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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
Na 2
C
M 2+
3 M 3+
2
T
Si 8(OH) 2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition (
A
0.5
B
Ca 1.5Na 0.5
C
M
2+
4.5 M
0.53+
T
Si 7.5Al 0.5(OH) 2) ( A = Na and/or K; M
2+ = Mg and Fe 2+; M
3+ = Fe 3+ 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)Mg 3,
M(1) M(3)Mg 2Fe 2+ and
M(1) M(3) Fe 2+
3, respectively, an empty A site and
T
Si 8 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
2.
The paper presents data on the thermochemical study (high-temperature melt calorimetry in a Tian–Calvet microcalorometer) of two natural Mg–Fe amphiboles: anthophyllite Mg 2.0(Mg 4.8Fe 0.2 2+)[Si 8.0O 22](OH) 2 from Kukh-i-Lal, southwestern Pamirs, Tajikistan, and gedrite Na 0.4Mg 2.0(Mg 1.7Fe 0.2 2+Al 1.3)[Si 6.3Al 1.7O 22](OH) 2 from the Kola Peninsula, Russia. The enthalpy of formation from elements is obtained as–12021 ± 20 kJ/mol for anthophyllite and as–11545 ± 12 kJ/mol for gedrite. The standard entropy, enthalpy, and Gibbs energy of formation are evaluated for Mg–Fe amphiboles of theoretical composition. 相似文献
3.
The thermoelastic behaviour of anthophyllite has been determined for a natural crystal with crystal-chemical formula ANa 0.01
B(Mg 1.30Mn 0.57Ca 0.09Na 0.04) C(Mg 4.95Fe 0.02Al 0.03) T(Si 8.00)O 22
W(OH) 2 using single-crystal X-ray diffraction to 973 K. The best model for fitting the thermal expansion data is that of Berman
(J Petrol 29:445–522, 1988) in which the coefficient of volume thermal expansion varies linearly with T as α
V,T
= a
1 + 2 a
2 ( T − T
0): α 298 = a
1 = 3.40(6) × 10 −5 K −1, a
2 = 5.1(1.0) × 10 −9 K −2. The corresponding axial thermal expansion coefficients for this linear model are: α
a
, 298 = 1.21(2) × 10 −5 K −1, a
2,a
= 5.2(4) × 10 −9 K −2; α
b
, 298 = 9.2(1) × 10 −6 K −1, a
2,b
= 7(2) × 10 −10 K −2. α
c
, 298 = 1.26(3) × 10 −5 K −1, a
2,c
= 1.3(6) × 10 −9 K −2. The thermoelastic behaviour of anthophyllite differs from that of most monoclinic ( C2/ m) amphiboles: (a) the ε
1 − ε
2 plane of the unit-strain ellipsoid, which is normal to b in anthophyllite but usually at a high angle to c in monoclinic amphiboles; (b) the strain components are ε
1 ≫ ε
2 > ε
3 in anthophyllite, but ε
1 ~ ε
2 ≫ ε
3 in monoclinic amphiboles. The strain behaviour of anthophyllite is similar to that of synthetic C2/ m
ANa B(LiMg) CMg 5
TSi 8 O 22
W(OH) 2, suggesting that high contents of small cations at the B-site may be primarily responsible for the much higher thermal expansion
⊥(100). Refined values for site-scattering at M4 decrease from 31.64 epfu at 298 K to 30.81 epfu at 973 K, which couples with similar increases of those of M1 and M2 sites. These changes in site scattering are interpreted in terms of Mn ↔ Mg exchange involving M1,2 ↔ M4, which was first detected at 673 K. 相似文献
4.
This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition IV(Si 2.76Al 1.24) VI(Al 0.64Mg 0.72 $ {\text{Fe}}_{1.45}^{2 + } $ Mn 0.03Ti 0.15) (K 0.96Na 0.05) O 10.67 (OH) 1.31 Cl 0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition IV(Si 3.14Al 0.86) VI(Al 0.75Mg 0.01 $ {\text{Fe}}_{1.03}^{2 + } $ $ {\text{Fe}}_{1.03}^{3 + } $ Mn 0.01Ti 0.01Li 1.09) (K 0.99Na 0.01) O 10.00 (OH) 0.65F 1.35]. Samples differ for Fe ordering in octahedral sites, Fe 2+/(Fe 2+?+?Fe 3+) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6?K in TK, where Fe is disordered in all octahedral sites, and 8?K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites. 相似文献
5.
This study is devoted to the physicochemical and mineralogical characterizations of palygorskite from Marrakech High Atlas, Morocco. The raw clay and its Na +-saturated <2 μm fraction were characterized using chemical, structural, and thermal analytical techniques. Measurements of specific surface area and porous volume are reported. The clay fraction was found to be made up of 95 % of palygorskite and 5 % of sepiolite. An original feature of this palygorskite is its deficiency in zeolitic H 2O. The half-cell structural formula of its dehydrated form was determined on the basis of 21 oxygens to be (Si 7.92Al 0.08)(Mg 2.15Al 1.4Fe 0.4Ti 0.05 $ \square_{1} $ )(Ca 0.03Na 0.08K 0.04)O 21, while the hydrated form could be formulated as (Si 7.97Al 0.03)(Mg 2.17Al 1.46Fe 0.40Ti 0.05)(Ca 0.03Na 0.07K 0,03)O 20.18(OH) 1.94(OH 2) 3.88·2.43 H 2O. These formulas show that the (Al 3++Fe 3+)/Mg 2+ ratio is around 0.84, revealing a pronounced dioctahedral character. Further, inside its octahedral sheet, it was determined that the inner M 1 sites are occupied by vacancies, whereas the M 2 sites are shared between 90 % of trivalent cations (78 % for Al 3+ and 22 % for Fe 3+), 7.5 % of Mg 2+, and 2.5 % of Ti 4+, all of them linked to 1.94 of structural hydroxyls. The two remaining Mg 2+ by half-cell occupy edge M 3 sites and are coordinated to 3.88 molecules of OH 2. Channels of this palygorskite are deficient in zeolitic H 2O since they contain only 2.43 H 2O molecules. A correlation was found between these results and the observation of very intense and well-resolved FTIR bands arising from dioctahedral domains (mainly Al 2OH, Fe 2OH, and AlFeOH) along with very small responses from a trioctahedral domain (Mg 3OH). Accordingly, a schematic representation of the composition of the octahedral sheet was proposed. The cation exchange capacity, specific surface area, and total pore volume were also assessed to be ca. 21.2 meq/100 g, 116 m 2/g, and 0.458 cm 3/g, respectively. 相似文献
6.
Hyperfine parameters of 57Fe in anthophyllites (Mg 2+, Fe 2+) 7 Si 8O 22(OH, F) 2 mainly depend on the amount of Al present in the structure. The quadrupole splitting of the doublet due to Fe 2+ in M1, M2 and M3 decreases systematically with the Al content, whereas that of the doublet due to Fe 2+ in M4 and the half-width of the combined M1, M2, M3 doublet increases. Structurally these variations suggest that, with the incorporation of Al (miscibility towards gedrite), the distortion of the M4 polyhedron decreases, whereas the M1, M2 and M3 polyhedra become more distorted and dissimilar. 相似文献
7.
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 Na 2O content (1.4–2.3 wt.%, Na A = 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 Na A and Al IV from the core (Na A = 0.40–0.51 pfu, Al IV = 1.6–1.9 pfu) to the rim (Na A = 0.49–0.61 pfu, Al IV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H 2O. 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.% Na 2O (Na A = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na 2O, 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 H 2O 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 CO 2-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
8.
A mica whose structural formula: (K 1.76Na 0.31)(Fe 2.22Mn 1.29Mg 0.99Ti 0.28Al 0.240.98) ·(Si 7.33Al 0.67)O 20.26(F 2.16OH 1.58) closely approximates that of tetrasilicic potassium mica K 2(M
5
2+
)Si 8O 20(OH,F) 4 where M 2+ represents Mg 2+, Fe 2+, Mn 2+, ..., has been discovered in the matrix of a peralkaline rhyolite (comendite) of the Mont-Dore massif (France). These micas had been obtained previously by synthesis only. In the groundmass of the rock, the micaceous phase is accompanied by a manganoan arfvedsonite, pyrophanite, magnetite, apatite, sphene, zircon and fluorite. The crystallographic properties of the mica are typically that of a tetrasilicic mica, with d
060 = 1.533Å and space group C2/m. There is a regular decrease of d
060 (parameter b) with the ionic radius of the octahedral cation in synthetic micas containing Fe 2+, Co 2+, Mg 2+, Ni 2+. The purely Mn 2+ end-member could not be synthesised; its instability is discussed on the basis of structural considerations. The conditions of crystallization of the micaceous phase are estimated to be 760 ° C, 800 bars with a f
o
2=10 –14.7 bar. This mica has crystallized from a residual liquid, with high activity of silica and low activity of alumina, whose origin is discussed. The name MONT-DORITE is proposed for this natural tetrasilicic mica having Fe/Fe+Mg >1/2 and Fe/Fe+Mn >1/2. This name is from the stratovolcano Mont-Dore. 相似文献
9.
Electron paramagnetic resonance (EPR) study of single crystals of chromium-doped forsterite grown by the Czochralski method
in two different research laboratories has revealed, apart from the known paramagnetic centers Cr 3+( M1), Cr 3+( M2) and Cr 4+, a new center
\text Cr 3+ ( M 1)- V\textMg 2+ ( M 2) {\text{Cr}}^{ 3+ } (M 1){-}V_{{{\text{Mg}}^{ 2+ } }} (M 2) formed by a Cr 3+ ion substituting for Mg 2+ at the M1 structural position with a nearest-neighbor Mg 2+ vacancy at the M2 position. For this center, the conventional zero-field splitting parameters D and E and the principal g values and A values of the 53Cr hyperfine splitting have been determined as follows: D = 33.95(3) GHz, E = 8.64(1) GHz, g = [1.9811(2), 1.9787(2), 1.9742(2)], A = [51(3), 52(2), 44(3)] MHz. The center has been identified by comparing EPR spectra with those of the charge-uncompensated
ion Cr 3+( M1) and the ion pair Cr 3+( M1)–Li +( M2) observed in forsterite crystals codoped with chromium and lithium. It has been found that the concentration of the new
center decreases to zero, whereas that of the Cr 3+( M1) and Cr 3+( M1)–Li +( M2) centers increases with an increase of the Li content from 0 up to ~0.03 wt% (at the same Cr content ~0.07 wt%) in the melt.
The known low-temperature luminescence data pertinent to the centers under consideration are also discussed. 相似文献
10.
Two pumpellyites with the general formula W
8
X
4
Y
8
Z
12O 56-n
(OH)
n
were studied using 57Fe Mössbauer spectroscopic and X-ray Rietveld methods to investigate the relationship between the crystal chemical behavior of iron and structural change. The samples are ferrian pumpellyite-(Al) collected from Mitsu and Kouragahana, Shimane Peninsula, Japan. Rietveld refinements gave Fe( X):Fe( Y) ratios (%) of 41.5(4):58.5(4) for the Mitsu pumpellyite and 46(1):54(1) for the Kouragahana pumpellyite, where Fe( X) and Fe( Y) represent Fe content at the X and Y sites, respectively. The Mössbauer spectra consisted of two Fe 2+ and two Fe 3+ doublets for the Mitsu pumpellyite, and one Fe 2+ and two Fe 3+ doublets for the Kouragahana pumpellyite. In terms of the area ratios of the Mössbauer doublets and the Fe( X):Fe( Y) ratios determined by the Rietveld refinements, Fe 2+( X):Fe 3+( X):Fe 3+( Y) ratios are determined to be 22:14:64 for the Mitsu pumpellyite and 27:8:65 for the Kouragahana pumpellyite. By applying the Fe 2+:Fe 3+-ratio determined by the Mössbauer analysis and the site occupancies of Fe at the X and Y sites given by the Rietveld method together with chemical analysis, the resulting formula of the Mitsu and Kouragahana pumpellyites are established as Ca 8(Fe
0.88
2+
Mg 0.68Fe
0.77
3+
Al 1.66) Σ3.99(Al 5.67Fe
2.34
3+
) Σ8.01Si 12O 42.41(OH) 13.59 and Ca 8(Mg 1.24Fe
0.65
2+
Fe
0.46
3+
Al 1.66) Σ4.01(Al 6.71Fe
1.29
3+
) Σ8.00Si 12O 42.14(OH) 13.86, respectively. Mean Y–O distances and volumes of the YO 6 octahedra increase with increasing mean ionic radii, i.e., the Fe 3+→Al substitution at the Y site. However, change of the sizes of XO 6 octahedra against the mean ionic radii at the X site is not distinct, and tends to depend on the volume change of the YO 6 octahedra. Thus, the geometrical change of the YO 6 octahedra with Fe 3+→Al substitution at the Y site is essential for the structural changes of pumpellyite. The expansion of the YO 6 octahedra by the ionic substitution of Fe 3+ for Al causes gradual change of the octahedra to more symmetrical and regular forms. 相似文献
11.
Electron probe and wet chemical analyses of amphibole pairs from the sillimanite zone of central Massachusetts and adjacent New Hampshire indicated that for a particular metamorphic grade there should be a restricted composition range in which three amphiboles can coexist stably. An unequivocal example of such an equilibrium three amphibole rock has been found in the sillimanite-orthoclase zone. It contains a colorless primitive clinoamphibole, space group P2 1/ m, optically and chemically like cummingtonite with blue-green hornblende exsolution lamellae on (100) and (¯101) of the host; blue-green hornblende, space group C2/m, with primitive cummingtonite exsolution lamellae on (100) and (¯101) of the host; and pale pinkish tan anthophyllite, space group Pnma, that is free of visible exsolution lamellae but is a submicroscopic intergrowth of two orthorhombic amphiboles. Mutual contacts and coarse, oriented intergrowths of two and three host amphiboles indicate the three grew as an equilibrium assemblage prior to exsolution. Electron probe analyses at mutual three-amphibole contacts showed little variation in the composition of each amphibole. Analyses believed to represent most closely the primary amphibole compositions gave atomic proportions on the basis of 23 oxygens per formula unit as follows: for primitive cummingtonite (Na 0.02Ca 0.21
– Mn 0.06Fe 2+
2.28Mg 4.12Al 0.28) (Al 0.17Si 7.83), for hornblende (Na 0.35Ca 1.56Mn 0.02Fe 1.71Mg 2.85Al 0.92) (Al 1.37Si 6.63), and for anthophyllite (Na 0.10Ca 0.06Mn 0.06Fe 2.25Mg 4.11Al 0.47) (Al 0.47Si 7.53). The reflections violating C-symmetry, on X-ray single crystal photographs of the primitive cummingtonite, are weak and diffuse, and suggest a partial inversion from a C-centered to a primitive clinoamphibole. Single crystal photographs of the anthophyllite show split reflections indicating it is an intergrowth of about 80% anthophyllite and about 20% gedrite which differ in their b crystallographic dimensions. Split reflections are characteristic of all analyzed orthorhombic amphiboles so far examined from Massachusetts and New Hampshire except the most aluminous gedrites, and the relative intensity of the gedrite reflections is roughly proportional to the degree of Na and Al substitution. Thin sections of a few of these anthophyllite specimens show lamellae parallel to (010) that are just resolved with a high power objective.Publication approved by the Director, U.S. Geological Survey. 相似文献
12.
The paper reports original thermochemical data on six natural amphibole samples of different composition. The data were obtained by high-temperature melt solution calorimetry in a Tian–Calvet microcalorometer and include the enthalpies of formation from elements for actinolite Ca 1.95(Mg 4.4Fe 0.5 2+ Al 01)[Si 8.0O 22](OH) 2(–12024 ± 13 kJ/mol) and Ca 2.0(Mg 2.9Fe 1.9 2+ Fe 0.2 3+ )[Si 7.8Al 0.2O 22](OH) 2, (–11462 ± 18 kJ/mol), and Na 0.1Ca 2.0(Mg 3.2Fe 1.6 2+ Fe 0.2 3+ )[Si 7.7Al 0.3O 22](OH) 2 (–11588 ± 14 kJ/mol); for pargasite Na 0.5K 0.5Ca 2.0-(Mg 3.4Fe 1.8 2+ Al 0.8)[Si 6.2Al 1.8O 22](OH) 2 (–12316 ± 10 kJ/mol) and Na 0.8K 0.2Ca 2.0(Mg 2.8Fe 1.3 3+ Al 0.9) [Si 6.1Al 1.9O 22](OH) 2 (–12 223 ± 9 kJ/mol); and for hastingsite Na 0.3K 0.2Ca 2.0(Mg 0.4Fe 1.3 2+ Fe 0.9 3+ Al 0.2) [Si 6.4Al 1.6O 22](OH) 2 (?10909 ± 11 kJ/mol). The standard entropy, enthalpy, and Gibbs free energy of formation are estimated for amphiboles of theoretical composition: end members and intermediate members of the isomorphic series tremolite–ferroactinolite, edenite–ferroedenite, pargasite–ferropargasite, and hastingsite. 相似文献
13.
Polarized absorption spectra of natural piemontite (Ca1.802Mn
2+0.178
Mg0.025) (Mn
3+0.829
Fe
3+0.346
Al1.825) [(Si2.992Al0.008) O12OH], viridine (Al1.945Mn
3+0.033
Fe
3+0.063
Mg0.003) [O|Si0.970 O4], and kanonaite (Al1.291Mn
3+0.682
Fe
3+0.019
) [O|Si1.006 O4] were measured at 295 and ca. 100 K. For piemontite, lowering the temperature resulted in a sharpening of broad bands in the 10 000–25 000 cm−1 region supporting their assignment to single ion Mn3+ in M3 non-centrosymmetric sites. Alternatively, in kanonaite, temperature behaviour pointed to a slightly stronger influence of vibronic coupling on strong bands near 16 000 and 22 000 cm−1, which supported an interpretation of Mn3+ in nearly centrosymmetric M1 sites. Measurements at ca. 100 K show pronounced fine structure in the viridine spectra which is attributed to Fe3+. The ɛ values for Mn3+ spin-allowed bands in the three minerals lie in the range 18 to 227 [1·g-atom−1·cm−1]. For the same band and polarisation, ɛ values in Mn3+-bearing andalusite-type minerals viridine and kanonaite are the same, which indicates an absence of strong magnetic coupling effects between Mn3+ ions in the andalusite type structure down to ca. 100 K. In silicates, the high ɛ values for Mn3+ spin-allowed bands, in comparison to those obtained for Fe2+ spin-allowed bands from sites of “similar distortion”, is attributed to a higher degree of covalency in the Mn3+-O bonds compared to the Fe2+-O bonds, as a result of the higher valence state of manganese. 相似文献
14.
The synthesis and the chemical, structural, magnetic, and Mössbauer spectral characterization of three synthetic alluaudites, Na 2Mn 2Fe(PO 4) 3, NaMn Fe 2(PO 4) 3 and Na 2MnFe IIFe III(PO 4) 3, and a natural sample with the nominal composition of NaMn Fe 2(PO 4) 3, collected in the Buranga pegmatite, Rwanda, are reported. All four compounds have the expected alluaudite monoclinic C2/ c structure with the general formula [ A(2) A(2)][ A(1) A(1) A(1) 2] M(1) M(2) 2(PO 4) 3 in which manganese(II) is on the M(1) site and manganese(II), iron(III) and, in some cases, iron(II) on the M(2) site. The X-ray structure of Na 2Mn 2Fe(PO 4) 3 also indicates a partially disordered distribution of Na I and Mn II on the M(1) and A(1) crystallographic sites. All four compounds are paramagnetic above 40 K and antiferromagnetically ordered below. Above 40 K the effective magnetic moments of NaMnFe 2(PO 4) 3 and Na 2MnFe IIFe III(PO 4) 3 are those expected of high-spin manganese(II) and iron(III) with the 6A1g electronic ground state and high-spin iron(II) with the 5T2g electronic ground state. In contrast, the effective magnetic moment of Na 2Mn 2Fe(PO 4) 3 is lower than expected as a result of enhanced antiferromagnetic exchange coupling by the manganese(II) on the M(2) site. The Mössbauer spectra of all four compounds have been measured from 4.2 to 295 K and have been found to be magnetically ordered below 40 K for Na 2Mn 2Fe(PO 4) 3 and 35 K for the remaining compounds. The Mössbauer spectra of Na 2Mn 2Fe(PO 4) 3 exhibit the two expected iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of manganese(II) and iron(III) ions on the M(2) site. Further, the Mössbauer spectra of Na 2MnFe IIFe III(PO 4) 3 exhibit the two iron(II) and two iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of iron(II) and iron(III) on the M(2) site. Surprisingly, the synthetic and natural samples of NaMnFe 2(PO 4) 3 have 19 and 10% of iron(II) on the M(2) site; apparently the presence of some iron(II) stabilizes the alluaudite structure through the reduction of iron(III)–iron(III) repulsion. The temperature dependence of the iron(II) quadrupole splitting yields a 440 to 600 cm –1 low-symmetry component to the octahedral crystal field splitting at the M(2) site. The iron(II) and iron(III) hyperfine fields observed at 4.2 K are consistent with the presence of antiferromagnetic ordering at low temperatures in all four compounds. 相似文献
15.
The Fe M
2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe 3(Al 1–xFe x) 2[SiO 4] 3 and andradite-skiagite (Fe 1–xCa x) 3Fe 2[SiO 4] 3), pyroxenes (acmite-hedenbergite (Ca 1–xNa x)(Fe 2+
1−xFe 3+
x)Si 2O 6), and spinels (magnetite-hercynite Fe(Al 1–xFe x) 2O 4) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission
electron microscope (TEM). The Fe M
2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV
for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge
feature varies depending on Fe 3+/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship
between Fe 3+/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M
2,3 edge is observed. Since the partial cross sections of the Fe M
2,3 edges are some orders of magnitude higher than those of the Fe L
2,3 edges, the Fe M
2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated
by Fe M
2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that
shows different edge shapes consistent with variations in the Fe 3+/ΣFe ratio over distances on the order of 100 nm.
Received: 14 April 1998 / Revised, accepted: 8 March 1999 相似文献
16.
Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and
after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the
presence of two substitutional Fe 3+ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E
1′. However, the two broad signals do not correspond to any known Fe 3+ centers in the quartz lattice, but are most likely attributable to Fe 3+ clusters on surfaces. The absolute numbers of spins of the Fe 3+ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used
to extract thermodynamic potentials, including Gibbs energy of activation Δ G, activation energy E
a, entropy of activation Δ S and enthalpy of activation Δ H for the Fe 3+ species in amethyst. In addition, magnetic susceptibilities ( χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and
temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe 3+ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects
of iron in silica particles. 相似文献
17.
The heat capacity of glaucophane from the Sesia-Lanza region of Italy having the approximate composition (Na 1.93Ca 0.05Fe 0.02) (Mg 2.60Fe 0.41) (Al 1.83Fe 0.15Cr 0.01) (Si 7.92Al 0.08)O 22(OH) 2 was measured by adiabatic calorimetry between 4.6 and 359.4 K. After correcting the C
p
0
data to values for ideal glaucophane, Na 2Mg 3Al 2Si 8O 22(OH) 2 the third-law entropy S
298
0
-S
0
0
was calculated to be 541.2±3.0 J·mol -1·K -1. Our value for S
298
0
- S
0
0
is 12.0 J·mol -1·K -1 (2.2%) smaller than the value of Likhoydov et al. (1982), 553.2±3.0, is within 6.2 J·mol -1·K -1 of the value estimated by Holland (1988), and agrees remarkably well with the value calculated by Gillet et al. (1989) from spectroscopic data, 539 J·mol -1·K -1. 相似文献
18.
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(Mg 2Li)Al 6(Si 6O 18)(BO 3) 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: Mg 2+ → Fe 2+, 2Mg 2+ → Li + + Al 3+, and Mg 2+ + OH → Al 3+ + O 2−. The composition of dark green marginal zones in tourmaline is characterized by the ideal formula Ca(Al 1.5Li 1.5)Al 6(Si 6O 18)(BO 3) 3 (OH 2O)(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
19.
A new coexisting amphibole pair was recently found in the Jianshan iron deposit, Loufan of Shanxi Province, China. Electron microprobe analysis shows that the coexisting pair is composed of grünerite K 0.001 (Na 0.027 Ca 0.073 Mn 0.031 Fe 1.801 2+ ) 1.932 (Fe 2.948 2+ Mg 1.964 Ti 0.002 Al 0.087) 5Si 8.069 O 22.10(OH) 2 and ferropargasite (K 0.135 Na 0.461) 0.596 (Na 0.088 Ca 1.853 Mn 0.005 Fe 0.072 2+ ) 2(Mn 0.005Fe 2.789 2+ Mg 0.875Ti 0.021Fe 0.499 3+ Al 0.812) 5(Si 6.103Al 1.897) 8O 22.00(OH) 2. The two kinds of amphiboles occur in amphibole schist not only as separate phenocrysts, but also are combined to form “single-crystal” phenocrysts in the form of topotactic intergrowths with the common c- and b-axes. The boundary between topotactic grünerite and ferropargasite is optically and chemically sharp. In comparison with the coexisting ferromagnesian amphibole and calcic amphibole pair discovered by predecessors, the newly discovered pair has lower Mg/Fe ratios and wider miscibility gaps. 相似文献
20.
Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO 2=NNO+2.3. The space group at room T is P2 1/ c [ a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3) ∘, V=415.2 ? 3]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi 2O 6, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The
crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding M2(Li 0.85Mg 0.09Fe 2+
0.06) M1(Fe 3+
0.85Mg 0.15)Si 2O 6. Li is ordered at the M2 site and Fe 3+ is ordered at the M1 site, whereas Mg (and Fe 2+) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200
°C) allowed characterization of a reversible displacive P2 1/ c→ C2/ c transition at 106 °C. Previous H T-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation.
For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition
temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate
radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian
P2 1/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and
thermodynamic behaviour in clinopyroxenes.
Received: 7 May 2002 / Accepted: 23 October 2002 相似文献
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