Characterization of Synthetic Hedenbergite (Cafesi2o6)–Petedunnite (Caznsi2o6) Solid Solution Series by X-Ray Powder Diffraction and 57Fe Mo¨Ssbauer Spectroscopy

$Characterization of Synthetic Hedenbergite (Cafesi2o6)–Petedunnite (Caznsi2o6) Solid Solution Series by X-Ray Powder Diffraction and 57Fe Mo¨Ssbauer Spectroscopy$ View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by ESC Publications - Cambridge Univesity Phys Chem Minerals (2004) 31: 67–79 Ó Springer-Verlag 2004 DOI 10.1007/s00269-003-0335-1 ORIGINAL PAPER A. L. Huber Æ M. Heuer Æ K. T. Fehr Æ K. Bente E. Schmidbauer Æ G. D. Bromiley Characterization of synthetic hedenbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) solid solution series by X-ray powder diffraction and 57Fe Mo¨ssbauer spectroscopy Received: 28 October 2002 / Accepted: 23 January 2003 Abstract Clinopyroxenes along the solid solution series solution containing 84 mole% petedunnite. The half- hedenbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) widths of intermediate solid solutions vary between were synthesized under hydrothermal conditions and 0.26 and 0.33 mm s)1, indicating, in accordance with different oxygen fugacities at temperatures of 700 to the microprobe analyses and X-ray diffraction, that 1200 °C and pressures of 0.2 to 2.5 GPa. Properties samples are homogeneous and well-crystallized. The were determined by means of X-ray diffraction, elec- data from this study demonstrate that the crystallinity tron microprobe analysis and 57Fe Mo¨ ssbauer spec- of hedenbergitic clinopyroxenes can be improved by troscopy at 298 K. Unit-cell parameters display a linear using oxide mixtures as starting materials. Crystal sizes dependency with changing composition. Parameters a0 for intermediate compositions range up to 70 lm, and b0 exhibit a linear decrease with increasing Zn suitable for standard single-crystal X-ray analysis. content while the monoclinic angle b increases linearly. Parameter c0 is not affected by composition and Keywords Petedunnite Æ Hedenbergite Æ Solid remains constant at a value of 5.248 A˚ . The molar solution Æ Synthesis Æ Crystal chemistry Æ Moessbauer volume can be described according to the equation spectroscopy Æ Unit-cell parameter Æ Clinopyroxene )1 Vmol (ccm mol ) ¼ 33.963(16))0.544(31)*Zn pfu. The isomer shifts of ferrous iron on the octahedral M1 site in hedenbergite are not affected by composition along Introduction the hedenbergite–petedunnite solid solution series and )1 remain constant at an average value of 1.18 mm s . Hedenbergite (CaFe2+Si O ) and petedunnite Quadrupole splittings of Fe2+ on the M1 are, however, 2 6 (CaZnSi2O6) are chain silicates belonging to the group of strongly affected by composition, and they decrease clinopyroxenes, crystallizing in the monoclinic space linearly with increasing petedunnite component in )1 group C2/c. Ca occupies the distorted eight-fold-coordi- hedenbergite, ranging from 2.25 mm s for pure he- 2+ 2+ )1 nated M2 polyhedra, whereas Fe and Zn occupy denbergite end member to 1.99 mm s for a solid the regular six fold-coordinated M1 octahedra in hedenbergite and petedunnite, respectively. The struc- tures of end members hedenbergite and petedunnite were This paper is dedicated to Prof. Dr. Georg Amthauer, Salzburg, on refined by Clark et al. (1969) and Ohashi et al. (1996), occasion of his 60th birthday respectively. Hydrothermal synthesis of hedenbergite was performed by Nolan (1969), Rutstein and Yund (1969), A. L. Huber Æ K. T. Fehr (&) Æ E. Schmidbauer Department of Earth- and Environmental Sciences, Turnock et al. (1973), Gustafson (1974), Kinrade et al. Ludwig-Maximilians University Munich, (1975), Burton et al. (1982), Haselton et al. (1987), Moe- Theresienstr.41, 80333 Munich, Germany cher and Chou (1990), Raudsepp et al. (1990), Perkins and e-mail: [email protected] Vielzeuf (1992), Kawasaki and Ito (1994), Zhang et al. Tel.: 0049-89-21804256 Fax: 0049-89-21804176 (1997) and Redhammer et al. (2000). The stability of hedenbergite at 0.2 GPa as a function of temperature and M. Heuer Æ K. Bente oxygen fugacity was determined by Gustafson (1974) and Institut fu¨ r Mineralogie, Kristallographie und Materialwissenschaft, Burton et al. (1982). Naturally occurring petedunnite was Scharnhorststraße 20, 04275 Leipzig, Germany first described by Essene and Peacor (1987) from Zn G. D. Bromiley skarns in Franklin, New Jersey. Petedunnite occurs as a Bayerisches Geo-Institut, major component in quaternary solid solution with Universitaetsstr. 30, 95447 Bayreuth, Germany hedenbergite, johannsenite (CaMnSi2O6) and diopside 68 2+ (CaMgSi2O6). Pure CaZnSi2O6 is not observed in nature was added after the sintering period to prevent oxidation of Fe . but was synthesized at 900 °C/ 2 GPa by Essene and The starting material for oxide mix (III) was a stoichiometric mixture of CaSiO3 glass, Fe, Fe2O3 and SiO2 (Table 1). During Peacor (1987). The stability field of end-member pete- sintering, oxide mix (I) crystallized to quartz, magnetite, haematite, dunnite is restricted to pressures >1 Gpa, as shown hardystonite, hedenbergite, wollastonite plus calcite (Ia) or zincite experimentally by Rothkopf and Fehr (1998) and Fehr (Ib), respectively. Oxide mix (II) crystallized to quartz, magnetite, and Huber (2001). First results on the synthesis of zincite, larnite and Ca2Fe7O11 in various amounts. All synthesis experiments were conducted in the presence of an hedenbergite–petedunnite solid solutions up to 30 mol% oxy-hydrous vapour phase. For oxide mix (Ia), synthesis took place petedunnite at ambient pressures of 0.5 GPa have been in the presence of a CO2–H2O vapour phase, as depicted in reported by Fehr and Hobelsberger (1997). Table 1. Hydrous experiments in the temperature range 700 to In nature, hedenbergitic clinopyroxenes and sulfides, 900 °C at pressures of 0.2 to 0.7 GPa were conducted using an such as sphalerite, are common constituents of skarns in internally heated gas media apparatus (Yoder 1950; Huckenholz et al. 1975) and a conventional cold-seal pressure vessel (Luth and phase assemblages with garnet and epidote (e.g. Nakano Tuttle 1963). Temperature and pressure fluctuations were typically et al. 1994). An intercrystalline exchange of Fe and Zn ±5°C and ± 0.002 GPa. Oxygen fugacity was controlled by the occurs between coexisting hedenbergite and sphalerite, redox condition of the pressure vessel (furnace buffered, ca.- resulting in chemical inhomogeneities within the rims of log fO2Ni/NiO buffer – 0.5, as proved by H2 sensor measurements). Experimental methods used here have been described elsewhere coexisting phases (Fehr and Heuss–Assbichler, 1994). (Huckenholz et al. 1974). Starting materials were sealed in platinum The corresponding intercrystalline exchange reaction capsules with 10 wt% water. can be described by the model reaction: Experiments at higher pressures were performed using a piston- cylinder solid media apparatus. Synthesis of petedunnite was ZnS þ CaFeSi2O6 (hedenbergite) performed in a piston-cylinder solid media apparatus with a low- friction NaCl cell designed by Fehr (1992), exhibiting friction as ¼ FeS þ CaZnSi2O6 (petedunnite) low as 1%. Temperature and pressure fluctuations were typically ±5°C and ± 0.002 GPa, respectively. Some intermediate solid Zinc contents in hedenbergite coexisting with sphalerite solutions (Hd3g, Hd4g, Hd5g, see Table 1) were synthesized at the up to 600 and 9000 ppm, respectively, were observed by Bayerisches Geoinstitut using an end-loaded piston-cylinder type Nakano et al. (1994) and Fehr and Heuss–Assbichler apparatus. Starting mixtures were loaded into 10-mm-long, 5-mm- (1994). o.d., 3-mm-i.d. graphite capsules for insertion into the sample assembly. Graphite was chosen for sample encapsulation to control In order to describe the equilibrium conditions and oxygen fugacity during the duration of the experiments. In exper- kinetics of the intercrystalline Zn–Fe exchange equilib- iments Hd3g and Hd5g, an additional platinum tube, 6 mm long, rium between zincian hedenbergite and ferroan sphal- 3 mm o.d., 2.8 mm i.d. was placed inside the graphite capsule to erite, the thermodynamic mixing properties of the solid separate the sample from the capsule. The platinum tube was open at both ends to allow buffering of the oxygen, but facilitated sample solution series hedenbergite–petedunnite have to be removal after the experiment. In experiment Hd4g, it was necessary known. Experimental determinations of thermodynamic to grind the recovered sample and fire the sample at 400 °C/144 h mixing properties and diffusion constants require to remove any remaining carbon. The loaded capsules were sur- homogeneous material with defined crystal-chemical rounded by pyrophyllite or Pyrex sample holders and inserted into talc-Pyrex cells with a tapered graphite resistance heater. Pressure properties. Therefore, the aim of this study is to was calibrated against the quartz–coesite and kyanite–sillimanite synthesize intermediate members of the solid solution transitions, as well as the melting point of diopside. Temperature series hedenbergite–petedunnite at defined oxygen fu- was measured with a Pt90Rh10-Pt thermocouple (s type). No cor- gacities and to determine their properties by X-ray dif- rection for the effects of pressure on thermocouple EMF was ap- fraction, electron microprobe analysis (EMPA) and 57Fe plied. Experiments were pressurized to 90% of the desired run pressure, heated to the desired temperature, and then further Mo¨ ssbauer spectroscopy. In addition, clinopyroxene pressurized to the final pressure. Pressure and temperature were crystals should be suitable for standard single-crystal monitored and controlled during the duration of the experiments. X-ray structure analysis. Experiments were quenched isobarically by shutting off the power whilst maintaining the pressure within ±

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