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1.
The crystal structure of four birefringent andradite samples (two from Arizona, one from Madagascar, and one from Iran) was refined with the Rietveld method, space group $Ia\overline{3} d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Each sample contains an assemblage of three different cubic phases. From the electron-microprobe (EMPA) results, fine-scale intergrowths in the Arizona-2 and Madagascar samples appear homogeneous with nearly identical compositions of {Ca2.99Mg0.01}Σ3[ ${\text{Fe}}_{1.99}^{3 + }$ ${\text{Mn}}_{0.01}^{3 + }$ ]Σ2(Si2.95Al0.03 ${\text{Fe}}_{0.02}^{3 + }$ )Σ3O12, Adr98 (Arizona-2), and Adr97 (Madagascar). Both samples are near-end-member andradite, ideally {Ca3}[ ${\text{Fe}}_{2}^{3 + }$ ](Si3)O12, so cation ordering in the X, Y, or Z sites is not possible. Because of the large-scale intergrowths, the Arizona-1 and Iran samples contain three different compositions. Arizona-1 has compositions Adr97 (phase-1), Adr93Grs4 (phase-2), and Adr87Grs11 (phase-3). Iran sample has compositions Adr86Uv12 (phase-1), Adr69Uv30 (phase-2), and Adr76Uv22 (phase-3). The crystal structure of the three phases within each sample was modeled quite well as indicated by the Rietveld refinement statistics of reduced χ2 and overall R (F 2) values of, respectively, 1.980 and 0.0291 (Arizona-1); 1.091 and 0.0305 (Arizona-2); 1.362 and 0.0231 (Madagascar); and 1.681 and 0.0304 (Iran). The dominant phase for each sample has the following unit-cell parameters (Å) and weight fractions (%): a = 12.06314(1), 51.93(9) (Arizona-1); 12.04889(1), 52.47(1) (Arizona-2); 12.06276(1), 52.21(8) (Madagascar); and 12.05962(2), 63.3(1) (Iran). For these dominant phases, the distances and site occupancy factors (sofs) in terms of neutral atoms at the Ca(X), Fe(Y), and Si(Z) sites are as follows: <Ca–O> = 2.4348, Fe–O = 2.0121(6), Si–O = 1.6508(6) Å; Ca(sof) = 0.955(2), Fe(sof) = 0.930(2), and Si(sof) = 0.917(2) (Arizona-1); <Ca–O> = 2.4288, Fe–O = 2.0148(7), Si–O = 1.6476(7) Å; Ca(sof) = 0.953(2), Fe(sof) = 0.891(2), and Si(sof) = 0.927(2) (Arizona-2); <Ca–O> = 2.4319, Fe–O = 2.0220(6), Si–O = 1.6460(6) Å; Ca(sof) = 0.955(2), Fe(sof) = 0.941(2), and Si(sof) = 0.939(2) (Madagascar); and <Ca–O> = 2.4344, Fe–O = 2.0156(8), Si–O = 1.6468(8) Å; Ca(sof) = 0.928(2), Fe(sof) = 0.908(2), and Si(sof) = 0.932(3) (Iran). The sofs based on the EMPA results are similar to those obtained from the Rietveld refinement. Each phase in the HRPXRD results can be correlated with a specific chemical composition. For example, the Iran sample composition Adr63Uv30 corresponds to phase-3 that has the smallest unit-cell parameter; Adr76Uv22 corresponds to phase-1 that has the intermediate cell value; and Adr86Uv13 corresponds to phase-2 that has the largest unit-cell parameter. The bond distances compare well with those obtained from radii sum. The three different cubic phases in each sample cause strain that arises from the mismatch of the cubic unit-cell parameters and give rise to birefringence.  相似文献   

2.
The crystal chemistry across the garnet series is examined, and several systematic trends are reported. The crystal structure of three different cubic phases intergrown in a birefringent near end-member andradite from Namibia was refined by the Rietveld method, space group $ Ia\bar{3}d, $ Ia 3 ¯ d , and monochromatic synchrotron high-resolution powder X-ray diffraction data. Electron microprobe results indicate three phases with distinct compositions. The sample is birefringent, indicating that it is not cubic when observed optically. The reduced χ 2 and overall R (F 2) Rietveld refinement values are 1.655 and 0.0284, respectively, so the multi-phase refinement is excellent. The composition, weight %, unit-cell parameter (Å), distances (Å), and site-occupancy factors (sofs) are as follows: phase-1, Adr99, 88.5(1)  %, a = 12.06259(1), average 〈Ca–O〉 = 2.4310, Fe–O = 2.0189(4), Si–O = 1.6490(4) Å, Ca(sof) = 0.948(1), Fe(sof) = 0.934(1), and Si(sof) = 0.940(1). For phase-2: Adr71Grs28, 7.1(1) %, a = 12.00361(5), average 〈Ca–O〉 = 2.440, Fe–O = 1.979(3), Si–O = 1.641(3) Å, Ca(sof) = 0.913(5), Fe(sof) = 0.767(4), and Si(sof) = 0.932(5). For phase-3: Grs79Adr17, 4.4(1) %, a = 11.89719(4), average 〈Ca–O〉 = 2.404, Al–O = 1.935(4), Si–O = 1.667(3) Å, Ca(sof) = 0.944(6), Al(sof) = 1.069(7), and Si(sof) = 0.887(5). The dominant phase-1 (89 %; Adr99) is nearly end-member andradite, Ca3Fe 2 3+ Si3O12, which contains no cation order in the Ca(X) or Fe(Y) sites. The intergrowth of the three cubic phases causes considerable strain in the minor phases-2 and phases-3 that arise from different structural parameters and gives rise to strain-induced birefringence. For comparison, the results for an isotropic, single-phase, grossular–andradite garnet (Grs76Adr21) are also presented. The strain in the minor phases is about 3–5 times more than the unstrained dominant phase-1, or the unstrained single-phase grossular–andradite.  相似文献   

3.
The crystal structure and chemical composition of a crystal of (Mg14?x Cr x )(Si5?x Cr x )O24 (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr2O4–Mg2SiO4 at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å3 and Z = 2. The structure was refined to R 1 = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe2+-bearing Anh-B. Taking into account that Cr3+ is not expected to be Jahn–Teller active, it appears that both the Cr3+–for–Mg and Cr3+–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg13.66(8)Si4.70(6)Cr0.62(4)O24 stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.  相似文献   

4.
Groundwater is a finite resource that is threatened by pollution all over the world. Shimabara City, Nagasaki, Japan, uses groundwater for its main water supply. During recent years, the city has experienced severe nitrate pollution in its groundwater. For better understanding of origin and impact of the pollution, chemical effects and surface–groundwater interactions need to be examined. For this purpose, we developed a methodology that builds on joint geochemical analyses and advanced statistical treatment. Water samples were collected at 42 sampling points in Shimabara including a part of Unzen City. Spatial distribution of water chemistry constituents was assessed by describing Stiff and Piper diagrams using major ions concentrations. The nitrate (NO3?+?NO2–N) concentration in 45% of water samples exceeded permissible Japanese drinking level of 10 mg L??1. Most of the samples showed Ca–HCO3 or Ca–(NO3?+?SO4) water types. Some samples were classified into characteristic water types such as Na–Cl, (Na?+?K)–HCO3, (Na?+?K)–(SO4?+?NO3), and Ca–Cl. Thus, results indicated salt water intrusion from the sea and anthropogenic pollution. At the upstream of Nishi River, although water chemistry was characterized as Ca–HCO3, ion concentrations were higher than those of other rivers. This is probably an effect of disinfection in livestock farming using slaked lime. Positive correlation between NO3? and SO42?, Mg2+, Ca2+, Na+, K+, and Cl? (r?=?0.32–0.64) is evidence that nitrate pollution sources are chemical fertilizers and livestock waste. Principal component analysis showed that chemistry of water samples can be explained by three main components (PCs). PC1 depicts general ion concentration. PC2 and PC3 share influence from chemical fertilizer and livestock waste. Cluster analyses grouped water samples into four main clusters. One of these is the general river chemistry mainly affected by PC1. The others reflect anthropogenic activities and are identified by the combination of the three PCs.  相似文献   

5.
The crystal structure of α-CaSi2O5 synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca1.02Si1.99O5 by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) Å3, yielding the density value, 3.55 g/cm3. The structure is closely related to that of titanite, CaTiSiO5 and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi2O5 at room pressure.  相似文献   

6.
A suite of 14 synthetic Ca-free P2 1 /c low-clinopyroxenes of compositions across the enstatite-ferrosilite (En-Fs) join have been studied by X-ray powder diffraction and Mössbauer spectroscopy. The crystal structure of one sample with composition X Fs=0.39 was determined by single-crystal X-ray diffraction. The powder diffraction data show that there is no significant (<0.25% at X Fs=0.5) excess volume of mixing on the enstatite-ferrosilite join. The molar volumes are described by V=31.261(17)+1.677(23) X Fs cm3.mol(MSiO3)–1. The linear behaviour of volume with composition arises from the compensating effects of non-linear changes in the unit-cell parameters with composition. Unweighted fits to the data yielded the relationships: a=9.6100(15)+0.105(2)X Fs, b=8.8156(28)+0.218(12)X Fs+0.0481(117)X 2 Fs , c?=5.1702(4)+0.0879(21)X Fs–0.0214(21)X 2 Fs , β=108.345(8)+0.947(30)X Fs–0.805(26)X 2 Fs . The strong positive deviation of β from linearity is directly correlated to the difference in site occupancies between M1 and M2 as determined by Mössbauer spectroscopy and arises directly from the expansion of M2-O1 and M2-O3 bonds upon initial substitution of Fe for Mg in enstatite. The hyperfine parameters from Mössbauer spectra are consistent with variations in the average local environment as recorded by the X-ray data. Asymmetric line broadening of the Mössbauer spectra provides evidence for next-nearest neighbour effects, and is consistent with no significant clustering of Fe or Mg within the samples.  相似文献   

7.
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   

8.
There are 59 springs at the Gevas–Gurp?nar–Güzelsu basins, 38 of these springs emerge from the fractured karst aquifers (recrystallized limestone and travertine) and 21 emerge from the Yuksekova ophiolites, K?rkgeçit formation and alluvium. The groundwater samples collected from 38 out of the total of 59 springs, two streams, one lake and 12 wells were analyzed physico-chemically in the year 2002. EC and TDS values of groundwater increased from the marble (high altitude) to the ophiolites and alluvium (toward Lake Van) as a result of carbonate dissolution and connate seawater. Five chemical types of groundwater are identified: Ca–Mg–HCO3, Mg–Ca–HCO3, Mg–Na–HCO3, Na–Ca–HCO3 and Mg–Ca–Na–HCO3. The calculations and hydrochemical interpretations show that the high concentrations of Ca2+, Mg2+ and HCO3 ? as predominant ions in the waters are mainly attributed to carbonate rocks and high pCO2 in soil. Most of the karst springs are oversaturated in calcite, aragonite and dolomite and undersaturated in gypsum, halite and anhydrite. The water–rock interaction processes that singly or in combination influence the chemical composition of each water type include dissolution of carbonate (calcite and dolomite), calcite precipitation, cation exchange and freshening of connate seawater. These processes contribute considerably to the concentration of major ions in the groundwater. Stable isotope contents of the groundwater suggest mainly direct integrative recharge.  相似文献   

9.
Water quality and hydrochemistry of Shariatpur district were evaluated in terms of hydrochemical composition and some important physico-chemical parameters. The groundwater of the study area is good for drinking, domestic as well as for irrigation purposes. Among the major ions, shallow tube well waters give higher concentration of Ca2+ which ranges from 24 to 260 mg/L. The deep tubewell waters show higher concentration of Na+ which varies from 74 to 582 mg/L during dry season. Among the trace elements most of the shallow aquifer samples show higher concentration of Fe2+, Mn2+ and As. Concentration of Fe2+ varies from 0.655 to 18.8 mg/L, and Mn2+ from trace to 0.868 mg/L during dry period. Hydrochemical analyses reveal significant seasonal variation in water quality of shallow aquifer. Both the shallow aquifer and the surface water of the study area are predominantly of Ca–Mg–HCO3 type, while the deep aquifer water is mainly of Na–K–Cl–SO4 type with slight inclination to Ca–Mg–HCO3 type. The study area is suitable for groundwater development if comprehensive and holistic approaches towards water resource management are taken into consideration.  相似文献   

10.
The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca2+, Mg2+, and Si(OH)4 and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M2 structural sites. Loss mainly from M2 sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy.From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca2+ and/or Mg2+ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on aH+ for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.  相似文献   

11.
The phase and melting relations of the C-saturated C–Mg–Fe–Si–O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = ?1.7 to ?1.2, where ΔIW is the deviation of the oxygen fugacity (fO2) from an iron-wüstite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < ?3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe–Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO2, whereas the solubility of Si decreased with increasing fO2. The carbon-bearing phases were graphite, Fe3C, SiC, and Fe alloy melt (Fe–C or Fe–Si–C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < ?6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe–Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth’s core.  相似文献   

12.
Single crystals of Li-aegirine LiFe3+Si2O6 were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe3+Si2O6 is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P21 /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO4 tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P21 /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From 57Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe3+Si2O6. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.  相似文献   

13.
Tremolite (CaxSr1–x)2Mg5[Si8O22/(OH)2] and diopside (CaxSr1–x)Mg[Si2O6] solid solutions have been synthesized hydrothermally in equilibrium with a 1 molar (Ca,Sr)Cl2 aqueous solution at 750°C and 200 MPa. The solid run products have been investigated by optical, electron scanning and high resolution transmission electron microscopy, electron microprobe, X-ray-powder diffraction and Fourier-transform infrared spectroscopy. The synthesized (Ca,Sr)-tremolites are up to 2000 µm long and 30 µm wide, the (Ca,Sr)-diopsides are up to 150 µm long and 20 µm wide. In most runs the tremolites and diopsides are well ordered and chain multiplicity faults are rare. Nearly pure Sr-tremolite (tr0.02Sr-tr0.98) and Sr-diopside (di0.01Sr-di0.99) have been synthesized. A continuous solid solution series, i.e. complete substitution of Sr2+ for Ca2+ on M4-sites exists for (Ca,Sr)-tremolite. Total substitution of Sr2+ for Ca2+ on M2-sites can be assumed for (Ca,Sr)-diopsides. For (Ca,Sr)-tremolites the lattice parameters a, b and β are linear functions of composition and increase with Sr-content whereas c is constant. For the diopside series all 4 lattice parameters are a linear function of composition; a, b, c increase and β decreases with rising Sr-content. The unit cell volume for tremolite increases 3.47% from 906.68 Å3 for tremolite to 938.21 Å3 for Sr-tremolite. For diopside the unit cell volume increases 4.87 % from 439.91 Å3 for diopside to 461.30 Å3 for Sr-diopside. The observed splitting of the OH stretching band in tremolite is caused by different configurations of the next nearest neighbors (multi mode behavior). Resolved single bands can be attributed to the following configurations on the M4-sites: SrSr, SrCa, CaCa and CaMg. The peak positions of these 4 absorption bands are a linear function of composition. They are shifted to lower wavenumbers with increasing Sr-content. No absorption band due to the SrMg configuration on the M4-site is observed. This indicates a very low or negligible cummingtonite component in Sr-rich tremolites, which is also supported by electron microprobe analysis.  相似文献   

14.
We present NMR spectroscopic data, obtained by 1H MAS, 1H static spin-echo, and 29Si{1H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg2SiO4, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow 29Si NMR peaks and display the most de-shielded SiVI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The 1H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The 1H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static 1H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved 1H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P21 mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg2SiO4, coexisting with phases B and superhydrous B, appears to give 29Si{1H} CP-MAS signal, indicating that it contains significant H concentration. The 29Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.  相似文献   

15.
Synthetic, flux-grown uvarovite, Ca3Cr2 [SiO4]3, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–Oa=2.3504(6), Ca–Ob= 2.4971(6) Å]. The structure of Ca3Cr2[SiO4]3 complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr3+ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are =9532 cm?1, =4650 cm?1, power law exponent t 4=6.7, Racah B35=703 cm?1 at 290 K (reference distance R 0=1.995 Å; fixed power law exponent t 2=3 and spin-orbit parameter ζ=135 cm?1). The interelectronic repulsion parameters Racah B 55=714 cm?1 and C=3165 cm?1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm?1 at ambient conditions, which shifts to 3510 cm?1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O4H4)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ?=60417 cm?2 l mol?1, is c H2O=33 ppm.  相似文献   

16.
Tremolite–tschermakite solid solutions have been synthesized between 700 and 850 °C and 200 and 2000 MPa. The starting materials were oxide–hydroxide mixtures and an additional 0.1–1.8 molal CaBr2 solution. The run products were characterized using SEM, HRTEM, EMP, XRD and FTIR. The synthesized Al tremolites formed needles and lath-shaped crystals of up to 300 × 20 μm. HRTEM investigations showed that the majority of the amphiboles were well ordered. The EMP analysis revealed that the Al tremolites were solid solutions in the ternary tremolite–tschermakite–cummingtonite. The highest observed Al content was close to the composition of magnesiohornblende (Xts=0.54). Different cummingtonite concentrations (Xcum=0.00–0.18) were observed, which generally increased with Al content. Rietveld refinements of the lattice constants showed a linear decrease of the cell parameters a and b with increasing Al content, whereas c and β increased. Small deviations from the linear behaviour were caused by variable amounts of the cummingtonite component. For pure tschermakite lattice parameters of a=9.7438(11) Å, b=17.936(14) Å, c=5.2995(3) Å, β=105.68(9)° and V=891.7 ± 1.4 Å3 were extrapolated by least-squares regression. Using the a and β lattice parameters for tremolite, tschermakite and cummingtonite, it was possible to derive amphibole compositions using powder XRD. IR spectra of the Al tremolites showed a total of 12 individual bands. The FWHMs of all bands increased with increasing Al content. According to their FWHMs, these bands were grouped into three band systems at 3664–3676 cm?1 (I), 3633–3664 cm?1 (II) and 3526–3633 cm?1 (III). Assuming [6]Al substitution at M2 and/or M3 and [4]Al at T1, three principal different configurational groups could be assigned as local environments for the proton. I: only Si4+ at T1 and one or two Al3+ at M2 and/or M3far, II: one Al3+ at T1 and one to three Al3+ at M2 and/or at M3far, III: either Al3+ on M3near and/or two Al3+ on T1 and additional one to four Al3+ at M2. It is assumed that these three configurational groups correspond to the three groups of observed bands. This was quantitativly supported by Monte-Carlo simulations. A model with random distribution at M2 and M3 including Al avoidance at tetrahedral and octahedral sites yielded the best agreement with the spectroscopical results.  相似文献   

17.
Babingtonite, Ca2Fe2+Fe3+[Si5O14(OH)] (Z?=?2, space group $ P\overline{1} $ ) from Yakuki mine (Japan), Grönsjöberget (Sweden), Kandivali Quarry (India), Baveno Quarry (Italy), Bråstad Mine (Norway), and Kouragahana (Japan), and manganbabingtonite, Ca2(Mn2+, Fe2+)Fe3+[Si5O14(OH)], from Iron Cap mine (USA) were studied using electron-microprobe analysis (EMPA), 57Fe Mössbauer analysis and single-crystal X-ray diffraction methods to determine the cation distribution at M1 and M2 and to analyze its effect on the crystal structure of babingtonite. Although all studied babingtonite crystals are relatively homogeneous, chemical zonation due to mainly Fe ? Mn substitution is observed in manganbabingtonite. Mössbauer spectra consist of two doublets with isomer shift (I.S.)?=?1.16–1.22 mm/s and quadrupole splitting (Q.S.)?=?2.33–2.50 mm/s and with I.S.?=?0.38–0.42 mm/s and Q.S.?=?0.82–0.90 mm/s, assigned to Fe2+ and Fe3+ at the M1 and M2 octahedral sites, respectively. The determined ratio of Fe2+/total Fe in manganbabingtonite (0.26) was smaller than that in the others (0.35–0.44) because of high Mn2+ content instead of Fe2+. The unit-cell parameters of babingtonite are a?=?7.466–7.478, b?=?11.624–11.642, c?=?6.681–6.690 Å, α?=?91.53–91.59, β?=?93.86–93.94, γ?=?104.20–104.34º, and V?=?560.2–562.3 Å3, and those of manganbabingtonite are a?=?7.4967(3), b?=?11.6632(4), c?=?6.7014(2) Å, α?=?91.602(2), β?=?93.989(2), γ?=?104.574(3)º, and V =565.09(5) Å3. Structural refinements converged to R 1 values of 1.64–3.16 %. The <M1-O> distance was lengthened due to the substitution of large octahedral cations such as Mn2+ for Fe2+. The increase of the M1-O8, M1-O8’ and M1-O13 lengths with mean ionic radii is slightly more pronounced than of the other M1-Oi lengths. The lengthened M1-O13 distance leads the positive correlation between Si5-O15-Si1 angle and M1-O13 distance. The increase of Si2-O3-Si1 and Si5-O12-Si4 angles due to the increase of mean ionic radius of M2 is also observed.  相似文献   

18.
The leaching processes along the flow path and over abstraction of the alluvial aquifer, the principal aquifer in delta Tokar, by the agricultural and domestic sectors and natural factors, have led to its salinization which may be due to interaction between geological formations and adjacent brackish and saline water bodies as well as seawater transgression. The main objectives of this study are to assess the hydrochemical characteristics of the groundwater and to delineate the locations and the sources of aquifer salinization. Water samples in the project area were chemically analyzed for major cations and anions at the laboratory by the standard analytical procedures. Chemical data and water level measurements were manipulated using GIS techniques for hydro chemical and flow direction maps and piper diagram for chemical facies and SPSS software for statistical analyses such as basic statistics (mean and standard deviation) and Spearman’s correlation matrix. The general flow direction of the groundwater is from Southwest towards East and Northeast. The hydraulic gradient is relatively steeper at the apex of the delta (0.06) and amounts to 0.005 at the distal part of the delta. The average transmissivity value of the water bearing formations was found to be about 4.5?×?103 m2/s, whereas, the storage coefficient was about 0.28. A hydrochemical study identified the locations and the sources of aquifer salinization and delineated their areas of influence. The investigation indicates that the aquifer water quality is significantly modified as groundwater flows from the southwestern parts of the study area, where the aquifer receives its water by lateral underflow from Khor Baraka flood plain, to the central and northeastern parts, with few exceptions of scattered anomalous concentration pockets in the deltaic plain. Significant correlation between TDS and/or EC with the major components of Na+, Cl?, and SO 4 ?2 ions is an indication of seawater influence on the groundwater salinity. Moreover, Cl?, SO 4 2? , and Na+ are predominant ions followed by Ca2+ and HCO 3 ? . Hence, four types of groundwater can be chemically distinguished: Na–Ca–SO4–Cl– facies, Na–Cl–SO4–HCO3– facies, Na–Ca–Mg–SO4–Cl–HCO3 facies, and Na–Ca–Mg–Cl–SO4 facies. The processes that govern changes in groundwater composition as revealed by chemical and statistical analyses are mainly associated with over-abstraction, biodegradation, marine intrusions, and carbonate saturation.  相似文献   

19.
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   

20.
Single crystals of C–Na2Si2O5 have been synthesized from the hydrothermal recrystallization of a glass. The title compound is monoclinic, space group P21/c with Z= 8 and unit-cell parameters a= 4.8521 (4)Å, b=23.9793(16)Å, c=8.1410(6)Å, β=90.15(1)° and V=947.2(2)Å3. The structure has been determined by direct methods and belongs to the group of phyllosilicates. It is based on layers of tetrahedra with elliptically six-membered rings in chair conformation. The sequence of directedness within a single ring is UDUDUD. The sheets are parallel to (010) with linking sodium cations in five- and sixfold coordination. Concerning the shape and the conformation of the rings, C–Na2Si2O5 is closely related to β-Na2Si2O5. However, both structures differ in the stacking sequences of the layers. A possible explanation for the frequently observed polysynthetic twinning of phase C is presented. In the 29Si MAS-NMR spectrum of C–Na2Si2O5 four well-resolved lines of equal intensity are observed at ?86.0, ?86.3, ?87.4, and ?88.2?ppm. The narrow range of isotropic chemical shifts reflects the great similarity of the environments of the different Si sites. This lack of pronounced differences in geometry renders a reliable assignment of the resonance lines to the individual sites on the basis of known empiric correlations and geometrical features impossible.  相似文献   

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