Günther J. Redhammer

Structural variations in the brownmillerite series Ca2(Fe2−xAlx)O5: Single-crystal X-ray diffraction at 25 °C and high-temperature X-ray powder diffraction (25 °C ≤ T ≤ 1000 °C)

Abstract A total of 30 synthetic samples of the Ca2Fe2-xAlxO5, 0.00 ≤ x ≤ 1.34 solid solution series have been investigated by single crystal X-ray diffraction at 25 °C. Pure Ca2Fe2O5 and samples up to x = 0.56 have space group Pnma, Z = 4, whereas samples with x > 0.56 show I2mb symmetry, Z = 4. The substitution of Fe3+ by the smaller Al3+ cation decreases unit-cell parameters and average octahedral and tetrahedral bond lengths and induces distinct changes in the O-atom coordination of the interstitial Ca atom. Discontinuities in the structural parameters vs. the Al3+tot content and changes in slope of these quantities are associated with the phase transition. The essential difference between the two modifications is the cation-O atom-cation angle within the planes of corner sharing octahedra, which is close to 180° in I2mb, but ≈184° in the Pnma phase, and the existence of two different orientations of the tetrahedral chains in Pnma as opposed to one in I2mb. At low overall Al3+ concentrations Al3+ preferentially enters the tetrahedral site until ≈2/3 of it is filled. Additional Al3+ cations, substituted for Fe3+, are equally distributed over octahedral and tetrahedral sites. At high temperature pure Ca2Fe2O5 transforms to a body-centered structure at 724(4) °C. Substituting Al3+ for Fe3+ linearly decreases the transition temperature by 15 °C per 0.1 Al3+ down to 623(5) °C for x = 0.65.

Spectroscopic and structural properties of synthetic micas on the annite-siderophyllite binary: Synthesis, crystal structure refinement, Mössbauer, and infrared spectroscopy

Abstract The effect of the incorporation of Al-Tschermak’s molecule to the trioctahedral potassium mica annite {K}[Fe3]O10(OH)2 on local and average structure has been investigated by hydrothermal synthesis, structure refinement of X-ray powder diffraction data, Mössbauer and infrared spectroscopy. The various types of brackets indicate different structural sites. Samples with compositions {K}[Fe3-xAlx]O10(OH)2 were prepared by hydrothermal techniques. The maximum solubility of Al3+ is limited to x = 0.92 at 500 °C and to x = 0.82 at 700 °C. The main factor controlling the substitution limits is the ditrigonal distortion of the tetrahedral rings. Lattice parameters decrease linearly with increasing Al3+ content of the mica. A considerable decrease of M2-O and nearly no change of M1-O bond lengths with increasing Al3+ contents is indicative of preferred occupation of the M2 site by [Al3+]. Changes in K-O distances are also very pronounced and reflect the ditrigonal distortion of the tetrahedral sheet. The bimodal ferrous quadrupole splitting distribution (QSD) in annite, extracted from Mössbauer spectra, becomes narrower and more centered around 2.60 mm/s with increasing Al3+ contents, and its evolution suggests an increasing deviation from ideal octahedral coordination of Fe by O, illustrated by the increasing octahedral flattening angle y. The population of individual QSD components proves that it is impossible to resolve cis and trans M-sites in micas by Mössbauer spectroscopy. In the hydroxyl stretching region, up to 7 bands are observed in the infra-red spectra which correspond to OH groups adjacent to 3 Fe2+ (N-bands), to OH groups coordinated by Fe2+, Al3+, and Fe3+ (I-bands) and to configurations having one octahedral vacancy (V-bands). N- and I-type bands are shifted toward lower wavenumbers with increasing Al3+ content because of increasing OH···Otet interactions.

Single-crystal structure refinements and crystal chemistry of synthetic trioctahedral micas KM3(Al3+,Si4+)4O10(OH)2, where M = Ni2+, Mg2+, Co2+, Fe2+, or Al3+

Abstract To study structural changes with changing chemical composition in the octahedral sheet of trioctahedral potassium-rich 1M micas, 3 natural and 12 synthetic micas were examined by singlecrystal X-ray diffraction. Samples with Ni2+, Mg2+, Co2+, Fe2+, and Al3+ in the octahedral sheet and with Si4+, Al3+ and one sample with Ga3+ in the tetrahedral sheet were prepared at high temperatures and pressures, which yielded crystallites ≤200 mm in size. For samples with approximately AlSi3 composition of the tetrahedral sheet and with no octahedral Al3+, mean M1-O and M2-O octahedral bond lengths correlate very well with the mean ionic radius of the octahedral cations. For these samples, the M1 and M2 sites are found to be very similar or identical within experimental error in terms of mean M-O bond lengths, bond angles, and polyhedral distortion parameters. Octahedral distortion is negatively correlated with the size of the octahedral cation. Octahedra in Ni2+-mica show the largest deviation from ideal octahedral geometry, whereas those in annite are closest to ideal octahedral geometry, but they are still significantly flattened. Octahedral Al3+ prefers the M2 site. This causes the mean M1-O bond length to decrease less with increasing Al3+ content as compared to the mean M2-O distance. This Al3+ preference for M2 also causes the M2 site to become smaller and more distorted than the M1 site. Refinement of the Mg-Fe ratios in the octahedral sites along the annite-phlogopite join shows that the two cations are statistically distributed over M1 and M2. Tetrahedra are regular and show only a small elongation along c*. Tetrahedral distortion parameters for the AlSi3 micas show no correlation with chemistry of the octahedral layer. However, the mean T-O bond lengths increase slightly with increasing size of the octahedral cation. With decreasing size of the lateral dimension of the octahedral sheet in the (001) plane, the tetrahedral sheet shows increasing ditrigonal distortion. Largest tetrahedral rotation angles are observed for synthetic near end-member siderophyllite with α = 11.5° and for tetra-gallium-phlogopite KMg3GaSi3O10(OH)2 with α = 10.8°.

A Mössbauer and X-ray diffraction study of annites synthesized at different oxygen fugacities and crystal chemical implications

A refined set of Mössbauer parameters (isomer shifts, quadrupole splittings, Fe2+/Fe3+ ratios) and lattice parameters were obtained from annites synthesized hydrothermally at pressures between 3 and 5 kbars, temperatures ranging from 250 to 780° C and oxygen fugacities controlled by solid state buffers (NNO, QMF, IM, IQF). Mössbauer spectra showed Fe2+ and Fe3+ on both the M1 and the M2 site. A linear relationship between Fe3+ content and oxygen fugacity was observed. Towards low Fe3+ values this linear relationship ends at ≈10% of total iron showing that the Fe3+ content cannot be reduced further even if more reducing conditions are used. This indicates that in annite at least 10% Fe2+ are substituted by Fe3+ in order to match the larger octahedral layer to the smaller tetrahedral layer. IR spectra indicate that formation of octahedral vacancies plays an important role for charge balance through the substitution 3 Fe2+ → 2 Fe3+ + ▪(oct).

Model pyroxenes III: Volume of C2/c pyroxenes at mantle P, T, and x

Abstract Variations in unit-cell volumes of mantle minerals as functions of P and T are important parameters in the description of the interior of the Earth and the behavior of materials. Recently, Thompson and Downs (2004) presented a model for the crystal structures of pyroxenes parameterized in terms of the O3-O3-O3 angle, θ, and the oxygen radius, r. This model has proven useful in the analysis of compression and expansion mechanisms in pyroxenes, providing an understanding of θ and r as functions of P and T. However, it did not provide a basis for analyzing changes in some properties that are strongly dependent on composition. In this paper, we show that ambient unit-cell volumes of the C2/c pyroxenes are correlated with M1 cation radius. This relationship can be used to calculate model ambient unit-cell volumes as a function of chemistry. From this starting point, pyroxene unit-cell volume variation with P and T can be modeled as a function of θ(P,T) and r(P,T). These relationships are investigated for diopside, hedenbergite, acmite, jadeite, and kosmochlor. The model reproduces observed unit-cell volumes of these phases recorded at P to within 0.09% and at T to within 0.10%, at simultaneous P and T for jadeite to within 0.57%, and at simultaneous P and T for diopside to within 1.20%. Ko and K’ from third-order Birch-Murnaghan fits to the observed volume vs. pressure relationships and those calculated from the Thompson-Downs model are statistically the same. The fit of the Thompson-Downs EOS to the observed data is compared to the fit of the third order Birch-Murnaghan. The model is used to create an algorithm that estimates volumes for C2/c pyroxenes as a function of P, T, and x.

High-pressure phase transformation in LiFeGe2O6 pyroxene

Abstract A synthetic pyroxene with composition LiFeGe2O6 and space group P21/c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, β, and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P21/c phase is estimated to be 110 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P21/c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.

Single-crystal X-ray diffraction and temperature dependent 57Fe Mössbauer spectroscopy on the hedenbergite-aegirine (Ca,Na)(Fe2+,Fe3+)Si2O6 solid solution

Abstract Synthetic samples with different chemical compositions along the hedenbergite-aegirine (CaFe2+Si2O6-NaFe3+Si2O6) solid-solution series have been investigated by single-crystal X-ray diffraction and 57Fe Mössbauer spectroscopy. All compounds show C2/c symmetry, both at 298 K and at low temperature (100 K). The structural changes within the hedenbergite-aegirine series are dominated by the M1 site while the M2 site plays a minor role. Replacement of Fe2+ by Fe3+ increases the polyhedral distortion of the M1 site and causes an increased repulsion between neighboring M1 sites. The changes in M1-site geometry also induce distinct alterations within the kinking state of the tetrahedral chains, but the changes in tetrahedral bond lengths and angles are small. In addition to the single-crystal X-ray diffraction experiments, a large number of synthetic samples were investigated by 57Fe Mössbauer spectroscopy at 298 K and, for three selected compositions, between 80 and 700 K. Here, substantial line broadening of the Fe2+ resonance absorption was observed as an aegirine component is substituted. Two different groups of local distortion environments were observed for Fe2+ within the solid-solution series, which change relative proportions and numeric value of the quadrupole splitting as a function of chemistry and temperature. This line broadening cannot be ascribed to discrete next-nearest-neighbor (NNN) configurations of Ca and Na as has been done in the literature. Above ~250 K, additional resonance absorption appears in the spectra of samples with aegirine components between 20 and 75 mol%. This absorption gains intensity with increasing temperature, while the 57Fe hyperfine parameters approach values intermediate between Fe2+ and Fe3+. This effect is ascribed to fast electron delocalization between Fe2+ and Fe3+ at elevated temperature.

Characterisation of synthetic trioctahedral micas by Mössbauer spectroscopy

Trioctahedral potassium micas |K}[M3]〈T4〉O10(OH)2 have been synthesized by hydrothermal techniques with various cationic substitutions in the octahedral and the tetrahedral sheet. Taking annite |K}[Fe32+]〈AlSi3〉O10(OH)2 as the reference mineral, [Fe2] was replaced by [Mg2] and [Ni2], 〈Al3+〉 by 〈Fe3+〉 and finally [Fe2+] + 〈Si4+〉 by [Al3+] + 〈Al3+〉. Mössbauer spectra were evaluated in terms of quadrupole splitting distributions (QSDs) using three generalized sites for 〈Fe3+〉, [Fe3+] and [Fe2]. Annites, nominally free of 〈Fe3+〉, show a lower limit of [Fe3+]/Fetot of 0.10, which stabilizes the structure. The ferrous iron, [Fe2], QSD consists of two main components. In some of the solid solution series, there is strong experimental evidence for a third ferrous component, particularly at higher [Al3+] contents. This third component is centered at low quadrupole splittings and may be assigned to a defect [Fe2] site, forming 1:2 structures with two neighbouring trivalent octahedral cations. For charge compensation one OH− is replaced by O2− for each [M3+] cation. The ferrous QSDs vary systematically with chemical composition. Compared to those of annite, the QSD parameters (mean quadrupole splitting 〈QS〉 and quadrupole splitting with maximum probability, QSpeak) are shifted towards higher values with increasing [Mg2] and [Ni2] contents, and decrease slightly with increasing content of trivalent cations. These trends can be interpreted in terms of changes in the local environment around the Fe probe nucleus, i.e., in terms of decreasing or increasing distortions from the ideal octahedral configurations.

Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3- SiO2-H2O as a function of fO2

Abstract This paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2- H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally ◻Na2Mg3Fe23+Si8O22(OH)2, were run at 700-800 °C, 0.4 GPa, and redox conditions varying from NNO (Nickel.Nickel Oxide) to NNO + 2.3 log fO₂. Powder XRD and SEM-EDX show a high (>85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)2]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reß ect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mössbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of fO₂: for increasing oxidation conditions, there is significant increase in M2Fe3+ and decrease in Fe2+, notably in M4Fe2+. Mössbauer spectra also show significant variation in M1Fe2+ and M3Fe2+ quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies <3680 cm-1, assigned to octahedral M1,3(Mg, Fe2+)-OH-A◻ configurations, and a broad band consisting of four overlapping components related to M1,3(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fO₂ of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the C(Mg,Fe2+)1B(Mg,Fe2+)1 CFe3+-1BNa-1 and the ANa1C(Mg,Fe2+)1 A◻-1 CFe3+-1 exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in fO₂ during crystallization

Mssbauer spectroscopic and x-ray powder diffraction studies of synthetic micas on the join annite KFe3AlSi3O10(OH)2-phlogopite KMg3AlSi3O10(OH)2

Micas of the composition K(Fe3−xMgx)AlSi3 O10(OH)2 (x=0.6, 1.2, 1.8, 2.4 and 3.0, corresponding to ann80phl20, ann60phl40, ann40phl60, ann20phl80 and ann0phl100) were synthesized hydrothermally under controlled oxygen fugacity conditions. Lattice parameters a0 and b0 show a distinct linear decrease with increasing Mg content. With increasing ferric iron content a deviation from this linear trend is observed especially within iron rich samples. The tetrahedral rotation angle α increases smoothly from 0° in annite to 9.1° in phlogopite. Mössbauer spectra show Fe2+ and Fe3+ on the octahedral M1 and M2 sites and partially also Fe3+ on the tetrahedral site. There is a smooth increase of the quadrupole splitting on both the M1 and the M2 site going from annite to phlogopite, probably due to changes in the lattice contribution to the electric field gradient, assuming a positive correlation between quadrupole splitting and distortion. Fe3+ contents, as determined by Mössbauer spectroscopy, versus oxygen fugacity shows that, depending on the composition of the micas, minimum amounts of Fe3+ are present. For ann80phl20 this minimum amount of Fe3+ is about 8% decreasing to about 1–2% Fe3+ for ann20phl80.The molar volume of each solid solution member has been estimated from the determined relations of the molar volume versus % Fe3+ contents, extrapolated back to 0% Fe3+. Plotting these volumes as a function of Xphl shows that negative excess volume occur in the annitephlogopite join, with the maximum deviation from ideality around Xphl=0.3. Margules volume parameters have been constrained as: Wv, AnnPhl=0.018±0.016 J/(bar.mol) and Wv, PhlAnn=-0.391±0.025 J(bar.mol) (three site basis).