M. Tribaudino

Single-crystal thermometric calibration of Fe-Mg order-disorder in pigeonites

Abstract The single-crystal X-ray technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ + Mn2+) between M1 and M2 sites of natural P21/c pigeonite. Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo6En76Fs18) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308- 2 (Wo10En47Fs43) from the BTS308 Paranà rhyodacite, were annealed at temperatures ranging from 600 to 1000 °C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2. The two geothermometric equations calculated by linear regression of ln KD* vs. 1/T are: ln KD* = -3291(± 269)/T(K) + 0.971(±0.253); (r2 = 0.974) ln KD* = -2816(± 83)/T(K) + 0.542(±0.083); (r2 = 0.995) respectively for PCA82506-3 (XFe* = 0.20) and BTS308-2 (XFe* = 0.49) [XFe* = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo10-.

A transmission electron microscope investigation of the C2/c →P21/c phase transition in clinopyroxenes along the diopside-enstatite (CaMgSi2O6-Mg2Si2O6) join

Abstract A transmission electron microscope investigation was performed on a series of synthetic clinopyroxenes with compositions between Ca0.7Mg1.3Si2O6 and Ca0.5Mg1.5Si206. For samples with Ca content lower than 0.6 atoms per formula unit (apfu) selected-area electron diffraction (SAED) patterns showed the presence of reflections violating the C2/c space group (b type, h+k odd), indicating a transition to the P21/c symmetry at room temperature. Antiphase domains induced by the transition could be imaged, with irregular boundaries and size decreasing with increasing Ca content. The antiphase domain size and shape appears unrelated to the non-periodic mottled texture that could be imaged in samples with Ca lower than about 0.65 apfu. A comparison with data reported in the literature suggests a different behavior in the cell parameters of the P21/c clinopyroxenes along the diopside-eustatite (Di-En) join. For compositions between Di60En40 to about Di40Eu60, cell parameters deviate little with respect to the trend shown by Ca-rich C2/c clinopyroxenes, whereas for compositions richer in Mg than about Di40En60, a significant deviation is present, which is marked by discontinuities in the c and β cell parameters.

Average and local structure in P21/c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6- Mg2Si2O6)

A single-crystal X-ray refinement has been performed on two synthetic P 2 1/c clinopyroxenes of composition Ca 0.15 Mg 1.85 Si 2 O 6 (Di 15 En 85 ) and Ca 0.23 Mg 1.77 Si 2 O 6 (Di 23 En 77 ). Splitting of the M2, O2B, O3A and O3B sites was inferred by the presence of significant residuals in the difference Fourier map and by an anomalous size and orientation of the displacement parameters. A refinement performed according to a split-model showed the coexistence of two M2 polyhedral configurations, one clinoenstatite-like, the other like Caricher C2/c clinopyroxenes. The two configurations have been interpreted assuming the coexistence of Ca and Mg centred polyhedra, possibly in connection with clustering preliminary to exsolution. The difference between the two configurations increases with increasing Ca content. A TEM investigation of the sample with composition Ca 0.15 Mg 1.85 Si 2 O 6 showed the lack of exsolution textures and antiphase domains larger than 1μm. Analysis of the average structural configurations shows that the substitution of Ca for Mg deforms differently Ca-poorer (up to 0.2–0.3 atoms p.f.u) and Ca-richer (between 0.3 and 0.6 Ca atoms p.f.u.) P2 1 / c iron-free clinopyroxenes. In Ca-poorer ones only minor changes in the β parameter were found, related to a lack of shift in tetrahedral chains, whereas Ca-richer undergo a significant decrease in the βparameter, possibly due to stronger chain shift.

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).

Antarctic FRO90011 lodranite: Cooling history from pyroxene crystal chemistry and microstructure

The Antarctic FRO90011 lodranite shows an achondritic coarse granoblastic texture with an approximately chondritic composition. The crystal chemistry of Pbca and C2/c pyroxene single crystals and the TEM texture of Pbca pyroxene were investigated to reconstruct the evolutionary history of the meteorite parent body. The structural behaviour of orthopyroxene (En85Fs12.4Wo2.6) is well constrained in the En-Fs trend. Orthopyroxene Fe2+ue5f8Mg ordering in the M1 and M2 sites yields a closure temperature (Tc) of 570 ± 20°C and a cooling rate 10°C/day near the Tc. Clinopyroxene is a low-iron augite (En51Fs5Wo44). Clinopyroxene Fe2+ue5f8Mg ordering in the M1 and M2 sites yields a Tc of 513 ± 50°C, which is compatible with the cooling rate derived from the orthopyroxene crystals. TEM investigation on orthopyroxene showed a few thin lamellae not larger than 20A parallel to (100) and rare one unit cell Guinier-Preston zones. TEM-EDS analysis did not reveal significant chemical differences between lamella-rich and lamella-free zones. The above data are interpreted as being due to fast cooling resulting from radiative heat loss of small fragments ejected from a parent body, which were subsequently reassembled into a larger body.

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.

The structure of Pbca orthopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6)

The crystal structure of two synthetic orthopyroxenes, with compositions Ca 0.04 Mg 1.96 Si 2 O 6 and Ca 0.07 Mg 1.93 Si 2 O 6 has been refined by single-crystal X-ray diffraction [R(4σ) = 2.7 and 3.6 %, respectively]. TEM observation has shown the presence of irregularly repeated P 2 1 / c clinopyroxene lamellae, oriented parallel to (100), in agreement with the presence of reflections with 1 = 2n violating the b glide plane. Significant electron density residuals close to the M2 and O2B sites were found, allowing split site refinement for the sample with composition Ca 0.07 Mg 1.93 Si 2 O 6. M2-O bond lengths for average and local configurations for Ca and Mg have been found to be very similar to those of clinopyroxenes along the join diopside-enstatite. The increasing presence, due to the substitution of Ca for Mg in the M2 site, of Ca configurations in orthopyroxene that are similar to the average ones in Ca-richer C 2/ c clinopyroxene reduces the stability of the orthopyroxene phase.

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).

Crystal structure of Di50CaTs50 synthetic clinopyroxene (CaMgo0.50AlSil1.5O6). Crystal chemistry along the Di-CaTs join

SummaryThe crystal structure of the Di50CaTs50 clinopyroxene Ca(Mg0.5Al0.5) (Al0,5Si1.5)O6, synthesized at T=1 100°C andP=14 kb, was refined in the C 2/c space group. Cell parameters are:a=9.655,b=8.771,c= 5.278 Å, β=106.22°. The finalR value is: 2.1%. Comparison with the Di and CaTs structures shows that the variation of the M 2-0, M 1-0 and T-0 bond lengths is continuous and almost linear in the clinopyroxene solid solutions along the Di-CaTs join. The modifications caused in M 2 and M 1 polyhedra by shifting in tetrahedral chains along z are significant. The different values of shifts with composition account for the anomalous trend for the β angle in the DiCaTs solid solution.RésuméLa structure cristalline du clinopyroxáne Ca(Mg0.5Al0.5)(Al0.5Si1.5)O6 (Di50CaTs50), cristallisé á T= 1100° etP=14 kb, a été affinée dans le groupe spatial C 2/c. Les paramétres de la maille sont:a=9.655,b=8.771,c=5.278 Å, β=106.22°. La valeur finale deR est 2.1%. La comparaison avec les structures de Di et de CaTs montre que le variation des longueures de liaison M 2-0, M 1-0 et T-0 est continue et presque linéaire pour les clinopyroxénes le long du joint Di-CaTs. Le déplacement des chaines tétraédriques le long de laxe z cause de remarquables modifications dans les polyèdres M 2 et M 1. Les différentes valeurs du déplacement le long de laxe z avec la composition expliquent la tendance anomale du paramétre β de la maille le long du joint.

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.