M2 Polyhedron Research Articles

Experimental and calculated Raman spectra in Ca–Zn pyroxenes and a comparison between (CaxM2+1−x)M2+Si2O6 pyroxenes (M2+ = Mg, Co, Zn, Fe2+)

The Raman spectra of the end member pyroxenes CaZnSi2O6 and Zn2Si2O6 are calculated by quantum mechanical modeling and compared with the experimental ones measured in synthetic (CaxZn1−x)ZnSi2O6 pyroxenes with x = 0, 0.2, 0.3, 0.5, 0.7, 1. The calculated spectra correctly predict the intensity and peak positions of the spectra recorded on the end members. The model provides also useful hints for the mode assignment at the intermediate compositions. The experimental peak positions are compared in (CaxM2+1−x)M2+Si2O6 pyroxenes, with M2+ = Mg, Co, Zn, Fe2+. These pyroxenes share a common charge and different mass and ionic radius; the relative contributions of the mass and ionic radius in the experimental spectrum are discussed in four of the most intense peaks. We found that the positions of the strongest peaks are related to the average bond distances of the polyhedra which most affect a given mode. Ca–Zn pyroxenes provide an exception, whereas the CaZnSi2O6 end member fits quite well in the bond-distance/peak positions relations found in other pyroxenes, and the same does not occur as Zn exchanges for Ca. Peak broadening occurs in Zn pyroxenes in intermediate compositions; it is related to the presence of polyhedral local configurations around Zn and Ca atoms in the M2 polyhedron. The broadening is higher in the ~ 1010 cm−1 peak (ν19), which, among the strongest peaks, shows the highest difference in the Raman wavenumber between end members. The different behaviours of Zn pyroxenes with respect to Mg, Co, and Fe2+ ones are likely related to the partially covalent bonding in the M2 cavity shown by Zn pyroxenes.

Co2+-doped diopside: crystal structure and optical properties

Synthetic clinopyroxenes along the CaMgSi2O6–CaCoSi2O6 join were investigated by a combined chemical-structural-spectroscopic approach. Single crystals were synthesized by flux growth methods, both from Ca-saturated and Ca-deficient starting compositions. Single crystal structure refinements show that the incorporation of Co2+ at the octahedrally coordinated cation sites of diopside, increases the unit-cell as well as the M1 and the M2 polyhedral volumes. Spectroscopic investigations (UV–VIS–NIR) of the Ca-rich samples reveal three main optical absorption bands, i.e. 4 T 1g → 4 T 2g(F), 4 T 1g → 4 A 2g(F) and 4 T 1g → 4 T 1g(P) as expected for Co2+ at a six-coordinated site. The bands arising from the 4 T 1g → 4 T 2g(F) and the 4 T 1g → 4 T 1g(P) electronic transitions, are each split into two components, due to the distortions of the M1 polyhedron from ideal Oh-symmetry. In spectra of both types, a band in the NIR range at ca 5000 cm−1 is caused by the 4 A 2g → 4 T 1g(F) electronic transition in Co2+ in a cubic field in the M2 site. Furthermore, an additional component to a band system at 14,000 cm−1, due to electronic transitions in Co2+ at the M2 site, is recorded in absorption spectra of Ca-deficient samples. No variations in Dq and Racah B parameters for Co2+ at the M1 site in response to compositional changes, were demonstrated, suggesting complete relaxation of the M1 polyhedron within the CaMgSi2O6–CaCoSi2O6 solid solution.

Structure refinement and crystal chemistry of tokkoite and tinaksite from the Murun massif (Russia)

AbstractThe structures of tokkoite, K2Ca4[Si7O18OH](OH,F) and tinaksite, K2Ca2NaTi[Si7O18OH]O from the Murun massif (Russia) were refined from single-crystal X-ray diffraction data in the triclinic space groupP1̄. Average crystallographic data area≈ 10.423,b≈ 12.477,c≈ 7.112 Å, α ≈ 89.92°, β ≈ 99.68°, γ ≈ 92.97°,V≈ 910.5 Å3for tokkoite;a≈ 10.373,b≈ 12.176,c≈ 7.057 Å, α ≈ 90.82°, β ≈ 99.22°, γ ≈ 92.80°,V≈ 878.5 Å3for tinaksite. The substantial similarities between the geometrical parameters of the tokkoite and tinaksite structures led us to conclude that the two minerals are isostructural. However, major differences of tokkoite with respect to tinaksite are larger lattice constants, especially concerning thebparameter, longer <M–O> distances, especially <M1–O>; larger values of the M1–M3 and O20–O2 bond lengths, and a stronger distortion of the M1 polyhedron. Mössbauer analysis showed that significant trivalent iron is present,VIFe3+40.0(7)% in tokkoite and 12.8(3)% in tinaksite. It is confirmed that 2Ca(M1+M2)2++ (F,OH)(O20)–↔ Ti(M1)4++ Na(M2)++ O(O20)–is the exchange reaction that describes the relation between tokkoite and tinaksite. In addition, this exchange reaction causes local stress involving mainly the M1 site and its interaction with the M2 and M3 sites.

Ca-Zn solid solutions inC2/cpyroxenes: Synthesis, crystal structure, and implications for Zn geochemistry

The effect of Zn substitution on a series of clinopyroxenes along the join CaZnSi2O6-Zn2Si2O6 was studied. The pyroxenes were synthesized at P = 4–5 GPa and T = 1000–1200 °C by a multi-anvil apparatus. SEM-EDS and XRD analysis showed complete solid solution; all of the samples have the C 2 /c space group. No miscibility gap between clino- and orthopyroxene nor phase transition to the P 21 /c space group was found. Moreover, the cell volume of Ca-Zn pyroxenes decreases less than expected from the decrease of the average cation size for the substitution of Zn for Ca. The crystal structures of three synthetic pyroxenes of composition (Ca0.5Zn0.5)ZnSi2O6, (Ca0.3Zn0.7)ZnSi2O6, and (Ca0.2Zn0.8)ZnSi2O6 were refined by single-crystal X-ray diffraction (R4σ between 3 and 4.5%). It was observed that the Ca-Zn substitution occurs in the M2 polyhedron, with a sub-site splitting of Zn in a position at approximately 0.7 A from Ca. In this position, Zn retains a highly distorted fourfold coordination; moreover, the tetrahedral chain configuration is little changed along the series, and the M1 polyhedral size increases with Zn substitution in M2. An implication of the present work is the interpretation of the partitioning of Zn between clinopyroxene and melt. The distribution coefficients of Zn and Co are quite different in rocks of the same composition, despite their very similar ionic radius, and the difference is related to the preference of Zn for the M2 site, where Zn may find a suitable atomic coordination.

Raman spectroscopy of CaM2+Ge2O6(M2+ = Mg, Mn, Fe, Co, Ni, Zn) clinopyroxenes

The Raman spectra of Ge-clinopyroxenes CaM2+Ge2O6 (M2+ = Mg, Mn, Fe, Co, Ni, Zn), general formula M2M1T2O6, are reported for the first time. Their spectral features are discussed in comparison with corresponding Si-pyroxenes. The vibrational wavenumbers of germanates may be roughly obtained by a scale factor of about ~0.8 by those of the corresponding silicates, due to the Ge-Si mass difference. The main peaks in the germanate Raman spectra at ~850 and ~540 cm−1 may be related to Ge-O tetrahedral stretching and chain bending, respectively; minor peaks between 200 and 400 cm−1 are ascribed to bending and stretching of the non-tetrahedral cations. Within Ge-pyroxenes, possible correlations between crystallographic parameters and the vibrational wavenumbers are investigated. The main stretching mode at ~850 cm−1 shows wavenumber changes with M2+ substitutions, but no simple correlation can be found with M2+ cation mass or size. On the other hand, the chain bending wavenumber linearly decreases with increasing ionic radius of the M2+ cation: the expansion of the M1 polyhedron reduces the chain kinking angle and the Ge-Ge distances correspondingly increase. Copyright © 2015 John Wiley & Sons, Ltd.

Crystal chemistry of Sc-bearing synthetic diopsides

Scandium substitution in the diopside structure was studied by single-crystal X-ray diffraction on a series of synthetic diopside samples. These diopsides were doped with increasing amounts of Sc3+ through a coupled substitution involving M1(Sc3+)1 M2[] 1 T (B)1 M1(Mg2+)−1 M2(Ca2+)−1 T(Si4+)−1 exchange, whereby charge compensation is achieved by vacancies at the M2 sites and B at the tetrahedral sites. The substitution of scandium for magnesium at the M1 site results in an increase in volume and distortion of the M1O6 polyhedron. The accompanying creation of vacancies at the M2 sites causes an increase in the M2 polyhedral volume. The modifications of the M1 and M2 polyhedra result in an increase of the polyhedral strip along the b lattice direction and a straightening of the tetrahedral chain. The geometrical modification of the M1 polyhedron due to scandium incorporation is comparable to those observed when similar amounts of Ti3+/Ti4+ substitute for Mg in the diopside structure, suggesting a structural control on the solubility of Sc and Ti in diopside that may influence the extent of the solid solutions between the Sc and Ti end-members.

The structure of (Ca,Co)CoSi2O6 pyroxenes and the Ca-M2+ substitution in (Ca,M2+)M2+Si2O6 pyroxenes (M2+ = Co, Fe, Mg)

The crystal structure of three C2/c clinopyroxenes with composition (Ca 0.8 Co 0.2 )CoSi 2 O 6 , (Ca 0.6 Co 0.4 ) CoSi 2 O 6 and (Ca 0.4 Co 0.6 )CoSi 2 O 6 was refined down to R4σ = 2.6% by single-crystal X-ray diffraction. The crystals were synthesized at P = 3 GPa by cooling from 1500 to 1200 °C in a piston-cylinder apparatus. At the end of the refinement cycles, electron density residuals (up to 2.1 e-) were observed close to the M2 site and related to the site splitting of Ca and Co in the M2 polyhedron in the two subsites M2 and M2′. Split refinement significantly improved the agreement factor and decreased the uncertainty in the atomic coordinates. Similar features were found in (Ca,Mg)MgSi 2 O 6 and (Ca,Fe) FeSi 2 O 6 intermediate pyroxenes. The average structural changes related to the cation substitution at the M2 site in (Ca,Co)CoSi 2 O 6 , (Ca,Mg)MgSi 2 O 6 , and (Ca,Fe)FeSi 2 O 6 pyroxenes are similar: the T tetrahedron becomes more regular, the difference between M1-O bond lengths increases, and the M2-O3 bond lengths with the furthermost O3 oxygen atoms become longer. The changes in the M2-O bond distances are, however, not linear, and they are higher for more increased substitution. The largest structural deformation occurs on the (010) plane, with higher deformation at about 60° from the c axis for any composition. The orientation of the deformation ellipsoid is most related to a shift in tetrahedral chains. The scalar deformation for the cation substitution, εs, is linearly related to the cation radius of the average M2 site (IRM2), i.e., the deformation is higher as the cation size decreases, following the equation: εs = -0.0072(12)IRM2 + 0.0082(13), R 2 = 0.75. Increasing deformation with cation substitution is supported as the major limiting factor for solid solution. The displacement parameters for unsplit M2, O2, and O3 atoms increase up to the intermediate composition, indicating a local configuration for the M2 polyhedron centered by Ca and Co. However no significant change in U eq of the O3 atom is observed up to 20% substitution of the smaller cation in the M2 site. Comparison with Raman spectral data suggests that local chain structural configurations occur only for the substitution of the smaller cation in the M2 site higher than 20%, and that the substitution mechanism is different for C2/c clinopyroxenes with lower and higher Ca content

Cation ordering in Pb2+-bearing, Mn3+-rich pargasite from Langban, Sweden

A multi-analytical approach using electron microprobe analysis, X-ray structural refinement, and polarized single-crystal Fourier transform infrared spectroscopy was used to characterize short-range and long-range structures of a Pb 2+ -bearing, Mn 3+ -rich pargasite. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, demonstrate that Pb 2+ is strongly ordered at the A-site in the monoclinic C 2/ m amphibole structure. This finding is in agreement with the observed ordering of Pb 2+ in the rare Pb 2+ -bearing P 2 /a amphibole joesmithite, but is in contrast to a Pb 2+ preference for the M4 site suggested by some studies on element partitioning between C 2/ m amphiboles and melts. Mn 3+ is strongly ordered at the M2 site in structure of the present amphibole. Contrasting results obtained for mean M2-O bond lengths and reported local Mn 3+ -O bond lengths as well as between bond-length distortion of the mean M2 polyhedron and the local Mn 3+ -centered M2O 6 octahedron in pargasite indicates that the structural relaxation of this polyhedron is complete or nearly so.

Raman spectroscopy of (Ca,Mg)MgSi2O6 clinopyroxenes

The Raman spectra of eight clinopyroxenes synthesized along the join diopside-clinoenstatite (Di-Cen, CaMgSi2O6-Mg2Si2O6) were measured. The splitting of the 670 cm−1 mode of the Ag symmetry, observed in the composition Di52En48 to clinoenstatite, was interpreted as evidence of a C 2/ c-P 21/ c phase transition. The transition was also revealed by deviation from the linear dependence of the peak position vs. composition and by the appearance of several new peaks in the samples richer in clinoenstatite. Analysis of peak positions vs. structural changes suggests that for the M2 polyhedron, in which Ca substitution for Mg occurs, a different deformation mechanism acts in Ca richer and poorer P 21/ c pyroxenes, and that Ca richer P 21/ c pyroxenes deform with the same mechanism as C 2/ c pyroxenes. The frequency of the peak at 670 cm−1 was found to change linearly with the kinking angle of the tetrahedral chains for C 2/ c and of the B chain for P 21/ c , whereas the position of the peak ascribed to the A chain was little affected by the kinking angle. Peak broadening in C 2/ c Ca-rich homogeneous pyroxenes was interpreted to be a consequence of the positional disorder of the Ca and Mg in the M2 cavity: peak broadening increases with increasing Mg content for peaks assigned to M2-O vibrations, but it changes little for peaks assigned to chain bending, which suggests that cation substitution in the M2 cavity occurs with little interaction with the silicate chain. Furthermore peak broadening was observed in intermediate pyroxenes as a consequence of mottled textures, antiphase domains, and compositional inhomogeneity.

Thermal expansion and high-temperature P21/c–C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6

A synthetic clinopyroxene with composition LiFe3+Ge2O6, monoclinic s.g. P21/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) A, β = 108.844(6)°, V = 442.75(16) A3, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P21/c to C2/c at a transition temperature Ttr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below Ttr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > Ttr the thermal volume expansion is due to dilatation of the structure in the \((\bar{1}\,0\,1) \) plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β. From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P21/c expands more than the C2/c phase. The evolution of Q2 (calculated as the normalized intensity of b-type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q02 = 0.538(2) at Ttr, with Tc of 481(7) K, b/a = −2,290 K, and c/a = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe3+Si2O6) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P21/c ↔ C2/c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13.3%, compared to ca. 1.3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T > Ttr ( Ttr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at Ttr of the kinking angle (O3–O3–O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T < Ttr [206.8(5)° at 703 K] and extends by 12° at the transition.

Lattice compression and structural behavior of NaVSi2O6 clinopyroxene to 11 GPa

High-pressure behavior of a synthetic single crystal of the C 2/ c clinopyroxene NaVSi 2 O 6 was investigated using X-ray diffraction and Raman spectroscopy in combination with diamond-anvil cell techniques. Lattice-parameters, single-crystal intensity data, and Raman spectra were collected up to pressures of about 11 GPa, revealing no discontinuity in the evolution of the lattice with increasing pressure. The fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields a bulk modulus of K 0 = 114.7 (1.8) GPa with its pressure derivative K ′ = 5.4 (4). Strain analysis reveals compressional anisotropy with unit strain axial ratios of 1:2.9:3.1. The direction of largest compression was found to be perpendicular to the (302) plane, which matches closely the direction of largest interlayer spacing between oxygen monolayers forming the structure of clinopyroxenes. The relatively large pressure dependency of the bulk modulus, compared to that of isostructural transition-metal compounds, was related to an increase in stiffness of the M2 polyhedron occurring above 4.6 GPa.

Synthesis, TEM characterization and thermal behaviour of LiNiSi2O6 pyroxene

A pyroxene with composition LiNiSi2O6 was synthesized at T = 1,473 K and P = 2.0 GPa; the cell parameters at T = 298 K are a = 9.4169(6) Å, b = 8.4465(7) Å, c = 5.2464(3) Å, β = 110.534(6)°, V = 390.78(3) Å3. TEM examination of the LiNiSi2O6 pyroxene showed the presence of h + k odd reflections indicative of a primitive lattice, and of antiphase domains obtained by dark field imaging of the h + k odd reflections. A HT in situ investigation was performed by examining TEM selected area diffraction patterns collected at high temperature and synchrotron radiation powder diffraction. In HTTEM the LiNiSi2O6 was examined together with LiCrSi2O6 pyroxene. In LiCrSi2O6 the h + k odd critical reflections disappear at about 340 K; they are sharp up to the transition temperature and do not change their shape until they disappear. In LiNiSi2O6 the h + k odd reflections are present up to sample deterioration at 650 K. A high temperature synchrotron radiation powder diffraction investigation was performed on LiNiSi2O6 between 298 and 773 K. The analysis of critical reflections and of changes in cell parameters shows that the space group is P-centred up to the highest temperature. The comparative analysis of the thermal and spontaneous strain contributions in P21/c and C2/c pyroxenes indicates that the high temperature strain in P-LiNiSi2O6 is very similar to that due to thermal strain only in C2/c spodumene and that a spontaneous strain contribution related to pre-transition features is not apparent in LiNiSi2O6. A different high-temperature behaviour in LiNiSi2O6 with respect to other pyroxenes is suggested, possibly in relation with the presence of Jahn–Teller distortion of the M1 polyhedron centred by low-spin Ni3+.

The thermal behavior of richterite

The thermal behavior of synthetic richterite A Na B (NaCa) C Mg 5 T Si 8 O 22 (OH) 2 , crystallized at 800 °C and 0.35 GPa under hydrothermal conditions, was studied by serial powder diffraction experiments using synchrotron radiation between 123 and 873 K. The a , b , and β cell parameters show a non-linear behavior, whereas the c parameter shows a linear trend; values of the saturation temperature 𝛉 s are 410(3), 215(2), and 300(2) K for a , b , and a sinβ, respectively. The axial expansion pattern below room temperature is α b > α c > α a , whereas above room T it is α b > α a > α c , the difference being related to the different saturation temperatures of the individual cell parameters. The thermal expansion was modeled following the Debye approximation for the density-of-state of phonons; the refined parameters are V 0 = 908.20 (0.04) A 3 , k = 1 (4), Q 0 =32 (2) MJ, and the Debye temperature 𝛉 D = 586(31) K. The non-linear behavior at low T is well described without systematic differences between the data and the model. The volume thermal expansion coefficient α V changes significantly with temperature, also at temperatures higher than room temperature. It is suggested that this may occur also in other amphiboles and pyroxenes, requiring critical re-examination of the available data. A comparison of the strain tensor for the thermal expansion between amphiboles and pyroxenes shows that in amphiboles the major deformation occurs onto the (010) plane, whereas in pyroxenes it occurs along the b axis. Moreover, the major deformation on (010) in Ca-bearing pyroxenes occurs along the bond of the M2 cation with the furthermost O3 atoms, whereas in compositionally related Ca-amphiboles (i.e., tremolite) it occurs in a direction rotated by 10 20° to a* , i.e., in a direction not corresponding to that of the M4-O5 bonds. It is proposed that the M4 polyhedron contributes less to the thermal deformation of amphibole than the M2 polyhedron contributes to the thermal deformation of pyroxene.

Low-temperature behavior of NaGaSi2O6

Synthetic NaGaSi2O6-pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, θD, and the Grüneisen parameter, γ, are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P1̅ phase transition observed in a previous work for NaTiSi2O6 at about 200 K did not occur for NaGaSi2O6 down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t2g orbitals, which do not exist in NaGaSi2O6.

Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin dia- mond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) A 3 , K0 = 129(1) GPa, K' = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compress- ibilities βb > βc ~ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show signiÞ cant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to Þ t into the pyroxene structure.

The effect of Ca substitution on the elastic and structural behavior of orthoenstatite

A single crystal of orthopyroxene with composition Ca 0.07 Mg 1.93 Si 2 O 6 (space group Pbca ) has been investigated at high pressure and room temperature by in-situ X-ray diffraction using a diamond anvil cell. The unit-cell parameters have been determined at ten different pressures up to 10.16(5) GPa. In the pressure range investigated no evidences of phase transitions have been found. The pressure-volume data have been fitted with a third-order Birch-Murnaghan equation of state resulting in the following parameters: V 0 = 838.26(8) A 3 , K T0 = 110(1) GPa, K ’ = 6.6(4). The Ca substitution in the pure orthoenstatite Mg 2 Si 2 O 6 structure causes a slight increase in K T0 and a decrease in K ’. The compressibility of a , b , and c unit-cell parameters is strongly anisotropic with a compressibility scheme β b >> β c >> β a . The structure evolution as a function of pressure has been determined at five different pressures up to 6.25(5) GPa. The M2 polyhedron undergoes the largest volume variation (~7.7%), whereas the volume variation of M1 is ~6.1%. The TA and TB tetrahedral volumes decrease by about 3% and 1.2%, respectively, without a discontinuity in the pressure range investigated.

High temperature single crystal investigation in a clinopyroxene of composition (Na0.5Ca0.5)(Cr0.5Mg0.5)Si2O6

The structure of a synthetic clinopyroxene of composition (Na 0.5 Ca 0.5 )(Cr 0.5 Mg 0.5 )Si 2 O 6 was investigated at room and high temperature. At room temperature a high resolution data collection (2𝛉 ≤ 110°) was done; at high temperature the data were collected up to 2𝛉 ≤ 50° and at T = 25, 200, 400 and 620°C. The observed symmetry was always C 2/ c , and no evidence of a reduction to the long range ordered P 2 1 / n symmetry was found. The analysis of the evolution of the B eq vs T suggests instead the presence of local clustering of NaCr and CaMg pairs. A comparison with the high temperature behaviour of diopside, kosmochlor and jadeite shows a significant decrease in volume expansion with the coupled NaM 3+ substitution for CaMg. This is mainly due to a decrease in the expansion along the b axis, and is related to a smaller expansion in the M1 polyhedron for the substitution of a trivalent cation in the M1 site and to the consequent higher stiffness of the O1A1 - O1B1 shared edge between the M1 polyhedra.

CRYSTAL CHEMISTRY OF THE CONSTITUENT PHASES OF ULTRAMAFIC XENOLITHS FROM HANNUOBA REGION (NE CHINA)

Because of the general inaccesibility of the lithospheric mantle, deep-seated xenoliths represent an important source of information on the earth’s interior. They are usually entrained by alkaline basalts in circumcratonic belts or by kimberlites and/or lamproites in cratonic areas. In the last decades the crystal chemistry of major mantle constituents has improved the knowledge of mantle mineralogy and has emerged as an additional petrological constraint for evaluating the cooling histories of the host rocks (Dal Negro et al., 1982; Saxena and Dal Negro, 1983; Cundari et al., 1986; Princivalle et al., 2000). In this work the crystal chemistry of coexisting phases in a suite of spinel peridotite xenoliths from Hannuoba region has been carried out by means of single crystal X-ray diffraction and electron microprobe to obtain site occupancies. The Hannuoba basaltic plateau, 200 km northwest of Beijing, large more than 1700 km2, lies within the North China Craton and consists of interlayered alkaline and tholeiitic basalts; the alkaline ones host deep-seated xenoliths while only very rare xenoliths have been found in the tholeiites. Volcanism is related to widespread Cenozoic rifting in the North China Craton. Along the southern margin of the Hannuoba plateau there are about forty xenolith-rich zones; in particular one of those, Damaping, is noteworthy for its abundant spinel peridotite xenoliths (approximately 85% of the outcrop), their large size (up to 80 * 60 * 45 cm), their coarse grain size and gem-quality olivine. The “peridotitic body” outcrops along a NE-trending tectonic belt. All the xenoliths investigated in this study were collected at Damaping. X-ray diffraction data were recorded on an automated KUMA-KM4 (K-geometry) diffractometer using MoK_ radiation, monochromatised by a flat graphite crystal. Data collection and structure refinement strategies were performed following procedures used for cpx by Dal Negro et al. (1982), for spinel by Della Giusta et al. (1986) and for olivine by Princivalle and Secco (1985). The same crystals used for X-ray single crystal data collection were also used for chemical analyses. These were performed by a CAMECA- CAMEBAX microprobe operating at 15 KV and 15 nA. A PAP-CAMECA program were used to convert X-ray counts into weight percent of the corresponding oxides. Results are considered accurate to within 2-3 % for major and better than 10 % for minor elements. Clinopyroxenes The Hannuoba clinopyroxenes (cpx), characterised by high Ca (0.775-0.872 atoms for formula unit, a.f.u.) and Mg (0.791-0.962 a.f.u.) contents, plot in the aluminous-diopside field (Burnham et al., 1967). Within the suite Vcell (cell volume) varies from 433.34 to 436.44 and parallels VM1 and VM2 (M1 and M2 site volume, respectively) increase. On the contrary, T site volume decreases with increasing Vcell, due to increasing Si content and this is related to the increase of mg# cpx and the general decrease of cpx modal content (i.e. increase of basalt component extraction). As is typical for pyroxenes from ultramafic inclusions T site is near full occupied by Si4+ (1.878-1.945 a.f.u.). M2 site is essentially filled by Ca and Na, with minor amounts of Mg and Fe2+. Substantial occupancy of this site by relatively large ions like Ca and Na tends to minimize the distortion of M2 polyhedron (Dal Negro et al., 1982). Mg and R3+ (AlVI+Fe3++Cr+Ti) cations fill M1 polyhedron, while Fe2+ is present only in small amount. M1 volume increase is controlled by R3+ decrease. Spinel Within the suite, unit cell parameter ao ranges from 8.1291 to 8.2265 A and Cr content increases from 0.1792 to 0.6191 a.f.u., whereas oxygen coordinate u remains virtually constant (0.26276-0.26310), therefore giving a quite constant ratio of octhahedral (M-O) and tetrahedral (T-O) bond distances. The increase in Cr causes progressive ordering of Mg and Al in T and M sites respectively. Princivalle et al., (1989) evidenced how the cation partitioning and the consequently oxygen parameter (u) of Cr-spinels from peridotites is related to the cooling history of the rock. Hannuoba spinels are characterised by higher u values (compared with analogues spinels described in literature), suggesting a slower cooling history. Olivine Hannuoba investigated olivines have forsteritic compositions (Fo 89.2-90.6 %) and Vcell varing from 291.6 to 291.8 A3. VM1 (11.876-11.907 A3) increases with decreasing mg#, due to larger ionic radius of Fe2+ relative to that of Mg, according to Shannon’s (1976) data. Mean M1-O and M2-O bond lengths increase slightly as Fo content decrease. Fe2+ and Mg entry in both the octahedral sites M1 and M2. No significant ordering of Fe2+ on M1 site has been detected (KD values near the unit).

Model pyroxenes II: Structural variation as a function of tetrahedral rotation

Model pyroxenes with regular tetrahedral and M1 octahedral coordination polyhedra have been derived. The M2 polyhedron is not constrained to be regular. These models are parameterized in terms of the O3-O3-O3 angle, θ, and the model O atom radius, r . Crystallographic parameters such as interatomic distances, unit cell volume, and packing distortion are determined as a function of the O3-O3-O3 angle. Results are compared with observed pyroxenes, providing insight into which interatomic interactions are important in determining pyroxene topology and behavior. Temperature is shown to favor polyhedral regularity in orthopyroxene and protopyroxene. Compression and expansion strain ellipsoids for observed and model pyroxenes are compared, demonstrating that a combination of tetrahedral rotation and isotropic compression approximately reproduces the compression ellipsoids of pyroxenes, but not the expansion ellipsoids.

Microtextures and crystal chemistry in P 2 1 / c pigeonites

¶A single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances.