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1.
 Amphiboles were synthesized from bulk compositions prepared along the join Ca1.8Mg5.2Si8O22(OH)2–Ca1.8Mg3Ga4Si6O22(OH)2 hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca2Mg5Si8O22F2–Ca2Mg3Ga4Si6O22F2, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu). The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: [6]Ga3++2[4]Ga3++1/2[A] Ca2+ = [6]Mg2++2[4]Si4++1/2[A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic amphiboles was sought but not observed in this study. Received: 5 March 2001 / Accepted: 31 July 2001  相似文献   

2.
A compressional study of (Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxenes was carried out at high pressures between 10−4 and 10.2 GPa using in situ single-crystal X-ray diffraction, Raman spectroscopy and optical absorption spectroscopy. Compressional discontinuities accompanied by structural changes, in particular, the appearance of two distinct Ti3+–Ti3+ distances within the octahedral chains at 4.37 GPa, provide evidence for the occurrence of a phase transition in NaTi3+Si2O6. Equation-of-state parameters are K 0 = 115.9(7) GPa with K′ = −0.9(3) and K 0 = 102.7(8) GPa with K′ = 4.08(5) for the low- and high-pressure range, respectively. The transition involves a C2/c–P [`1] \overline{1} symmetry change, which can be confirmed by the occurrence of new modes in Raman spectra. Since no significant discontinuity in the evolution of the unit-cell volume with pressure has been observed, the transition appears to be second-order in character. The influence of the coupled substitution Na+Ti3+↔Ca2+Mg2+ on the static compression behavior and the structural stability has been investigated using a sample of the intermediate composition (Na0.54Ca0.46)(Mg0.46Ti0.54)Si2O6. No evidence for a deviation from continuous compression behavior has been found, neither in lattice parameter nor in structural data and the fit of a third-order Birch–Murnaghan equation-of-state to the pressure–volume data yields a bulk modulus of K 0 = 109.1(5) GPa and K′ = 5.02(13). Raman and polarized absorption spectra have been compared to NaTiSi2O6 and reveal major similarities. The main driving force for the phase transition in NaTi3+Si2O6 is the localization of the Ti3+ d-electron and the accompanying distortion, which is suppressed in the (Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxene.  相似文献   

3.
The high-pressure behavior of three synthetic amphiboles crystallized with space group P21/m at room conditions in the system Li2O–Na2O–MgO–SiO2–H2O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions ANa B(Na x Li1 − x Mg1) CMg5 Si8 O22(OH)2 with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O–H dipoles in the P21/m structure, bonded to the same local environment M1M3Mg3–OH–ANa, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P21/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole B(Na0.2Li0.8Mg1) in the frequency range 50 to 1,400 cm−1 also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479–482, 2006a) regarding a new high-pressure amphibole polymorph.  相似文献   

4.
As a first step towards accurate quantification of the polysomatic states of biopyriboles, we have studied the polysomatic transformation between amphibole and hydrous triple-chain silicate (TCS) in the synthetic system Na2O-MgO-SiO2-H2O (NMSH). The reaction is: 4Na2Mg4Si6O16(OH)2 TCS 3Na2.67Mg5.33Si8O21.33(OH)2.67. Amphibole We have characterised a polysomatic intergrowth of amphibole and TCS (synthesized at 2 kbar/(653° C) by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), infrared spectroscopy and 29Si magic-angle-spinning (MAS) NMR spectroscopy. The sample is a fine-scale lamellar intergrowth of double- and triple-chain structures; lamellae are 27 Å to hundreds of Ångströms wide. The 29Si MAS NMR spectrum of the intergrowth is explicitly a superposition of the individual amphibole and TCS spectra. By ensuring that the recycle delay time used considers the longest spin-lattice relaxation time (ca. 900 s), the relative amounts of double- and triple-chain structures can be quantified by simple deconvolution of the spectrum. The relative amounts of double- and triple-chain structures are 42 ± 5 and 58 ± 5 mol%, respectively. With regard to quantifying populations of chain multiplicities in biopyriboles, we believe that 29Si NMR is more accurate than the conventional HRTEM fringe-counting method (Maresch and Czank 1983, 1988), and is far superior to XRD and infrared spectroscopy, which suffer from high sensitivity to particle size and calibration problems. 29Si MAS NMR can provide an accurate means of monitoring the progress of polysomatic reactions in biopyriboles. It is likely to be most effective for samples containing only a few different chain multiplicities (e.g. m = 1, 2, 3 and perhaps 4), such as occur in natural pyroxenes and amphiboles.  相似文献   

5.
OH in zoned amphiboles of eclogite from the western Tianshan,NW-China   总被引：1，自引：0，他引：1  
Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member ( B Na2 C M2+ 3 M3+ 2 T Si8(OH)2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition ( A 0.5 B Ca1.5Na0.5 C M 2+ 4.5 M 0.53+ T Si7.5Al0.5(OH)2) (A = Na and/or K; M 2+ = Mg and Fe2+; M 3+ = Fe3+ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm−1) which can be attributed to a local O(3)-H dipole surrounded by M(1) M(3)Mg3, M(1) M(3)Mg2Fe2+ and M(1) M(3) Fe2+ 3, respectively, an empty A site and T Si8 environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm−1) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.  相似文献   

6.
Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K2ZrSi3O9 (wadeite) and K2TiSi3O9 cyclosilicates under upper mantle conditions. The K2ZrSi3O9–K2TiSi3O9 join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K2ZrSi3O9 is the more refractory end member. At 2 GPa, in the more complex K2ZrSi3O9–K2TiSi3O9–K2Mg6Al2Si6O20(OH)4 system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO2 partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO2 changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K2(Zr,Ti)Si3O9 phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution VITi+VI□=2VIMg. Received: 26 January 1999 / Accepted: 10 January 2000  相似文献   

7.
The synthetic amphibole Na0.95(Li0.95Mg1.05)Mg5Si8O22(OH)2 was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P21 /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm−1, respectively, and two minor bands at 3,667 and 3,687 cm−1. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site (A□0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P21 /m ⇔ C2/m phase transition at 255°C for synthetic amphibole ANa0.8B(Na0.8Mg1.2)CMg5Si8O22(OH)2 indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.  相似文献   

8.
Summary The phase relations of K-richterite, KNaCaMg5Si8O22(OH)2, and phlogopite, K3Mg6 Al2Si6O20(OH)2, have been investigated at pressures of 5–15 GPa and temperatures of 1000–1500 °C. K-richterite is stable to about 1450 °C at 9–10 GPa, where the dp/dT-slope of the decomposition curve changes from positive to negative. At 1000 °C the alkali-rich, low-Al amphibole is stable to more than 14 GPa. Phlogopite has a more limited stability range with a maximum thermal stability limit of 1350 °C at 4–5 GPa and a pressure stability limit of 9–10 GPa at 1000 °C. The high-pressure decomposition reactions for both of the phases produce relatively small amounts of highly alkaline water-dominated fluids, in combination with mineral assemblages that are relatively close to the decomposing hydrous phase in bulk composition. In contrast, the incongruent melting of K-richterite and phlogopite in the 1–3 GPa range involves a larger proportion of hydrous silicate melts. The K-richterite breakdown produces high-Ca pyroxene and orthoenstatite or clinoenstatite at all pressures above 4 GPa. At higher pressures additional phases are: wadeite-structured K2SiVISiIV 3O9 at 10 GPa and 1500 °C, wadeite-structured K2SiVISiIV 3O9 and phase X at 15 GPa and 1500 °C, and stishovite at 15 GPa and 1100 °C. The solid breakdown phases of phlogopite are dominated by pyrope and forsterite. At 9–10 GPa and 1100–1400 °C phase X is an additional phase, partly accompanied by clinoenstatite close to the decomposition curve. Phase X has variable composition. In the KCMSH-system (K2CaMg5Si8O22(OH)2) investigated by Inoue et al. (1998) and in the KMASH-system investigated in this report the compositions are approximately K4Mg8Si8O25(OH)2 and K3.7Mg7.4Al0.6Si8.0O25(OH)2, respectively. Observations from natural compositions and from the phlogopite-diopside system indicate that phlogopite-clinopyroxene assemblages are stable along common geothermal gradients (including subduction zones) to 8–9 GPa and are replaced by K-richterite at higher pressures. The stability relations of the pure end member phases of K-richterite and phlogopite are consistent with these observations, suggesting that K-richterite may be stable into the mantle transition zone, at least along colder slab geotherms. The breakdown of moderate proportions of K-richterite in peridotite in the upper part of the transition zone may be accompanied by the formation of the potassic and hydrous phase X. Additional hydrogen released by this breakdown may dissolve in wadsleyite. Therefore, very small amounts of hydrous fluids may be released during such a decomposition. Received April 10, 2000; revised version accepted November 6, 2000  相似文献   

9.
The distribution of Rb-Na and Rb-K between richterite and a 2-molal aqueous (Na, K, Rb)-chloride solution has been investigated with hydrothermal experiments at 800∘C and 200 MPa. Experiments were performed as syntheses in which amphiboles grew in the presence of an excess fluid containing the exchangeable cations Na+-Rb+ or Na+-K+-Rb+. The obtained amphiboles were large enough (up to 20 m in width) for reliable EMP analysis. They were chemically homogeneous and HRTEM investigations showed that they were structurally well ordered. The Rb, Na, K, Ca and Mg concentrations in coexisting fluids were measured by ICP-AES. According to the possible incorporation of Na, K and Rb on the A-site, solid solutions in the ternary Na(NaCa) Mg5[Si8O22/(OH)2] (richterite)-K(NaCa)Mg5[Si8O22/(OH)2] (K-richterite)-Rb(NaCa)Mg5[Si8O22/(OH)2] (Rb-richterite) were expected. However, Rb-rich richterites always had significant amounts of A-site vacancy concentrations (X □ amph=□ A /(RbA+KA +NaA+□A) of up to 0.42 in the K-free (Na,Rb)-richterites and of up to 0.67 in the (Na, K, Rb)-richterites which corresponds to the same content of tremolite+cummingtonite-component. Amphiboles containing practically only Rb besides vacancies and no Na and/or K on the A-site were also synthesized, however. The Rb-Na and Rb-K exchange coefficients between fluid and richterites are similar. Rubidium always fractionated strongly into the fluid phase. For low Rb-concentrations in richterite (X Rb amph<0.1) a linear correlation between X Rb fluid and X Rb amph exists. In this concentration range, the derived exchange coefficients K D(Rb−K) amph−fluid and K D(Rb−Na) amph−fluid were 0.08 ± 0.04 and 0.04 ± 0.02, respectively. These low exchange coefficients show that significant amounts of Rb in amphiboles require a Rb-rich fluid phase. The results indicate that K-Rb fractionation between alkali amphiboles and fluids is significantly different from K-Rb fractionation between alkali feldspar/ phlogopite and fluid, with KDs of about 0.5 and 1.2, respectively. Formation of richterites will drastically alter the K/Rb-ratios of fluids or melts. These results may have important implications for the genetical interpretation of various geological settings, e.g., MARID-type rocks. Received: 6 October 1997 / Accepted: 7 July 1998  相似文献   

10.
The thermoelastic behaviour of a natural gedrite having the crystal-chemical formula ANa0.47 B(Na0.03 Mg1.05 Fe0.862+ Mn0.02 Ca0.04) C(Mg3.44 Fe0.362+ Al1.15 Ti0.054+) T(Si6.31 Al1.69)O22 W(OH)2 has been studied by single-crystal X-ray diffraction to 973 K (Stage 1). After data collection at 973 K, the crystal was heated to 1,173 K to induce dehydrogenation, which was registered by significant changes in unit-cell parameters, M1–O3 and M3–O3 bond lengths and refined site-scattering values of M1 and M4 sites. These changes and the crystal-chemical formula calculated from structure refinement show that all Fe2+ originally at M4 migrates into the ribbon of octahedrally coordinated sites, where most of it oxidises to Fe3+, and there is a corresponding exchange of Mg from the ribbon into M4. The resulting composition is that of an oxo-gedrite with an inferred crystal-chemical formula ANa0.47 B(Na0.03 Mg1.93 Ca0.04) C(Mg2.56 Mn0.022+ Fe0.102+ Fe1.223+ Al1.15 Ti0.054+) T(Si6.31 Al1.69) O22 W[O1.122− (OH)0.88]. This marked redistribution of Mg and Fe is interpreted as being driven by rapid dehydrogenation at the H3A and H3B sites, such that all available Fe in the structure orders at M1 and M3 sites and is oxidised to Fe3+. Thermoelastic data are reported for gedrite and oxo-gedrite; the latter was measured during cooling from 1,173 to 298 K (Stage 2) and checked after further heating to 1,273 K (Stage 3). The thermoelastic properties of gedrite and oxo-gedrite are compared with each other and those of anthophyllite.  相似文献   

11.
A detailed study of the chemical composition and substitutions in calcium tourmalines from a scapolite-bearing rare-metal pegmatite vein from the Sol’bel’der River basin has shown that their species attribution is determined by occupancy of octahedral site Y. The composition of the yellow tourmaline most abundant in the central part of the pegmatite bodyis rather constant and characterized by the ideal formula Ca(Mg2Li)Al6(Si6O18)(BO3)3(OH)3F. Variations in the chemical composition of zonal tourmaline crystals from the contact part of the pegmatite are controlled by abrupt change in the chemical medium during their formation. The yellow cores of these crystals are close in composition to tourmaline from the central part of the pegmatite vein. The Mg content abruptly decreases toward the crystal margin: Mg2+ → Fe2+, 2Mg2+ → Li+ + Al3+, and Mg2+ + OH → Al3+ + O2−. The composition of dark green marginal zones in tourmaline is characterized by the ideal formula Ca(Al1.5Li1.5)Al6(Si6O18)(BO3)3 (OH2O)(F). The results indicate specific formation conditions of pegmatite. The crystallochemical formulas of the studied tourmalines allow us to regard them as new mineral species in the tourmaline group.  相似文献   

12.
The pseudo-binary system Mg3Al2Si3O12–Na2MgSi5O12 modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet is a liquidus phase of the system up to 60 mol% Na2MgSi5O12 (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases. Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na2O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na2O in garnet is 0.7–0.8 wt% (∼6 mol% Na2MgSi5O12). With the temperature decrease, Na concentration in Grt increases, and the maximal Na2MgSi5O12 content of ∼12 mol% (1.52 wt% Na2O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing <0.4 wt% Na2O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations (>1 wt% Na2O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring Na-bearing garnets is restricted by the eclogite/garnetite bulk composition.  相似文献   

13.
High PT experiments were performed in the range 2.5–19 GPa and 800–1,500°C using a synthetic peridotite doped with trace elements and OH-apatite or with Cl-apatite + phlogopite. The aim of the study was (1) to investigate the stability and phase relations of apatite and its high PT breakdown products, (2) to study the compositional evolution with P and T of phosphate and coexisting silicate phases and (3) to measure the Cl-OH partitioning between apatite and coexisting calcic amphibole, phlogopite and K-richterite. Apatite is stable in a garnet-lherzolite assemblage in the range 2.5–8.7 GPa and 800–1,100°C. The high-P breakdown product of apatite is tuite γ-Ca3 (PO4)2, which is stable in the range 8–15 GPa and 1,100–1,300°C. Coexisting apatite and tuite were observed at 8 GPa/1,050°C and 8.7 GPa/1,000°C. MgO in apatite increases with P from 0.8 wt% at 2.5 GPa to 3.2 wt% at 8.7 GPa. Both apatite and tuite may contain significant Na, Sr and REE with a correlation indicating 2 Ca2+=Na+ + REE3+. Tuite has always higher Sr and REE and lower Fe and Mg than apatite. Phosphorus in the peridotite phases decreases in the order Pmelt ≫ Pgrt ≫ PMg2SiO4 > Pcpx > Popx. The phosphate-saturated P2O5 content of garnet increases from 0.07 wt% at 2.5 GPa to 1.5 wt% at 12.8 GPa. Due to the low bulk Na content of the peridotite, [8]Na[4]P[8]M2+ −1 [4]Si−1 only plays a minor role in controlling the phosphorus content of garnet. Instead, element correlations indicate a major contribution of [6]M2+[4]P[6]M3+ −1 [4]Si−1. Pyroxenes contain ~200–500 ppm P and olivine has 0.14–0.23 wt% P2O5 in the P range 4–8.7 GPa without correlation with P, T or XMg. At ≥12.7 GPa, all Mg2SiO4 polymorphs have <200 ppm P. Coexisting olivine and wadsleyite show an equal preference for phosphorus. In case of coexisting wadsleyite and ringwoodite, the latter fractionates phosphorus. Although garnet shows by far the highest phosphorus concentrations of any peridotite silicate phase, olivine is no less important as phosphorus carrier and could store the entire bulk phosphorus budget of primitive mantle. In the Cl-apatite + phlogopite-doped peridotite, apatite contains 0.65–1.35 wt% Cl in the PT range 2.5–8.7 GPa/800–1,000°C. Apatite coexists with calcic amphibole at 2.5 GPa, phlogopite at 2.5–5 GPa and K-richterite at 7 GPa, and all silicates contain between 0.2 and 0.6 wt% Cl. No solid potassic phase is stable between 5 and 8.7 GPa. Cl strongly increases the solubility of K in hydrous fluids. This may lead to the breakdown of phlogopite and give rise to the local presence in the mantle of fluids strongly enriched in K, Cl, P and incompatible trace elements. Such fluids may get trapped as micro-inclusions in diamonds and provide bulk compositions suitable for the formation of unusual phases such as KCl or hypersilicic Cl-rich mica.  相似文献   

14.
Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 Å phase component ([v]XIISi1K?1Al ?1 IV , where [v]XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg3Si4O10(OH)2 nH2O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa?1) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.  相似文献   

15.
Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases, stable during T = 900–990°C ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe + Mg] = X Mg = 0.69–0.80) and shows wide variation in Na2O content (1.4–2.3 wt.%, NaA = 0.33–0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal to garnet shows an increase in NaA and AlIV from the core (NaA = 0.40–0.51 pfu, AlIV = 1.6–1.9 pfu) to the rim (NaA = 0.49–0.61 pfu, AlIV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H2O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched in Na to form sodicgedrite, which is regarded as a unique feature of high-grade rocks with Mg-Al-rich and K–Si-poor bulk chemistry. We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1,000°C. However, the product gedrite is Na-poor with only <0.13 wt.% Na2O (NaA = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na2O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite. The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H2O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite). The occurrence of abundant primary CO2-rich fluid inclusions in this rock, which possibly infiltrated along the collisional suture during the final amalgamation of the Gondwana supercontinent, strengthens the inference of low water activity.  相似文献   

16.
The solubility of the albite-paragonite-quartz mineral assemblage was measured as a function of NaCl and fluorine concentration at 400°C, 500 bars and at 450°C, 500 and 1000 bars. Decreasing Al concentrations with increasing NaCl molality in F-free fluids of low salinity (mNaCl < 0.01) demonstrates that Al(OH)4− dominates Al speciation and is formed according to the reaction 0.5 NaAl3Si3O12H2(cr)+2 H2O = 0.5 NaAlSi3O8(cr)+Al(OH)4−+H+. Log K results for this reaction are −11.28 ± 0.10 and −10.59 ± 0.10 at 400°C, 500 bars and 450°C, 1000 bars, respectively. Upon further salinity increase, Al concentration becomes constant (at 400°C, 500 bars) or even rises (at 450°C, 1000 bars). The observed Al behavior can be explained by the formation of NaAl(OH)40(aq) or NaAl(OH)3Cl(aq)0. The calculated constant for the reaction Al(OH)4−+Na+=NaAl(OH)40(aq) expressed in log units is equal to 2.46 and 2.04 at 400°C, 500 bars and 450°C, 1000 bars, respectively. These values are in good agreement with the predictions given in Diakonov et al. (1996). Addition of fluoride at m(NaCl) = const = 0.5 caused a sharp increase in Al concentration in equilibrium with the albite-paragonite-quartz mineral assemblage. As fluid pH was also constant, this solubility increase indicates strong aluminum-fluoride complexation with the formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq), according to 0.5 NaAl3Si3O12H2(cr)+Na++HF(aq)0+H2O = 0.5 NaAlSi3O8(cr)+ NaAl(OH)3F(aq)0+H+, log K = −5.17 and −5.23 at 400°C and 450°C, 500 bars, respectively, and 0.5 NaAl3Si3O12H2(cr)+Na++2 HF(aq)0 = 0.5 NaAlSi3O8(cr)+NaAl(OH)2F20(aq)+H+, log K = −2.19 and −1.64 at the same P-T conditions. It was found that temperature increase and pressure decrease promote the formation of Na-Al-OH-F species. Stability of NaAl(OH)2F20(aq) in low-density fluids also increases relative to NaAl(OH)3F(aq)0. These complexes, together with Al(OH)2F(aq)0 and AlOHF20(aq), whose stability constants were calculated from the corundum solubility measured by Soboleva and Zaraisky (1990) and Zaraisky (1994), are likely to dominate Al speciation in metamorphic fluids containing several ppm of fluorine.  相似文献   

17.
 中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg 1Fe+2和Fe-3+2(RⅥ+3)-2□Ⅵ组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg 1Fe+2+Fe-3+2(RⅥ+3)2□Ⅵ.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。  相似文献   

18.
Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope [(Mg2.84Fe0.10Ca0,06) Al2Si3O12], and four synthetic specimens with octahedrally-coordinated silicon: majorite [Mg3(MgSi)Si3O12], calcium-bearing majorite [(Ca0.49Mg2.51)(MgSi)Si3012], sodium majorite [(Na1.88Mgp0.12)(Mg0.06Si1.94)Si3O12], and an intermediate composition [(Na0.37Mg2.48)(Mg0.13Al1.07 Si080) Si3O12]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.  相似文献   

19.
 The crystal structure of MgFe2O4 was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite undergoes a phase transformation at about 25 GPa, which leads to a CaMn2O4-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe2O4 form on the high-pressure phase transformations in the (MgSi)0.75(FeIII)0.5O3 system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe2O4 and Mg2SiO4. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure. We thus conclude for the formation of Mg3Fe2Si3O12 perovskite. Received: 27 March 2000 / Accepted: 1 October 2000  相似文献   

20.
Titanium in phengite: a geobarometer for high temperature eclogites   总被引：1，自引：1，他引：0  
Phengite chemistry has been investigated in experiments on a natural SiO2–TiO2-saturated greywacke and a natural SiO2–TiO2–Al2SiO5-saturated pelite, at 1.5–8.0 GPa and 800–1,050°C. High Ti-contents (0.3–3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si + R2+ = AlIV + AlVI) coupled with the substitution: AlVI + Si = Ti + AlIV. The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si3Al]O10(OH)2, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel + Rt = TiP + Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T–P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust.  相似文献   

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