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1.
Summary Pyroxene samples, from the Greek island of Syros, taken from a blueschist-eclogite facies Mg-rich metagabbro, were investigated by chemical and XRD analyses and M?ssbauer spectroscopy. Single-crystal XRD and microprobe analysis showed that the natural sample is a typical omphacite of intermediate composition in the Ca–Na pyroxene solid solutions. The space group P2/n was confirmed and the cations Mg, Al and Ca, Na were found to be ordered in the M1 and M2 positions, respectively. M?ssbauer spectroscopy showed that there is both ferrous and ferric iron in the structure, with the ratio 1.38:1. The M2 sites are fully occupied by Ca and Na, thus the iron (Fe2+ and Fe3+) can substitute only for Mg and Al in the M1 sites. Partial disorder was attained by annealing the sample at 850 °C, 20 kbar for 7 days, as confirmed by decrease of intensity of reflections affected by the C-type extinction.  相似文献   

2.
Solid solution in vesuvianite is elucidated by examining chemical trends and cation abundances in 22 microprobe analyses of samples from the Big Maria Mountains, southeastern California. Two recent structure refinements indicate 50 filled cation sites per formula, providing the basis for data normalization. Previous optical absorption and Mössbauer studies help clarify site occupancies. Stoichiometric abundances of Si and Ca + Na indicate 18 and 19 per formula, filling all 4- and 8-fold sites respectively. The four 6-fold A-sites are filled with Al. The solid solution occurs mainly within the eight 6-fold AlFe-sites (Al, Mg, Fe2+, Fe3+, Ti) and one 5-fold B-site (Mg, Fe2+, Fe3+). Chemical trends and crystal chemical constraints delineate eight independent substitutions.An extensive solid solution in the elements Mg, Fe, Al, and Ti suggests considerable potential as a petrogenetic indicator. In order to treat equilibria involving vesuvianite thermodynamically, a reference composition must be chosen and activity-composition relations modeled. For a reference composition, Mg-vesuvianite (Fe, Ti, Na-free) was chosen because of its chemical simplicity, but problems in ascertaining its stoichiometry have led previous workers to propose at least six different formulas. In this study, its formula is determined from the microprobe analyses by applying exchange vectors to substitute components of pure Mg-vesuvianite for Fe and Ti. This yields Ca19Mg2Al11-Si18 O69(OH)9, with AlFe-sites=MgAl7, and B-sites=Mg. Subdivision of the AlFe-sites into at least two distinct sites is suggested by observed chemical trends which are explanable only when different substitutions are considered to operate within different AlFe-sites.A thermodynamic mole fraction is formulated for Mg-vesuvianite based on an ideal mixing-on-sites solution model. A method is provided for estimating the distribution of Fe between the AlFe- and B-sites. Thermodynamic mole fractions calculated using Fe site distributions estimated from microprobe data yield results similar to those calulated using Fe site distributions determined from Mössbauer analysis.  相似文献   

3.
A garnet-omphacite-kyanite-coesite geothermobarometer is constructed to estimate the peak metamorphic pressure of ultrahigh-pressure (UHP) eclogites. For this we use a quaternary regular solution model for garnet, and propose a new model for sodic pyroxene that assumes double site solid-solution (Na,Ca)M2(Al,Fe3+,Fe2+,Mg)M1Si2O6, where cations in the M2- and the M1-sites mix randomly but with inter-site interactions, not only between nearest neighboring cations but also between next nearest neighboring cations. In the new model, overall excess interaction parameters are quantitatively deduced from the previous experimental data and critical temperature, and the activity of the jadeite component agrees reasonably with that based on the Landau theory. Furthermore, the relations of the calculated order parameter to temperature and composition are consistent with other experimental works. These facts support the usefulness of the new model. Application of our geothermobarometer to kyanite eclogite gave metamorphic conditions around 3.4 GPa and 800 ∘C for both samples from the Dora Maira Massif, Western Alps, and from Rongcheng County, in the Su-Lu region of eastern China. Received: 30 December 1996 / Accepted: 25 August 1997  相似文献   

4.
Lithian ferrian enstatite with Li2O = 1.39 wt% and Fe2O3 7.54 wt% was synthesised in the (MgO–Li2O–FeO–SiO2–H2O) system at P = 0.3 GPa, T = 1,000°C, fO2 = +2 Pbca, and a = 18.2113(7), b = 8.8172(3), c = 5.2050(2) Å, V = 835.79(9) Å3. The composition of the orthopyroxene was determined combining EMP, LA-ICP-MS and single-crystal XRD analysis, yielding the unit formula M2(Mg0.59Fe 0.21 2+ Li0.20) M1(Mg0.74Fe 0.20 3+ Fe 0.06 2+ ) Si2O6. Structure refinements done on crystals obtained from synthesis runs with variable Mg-content show that the orthopyroxene is virtually constant in composition and hence in structure, whereas coexisting clinopyroxenes occurring both as individual grains or thin rims around the orthopyroxene crystals have variable amounts of Li, Fe3+ and Mg contents. Structure refinement shows that Li is ordered at the M2 site and Fe3+ is ordered at the M1 site of the orthopyroxene, whereas Mg (and Fe2+) distributes over both octahedral sites. The main geometrical variations observed for Li-rich samples are actually due to the presence of Fe3+, which affects significantly the geometry of the M1 site; changes in the geometry of the M2 site due to the lower coordination of Li are likely to affect both the degree and the kinetics of the non-convergent Fe2+-Mg ordering process in octahedral sites.  相似文献   

5.
A model is proposed for the thermodynamic properties of multicomponent pyroxenes in the composition space defined by the end-member component CaMgSi2O6 and the exchange components Fe(Mg)-1, TiAl2(MgSi2)-1, Fe3+(Al)-1, Fe3+Al(MgSi)-1, and Mg(Ca)-1. It is formulated for the simplifying assumptions that: (1) a molecular mixing type approximation describes changes in the molar configurational entropy associated with the coupled exchange substitutions TiAl2MgSi2, Fe3+AlMgSi, and Al2MgSi (and their ferroan equivalents), and (2) Fe2+ and Mg2+, and Al3+ and Fe3+ display long-range non-convergent ordering between M2 and octahedral M1 sites, and octahedral M1 and tetrahedral sites, respectively. The molar vibrational Gibbs energy is described by a Taylor expansion of second degree in seven linearly independent composition and ordering variables, which is extended to third degree to account for asymmetry in the mixing of Ca and Mg, and Ca and Fe on the M2 site, and is further modified for the assumption that the standard state properties of Ca end-member components of clinopyroxenes are linearly dependent on the coordination number of Ca2+ on the M2 site. The model is shown to be consistent with miscibility gap feaures of pyroxenes in the system CaMgSi2O6–CaTiAl2O6–CaAl2SiO6. In subsequent papers, the model is calibrated for the simplifying assumptions that: (1) all regular-solution-type parameters are constants independent of temperature, (2) Pbca and C2/c end-members have identical heat capacities and coefficients of thermal expansion and compressibility, and (3) the heat capacities and coefficients of thermal expansion and compressibility are zero for all reciprocal reactions relating Pbca and pigeonite or high-calcium pyroxene C2/c endmember components.  相似文献   

6.
Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe3+-rich epidote-group minerals suggests the presence of Fe3+ in most of these amphiboles. An empirical Fe3+ estimation for the microprobe analyses is based on two constraints: the Na?Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) c =-1.479 Na(M4) c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) u and ΣFM(M4) u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) u +1.479 ΣFM u ). From the deficient oxygen the Fe3+ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe3+ contents of four hornblende separates by Mössbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe2+/(Fe2++Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe2+/(Fe2++Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X Ab/X Ed, where X Ab=Na/(Na+Ca) in plagioclase and X Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe?Mg clinopyroxene+anorthite+(NaAlSi-1)Hbl+H2O.  相似文献   

7.
A thermodynamic model for the Gibbs free energy of igneous pyroxenes with the general formula [Na, Ca, Fe2+, Mg]M2[Fe2+, Mg, Ti, Al, Fe3+]M1[Al, Fe3+, Si]TetSiO6 is calibrated from experimentally determined compositions of coexisting pyroxene and silicate melt. The model is based upon the general formulation, and relies upon the calibration of the “quadrilateral” subsystem, previously published by the present authors. The calibration database of pyroxene-liquid equilibria spans a broad spectrum of temperature, pressure and oxygen fugacity conditions, ranging from 1000°–1600°C, 0.001–30 kbar and iron-wüstite to air. Chemical potentials of endmember pyroxene components as well as exchange potentials between pyroxenes and coexisting liquids are defined utilizing the present authors' thermodynamic melt model. Model parameters are extracted from these relations by regression analysis. The resulting model and derivative endmember properties are internally consistent with an existing standard state thermodynamic database. The success of the model and its applicability to igneous petrogenesis are demonstrated by comparing calculated and experimentally determined liquidus compositions, temperatures and symmetry states for pyroxenes crystallizing from a variety of silicate melts, ranging in composition from tholeiites and angrites through rhyolites to potash ankaratrites.  相似文献   

8.
It is well known that in pyroxene structure,there are two metal sites,M1 and M2.Generally speaking,Ferrous iron in each of these sites would normally be expected to give rise to a doublet,However,anomalies have been found in the relative areas of the peaks in the room temperature spectra of some clinopyroxene(CPX)when the above assignment is followed.According to the calculation of Next Nearest Neighbor configurations of divalent cations in M1,we found that the four configurations of M1 can be divided into two groups.One group is 3Ca configuration that increases with the content of Ca(p.f.u);the other group is made up of three No-3Ca configurations that decrease with the content of Ca.The two groups contribute to the spectrum structure of M1.so in this study we fit two doublets for ferrous iron in M1.Though there were several reports on Fe^3 in tetrahedral site previously,it was not sure that Fe^3 occupies the T site is a universal fact in CPX,despite of the content of Al.We found that the Fe^3 in the T site fitted by Moessbauer spectroscopy is negatively correlated to the Si content in the T site and positively correlated to the Fe^3 in the T site estimated on the supposition that Fe^3 and Al occupy the T site randomly.If it is true.it is important in the modeling of ion exchange geobarometries and geothermomeries.  相似文献   

9.
Crystallographic determinations on natural olivine single crystals of mantle composition and origin, carried out by in situ neutron diffraction at high temperature, show that the octahedrally co-ordinated Fe2+ and Mg cations undergo two successive trends of cation ordering with increasing temperature. An initial slight preference of Fe2+ for site M1, up to a temperature of about 850?°C, is followed by a reverse-ordering reaction with a site preference exchange between the two cations. The cross-over between the two regimes of ordering, corresponding to a situation of complete disorder, occurs at about 900?°C. Above this temperature Fe2+ progressively and strongly segregates into site M2 up to 1300?°C, the practical limit of the experimental setup utilized in the experiments. Care was taken to ensure that no chemical changes occurred in the crystals (i.e. oxidation), as testified by Mössbauer spectroscopy determinations carried out before and after the heat treatment. The cation-ordering behaviour is reflected in temperature-dependent changes of geometrical and atomic displacement parameters occurring in the octahedral sites M1 and M2. A thermodynamical explanation of this behaviour is proposed in terms of a prevailing vibrational contribution to entropy.  相似文献   

10.
The partition of Si, Al, Ti, Fe3+, Mg, Fe2+, Mn, Ca and Na between coexisting Ca-rich and Ca-poor pyroxenes from a wide variety of igneous and metamorphic rocks have been investigated systematically. Many of the distributions, and especially those for the partition of Ti, Mg, Fe2+, Mn and Na, indicate characteristic trends for pyroxenes from the various petrologic groups identified. The partition of Mg, Fe2+ and Mn correlate with inferred cooling rates, the partition co-efficients of pyroxenes from extruded and other quickly cooled rocks most nearly approaching unity. In contrast, the partition of Si and Ti and the absolute amounts of Al may be related to the physicochemical conditions prevailing during original crystallisation; Ti being particularly relatively enriched in Ca-rich pyroxenes of ultramafic associations. The trends of the compositions of the Ca-rich pyroxenes plotted in the pyroxene quadrilateral also correlate with cooling rates and comparison with the limited data available on the phase relations of coexisting pyroxenes suggests that sub-solidus chemical readjustments have occurred in both phases.  相似文献   

11.
Oxidation and dehydrogenation processes for heat-treated anthophyllites were investigated using Mössbauer and infrared spectroscopy. At temperatures from 350°C to about 650°C, Fe2+ at the M1 and M3 sites oxidizes, yielding Fe3+ + one electron. A proton from the (OH)– is liberated and combines with this electron to form a hydrogen atom; and some Fe2+ ions at the M2 and M4 sites exchange with Mg at the M1 and M3 sites and then are oxidized in a similar way; at higher temperature, OH remaining in the (MgMgMg/Fe3+)-(OH)-configuration are dehydrogenated by decomposition of the amphibole to orthopyroxene and quartz. During oxidation and dehydrogenation of anthophyllite, there is disordering of Mg and Fe at the M1, M2, M3 and M4 sites in all samples studied. When all Fe2+ is oxidized, the site occupancies of at the M4 and M1, M2, M3 sites become identical, indicating that Mg and Fe3+ are completely disordered at these sites.  相似文献   

12.
Low‐T eclogites in the North Qilian orogen, NW China share a common assemblage of garnet, omphacite, glaucophane, epidote, phengite, quartz and rutile with or without paragonite. Phase relations for the low‐T eclogites can be modelled well in the system NCKFMASHO with the updated solid‐solution models for amphibole and clinopyroxene. Garnet in the eclogite typically exhibits growth zonations in which pyrope increases while grossular somewhat decreases from core to rim, which is modelled as having formed mainly in the P–T conditions of lawsonite‐eclogite facies at the pre‐peak stage. Omphacite shows an increase in jadeite component as aegirine and also total FeO decrease in going from the inclusions in garnet to grains in the matrix, and from core to rim of zoned crystals, reflecting an increase in metamorphic P–T conditions. Glaucophane exhibits a compositional variation in X(gl) (= Fe2+/(Fe2+ + Mg)) and F(gl) (= Fe3+/(Fe3+ + Al) in M2 site), which decrease from the inclusions in garnet to crystals in the matrix, consistent with an increase in P–T conditions. However, for zoned matrix crystals, the X(gl) and F(gl) increase from core to rim, is interpreted to reflect a late‐stage decompression. Using composition isopleths for garnet rim and phengite in P–T pseudosections, peak P–T conditions for three samples Q5–45, Q5–01 and Q7–28 were estimated as 530–540 °C at 2.10–2.25 GPa, 580–590 °C at 2.30–2.45 GPa and 575–590 °C at 2.50–2.65 GPa, respectively, for the same assemblage garnet + omphacite + glaucophane + lawsonite (+ phengite + quartz + rutile) at the peak stage. The eclogites suggest similar P–T ranges to their surrounding felsic–pelitic schists. During post‐peak decompression of the eclogites, the most distinctive change involves the transformation of lawsonite to epidote, releasing large amount of water in the rock. The released fluid promoted further growth of glaucophane at the expense of omphacite and, in appropriate bulk‐rock compositions, paragonite formed. The decompression of eclogite did not lead to pronounced changes in garnet and phengite compositions. Peak P–T conditions of the North Qilian eclogite are well constrained using both the average P–T and pseudosection approaches in Thermocalc. Generally, the conventional garnet–clinopyroxene geothermometer is too sensitive to be used for constraining the temperature of low‐T eclogite because of the uncertainty in Fe3+ determination in omphacite and slight variations in mineral compositions because of incomplete equilibration.  相似文献   

13.
Fifteen samples of (Mg,Fe)SiO3 majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O3 majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe3+ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe2+–Fe3+ charge transfer, where band intensity increases with increasing Fe3+ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe3+ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO3 majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe3+ concentration of (Mg,Fe)(Si,Al)O3 majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O3 perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.  相似文献   

14.
Abstract Chemical analysis (including H2, F2, FeO, Fe2O3) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, Ca18.92Mg1.88Fe3+0.40Al10.97Si17.81- O69.0.1(OH)8.84F0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg-vesuvianite to be Ca19Mg(MgAl7)Al4Si18O69(OH)9, which is related to Fe3+-vesuvianite by the substitutions Mg + OH = Fe3++ O in the B and O(10) sites and Fe3+= Al in the AlFe site. Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgO-Al2O3-SiO2-H2O-CO2 shows that assemblages with Mg-vesuvianite are stable to high T in the absence of quartz and require water-rich conditions (XH2O > 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH2O > 0.97).  相似文献   

15.
The NMR spectra of 27Al and 23Na and the EPR spectra of Fe3+ and Mn2+ have been studied in a range of plagioclase feldspars, of intermediate composition. Many of these crystals have previously been investigated by TEM methods (McLaren and Marshall, 1974). The broad, central 27Al signals observed in labradorites have been interpreted in terms of an overall order/disorder of the Al/Si distribution. Spin Hamiltonian parameters have been determined for the Fe3+ spectra observed in labradorites and bytownites; these spectra were similar and yielded evidence of disordering of the Al/Si distribution. The severe limitations imposed by the variety of sites, the complex microstructures and the disordering, limit the structural information available from the resonance methods.  相似文献   

16.
The oxidation of a synthetic hercynite with composition Fe2+ 0.699Mg0.301Al1.941Fe3+ 0.059O4 was investigated by X-ray single-crystal diffraction. Heating runs at 500°C up to 212 h did not produce detectable oxidation, but only a small variation in oxygen coordinate u, consistent with very limited reordering of Mg and Al in tetrahedral (T) and octahedral (M) sites, respectively. Oxidation began after the first run at 600°C, producing progressive decreases in u, cell parameter a and the mean atomic number in T site. After 1,842 h at 600°C, the transformation was close to equilibrium, with about 70% of ferrous iron transformed into ferric. This produced about 0.17 vacancies per formula unit, and caused a great increase in the displacement parameters of oxygen and M sites. Vacancies were strongly ordered in M sites, and the oxygen displacement parameter becames anisotropic, unlike stoichiometric spinels—because some of the oxygen coordination polyhedra have a vacancy instead of a cation in one vertex. The behaviour of displacement parameters in this case supports the calculated point defect distribution.  相似文献   

17.
A series of clinopyroxenes (Cpx) in peridotitic spinel nodules from Rio Grande do Norte, North-Eastern Brazil, was investigated by X-ray diffraction and electron microprobe analyses and compared with an analogous series from spinel peridotite nodules from Mt. Leura, Victoria, Australia. The examined Cpx series is mainly characterized by increasing Mg/(Mg+Fe2+) ratio (i.e. increase of refractory character of the host nodules), by the substitutions of Alvi vs Mg in the M1 site and Na vs (Ca+Mg) in the M2 site, whereas Aliv in the T site remains substantially constant. The strong Na depletion in the Brazilian Cpx causes a different charge balance from that in the Australian Cpx, in which Na remains quite constant. This feature, associated with the differences in polyhedral and cell volumes which are significantly higher in the Brazilian Cpx than in those of the Australian Cpx, suggests that the Brazilian Cpx crystal chemistry may be related to a lower pressure regime, compared with that of the Australian Cpx series.  相似文献   

18.
This work reports the synthesis of ferri-clinoholmquistite, nominally Li2(Mg3Fe3+2)Si8O22(OH)2, at varying fO2 conditions. Amphibole compositions were characterized by X-ray (powder and single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Mössbauer and Raman) techniques. Under reducing conditions ( NNO+1, where NNO = Nickel–Nickel oxide buffer), the amphibole yield is very high (>90%), but its composition, and in particular the FeO/Fe2O3 ratio, departs significantly from the nominal one. Under oxidizing conditions ( NNO+1.5), the amphibole yield is much lower (<60%, with Li-pyroxene abundant), but its composition is close to the ideal stoichiometry. The exchange vector of relevance for the studied system is M2(Mg,Fe2+) M4(Mg,Fe2+) M2Fe3+–1 M4Li–1, which is still rather unexplored in natural systems. Amphibole crystals of suitable size for structure refinement were obtained only at 800 °C, 0.4 GPa and NNO conditions (sample 152), and have C2/m symmetry. The X-ray powder patterns for all other samples were indexed in the same symmetry; the amphibole closest to ideal composition has a = 9.428(1) Å, b = 17.878(3) Å, c = 5.282(1) Å, = 102.06(2)°, V = 870.8(3) Å3. Mössbauer spectra show that Fe3+ is strongly ordered at M2 in all samples, whereas Fe2+ is disordered over the B and C sites. FTIR analysis shows that the amount of CFe2+ increases for increasingly reducing conditions. FTIR data also provide strong evidence for slight but significant amounts of Li at the A sites.  相似文献   

19.
Four pairs of associated calcic and sodic amphiboles from blueschistfacies metamorphic rocks were analyzed with the electron microprobeand studied by single-crystal X-ray diffraction techniques.Except for ranges in the ratios Mg/(Mg+Fe) and Fe3+/(Fe3++Al+Ti),the sodic amphiboles are similar in chemical composition. Theamount of calcium in the M(4)-site ranges only from 0·18to 0·21 ion per formula unit. The calcic amphiboles,in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio(0·29–0·48). Three of the calcic amphibolescontain less than 1·5 calcium ions per formula unit,indicating a significant solid solution of sodic amphibole componentsin the calcic amphibole phase. The a and b unit-cell parametersof the calcic amphiboles decrease with increased content ofthe sodic component.  相似文献   

20.
Tourmalines from the Kalinovka porphyry copper deposit with epithermal bismuth-gold-basemetal mineralization and the Michurino gold-silver-base-metal prospect have been studied in the South Urals. Tourmaline from the Kalinovka deposit occurs as pockets and veinlets in quartz-sericite metasomatic rock and propylite. The early schorl-“oxy-schorl” [Fetot/(Fetot + Mg) = 0.66?0.81] enriched in Fe3+ is characterized by the homovalent isomorphic substitution of Fe3+ for Al typical of propylites at porphyry copper deposits. The overgrowing tourmalines of the second and third generations from propylite and quartz-sericite metasomatic rock are intermediate members of the dravite-magnesio-foitite solid solution series [Fetot/(Fetot + Mg) = 0.05?0.46] with homovalent substitution of Mg for Fe2+ and coupled substitution of X ? + YAl for XNa + YMg. These substitutions differ from the coupled substitution of YAl + WO2? for YFe2+ + WOH? in tourmaline from quartz-sericite rocks at porphyry copper deposits. At the Michurino prospect, the tourmaline hosted in the chlorite-pyrite-quartz veins and veinlets with Ag-Au-Cu-Pb-Zn mineralization is an intermediate member of the dravite-magnesio-foitite solid solution series [Fetot/(Fetot + Mg) = 0.20?0.31] with homovalent substitution of Mg for Fe2+ and coupled substitutions of X ? + YAl for XNa + YMg identical to that of late tourmaline at the Kalinovka deposit. Thus, tourmalines of the porphyry and epithermal stages are different in isomorphic substitutions, which allow us to consider tourmaline as an indicator of super- or juxtaposed mineralization.  相似文献   

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