Emiliano Bruno

Crystal structures of Ca-rich clinopyroxenes on the CaMgSi2O6-Mg2Si2O6 join

The structural changes occurring in the clinopyroxenes with composition Di100, Di90En10 and Di80En20, due to the Ca-Mg substitution in the M2 site, have been studied. Evidence is given that with increasing Mg content a small percentage of the atoms converts from the M2 position to a new M2′ position which is solely occupied by Mg. The maximum conversion of M2 to M2′ found in this study is 7%. The closest parallel to the M2′ geometry is found in the ZnSiO3 pyroxene (C2/c). The presence of this new site causes significant changes in the tetrahedral configuration, because the M2′ atoms are not bonded to 03. The intermediate compositions, Di90En10 and Di80En20, may be thought of as the coexistence of two structural models: diopside and ZnSiO3 pyroxene (C2/c).

Average structure and M2 site configurations in C2/c clinopyroxenes along the Di-En join

In order to clarify the structural configurations observed in Diss in the Ca-rich region of the Di-En join (in which TEM observations show neither exsolution microstructures nor evidence of spinodal decomposition) single crystals large enough for X-ray diffraction analyses, with composition (Ca0.66Mg0.34)MgSi2O6, have been equilibrated close to the solvus atT=1350° C for 317 h, and quenched at room temperature. The refinement in C2/c space group shows that in the M2 site Ca and Mg are fully ‘ordered’ in two split positions (M2occ: 0.66 Ca; M2′occ: 0.34 Mg). Since the average structure shows a relevant elongation of anisotropic thermal ellipsoids of the O2 and O3 oxygen atoms, the refinement has been carried out according to a split model for O2 and O3 atoms: Ca appears 8-coordinated (as in diopside) and Mg shows a sixfold coordination similar to that of high-pigeonite. This coordination for Mg is significantly different from the fourfold coordination (Zn-like in Zn-cpx) proposed previously and it is a more probable coordination for Mg from a crystalchemical point of view. The same results were obtained refining a Di80En20 cpx, equilibrated atT=1230° C, according to the same O-split model. The data support the coexistence of a Di-like configuration for Ca and of a highPig-like configuration for Mg away from the solvus also. AtT very near toTsolidus the different configurations, observed at room temperature in the quenched samples, should converge and Ca and Mg should retain a single disordered configuration in the M2 site.

THE STRUCTURE OF ORDERED AND DISORDERED LEAD FELDSPAR (PBAL2SI2O8)

Abstract The Al-Si configuration in ordered and disordered lead feldspar (PbAl2Si2O8) has been investigated by single-crystal X-ray diffraction. Single crystals synthesized from melt and cooled from T = 1280 to 1000 °C in 1 h and then to T= 700 °C in 15 h show a completely disordered Al-Si configuration with no b>-type superstructure reflections (C2/m, a = 8.428, b = 13.054, c = 7.174 Å, p = 115.32°; R = 5.8%). Subsequent structure refinement of a crystal hydrothermally annealed at T = 500 °C and PH₂O= 2 kbar for 216 h shows an ordered Al-Si configuration (I2/c, a = 8.388, b = 13.067, c = 14.327 Å , p = 115.19°; R = 4.7%; Qod ≈ 0.9; 950 b reflections with Fo ≥ 4σ). In the ordered lead feldspar the Pb polyhedron is significantly distorted in comparison with the Sr polyhedron in strontium feldspar. The coordination of the Pb site is reduced from sevenfold, as in strontium feldspar, to a rather irregular sixfold configuration. Such distortion could be related to the lone-pair effect in Pb2+. In the disordered lead feldspar the Fourier map shows an anomalous electron-density distribution around the Pb site and consequently a split-site model for Pb was assumed in the refinement (Pb-Pb′ split = 0.557 Å).

Single-crystal in situ high-temperature structural investigation on strontium feldspar

Abstract A single crystal of ordered strontium feldspar (SrAl2Si2O8) was used for in situ X-ray intensity data collection at T = 20, 160, 330, 510, and 670 °C. The crystal was synthesized from the melt and thermally treated at T = 1450 °C for 146 h (a = 8.379, b = 12.963, c = 14.245 Å, β = 115.46 °, V = 1397.0 Å3; Qod = 0.82). At room temperature 1517 reflections of a-type and 988 reflections of b-type with Fo ≥ 2σ (Fo) were observed with R = 4.0% for refinement in space group I2/c. The dimensions of the tetrahedra do not change significantly with increasing temperature implying that the Al-Si configuration remains unchanged throughout the experimentally investigated temperature range. The Sr-coordination polyhedron expands regularly with temperature. The linear coefficient of volume expansion (αv = 1.69 × 10-5 /°C) is close to that observed for the other feldspars. The thermal expansion ellipsoid shows a remarkable anisotropy and the main expansion occurs close to a*, as observed in the other monoclinic K-, Ba-, and Pb-feldspars. The variation along a* is related to the flexing of the double-crankshaft chains in response to the expansion of the Sr-polyhedron. As in Pb-feldspar, a progressive displacement of the non-tetrahedral cation towards the c-glide plane with increasing temperature is observed. However, in Sr-feldspar, the temperature increase does not cause the atoms of the M polyhedron to approach C2/m symmetry. These results suggest that the atoms of the Srpolyhedron retain I2/c symmetry at elevated temperatures and the Sr-polyhedron does not assume a configuration that may significantly favor Al-Si disorder.

Cell parameters of thermally treated anorthite. Al,Si configurations in the average structures of the high temperature calcic plagioclases

Thermal treatments of anorthite carried out at up to 1,547° C show that the γ unit cell parameter changes as a function of the treatment temperature. The best fit curve found by non-linear least squares analysis is: γ=91.419-(0.327·10-6)T2+(0.199·10-12)T4-(0.391·10)T6. The results obtained support significant Al,Si disorder (ΔAl∼0.10, where ΔAl=t1(0)-1/3 [t1(m)+t2(0)+t2(m)], Ribbe 1975), in anorthite equilibrated near the melting point and confirm a “high temperature” series differentiated from the “low temperature” series for calcic plagioclases in the An85–An100 range also. In the plot γ vs. An-content the “high” and “low temperature” curves intersect at An85 composition and progressively diverge in the An85–An100 range. The trends of the “high” and “low temperature” curves in this range are interpretable on the basis of the degree of Al, Si order in the average structures of calcic plagioclases.

Al-Si ordering in Sr-feldspar SrAl2Si2O8: IR, TEM and single-crystal XRD evidences

Al-Si ordering in Sr-feldspar has been followed by isothermal annealing, starting from a disordered metastable configuration. Ordering could not be followed by changes in the spontaneous strain as cell parameters did not show significant changes with thermal treatment from 0.016 h to 452 h at T=1350° C, while, on the contrary, significant changes in IR spectra are observed. A single crystal obtained from melt (Qod≈ 0) has been progressively heated up to 678 h at T=1350° C and the relevant structural refinements enabled to monitor changes in degree of Al-Si order up to Qod = 0.86. In isothermal treatment for Sr-feldspar it is observed a significantly lower Qod than in anorthite after the same annealing time. TEM observation has shown in Sr-feldspar, also for shortest annealing, ‘b’ type reflections, while in anorthite, in the same conditions, ‘e’ type reflections have been observed (Carpenter 1991a). In the first stages of ordering ‘b’ APDs sized ≈ 100 Å (at T=1350° C, 0.33 h) have been observed in Sr-feldspar; APD coarsening occurs with an activation energy of 120±7 kcal mol-1, not significantly different from anorthite. The ordering process seems to be a slower process in Sr-feldspar than in anorthite, even though data from longer annealing suggest that the Qod close to the equilibrium is the same in Sr and Ca-feldspar (Qod = 0.86 at T=1350° C).

Single-crystal in situ high-temperature structural investigation of the I1̅–I2/c phase transition in Ca0.2Sr0.8Al2Si2O8 feldspar

Abstract Structural modifications induced by the I1̅-I2/c displacive transition in Ca0.2Sr0.8Al2Si2O8 feldspar (An20SrF80) have been investigated in situ by single-crystal X-ray diffraction at 20, 200, 400, 500, and 620 °C. Crystals were synthesized from the melt, cooled slowly to 1300 °C, and then quenched in air. At room temperature (a = 8.361, b = 12.973, c = 14.259 Å, α = 90.79, β = 115.55, γ = 90.62°, V = 1394.9 Å3; space group: I1̅; Qod = 0.88), the polyhedra of the non-tetrahedral cation have different configurations at the Ca/Sr(0) and Ca/Sr(z) sites. In monoclinic Sr feldspar, the Sr-OB and Sr-OD distances are regular, but in triclinic An20SrF80 feldspar the OB(m0) atom in the Ca/Sr(0)-polyhedron and the OD(mz) atom in the Ca/Sr(z)-polyhedron are displaced. The topochemical symmetry of the framework is essentially monoclinic and the average dimensions of the pseudo-symmetrical tetrahedra do not change within the error limits. With increasing temperature, the distances between the Ca/Sr cations and the pseudo-related O atom pairs converge on the values adopted at the I1̅-I2/c transition. At the transition point, the M-polyhedron assumes a regular coordination, similar to that observed in monoclinic Sr feldspar at room temperature. The results obtained indicate Ttr = 520 ± 10 °C, in agreement with the transition temperature obtained from the changes of cell dimensions. The variation of cos2α* with temperature is consistent with the solution to Landau 2-4-6 potential with Tc = 506 ± 7 °C.

Iī-I2/c phase transition in alkaline-earth feldspars along the CaAl2Si2O8-SrAl2Si2O8 join: Thermodynamic behaviour

Feldspars crystallized in the range SrF80An20-SrF60An40 of the join CaAl2Si2O8-SrAl-2Si2O8 from gels at T = 1350–1500° C for short time show cell parameters significantly different from those obtained from longer heating. This difference has been interpreted as due to higher metastable Al/Si disorder in short heated samples and to more ordered configurations with longer heat treatments. HT powder spectra show that the Tc of the Iī-I2/c phase transition of feldspars changes with composition and with the degree of order at constant composition; the evolution of the spontaneous strain (∼cosα*) versus T for the samples examined shows a variation in the order of the transition from continuous (II order) in more ordered and Sr-rich samples to a discontinuous behaviour (presumably I order) for more disordered and Sr-poor samples. A biquadratic coupling between displacive and OD parameters is therefore suggested.

High-pressure structural evolution and equation of state of analbite

Abstract The volume and unit-cell parameters of analbite (i.e., NaAlSi3O8 with complete Al,Si disorder) have been determined by single-crystal X-ray diffraction to a maximum pressure of ~8.71 GPa. The volume variation with pressure is described by a fourth-order Birch-Murnaghan equation of state with K0T = 50.3(5) GPa, K0′ = 8.9(5), and K0″ = -2.4(3) GPa-1. The value of the room-pressure bulk modulus is ~4% lower than that of low albite, and the onset of volume softening in analbite is at ~6.7 GPa, some 1.7 GPa higher than the onset in albite. The anisotropy of compression of analbite is less than that in albite. Single-crystal structure determinations of analbite to ~9.4 GPa show that there is no significant detectable compression of the T-O bonds within the structure, and the compression of the framework of tetrahedra is therefore accommodated by changes in the T-O-T angles, which result in significant compression of the “crankshaft chains” within the framework. No significant shear of the tetrahedral rings of analbite was detected, in contrast to the structural compression of albite. Overall, the structural changes that occur in analbite from 0.0001 to 9.4 GPa resemble those seen in ordered albite over the pressure range 0.0001-4 GPa. Therefore analbite shows a significantly greater structural rigidity than low albite up to pressures of 9.4 GPa

The high-pressure structural configurations of Ca0.2Sr0.8Al2Si2O8 feldspar: The I1̅-I2/c and I2/c-P21/c phase transitions

Abstract Single-crystal in situ high-P X-ray diffraction was performed at P = 0.0001, 3.2, 4.4, 6.2, and 7.4 GPa on synthetic Ca0.2Sr0.8Al2Si2O8 feldspar (An20SrF80). Data collections confirmed the displacive first-order triclinic I1̅-monoclinic I2/c phase transition at P ~4.3 GPa found in a previous high-P investigation and defined the first-order monoclinic-monoclinic transformation at P ~7.3 GPa as an I2/c-P21/c symmetry change. The structural modifications induced by the increase of pressure inside the stability fields of I1̅ and I2/c configurations as well as the structural behavior of the two phase transitions were detailed. The I1̅-I2/c transition is similar to the displacive ferroelastic I1̅-I2/c phase transition observed for the same composition with increasing temperature and is related to the increase of the M-site coordination number, with the consequent regularization of Ca/Sr polyhedra and framework. Variations of the T-O-T bond angles are observed, whereas the O-T-O angles do not change significantly. The I2/c-P21/c transition involves a significant modification of the M-polyhedra. In the M(0)- polyhedron, the two OC atoms, that were not coordinated in I1̅ and I2/c space groups, are now bonded more strongly than either OB or OD atoms [the M(0)-OC(0i) and M(0)-OC(zi) distances are 2.54 and 2.57 Å, respectively], whereas in the M(i)-polyhedron, it is the second OA(20) oxygen that becomes coordinated and, at the same time, one of the bonds to OD atoms is broken. Moreover, a significant deformation of the framework is obtained due to the decrease of the symmetry from the loss of the two-fold axis and of half of the centers of symmetry. The transition induces, besides a sign cant distortion in the T-O-T bond angles, a deformation of the internal O-T-O angles within the tetrahedra. The T2(00) tetrahedron, with the OA(20)-T2(00)-OC(0i) angle of 91° and the OC(0i)-T2(00)-OD(0i) angle of 140°, is the most deformed.