on Elephant Island delineate a gradual increase in metamorphic grade from the pumpellyite–actinolite facies, through the crossite–epidote blueschist facies, to the lower amphibolite facies. Geothermometry in garnet–amphibole and garnet–biotite pairs yields temperatures of about 350 °C in zone III to about 525 °C in zone VII. Pressures were estimated on the basis of Si content in white mica, Al₂O₃ content in alkali amphibole, Na^M4/Al^IV in sodic-calcic and calcic amphibole, Al^VI/Si in calcic amphibole, and jadeite content in clinopyroxene. Mean values vary from about 6–7.5 kbar in zone II to about 5 kbar in zone VII. Results from the other islands of the Elephant Island group are comparable to those from the main island; Smith Island yielded slightly higher pressures, up to 8 kbar, with temperatures estimated between 300 and 350 °C. Zoned minerals and other textural indications locally enable inference of P–T –t trajectories, all with a clockwise evolution. A reconstruction in space and time of these P–T –t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases (D₁ & D₂) that affected the area. This thermal structure is in good agreement with the one expected for a subduction zone. The arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex. In this model, D₁ represents the subduction movements expressed by the first vector of the clockwise P–T–t path, D₂ reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure, and D₃ corresponds to isostatic uplift accompanied by erosion, under circumstances of decreasing temperature and pressure.     

The effect of Fe-Mg substitution on phase relations in marly rocks of the Western Hohe Tauern (Austria)

Based on mineral assemblages and compositions in metamorphic marly rocks of the Western Hohe Tauern, the effect of Fe-Mg substitution on previously deduced phase relations in a simplified marly rock system (Hoschek 1980) is estimated. Fe-Mg partitioning in coexisting minerals is influenced, amongst others, by the F-OH substitution. Calculations with extrapolated mean K _D ^Fe-Mg values for F-free minerals and with the assumption of ideal Fe-Mg solid solution show similar effects of the Fe-Mg and the F-OH substitution on phase relations. The consideration of the Fe²⁺ and F distribution leads to a better compatibility between experimentally determined mineral stabilities and observed mineral assemblages in the marly rocks of the Western Hohe Tauern.     

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.     

鞍山和歪头山前寒武纪铁建造中普通辉石—阳起石组合的成因研究

鞍山和歪头山前寒武纪铁建造中普通辉石和阳起石共存。经镜下、化学成份和矿物相律的综合研究,作者认为,它们是平衡共生关系。这一共生组合的形成取决于两个因素:(1)原岩成份中的ca/(Mg+Fe~(2+))比值高,(2)角闪岩相下限的变质条件。因此它们形成的范围较窄,在自然界分布极为有限。     

Composition of plagioclase and associated minerals in some schists from Vermont,U.S.A., and South Westland,New Zealand,with inferences about the peristerite solvus

Two suites of regionally metamorphosed semi-pelitic schists were studied in order to investigate the paragenesis of low temperature plagioclase, from which something may be inferred as to the nature of the peristerite solvus at the temperatures and pressures of formation of these rocks: one from the Gile Mountain Formation in the Hanover and Mt. Cube quadrangles, eastern Vermont, U.S.A.; the other from the Alpine schists along the Haast River, South Westland, New Zealand. Plagioclase, muscovite, biotite, chlorite, carbonate, and garnet compositions were determined with an ARL EMX electron probe microanalyzer. The variation in plagioclase composition with increasing grade in the Vermont schists suggests that the peristerite solvus is asymmetrical with a near vertical albite-rich side and a sloping oligoclase-rich side. The top of the solvus appears to lie slightly above the temperature expressed by the almandine isograd in these schists. The compositions of the coexisting albite and oligoclase in the New Zealand rocks suggest a lower geothermal gradient than in Vermont, creating a different pattern of variation in plagioclase composition. Distribution diagrams of Mg, Ti, and Al^IV for muscovite-biotite and chlorite-biotite pairs in both suites of rocks support the hypothesis that the plagioclase relations observed represent equilibrium.     

Discussion of “Thermodynamic parameters of CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-Mg<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>6</Subscript> pyroxenes based on regular solution and cooperative disordering models” by Holland,Navrotsky, and Newton

Phase equilibria in the join CaMgSi₂O₆-CaFeAlSiO₆-CaTiAl₂O₆ have been determined in air at 1 atm by the ordinary quenching method. Clinopyroxene_ss, forsterite, perovskite, magnetite_ss, spinel_ss, hibonite and an unknown phase X are present at liquidus temperatures (ss: solid solution). At subsolidus temperatures the following phase assemblages were encountered; clinopyroxene_ss+perovskite, clinopyroxene_ss +perovskite+spinel_ss, clinopyroxene_ss +perovskite+melilite (+anorthite), clinopyroxene_ss +perovskite+melilite+spinel_ss+anorthite, clinopyroxene_ss +perovskite+anorthite+spinel_ss, and clinopyroxene_ss +perovskite+anorthite+hibonite. At subsolidus temperatures the single phase field of clinopyroxene_ss extends up to 19 wt.% CaTiAl₂O₆. Even in the field of clinopyroxene_ss+perovskite, the TiO₂ content in clinopyroxene_ss continues to increase and attains 9.2 wt.% TiO₂ with 24.8 wt.% Al₂O₃. An interesting fact is that unusual clinopyroxenes which contain more Al^IV than Si^IV are present in the CaFe-AlSiO₆-rich region. The liquid coexisting with pyroxene is richer in Ti, Al, and Fe³⁺ than the coexisting pyroxene. The clinopyroxenes_ss coexisting with liquid contain less TiO₂, Al₂O₃ and Fe₂O₃ than those crystallized at subsolidus temperatures. The petrological significance of the join and the crystallization of Ti- and Al-rich clinopyroxenes are discussed on the basis of the experimental results of the join.