2.1 SYNTHESIS OF SINGLE PYROXENE CRYSTALS
Although single crystals of some of the pyroxene phases studied in this thesis were available from either public collections (for example, the synthetic MgSiO^ orthoenstatite was acquired from Smithsonian Collection, Washington, U.S.A.) or private collections (eg, the natural orthopyroxenes were donated by Dr. Ann Chopelas (OPXl) and Dr. Steve Mackwell (0PX2)), crystals of the remaining compositions and/or structure-types had to be synthesised using either the piston-cylinder apparatus or the multi-anvil press at the Bayerisches Geoinstitut, Bayreuth, Germany.
Starting materials for all such syntheses were mixtures of fay alite (Fe2SiOJ, MgO and quartz (SiO^); the relative quantities of each of these were calculated so as to give the required bulk composition of the product. Approximately 5 % by weight BaO - B2O3 (78 wt% BaO, 22 wt% B2O3) was added to each sample capsule to act as a flux. It was intended that this flux would melt by the target pressure and temperature of the run in order to promote the growth of large single crystals of the pyroxene phase. A slight excess of quartz was also added in all runs to compensate for the dissolution of Si02 into the flux. Silver was used as the capsule material to prevent loss of Fe-^ to the capsule during the experiment. The sample assembly and experimental details for all ortho- and ciinopyroxene synthesis experiments were essentially the same as those reported in Woodland and O'Neill (1993).
The piston cylinder apparatus was used to synthesise orthopyroxenes at pressures below ~ 5 GPa. Talc-Pyrex pressure cells (diameter 1.905 cm) were used to enclose the sample capsule. The temperature was controlled by means of Pt - PtgoRhio
thermocouples, and the experiments were carried out at 1100°C. Although no correction was made for the effect of pressure on the emf, this uncertainty was estimated to be approximately ±_ 20°C. The duration of the experiments at pressure and temperature was in the range 1 0 - 1 2 hours. The pressure had previously been calibrated from the position of several well-characterised phase equilibria: a) the olivine - spinel transition in Mg^GeO^ (Ross and Navrotsky, 1987), b) the albite + jadeite + quartz equilibrium (Holland, 1980), c) the ferrosilite + fayalite + quartz equilibrium (Bohlen et al., 1980), and d) the pyroxene - garnet transition in CaOeOj (Ross et al., 1986). Based on the widths of these calibration brackets, uncertainties in the pressure measurements are estimated to be approximately jT 0.1 GPa.
To obtain pressures in excess of 5.5 GPa, necessary for the synthesis of FeSiOa ciinopyroxene, experiments were conducted in the Sumitomo 12(X) multi-anvil press at the Bayerisches Geoinstitut, using Toshiba F grade tungsten carbide cubes with an 11mm truncated edge. The pressure cells were MgO octahedra, measuring 18mm along their edges. The silver sample capsule was heated by a graphite resistance heater with the temperature being monitored using axially inserted Pt - PtgoRhio thermocouples; again no correction was made for the effect of pressure on the emf. The multi-anvil press had been calibrated using both of the transitions in Bi (I-II, III-IV) at ambient temperature, and the following transitions at 1000°C and 1450°C: a) quartz - coesite (Bohlen and Boettcher, 1982), b) coesite - stishovite (Yagi and Akimoto, 1976), and c) fayalite - y spinel (Yagi et al., 1987). The accuracy of the pressure measurements, based on the widths of the reversal brackets and repeated checks of these reversals, is estimated to be ±_ 0.3 GPa. The duration of the experiments at pressure and temperature were in the range 10.5 - 12.5 hours. After this time, the temperature was quenched to the ambient temperature while the pressure was released slowly over a period of about 10 hours. The synthesis conditions of the Mgo 3Feo.7Si03 and FeSi03 orthopyroxenes and the FeSi03 clinoferrosilite are given in Table 2.1.
Composition Phase Equipment Pressure Temperature
^ 8o. 3^®0.7^ ortho Piston Cylinder 3.0 GPa H O O T
FeSiOs ortho Piston Cylinder 3.0 GPa H O O T
FeSiOg clino Multi-Anvil 8.0 GPa 1200 T
Table 2.1: Synthesis conditions for Mgo gFeoyySiOs and FeSiO^ orthopyroxenes and FeSiO) ciinopyroxene.
2.2 INITIAL CRYSTAL SELECTION
The compositions of several crystals from each sample were analyzed using energy-dispersive X-ray analysis with a Phillips CM20-FEG transmission electron microscope (at the Bayerisches Geoinstitut), which gave the relative proportion of elements in the sample to a precision of about i i 1 % (see also Section 2.5). Crystals of both the orthorhombic and monoclinic phases of the MgSiOj and FeSiO^ end- members showed no impurities; compositions of all other pyroxenes studied are reported in Table 2.2. Mossbauer experiments on representative samples of the orthoferrosilite and clinoferrosilite crystals (McCammon, personal communication) displayed no peaks due to the presence of Fe^^, implying that the concentration of Fe^^ in both samples was less than 0 .8 %.
Sample Average AMg^"^ AFe^"^ ACa^^ AaF‘
composition ^êo.59^®0.41^^^3 1-3 % 1 - 3 % - ^§0.29f‘®0.71^^^3 2 -7 % 2 - 7 % - OPXl ( ^ 80.85^^0.13^^ .02) 1 -5 % 1 - 2 % < 0 .5 % 0.5 - 1% (^^.96A1q.04)^3 OPX2 (^ 80.83^^0.12(-^.006 1 - 2 .5 % 0.5-1.5% < 0.5 % < 1% A^0.04)(S^.97A1o.03)03
Table 2.2: Compositional variation in natural and synthetic (Mg,Fe)Si03 samples determined using energy-dispersive X-ray analysis with the TEM. The A s represent the percentage variation in the concentrations of the respective cations.
Crystal fragments from all the samples were selected initially according to their dimensions and optical quality; in all cases they measured between about 125 x 80 x 40 fim and 60 x 30 x 20 /xm, and were free from obvious twins and optical imperfections. In order to check that they were also of a suitable quality for high pressure X-ray diffraction, preliminary data collections were carried out at ambient pressure and temperature with a conventional glass-fibre mount. The crystal was accepted for high-pressure experiments if the widths of the diffraction profiles were sufficiently narrow (ie, having a width of less than approximately 0.35° in 00-scans), if the estimated standard deviations (esds) of the unconstrained unit cell parameters were within —0.01 % of their full value, if the unconstrained unit cell angles, ttand y, were within — 0.02° of the expected values of 90° (for both ortho- and clinopyroxenes), and if the intensity data refined easily to a structure with an R-value (denoted R(avg) in Tables 2. 3a-e) of less than — 4 % ; within the experimental uncertainties, the positional parameters of the MgSiO^ and FeSiO^ end-members at ambient conditions had to be equal to the literature values.
Figures 2.1a and b show the relative widths of ’’narrow” and ”broad” X-ray peak profiles respectively, collected from different orthopyroxene samples at ambient conditions using conventional glass fibre mounts. It was not possible to obtain reasonable structure refinements from pyroxenes which exhibited peak profiles much broader than the "broad” one in Figure 2. lb (ie., having peaks with full widths at half maximum height (FWHM) much greater than —0.35° on (0). For this reason, intensity data was not collected from either the natural orthopyroxene (OPXl) or the synthetic orthopyroxene with approximate composition Mg^^Fco^SiO^. Subsequent microstructural analyses of all the pyroxene phases studied in this thesis using the transmission electron microscope (see Section 2.5) have revealed a possible explanation for this variation in the widths of the X-ray peak profiles between the samples.