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1.
The crystal chemistry of a ferroaxinite from Colebrook Hill, Rosebery district, Tasmania, Australia, was investigated by electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma–atomic emission spectroscopy (ICP–AES), 57Fe Mössbauer spectroscopy and single-crystal neutron diffraction at 293 K. The chemical formula obtained on the basis of the ICP–AES data is the following: \( ^{X1,X2} {\text{Ca}}_{4.03} \,^{Y} \left( {{\text{Mn}}_{0.42} {\text{Mg}}_{0.23} {\text{Fe}}^{2 + }_{1.39} } \right)_{\varSigma 2.04} \,^{Z1,Z2} \left( {{\text{Fe}}^{3 + }_{0.15} {\text{Al}}_{3.55} {\text{Ti}}_{0.12} } \right)_{\varSigma 3.82} \,^{T1,T2,T3,T4} \left( {{\text{Ti}}_{0.03} {\text{Si}}_{7.97} } \right)_{\varSigma 8} \,^{T5} {\text{B}}_{1.96} {\text{O}}_{30} \left( {\text{OH}} \right)_{2.18} \). The 57Fe Mössbauer spectrum shows unambiguously the occurrence of Fe2+ and Fe3+ in octahedral coordination only, with Fe2+/Fe3+ = 9:1. The neutron structure refinement provides a structure model in general agreement with the previous experimental findings: the tetrahedral T1, T2, T3 and T4 sites are fully occupied by Si, whereas the T5 site is fully occupied by B, with no evidence of Si at the T5, or Al or Fe3+ at the T1–T5 sites. The structural and chemical data of this study suggest that the amount of B in ferroaxinite is that expected from the ideal stoichiometry: 2 a.p.f.u. (for 32 O). The atomic distribution among the X1, X2, Y, Z1 and Z2 sites obtained by neutron structure refinement is in good agreement with that based on the ICP–AES data. For the first time, an unambiguous localization of the H site is obtained, which forms a hydroxyl group with the oxygen atom at the O16 site as donor. The H-bonding scheme in axinite structure is now fully described: the O16–H distance (corrected for riding motion effect) is 0.991(1) Å and an asymmetric bifurcated bonding configuration occurs, with O5 and O13 as acceptors [i.e. with O16···O5 = 3.096(1) Å, H···O5 = 2.450(1) Å and O16–H···O5 = 123.9(1)°; O16···O13 = 2.777(1) Å, H···O13 = 1.914(1) Å and O16–H···O13 = 146.9(1)°].  相似文献   

2.
Genesis of metaautinute [Ca(UO2/PO4)2 · 7H2O] starting from curite hints at the existence of an intermediate hydrogen autunite stage [HUO2PO4 · 4H2O]. The substitution of protons in hydrogen autunite by Ca2+ ions is proved by electrokinetic measurements. As a consequence of the similarity between X-ray powder patterns of hydrogen autunite and meta-autunite a glycolation method has been applied in order to distinguish the two species. The cell dimensions have been determined from Guinier X-ray diffraction patterns. Both minerals are tetragonal with a=6.981±0.005 Å and c=8.448±0.005 Å for metaautunite and a=7.084±0.005 Å and c=8.777±0.005 Å for hydrogen autunite. For both minerals, the zeta-potential is mostly negative and is strongly influenced by temperature, pH and concentration of cations in the suspension. The surface conductivity has been calculated from the value of the zetapotential. The electrokinetic properties of metaautunite are very similar to those of metatorbernite.  相似文献   

3.
Ephesite, Na(LiAl2) [Al2Si2O10] (OH)2, has been synthesized for the first time by hydrothermal treatment of a gel of requisite composition at 300≦T(° C)≦700 and \(P_{H_2 O}\) upto 35 kbar. At \(P_{H_2 O}\) between 7 and 35 kbar and above 500° C, only the 2M1 polytype is obtained. At lower temperatures and pressures, the 1M polytype crystallizes first, which then inverts to the 2M1 polytype with increasing run duration. The X-ray diffraction patterns of the 1M and 2M1 poly types can be indexed unambiguously on the basis of the space groups C2 and Cc, respectively. At its upper thermal stability limit, 2M1 ephesite decomposes according to the reaction (1) $$\begin{gathered} {\text{Na(LiAl}}_{\text{2}} {\text{) [Al}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{{\text{10}}} {\text{] (OH)}}_{\text{2}} \hfill \\ {\text{ephesite}} \hfill \\ {\text{ = Na[AlSiO}}_{\text{4}} {\text{] + LiAl[SiO}}_{\text{4}} {\text{] + }}\alpha {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}} {\text{ + H}}_{\text{2}} {\text{O}} \hfill \\ {\text{nepheline }}\alpha {\text{ - eucryptite corundum}} \hfill \\ \end{gathered}$$ Five reversal brackets for (1) have been established experimentally in the temperature range 590–750° C, at \(P_{H_2 O}\) between 400 and 2500 bars. The equilibrium constant, K, for this reaction may be expressed as (2) $$log K{\text{ = }}log f_{{\text{H}}_{\text{2}} O}^* = 7.5217 - 4388/T + 0.0234 (P - 1)T$$ where \(f_{H_2 O}^* = f_{H_2 O} (P,T)/f_{H_2 O}^0\) (1,T), with T given in degrees K, and P in bars. Combining these experimental data with known thermodynamic properties of the decomposition products in (1), the following standard state (1 bar, 298.15 K) thermodynamic data for ephesite were calculated: H f,298.15 0 =-6237372 J/mol, S 298.15 0 =300.455 J/K·mol, G 298.15 0 =-5851994 J/mol, and V 298.15 0 =13.1468 J/bar·mol.  相似文献   

4.
We have collected high resolution neutron powder diffraction patterns from Na2SO4·10D2O over the temperature range 4.2–300 K following rapid quenching in liquid nitrogen, and over a series of slow warming and cooling cycles. The crystal is monoclinic, space-group P21/c (Z = 4) with a = 11.44214(4) Å, b = 10.34276(4) Å, c = 12.75486(6) Å, β = 107.847(1)°, and V = 1436.794(8) Å3 at 4.2 K (slowly cooled), and a = 11.51472(6) Å, b = 10.36495(6) Å, c = 12.84651(7) Å, β = 107.7543(1)°, V = 1460.20(1) Å3 at 300 K. Structures were refined to R P (Rietveld powder residual, \( R_{P} = {{\sum {\left| {I_{\text{obs}} - I_{\text{calc}} } \right|} } \mathord{\left/ {\vphantom {{\sum {\left| {I_{\text{obs}} - I_{\text{calc}} } \right|} } {\sum {I_{\text{obs}} } }}} \right. \kern-\nulldelimiterspace} {\sum {I_{\text{obs}} } }} \)) better than 2.5% at 4.2 K (quenched and slow cooled), 150 and 300 K. The sulfate disorder observed previously by Levy and Lisensky (Acta Cryst B34:3502–3510, 1978) was not present in our specimen, but we did observe changes with temperature in deuteron occupancies of the orientationally disordered water molecules coordinated to Na. The temperature dependence of the unit-cell volume from 4.2 to 300 K is well represented by a simple polynomial of the form V = ? 4.143(1) × 10?7 T 3 + 0.00047(2) T2 ? 0.027(2) T + 1437.0(1) Å3 (R 2 = 99.98%). The coefficient of volume thermal expansion, α V , is positive above 40 K, and displays a similar magnitude and temperature dependence to α V in deuterated epsomite and meridianiite. The relationship between the magnitude and orientation of the principal axes of the thermal expansion tensor and the main structural elements are discussed; freezing in of deuteron disorder in the quenched specimen affects the thermal expansion, manifested most obviously as a change in the behaviour of the unit-cell parameter β.  相似文献   

5.
Calcium and magnesium‐bearing sabugalite occurs as aggregations of yellowish platy crystals in veinlets or druses in conglomerate from the oxidized parts of the Tono uranium deposit, Central Japan. X‐ray powder diffractometry of this mineral has reflections consistent with previous powder diffraction data of sabugalite. It is included in the monoclinic system with space group C2/m and calculated cell parameters of a = 19.68Å, b = 9.89Å, c = 9.82Å α = γ = 90°, β‐96.93° and V = 1897.83Å3. Chemical analysis yields a formula of (Ca0.10 Mg0.09)Σ0.19Al0.53(UO2)2.04((PO4)1.99(AsO4)0.01)Σ2.00·11.22H2O. EMPA mapping shows that the mineral is compositionally uniform with no micron‐scale layering. Charge of cations including Ca and Mg in the cation‐H2O layer is 1.98 being identical to that of autunite group minerals. This suggests that the charge balance in the cation‐H2O layer of the mineral could be made by the alkaline earth or alkaline elements rather than by hydrogen ions.  相似文献   

6.
A Ca- and Mn-poor clino-amphibole with Mg/Mg+Fetot+Mn (atomic ratio)=0.81 is described. The structural formula is $$Na_{0.09} (Ca_{0.19} Mg_{5.45} Fe_{1.23}^{2 + } Mn_{0.04} Fe_{0.00}^{3 + } Ti_{0.01} Al_{0.07} )_{6.99} [(Si_{7.83} Al_{0.17} )_{8.00} O_{22} /(OH)_2 ].$$ The unit-cell constants area 0=9.49 Å,b 0=18.00 Å,c 0=5.30 Å, β=102.0°,V 0=886 Å3, the refractive indices αNa=1.621, βNa=1.632, and γNa=1.643. These values, when plotted against the Mg/Mg+Fe ratio, fit the extrapolations towards Mg7[Si8O22/(OH)2] from recently published determinative curves for the cummingtonite series. The clino-amphibole, or part of it, has space groupP21/m rather thanC2/m. The most magnesian cummingtonites reported thus far have Mg/Mg+Fe+Mn ratios around 0.7, but recently more magnesian Ca-poorP21/m clino-amphiboles have been reported. Although Ca and Mn have been claimed to stabilize cummingtonite as against anthophyllite, most magnesian cummingtonites appear to have <0.24 Ca, and <0.1 Mn per formula unit. The nomenclature of the cummingtonite series is discussed. Retaining the subdivision of the cummingtonite series at Mg/Mg+Fe=0.5, the author proposes to reviveTilley’s (1939) name magnesiocummingtonite for members with Mg/Mg+Fe >0.5. Grunerite is reserved for members with Mg/Mg+Fe <0.5. The space group,C2/m orP21/m, may be indicated with a suffix, if known.  相似文献   

7.
The crystal structure of a new compound, [(H5O2)(H3O)(H2O)][(UO2)(SeO4)2] (monoclinic, P21/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) Å, β = 103.880(13)°, V = 1182.4(3) Å3), has been solved by direct methods and refined to R 1 = 0.036. The structure is based on [(UO2)(SeO4)2]2? sheet complexes formed by corner-shared UO7 pentagonal bipyramids and SeO4 tetrahedrons. The sheets are parallel to the ( $ \bar 1 The crystal structure of a new compound, [(H5O2)(H3O)(H2O)][(UO2)(SeO4)2] (monoclinic, P21/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) ?, β = 103.880(13)°, V = 1182.4(3) ?3), has been solved by direct methods and refined to R 1 = 0.036. The structure is based on [(UO2)(SeO4)2]2− sheet complexes formed by corner-shared UO7 pentagonal bipyramids and SeO4 tetrahedrons. The sheets are parallel to the (01) plane. Oxonium ions and water molecules forming [(H3O)·(H2O)·(H5O2)]2+ complexes are interlayer. Among minerals, the existence of (H5O2)+ has been unambiguously confirmed only in rhomboclase, (H5O2)+[Fe2(SO4)2(H2O)2]. Original Russian Text ? S.V. Krivovichev, 2008, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2008, No. 2, pp. 123–130.  相似文献   

8.
The crystal structures of two new compounds (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) Å3) and (H3O)2[(UO2)(SeO4)2(H2O)](H2O) (2, monoclinic, P21/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) Å3) have been solved by direct methods and refined to R 1 = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO2)(SeO4)2]2? formed by cornershared [(UO2)O4(H2O)] bipyramids and SeO4 tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO2)(SeO4)2(H2O)]2? layers are linked by hydrogen bonds via interlayer groups H2O and H3O+. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H2O2)+[Fe(SO4)2(H2O)2] and goldichite K[Fe(SO4)2(H2O)2](H2O)2, respectively.  相似文献   

9.
Meridianiite, MgSO4·11H2O, is the most highly hydrated phase in the binary MgSO4–H2O system. Lower hydrates in the MgSO4–H2O system have end-member analogues containing alternative divalent metal cations (Ni2+, Zn2+, Mn2+, Cu2+, Fe2+, and Co2+) and exhibit extensive solid solution with MgSO4 and with one another, but no other undecahydrate is known. We have prepared aqueous MgSO4 solutions doped with these other cations in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. The solid products have been characterised by X-ray powder diffraction, and the onset of partial melting has been quantified using a thermal probe. We have established that of the seven end-member metal sulfates studied, only MgSO4 forms an undecahydrate; ZnSO4 forms an orthorhombic heptahydrate (synthetic goslarite), MnSO4, FeSO4, and CoSO4 form monoclinic heptahydrates (syn. mallardite, melanterite, bieberite, respectively), and CuSO4 crystallises as the well-known triclinic pentahydrate (syn. chalcanthite). NiSO4 forms a new hydrate which has been indexed with a triclinic unit cell of dimensions a = 6.1275(1) Å, b = 6.8628(1) Å, c = 12.6318(2) Å, α = 92.904(2)°, β = 97.678(2)°, and γ = 96.618(2)°. The unit-cell volume of this crystal, V = 521.74(1) Å3, is consistent with it being an octahydrate, NiSO4·8H2O. Further analysis of doped specimens has shown that synthetic meridianiite is able to accommodate significant quantities of foreign cations in its structure; of the order 50 mol. % Co2+ or Mn2+, 20–30 mol. % Ni2+ or Zn2+, but less than 10 mol. % of Cu2+ or Fe2+. In three of the systems we examined, an ‘intermediate’ phase occurred that differed in hydration state both from the Mg-bearing meridianiite end-member and the pure dopant end-member hydrate. In the case of CuSO4, we observed a melanterite-structured heptahydrate at Cu/(Cu + Mg) = 0.5, which we identify as synthetic alpersite [(Mg0.5Cu0.5)SO4·7H2O)]. In the NiSO4- and ZnSO4-doped systems we characterised an entirely new hydrate which could also be identified to a lesser degree in the CuSO4- and the FeSO4-doped systems. The Ni-doped substance has been indexed with a monoclinic unit-cell of dimensions a = 6.7488(2) Å, b = 11.9613(4) Å, c = 14.6321(5) Å, and β = 95.047(3)°, systematic absences being indicative of space-group P21/c with Z = 4. The unit-cell volume, V = 1,176.59(5) Å3, is consistent with it being an enneahydrate [i.e. (Mg0.5Ni0.5)SO4·9H2O)]. Similarly, the new Zn-bearing enneahydrate has refined unit cell dimensions of a = 6.7555(3) Å, b = 11.9834(5) Å, c = 14.6666(8) Å, β = 95.020(4)°, V = 1,182.77(7) Å3, and the new Fe-bearing enneahydrate has refined unit cell dimensions of a = 6.7726(3) Å, b = 12.0077(3) Å, c = 14.6920(5) Å, β = 95.037(3)°, and V = 1,190.20(6) Å3. The observation that synthetic meridianiite can form in the presence of, and accommodate significant quantities of other ions increases the likelihood that this mineral will occur naturally on Mars—and elsewhere in the outer solar system—in metalliferous brines.  相似文献   

10.
The effects of temperature on the crystal structure of a natural epidote [Ca1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) Å and β = 115.36(5)°, Sp. Gr. P21 /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe3+ at the M3 site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)–H(1)···O(2) and O(10)–H(1)···O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)–H(1)···O(4) is energetically more favourable, as H(1)···O(4) = 1.9731(28) Å, O(10)···O(4) = 2.9318(22) Å and O(10)–H(1)···O4 = 166.7(2)°, whereas H(1)···O(2) = 2.5921(23) Å, O(10)···O(2) = 2.8221(17) Å and O(10)–H(1)···O2 = 93.3(1)°. The O(10)–H(1) bond distance corrected for “riding motion” is 0.9943 Å. The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe3+ share the M(3) site along with Al3+ [%Fe(M3)1,070K = 74(2)%]. The refined amount of Fe3+ at the M(1) is not significant [%Fe(M1)1,070K = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for “rigid body motions”. The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)–O(9)–Si(1) of the Si2O7 group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)···O(4) and H(1)···O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe3+ between different octahedral sites and/or convergent ordering on M(3) are discussed.  相似文献   

11.
Sabugalite has been synthesized directly from pure chemicals. From chemical, differential thermal and thermogravimetric analyses, its formula is calculated as HA1(UO2/PO4)2·16H2O. The natural relationship between hydrogen autunite, autunite and sabugalite was investigated by means of ion exchange experiments, and its infrared spectrum, electrokinetic properties and solubility studied. An increase in solubility results in a more positive zeta-potential. The cell dimensions have been determined from Guinier-Hägg diffraction data. Synthetic sabugalite crystallizes in the monoclinic system with space group C2/m and cell parameters: a=19.426 Å; b=9.843 Å; c=9.850 Å; α=γ=90°; β=96.161°; V=1,872.54 Å3 and Z=2.  相似文献   

12.
We have collected high-resolution neutron powder diffraction patterns from MgSO4·11D2O over the temperature range 4.2–250 K. The crystal is triclinic, space-group \( \text{P} \bar{1} \) (Z = 2) with a = 6.72746(6) Å, b = 6.78141(6) Å, c = 17.31803(13) Å, α = 88.2062(6)°, β = 89.4473(8)°, γ = 62.6075(5)°, and V = 701.140(6) Å3 at 4.2 K, and a = 6.75081(3) Å, b = 6.81463(3) Å, c = 17.29241(6) Å, α = 88.1183(3)°, β = 89.4808(3)°, γ = 62.6891(3)°, and V = 706.450(3) Å3 at 250 K. Structures were refined to wRp = 3.99 and 2.84% at 4.2 and 250 K, respectively. The temperature dependence of the lattice parameters over the intervening range have been fitted with a modified Einstein oscillator model which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion, αV, is considerably smaller than ice Ih at all temperatures, and smaller even than MgSO4·7D2O (although ?αV/?T is very similar for both sulfates); MgSO4·11D2O exhibits negative αV below 55 K (compared to 70 K in D2O ice Ih and 20 K in MgSO4·7D2O) The relationship between the magnitude and orientation of the principal axes of the expansion tensor and the main structural elements are discussed.  相似文献   

13.
Single-phase K-cymrite, K[AlSi3O8]·H2O, has been synthesized in the P-T range 3≤P(GPa)≤4 and 350≤T(°C)≤650, and characterized by a variety of techniques like SEM, FTIR, and 29Si MAS-NMR. Its thermal expansivity and compressibility have been measured up to 375?°C and 6.0?GPa, respectively. Within the uncertainty of the microchemical determination of H2O by Karl-Fischer titration, it invariably contains 1?mol of H2O per mol of KAlSi3O8. Under the SEM, it appears a small idiomorphic prisms. It is optically negative, with n o=1.553(1) and n e=1.521(1). FTIR spectrum identifies the water in its structure as molecular H2O. Its lattice constants are a=5.3348(1)?Å, c=7.7057(1) Å, V= 189.924 Å3, the space group being P6/mmm. The 29Si MAS-NMR suggests a weak short-range order of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural with cymrite (BaAl2Si2O8·H2O), the structure comprising double tetrahedral sheets with H2O molecules residing in their cavities, K serving as an interlayer cation. Whereas cymrite, with its ordered tetrahedral Al/Si distribution, shows a Pm symmetry, the weak short-range Al/Si order in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[AlSi3O8]·H2O (KCym)=K[AlSi3O8] (Kfs)+H2O, executed in this study, confirm the earlier results of Thompson (1994) and supplement her data. A simultaneous treatment of those reversals, together with the thermodynamic data for Kfs and H2O available in the literature, helps derive the standard enthalpy of formation (?4233±9.4?kJ/mol) and standard entropy (276.3±10.2 J/K·mol) for K-cymrite. The computed phase relations of KCym in the KAlSi3O8-H2O binary are shown in Figure 4 for three different values of aH 2O. Given a 5?°C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphic environment, KCym will be expected to grow by hydration of Kfs, unless the aH 2O had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend critically on the rate of uplifting and on the aH 2O prevailing during that process.  相似文献   

14.
K-substituted gonnardite, K2.18Na0.04Ca0.02[Al2.26Si2.74O10]·2.2H2O, was studied by X-ray powder diffraction method. The structure was refined with the Rietveld technique in the tetragonal space group $I\overline{4} 2d$ with a = 13.65409(16), c = 6.56928(11) Å, V = 1224.74(2) Å3, Z = 4. Most of K+ cations (1.94 apfu) statistically occupy three nearest positions to be considered as the split one. “Excess” cations are located in the position non-typical for K+. Statistics in the cation distribution is defined by the occupation of the additional position. Based on a crystal chemical positional model (C2R2A2) [T5O10], the separation of the local water–cation assemblages from an average statistical pattern has been suggested.  相似文献   

15.
A single crystal of scolecite, CaAl2Si3O10· 3H2O, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoKα radiation (λ=0.71069?Å) in a complete sphere of reflection up to sinθ/λ=0.9?Å?1. The structure was refined in the pseudo-orthorhombic setting of space group F1d1 instead of the conventional setting Cc for better comparison with natrolite (Fdd2). The cell parameters are: a=18.502(1)?Å, b=18.974(2)?Å, c=6.525(1)?Å, β=90.615(7)°, V=2290.6(3)?Å3, Z=8. A refinement of high-order diffraction data yielded residuals of R(F)=0.9%, R w (F)=0.9%, GoF=1.73 for 1831 high-angle reflections (0.7≤sinθ/λ≤0.9?Å?1) and R(F)=1.2%, R w (F)=1.4%, GoF=3.22 for all 3478 independent reflections. In comparison with natrolite, the replacement of 2 Na+ by 1 Ca2+ and 1 H2O leads to a reduction of symmetry from Fdd2 to F1d1. Each general atomic position in natrolite (except of Na) splits into two crystallographically independent positions in scolecite. The T?O distances and T?O?T angles of these two sites differ distinctly from each other due to the influence of the calcium ions on the framework. An unexpected result of our detailed analysis of the data is that the additional water molecule (O7) disturbs the symmetry of the framework to a greater extent than the replacement of Na+ by Ca2+. As a comparison of the displacement parameters indicates, the bonds within the tetrahedral framework and to the extraframework cations are stronger in scolecite than in natrolite. The isotropic U(equ) values of the framework atoms and extraframework cations are about 10% smaller in scolecite compared to natrolite. The same tendency is shown by the analysis of the internal vibrational amplitudes ΔU. The corresponding force constants are in the range of F=358 to 3367?Nm?1 for the T?O bonds in scolecite (in natrolite: F=354 to 824?Nm?1). The values of the force constants which determine the vibrations of the Ca ions and water molecules against the framework oxygen atoms lie in the range of F=33 to 1757?Nm?1 (in natrolite: F=57 to 293?Nm?1).  相似文献   

16.
A new mineral aklimaite, Ca4[Si2O5(OH)2](OH)4 · 5H2O, has been found near Mount Lakargi, Upper Chegem caldera, Kabardino-Balkaria, the Northern Caucasus, Russia, in the skarnified limestone xenolith in ignimbrite. This hydrothermal mineral occurs in a cavity of altered larnite skarn and is associated with larnite, calcium humite-group members, hydrogarnets, bultfonteinite, afwillite, and ettringite. Aklimaite forms transparent, colorless (or occasionally with pinkish tint) columnar or lath-shaped crystals up 3 × 0.1 × 0.01 mm in size, flattened on {001} and elongated along {010}; they are combined in spherulites. The luster is vitreous; the cleavage parallel to the {001} is perfect. D calc = 2.274 g/cm3. The Mohs’ hardness is 3–4. Aklimaite is optically biaxial, negative, 2V meas > 70°, 2V calc = 78°, α = 1.548(2), β = 1.551(3), γ = 1.553(2). The IR and Raman spectra are given. The chemical composition (wt %, electron microprobe) is as follows: 0.06 Na2O, 0.02 K2O, 45.39 CaO, 0.01 MnO, 0.02 FeO, 24.23 SiO2, 0.04 SO3, 3.22 F, 27.40 H2O(calc.), ?1.36 -O=F2; the total is 99.03. The empirical formula calculated on the basis of 2Si apfu with O + OH + F = 16 is as follows: (Ca4.02Na0.01)Σ4.03[Si2.00O5.07(OH)1.93][(OH)3.16F0.84] Σ4.00 · 5H2O. The mineral is monoclinic, space group C2/m, a = 16.907(5), b = 3.6528(8), c = 13.068(4) Å, β = 117.25(4)·, V= 717.5(4) Å3, Z = 2. Aklimaite is representative of the new structural type, the sorosilicate with disilicate groups [Si2O5(OH)2]. The strongest reflections in the X-ray powder patterns [d, Å (hkl)] are: 11.64(100)(001), 2.948(32)(310, 203), 3.073(20) ( $\bar 404$ , $\bar 311$ ), 2.320(12)(005, 510), 2.901 (11)(004), 8.30(10) $\left( {\bar 201} \right)$ . The type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.  相似文献   

17.
Hydrothermal scheelite was synthesized using Na2WO4 · 2 H2O mixed with CaCl2 · H2O, CaSO4 · 2 H2O or CaF2 at different temperatures (270–720° C) and 108 Pa. The morphology of the crystals depends on the starting products. The observed faces include the {112}, {114}, {011}, and {013} forms. Pure or REE doped scheelites were studied by thermoluminescence (TL), fluorescence and electron paramagnetic resonance (EPR). The main TL peaks are located near 88, 149, 216, 277, and 315 K. Results obtained with EPR or optical fluorescence have been correlated with TL measurements and show that the trivalent lanthanide elements substitute for calcium ions without site distortion. The differences in TL observed between Eu and the other doping elements are related to the greater stability of Eu2+ caused by X-irradiation.  相似文献   

18.
Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO3 perovskite more accurately, we synthesized single crystals of Mg0.946(17)Fe0.056(12)Si0.997(16)O3 perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe2+ and Fe3+) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O3 perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations.  相似文献   

19.
The heat capacity of synthetic ferrosilite, Fe2Si2O6, was measured between 2 and 820 K. The physical properties measurement system (PPMS, Quantum Design®) was used in the low-temperature region between 2 and 303 K. In the temperature region between 340 and 820 K measurements were performed using differential scanning calorimetry (DSC). The C p data show two transitions, a sharp λ-type at 38.7 K and a small shoulder near 9 K. The λ-type transition can be related to collinear antiferromagnetic ordering of the Fe2+ spin moments and the shoulder at 10 K to a change from a collinear to a canted-spin structure or to a Schottky anomaly related to an electronic transition. The C p data in the temperature region between 145 and 830 K are described by the polynomial $C_{p} {\left[ {\hbox{J\,mol}^{{ - 1}}\,{\hbox{K}}^{{ - 1}} } \right]} = 371.75 - 3219.2T^{{ - 1/2}} - 15.199 \times 10^{5} T^{{ - 2}} + 2.070 \times 10^{7} T^{{ - 3}} $ The heat content [H 298 – H 0] and the standard molar entropy [S 298 – S 0] are 28.6 ± 0.1 kJ mol?1 and 186.5 ± 0.5 J mol?1 K?1, respectively. The vibrational part of the heat capacitiy was calculated using an elastic Debye temperature of 541 K. The results of the calculations are in good agreement with the maximum theoretical magnetic entropy of 26.8 J mol?1 K?1 as calculated from the relationship 2*Rln5.  相似文献   

20.
Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite ( $ P {\bar 3}1c $ , a?=?10.924(2)/10.882(2) Å, c?=?17.086(3) / 17.154(3) Å, V?=?1765.8(3)/1759.2(5) Å3, at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O–H distances in coquimbite at 50 K vary between 0.98 and 1.01 Å. In addition to H2O molecules coordinated to the Al3+ and Fe3+ ions, there are rings of six “free” H2O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 Å. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its “dimorph” paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe2(SO4)3·9H2O. Coquimbite, on the other hand, has a composition approximating Fe1.5Al0.5(SO4)3·9H2O. Hence, even a “simple” sulfate Fe2-x Al x (SO4)3·9H2O may be structurally rather complex.  相似文献   

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