Structural control of polyhedral compression in synthetic braunite, Mn2+Mn3+ 6O8SiO4

The compression of synthetic braunite, Mn²⁺Mn³⁺ ₆O₈SiO₄, was studied by high-pressure single-crystal X-ray diffraction carried out in a diamond-anvil cell. The equation of state at room temperature (third-order Birch-Murnaghan equation of state: V ₀=1661.15(8) ų, K _0,298=180.7±0.9 GPa, K′=6.5±0.3) was determined from unit-cell volume data to 9.18 GPa. Crystal structures were determined at 6 different pressures to 7.69 GPa. Compression of the structure (space group I4₁/acd) was found to be slightly anisotropic (a ₀=9.4262(4) Å, K _a =499±4 GPa, K _a ′=19.7±0.9; c ₀=18.6964(6) Å, K _c =657±6 GPa, K _c ′=15.7±1.4) which can be attributed to the fact that the Mn³⁺-O bonds, which are the most compressible bonds, are aligned closer to the (001) plane than to the c axis. The large bulk modulus is the result of the structural topology in which 2/3 and 1/2 of the edges of the Mn²⁺O₈ and Mn³⁺O₆ polyhedra share edges with other polyhedra. The Mn²⁺O₈ polyhedra were found to compress isotropically, whereas anisotropic compressional behaviour was observed for all three Mn³⁺O₆ octahedra. Although the polyhedral geometry of all three crystallographically independent Mn³⁺ sites shows the same type of uniaxially elongated distortion, the compression of the individual octahedral configurations was found to be strongly dependent upon both the geometry of the polyhedron itself and the types of, and the connectivity to, the neighbouring polyhedra. The differences in the configuration of the different oxygen atoms, and therefore the structural topology, is one of the major factors determining the type and degree of the pressure-induced distortion, while the Jahn-Teller effect plays a subordinate role.     

Synthetic Mn3+-kyanite and viridine, (Al2-xMn x 3+ ) SiO5, in the system Al2O3-MnO-MnO2-SiO2

Experiments on the join Al₂SiO₅-“Mn₂SiO₅” of the system Al₂O₃-SiO₂-MnO-MnO₂ in the pressure/temperature range 10–20 kb/900–1050° C with gem quality andalusite, Mn₂O₃, and high purity SiO₂ as starting materials and using /_O2-buffer techniques to preserve the Mn³⁺ oxidation state had following results: At 20 kb/1000°C orange-yellow kyanite mixed crystals are formed. The kyanite solid solubility is limited at about (Al_1.88Mn _0.12 ³⁺ )SiO₅ and, thus, equals approximately that on the join Al₂SiO₅-“Fe₂SiO₅” (Langer and Frentrup, 1973) indicating that there is no Jahn-Teller stabilisation of Mn³⁺ in the kyanite matrix. 5 mole % substitution causes the kyanite lattice constants a _o, b _o, c _o, and V _o to increase by 0.015, 0.009, 0.014 Å, and 1.6 ų, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn³⁺-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusite_ss (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al_1.86Mn _0.14 ³⁺ )SiO₅-(Al_1.56Mn _0,44 ³⁺ )SiO₅. Thus, Al→Mn³⁺ substitutional degrees are appreciably higher in andalusite than in kyanite, proving a strong Jahn-Teller effect of Mn³⁺ in the andalusite structure, which stabilises this structure type at the expense of kyanite and sillimanite and, thus, enlarges its PT-stability range extremely. 17 mole % substitution cause the andalusite constants a _o, b _o, c _o, and V _o to increase by 0.118, 0.029, 0.047 Å and 9.4 ų, resp. At “Mn₂SiO₅”-contents smaller than about 7 mole %, viridine coexists with Mn-poor kyanite. At “Mn₂SiO₅”-concentrations higher than the maximum kyanite or viridine miscibility, braunite (tetragonal, ideal formula Mn²⁺Mn³⁺[O₈/Si0₄]), pyrolusite and SiO₂ were found to coexist with the Mn³⁺-saturated ky _ss or and _ss, respectively. In both cases, braunites were Al-substituted (about 1 Al for 1 Mn³⁺). Pure synthetic braunites had the lattice constants a _o 9.425, c _o, 18.700 Å, V _o 1661.1 ų (ideal compn.) and a _o 9.374, c _o 18.593 ų, V _o 1633.6 ų (1 Al for 1 Mn³⁺). Stable coexistence of the Mn²⁺-bearing phase braunite with the Mn⁴⁺-bearing phase pyrolusite was proved by runs in the limiting system MnO-MnO₂-SiO₂.     

Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7])

Crystals of sodium trisilicate (Na₂Si₃O₇) have been grown in the presence of melt at 9 GPa, 1200 °C using the MA6/8 superpress at Edmonton, and the X-ray structure determined at room pressure (R=2.0%). Na₂Si₃O₇ is monoclinic with a=8.922(2) Å, b= 4.8490(5) Å, c=11.567(1) Å, β=102.64(1)° (C2/c), D _x = 3.295 g·cm^-3. Silicon occurs in both tetrahedral and octahedral coordination (^[6]Si∶^[4]Si = l∶2). The SiO₄ tetrahedra form a diorthosilicate [Si₂O₇] group and are linked by the isolated SiO₆ octahedra via shared corners into a framework of 6-membered (^[4]Si-^[4]Si-^[6]Si^[4]Si-^[4] Si-^[6]Si) and 4-membered (^[4]Si-^[6]Si-^[4]Sr-^[6]Si) rings: 〈^[6]Si-O〉=1.789 Å, 〈^[4]Si-O〉= 1.625 Å, ^[4]Si-O-^[4]Si=132.9° and the bridging oxygen is overbonded (s = 2.22). Channels parallel to b-axis and [110] accommodate Na in irregular 6-fold coordination: 〈Na-O〉 = 2.511 Å.     

Kanonaite, (Mn 0.76 3+ Al0.23Fe 0.02 3+ )[6]Al[5][O¦SiO4], a new mineral isotypic with andalusite

Kanonaite forms rare porphyroblasts up to 12mm long in a gahnite— Mg-chlorite — coronadite — quartz schist occurring near Kanona, Zambia. The composition is (microprobe analysis): SiO₂ 32.2, Al₂O₃ 33.9, Mn as Mn₂O₃ 32.2, Fe₂O₃ 0.66, ZnO 0.13, MgO 0.04, BaO 0.04, TiO₂ 0.01, CaO 0.01, PbO 0.01, CuO 0.01, total 99.21, corresponding to $$\left( {{\text{Mn}}_{{\text{0}}{\text{.76}}}^{{\text{3 + }}} {\text{Al}}_{{\text{0}}{\text{.23}}} {\text{Fe}}_{{\text{0}}{\text{.015}}}^{{\text{3 + }}} } \right)_{1.005}^{\left[ 6 \right]} {\text{AL}}_{1.00}^{\left[ 5 \right]} \left[ {{\text{O}}_{{\text{1}}{\text{.00}}} |{\text{Si}}_{{\text{0}}{\text{.99}}} {\text{O}}_{{\text{4}}{\text{.00}}} } \right]$$ The mineral is greenish black, strongly pleochroic with X(∥a) yellow green, Y(∥b) bluish green, Z(∥c) deep golden yellow, biaxial positive, with 2V = 53°(3°), α = 1.702, β = 1.730, γ = 1.823. Vickers microhardness (100 gram load) ranges between 906 and 1017kp/mm². The structure is orthorhombic, isotypic with andalusite, space group Pnnm, a = 0.7953(2), b = 0.8038(2), c = 0.5619(2) nm, V = 0.3592(1) nm³, a/b = 0.9895(3), c/b = 0.6990(3), S.G.(_x) = 3.395 g/cm³, Z = 4. The strongest X-ray powder lines are (d in nm, I, hkl):0.5669, 100, 110; 0.4590, 75, 011 and 101; 0.3577, 90, 120 and 210; 0.2827, 94, 220; 0.2517, 90, 310 and 112; 0.2212, 83, 320, 122 and 212. Comparison of the intensities of 373 observed X-ray reflections with those calculated for several models of Mn³⁺-distribution indicates octahedral coordination of all or most of the manganese present. Interpretation of magnetic measurements (μ_eff = 3.15B.M. per Mn atom at 25 ° C) indirectly supports octahedral coordination of Mn³⁺. The name of the mineral is for Kanona, a town near the type locality. The name is proposed for the end member Mn^{3+ [6]}Al^[5][O¦SiO₄] and for members of the solid-solution series towards andalusite with octahedral Mn³⁺>Al. The presently described mineral may be referred to as aluminian kanonaite.     

Mendigite,Mn2Mn2MnCa(Si3O9)2, a new mineral species of the bustamite group from the Eifel volcanic region,Germany

A new mineral, mendigite (IMA no. 2014-007), isostructural with bustamite, has been found in the In den Dellen pumice quarry near Mendig, Laacher Lake area, Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany. Associated minerals are sanidine, nosean, rhodonite, tephroite, magnetite, and a pyrochlore-group mineral. Mendigite occurs as clusters of long-prismatic crystals (up to 0.1 × 0.2 × 2.5 mm in size) in cavities within sanidinite. The color is dark brown with a brown streak. Perfect cleavage is parallel to (001). D _calc = 3.56 g/cm³. The IR spectrum shows the absence of H₂O and OH groups. Mendigite is biaxial (–), α = 1.722 (calc), β = 1.782(5), γ = 1.796(5), 2V _meas = 50(10)°. The chemical composition (electron microprobe, mean of 4 point analyses, the Mn²⁺/Mn³⁺ ratio determined from structural data and charge-balance constraints) is as follows (wt %): 0.36 MgO, 10.78 CaO, 37.47 MnO, 2.91 Mn₂O₃, 4.42 Fe₂O₃, 1.08 Al₂O₃, 43.80 SiO₂, total 100.82. The empirical formula is Mn_2.00(Mn_1.33Ca_0.67) (Mn_0.50 ²⁺ Mn_0.28 ³⁺ Fe_0.15 ³⁺ Mg_0.07)(Ca_0.80 (Mn_0.20 ²⁺)(Si_5.57 Fe_0.27 ³⁺ Al_0.16O₁₈). The idealized formula is Mn₂Mn₂MnCa(Si₃O₉)₂. The crystal structure has been refined for a single crystal. Mendigite is triclinic, space group \(P\bar 1\); the unit-cell parameters are a = 7.0993(4), b = 7.6370(5), c = 7.7037(4) Å, α = 79.58(1)°, β = 62.62(1)°, γ = 76.47(1)°; V = 359.29(4) ų, Z = 1. The strongest reflections on the X-ray powder diffraction pattern [d, Å (I, %) (hkl)] are: 3.72 (32) (020), 3.40 (20) (002, 021), 3.199 (25) (012), 3.000 (26), (\(01\bar 2\), \(1\bar 20\)), 2.885 (100) (221, \(2\bar 11\), \(1\bar 21\)), 2.691 (21) (222, \(2\bar 10\)), 2.397 (21) (\(02\bar 2\), \(21\bar 1\), 203, 031), 1.774 (37) (412, \(3\bar 21\)). The type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, registration number 4420/1.     

Structural variation of babingtonite depending on cation distribution at the octahedral sites

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) ų. 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.     

The crystal and magnetic structure of Li-aegirine LiFe3+Si2O6: a temperature-dependent study

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.     

Structural change associated with the incommensurate-normal phase transition in akermanite, Ca2MgSi2O7, at high pressure

The structural changes associated with the incommensurate (IC)-normal (N) phase transition in akermanite have been studied with high-pressure single-crystal X-ray diffraction up to 3.79?GPa. The IC phase, stable at room pressure, transforms to the N phase at ～1.33?GPa. The structural transformation is marked by a small but discernable change in the slopes of all unit-cell parameters as a function of pressure. It is reversible with an apparent hysteresis and is classified as a tricritical phase transition. The linear compressibility of the a and c axes are 0.00280(10) and 0.00418(6)?GPa^?1 for the IC phase, and 0.00299(11) and 0.00367(8)?GPa^?1 for the N phase, respectively. Weighted volume and pressure data, fitted to a second-order Birch-Murnaghan equation of state (K′≡4.0), yield V₀=307.4(1)?ų and K₀=100(3)?GPa for the IC phase and V₀=307.6(2)?ų and K₀=90(2)?GPa for the N phase. No significant discontinuities in Si–O, Mg–O and Ca–O distances were observed across the transition, except for the Ca–O1 distance, which is more compressible in the IC phase than in the N phase. From room pressure to 3.79?GP the volume of the [SiO₄] tetrahedron is unchanged (2.16?ų), whereas the volumes of the [MgO₄] and [CaO₈] polyhedra decrease from 3.61 to 3.55(1)?ų and 32.8 to 30.9(2)?ų, respectively. Intensities of satellite reflections are found to vary linearly with the isotropic displacement parametr of Ca and the librational amplitude of the [SiO₄] tetrahedron. At room pressure, there is a mismatch between the size of the Ca cations and the configuration of tetrahedral sheets, which appears to be responsible for the formation of the modulated structure; as pressure increases, the misfit is diminished through the relative rotation and distortion of [MgO₄] and [SiO₄] tetrahedra and the differential compression of individual Ca–O distances, concurrent with a displacement of Ca along the (110) mirror plane toward the O1 atom. We regard the high-pressure normal structure as a result of the elimination of microdomains in the modulated structure.     

Crystal structure refinement and reflectance measurements of hausmannite,Mn3O4

Summary The crystal structure of hausmannite from Langbån, Sweden, was refined with 462 independent X-ray diffraction data toR = 0.036 (R_W = 0.034). The Jahn-Teller distortion of the Mn³⁺O6 octahedron is relatively strong: Mn³⁺ -O = 1.930 (1) Å (4 x), 2.282 (1) Å (2 x), and the anisotropies of the thermal vibrations of Mn^3- and O differ clearly from those of the corresponding atoms in cubic spinels. It is shown that, from a stereochemical point of view, the hausmannite structure-type is not the only geometrically possible Jahn-Teller distortion of the spinel-type. The reflectance of hausmannite was measured on an oriented cut plate with polarized light (k = 400–700 nm) in air and in oil immersion, and the refractive indices and absorption coefficients were derived.

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Kristallstrukturverfeinerung und reflexionsmessungen am Hausmannit, Mn₃O₄

Zusammenfassung Die Kristallstruktur eines Hausmannits von Langbån, Schweden, wurde an Hand von 462 unabhängigen Einkristall-Röntgendiffraktometerdaten zu einemR = 0.036 (R _w = 0.034) verfeinert. Die Jahn-Teller-Verzerrung des Mn3+O_6-Oktaeders ist relativ stark: Mn^3--O = 1.930 (1) Å (4 x); 2.282 (1) Å (2 x), und die Anisotropien der thermischen Schwingungen von Mn3 und O unterscheiden sich deutlich von denen analoger Atome in kubischen Spinellen. Es wird gezeigt, daß aus einer stereochemischen Sicht der Strukturtyp des Hausmannits nicht die einzig geometrisch mögliche JahnTeller-Verzerrung des Spinell-Typs ist. Das Reflexionsvermögen des Hausmannits wurde an einer orientiert geschnittenen Platte mit polarisiertem Licht ( = 400–700 nm) in Luft und in Ölimmersion gemessen und Brechungsindizes und Absorptionskoeffizienten wurden bestimmt.

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With 2 Figures     

High-pressure perovskites on the join CaTiO3-FeTiO3

An exploratory high-pressure study of the join CaTiO₃-FeTiO₃ has uncovered two intermediate perovskites with the compositions CaFe₃Ti₄O₁₂ and CaFeTi₂O₆. These perovskites have ordering of Ca²⁺ and Fe²⁺ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe²⁺ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe₃Ti₄O₁₂, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn₇O₁₂. Fe²⁺ is in a square-planar A site (similar to Mn³⁺ in NaMn₇O₁₂) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi₂O₆ crystallizes in a unique tetragonal double perovskite structure (SG P4₂/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO₃ recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe₃Ti₄O₁₂ is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.     

Synthesis and physical properties of piemontite Ca2Al3-pMn p 3+ (Si2O7/SiO4/O/OH)

Mn³⁺-bearing piemontites and orthozoisites, Ca₂(Al_3-pMn³⁺ _p)-(Si₂O₇/SiO₄/O/OH), have been synthesized on the join Cz (p = 0.0)-Pm (p = 3.0) of the system CaO-Al₂O₃-(MnO·MnO₂)-SiO₂-H₂O atP = 15 kb,T= 800 °C, and \(f_{O_2 } \) of the Mn₂O₃/MnO₂ buffer. Pure Al-Mn³⁺-piemontites were obtained with 0.5≦p≦1.75, whereas atp=0.25 Mn³⁺-bearing orthozoisite (thulite) formed as single phase product. The limit of piemontite solid solubility is found near p=1.9 at the above conditions. Withp>1.9, the maximum piemontite coexisted with a new high pressure phase CMS-X1, a Ca-bearing braunite (Mn _0.2 ²⁺ Ca_0.8)Mn ₆ ³⁺ O₈(SiO₄), and quartz. Al-Mn³⁺-piemontite lattice constants (LC),b ₀,c ₀,V ₀, increase with increasingp:

Single crystal hydrostatic compression of (Mg,Mn,Fe,Co)2SiO4 olivines

Four synthetic endmember olivines (Mg,Mn, Fe,Co)₂SiO₄ with space group Pbnm were loaded together in one diamond cell mount. Their unit-cell parameters were determined by single crystal X-ray diffraction to 10 GPa. The linear compressibilities β_a, β_b, β_c were 1.53, 2.90, 2.32; 1.45, 3.48, 1.98; 1.35, 3.29, 1.76; and 1.25, 2.82, 2.01×10⁻³ GPa⁻¹ for Mg₂SiO₄, Mn₂SiO₄, Fe₂SiO₄ and Co₂SiO₄, respectively. The b axis is the most compressible direction in all crystals studied. Bulk modulus K_T0 and its first pressure derivative were simultaneously determined for Mg₂SiO₄, Fe₂SiO₄ and Co₂SiO₄ crystals respectively by fitting volume data to a third order Birch-Murnaghan equation of state. They are 127(4) and 4.2(8), 136(3) and 4.1(7), and 144(2) and 4.1(5). The K_T0 and could not simultaneously be determined unambiguously for Mn₂SiO₄. Direct comparisons of unit-cell volumes at high pressure among pairs of olivines reveal anomalous compression behavior of the Mg₂SiO₄ crystal regarding the bulk modulus-volume relationship. This behavior, however, could not be observed in the transition metal olivines (Mn,Fe,Co)₂SiO₄. The distinct electronic configurations of Mg²⁺ and the transition metal cations Mn²⁺, Fe²⁺, and Co²⁺ result in the different compression behaviors of Mg₂SiO₄ and (Mn,Fe,Co)₂SiO₄. Received: 14 April 1997 / Revised, accepted: 29 July 1997     

Miscibility and compatibility of braunite,Mn2+Mn 6 3+ O8/SiO4, in the system Mn-Si-O at 1 atm in air

Experimental investigations between 800 ° to 1,100 ° C yielded no evidence for extensive substitution of Mn²⁺+Si⁴⁺2Mn³⁺ in braunite, leading to a complete solid solution series between partridgeite (Mn₂O₃) and braunites with silica contents up to 40 wt. % as proposed by Muan (1959a, b). In the presence of excess manganese braunite of nearly ideal composition coexists at 800 ° C with partridgeite and at T1,000 ° C with hausmannite (Mn₃O₄). At 800 ° C and 1,000 ° C braunite coexists, in the presence of excess silica, with a SiO₂-polymorph and at 1,100 ° C with rhodonite (MnSiO₃). Quantitative analysis of the X-ray patterns of coexisting cristobalite and braunite confirms a maximum silica-excess in braunite of only about 2 wt.% over the ideal composition, Mn²⁺Mn ₆ ³⁺ SiO₁₂.     

The Behavior of Mixed Ca–Mn Carbonates in Water and Seawater: Controls of Manganese Concentrations in Marine Porewaters

The dissolution behavior of natural, ordered kutnahorite (Mn_1.14Ca_0.82Mg_0.04Fe_0.012(CO₃)₂) and a disordered, calcian rhodochrosite (Mn_1.16Ca_0.78Mg_0.06(CO₃)₂) precipitated in the laboratory was investigated in deionized distilled water and artificial seawater in both open and closed systems at 25 °C, one atmosphere total pressure, and various pCO₂s. Both solids dissolved congruently in distilled water in an open system and yielded identical long-term equilibration or extrapolated ion activity products, IAP_pkt = a_Ca ²⁺a_Mn ²⁺(a_{CO 3} ^2?)² = 1.7 (±0.12)× 10^?21 or pIAP_pkt = 20.77 (±0.03). This value is believed to be the thermodynamic solubility product of pseudokutnahorite. In contrast, the steady state ion concentration products, ICP_pkt = [Ca²⁺][Mn²⁺][CO₃ ^2?]², measured following the dissolution of both minerals in artificial seawater increase as the CO₂ partial pressure decreases and the [Mn²⁺]:[Ca²⁺] ratio increases. These observations are interpreted as resulting from the formation of phases of different stoichiometry in response to large variations of the [Mn²⁺]:[Ca²⁺] ratio in solution. These data and results of calcite-seawater equilibration experiments in the presence of various dissolved Mn(II) concentrations define the fields of stability of manganoan calcites and calcian rhodochrosites in seawater within Lippmann phase diagrams for the CaCO₃–MnCO₃–H₂O system. Results of this study reveal that the nature (i.e., mineralogy) and composition of manganese-rich carbonate phases that may form under suboxic/anoxic conditions in marine sediments are dictated by the porewater [Mn²⁺]:[Ca²⁺] ratio, the abundance of calcite surfaces and reaction kinetics.     

Clinopyroxene-perovskite phase transition of FeGeO3 under high pressure and room temperature

In order to confirm the possible existence of FeGeO₃ perovskite, we have performed in situ X-ray diffraction measurements of FeGeO₃ clinopyroxene at pressures up to 40 GPa at room temperature. The transition of FeGeO₃ clinopyroxene into orthorhombic perovskite is observed at about 33GPa. The cell parameters of FeGeO₃ perovskite are a=4.93(2) Å, b=5.06(6) Å, c=6.66(3) Å and V=166(3) ų at 40 GPa. On release of pressure, the perovskite phase transformed into lithium niobate structure. The previously reported decomposition process of clino-pyroxene into Fe₂GeO₄ (spinel)+GeO₂ (rutile) or FeO (wüstite) +GeO₂ (rutile) was not observed. This shows that the transition of pyroxene to perovskite is kinetically accessible compared to the decomposition processes under low-temperature pressurization.     

Mössbauer,electrokinetic and refined lattice parameters study of synthetic manganoan lipscombite

Manganoan lipscombite (Fe _x ^/2+ , M _y ^/2+ ) Fe _3?(x ^+y)/3+ [OH)_3?(x+y)(PO₄)₂] was synthesized from pure chemicals. From the study of the Mn²⁺/Fe²⁺ atomic ratio by Mössbauer spectra, solubility, and electrokinetic properties, it was found that the crystal structure of lipscombite is not changed substantially by the manganese substitution. The unit cell parameters were determined from Guinier-Hägg X-ray diffraction patterns, which are identical for both synthetic ferrous-ferric and manganoan lipscombite. The two compounds crystallize in the tetragonal system with a=5.3020±0.0005 Å and c=12.8800±0.0005 Å.     

The crystal chemistry of birefringent natural uvarovites. Part III. Application of the superposition model of crystal fields with a characterization of synthetic cubic uvarovite

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–O_a=2.3504(6), Ca–O_b= 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.     

A Stucture—mineralogical Study of Ringwoodite

The powder XRD analysis of ringwoodite(γ-Fe2SiO4),which was synthesized in a II-stage anvil high-pressure capsule,was made,Its unit-cell parameter was calculated:a=8.219A,After the refinements,for several cycles,of the oxygen parameter x and the occupancy rate of Si in octahedron site,i.e.,the iversion degree u,the final result is R=0.077,when x=0.379A and u=27.5%,with the structural formula (Fe1.725 Si0.275)VI(Si0.725Fe0.275)IV O4 and atomic distances(Fe,Si)VI-O=2.022 A and (Si,Fe)IV-O=1.836A,Meanwhile,the Moessbauer spectroscopic analysis of the sample was conducted and the results obtained are in good agreement with those of X-ray structural analysis ,This paper focuses on the phase transformation and the properties of bonds of α－Fe2SiO4→γ-Fe2SiO4.     

Auriacusite, Fe3+Cu2+AsO4O, the first M 3+ member of the olivenite group, from the Black Pine mine, Montana, USA

Auriacusite, ideally Fe³⁺Cu²⁺AsO₄O, is a new arsenate mineral (IMA2009–037) and the Fe³⁺ analogue of olivenite, from the Black Pine mine, 14.5 km NW of Philipsburg, Granite Co., Montana, USA. It occurs lining quartz vughs and coating quartz crystals and is associated with segnitite, brochantite, malachite, tetrahedrite and pyrite. Auriacusite forms fibrous crystals up to about 5?µm in width and up to about 100?µm in length, which are intergrown to form fibrous mats. Individual crystals are a brownish golden yellow, whilst the fibrous mats are ochreous yellow. The crystals have a silky lustre and a brownish yellow streak. Mohs hardness is about 3 (estimated). The fracture is irregular and the tenacity is brittle. Auriacusite crystals are biaxial (+), with α?=?1.830(5), β?=?1.865(5) and γ?=?1.910(5), measured using white light, and with 2V _meas.?=?83(3)º and 2V _calc. = 84.6º. Orientation: X?=?a, Y?=?c, Z?=?b. Crystals are nonpleochroic or too weakly so to be observed. The empirical formula (based on 5 O atoms) is (Fe _1.33 ³⁺ Cu_0.85Zn_0.03)_Σ2.21(As_0.51Sb_0.27Si_0.04?S_0.02Te_0.01)_Σ0.85O₅. Auriacusite is orthorhombic, space group Pnnm, a?=?8.6235(7), b?=?8.2757(7), c?=?5.9501(5) Å, V?=?424.63(6) ų, Z?=?4. The five strongest lines in the powder X-ray diffraction pattern are [d _obs in Å / (I) / hkl]: 4.884 / (100) / 101, 001; 2.991 / (92) / 220; 2.476 / (85) / 311; 2.416 / (83) / 022; 2.669 / (74) / 221. The crystal structure was solved from single-crystal X-ray diffraction data utilising synchrotron radiation and refined to R ₁?=?0.1010 on the basis of 951 unique reflections with F _o?>?4σF. Auriacusite is identified as a member of the olivenite group with Fe³⁺ replacing Zn²⁺ or Cu²⁺ in trigonal bipyramidal coordination. Evidence suggests that auriacusite is an intermediate member between olivenite and an as yet undescribed Fe³⁺Fe³⁺-dominant member. The name is derived from the Latin auri (golden yellow) and acus (needle), in reference to its colour and crystal morphology.