Phys Chem Minerals (2005) 32: 552–563 DOI 10.1007/s00269-005-0025-2 ORIGINAL PAPER Matthias Heuer Æ Alexandra L. Huber Geoffrey D. Bromiley Æ Karl Thomas Fehr Æ Klaus Bente Characterization of synthetic hedenbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) solid solution series by X-ray single crystal diffraction Received: 6 January 2005 / Accepted: 12 July 2005 / Published online: 11 November 2005 Ó Springer-Verlag 2005 Abstract Clinopyroxenes of the solid solution series he- Introduction denbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) were synthesized at temperatures of 825–1200°C and pres- Hedenbergite (CaFeSi O ) and petedunnite (Ca- sures of 0.5–2.5 GPa. Compositions were determined by 2 6 ZnSi O ) are chain silicates belonging to the group of electron microprobe analysis. Selected crystals were 2 6 clinopyroxenes crystallizing in the monoclinic space investigated by means of single crystal diffraction and group C2/c. Ca occupies the distorted eightfold-coordi- structure refinement and the structural distortion was nated M2 polyhedra, whereas Fe2+ and Zn2+ occupy studied depending on the substitution of iron by zinc on the sixfold-coordinated M1 octahedra in hedenbergite the octahedral M1 site. It is shown that the coordination and petedunnite, respectively. The structures of end of the M1 site has the most significant effect on M–O member hedenbergite and petedunnite were refined by bond lengths, with changes on the other sites accom- Clark et al. (1969) and Ohashi et al. (1996), respectively. modating this distortion. The mean quadratic elonga- Hydrothermal synthesis of hedenbergite was per- tion and the octahedral angle variance as quantitative formed by Nolan (1969), Rutstein and Yund (1969), measures of the distortion of the coordination polyhe- 57 Turnock et al. (1973), Gustafson (1974), Kinrade et al. dron were correlated with former results of Fe Mo¨ ss- (1975), Burton et al. (1982), Haselton et al. (1987), bauer spectroscopy at 298 K. The results presented now Moecher and Chou (1990), Raudsepp et al. (1990), complete an earlier work on synthetic, crystalline pow- Perkins and Vielzeuf (1992), Kawasaki and Ito (1994), ders of the same material and deliver exact structural Zhang et al. (1997) and Redhammer et al. (2000). The data that were not possible to obtain by Rietveld stability of hedenbergite at 0.2 GPa as a function of refinements on powder data. temperature and oxygen fugacity was determined by Gustafson (1974) and Burton et al. (1982). Keywords Petedunnite Æ Hedenbergite Æ Solid Naturally occurring petedunnite was first described solution Æ Synthesis Æ Crystal chemistry Æ Single crystal by Essene and Peacor (1987) from Zn skarns in Frank- diffraction Æ Clinopyroxene lin, New Jersey. Petedunnite occurs as a major compo- nent in quaternary solid solution with hedenbergite, johannsenite (CaMnSi2O6) and diopside (CaMgSi2O6) (Huber et al. 2000). Pure CaZnSi2O6 has not been ob- served in nature but was synthesized at 900°C, 2 GPa by Essene and Peacor (1987), at 900, 970 and 1,000°C/ M. Heuer (&) Æ K. Bente Institut fu¨ r Mineralogie, Kristallographie und 2 GPa by Huber et al. (2004a, b) and at 1350°C/2.5 GPa Materialwissenschaft, Scharnhorststraße 20, by Redhammer and Roth (2005). The stability field of 04275 Leipzig, Germany end-member petedunnite is restricted to pressures E-mail: [email protected] greater then 0.8 GPa, as shown experimentally by Huber A. L. Huber Æ K. T. Fehr et al. (2004b). Department of Earth- and Environmental Sciences, First results on the synthesis of hedenbergite–pete- Ludwig-Maximilians University Munich, Theresienstr.41, dunnite solid solutions upto 30 mol% petedunnite at 80333 Munich, Germany 0.5 GPa have been reported by Fehr and Hobelsberger (1997). The complete hedenbergite–petedunnite solid G. D. Bromiley Department of Earth Sciences, University of Cambridge, solution series was synthesized over a wide temperature Downing Street, CB2 3EQ Cambridge, UK and pressure range by Huber et al. (2004a). Preliminary 553 studies on the characterisation of the solid solution CaCO3 (99.999%), Fe2O3 (99.99%), Fe (99.999%), FeO series hedenbergite–petedunnite were conducted by (99.9%) and ZnO (99.99%). All oxides, except FeO, Heuer et al. (2002a, b), Huber and Bromiley (2001), were annealed at high temperatures. Three different Huber and Fehr (2002, 2003a, b), Huber et al. (2004a). mixtures were prepared by sintering oxides in appro- In nature, hedenbergitic clinopyroxenes and sulfides, priate portions according to the hedenbergite–petedun- such as sphalerite, are common constituents of skarns in nite solid solution’s bulk compositions as described in phase assemblages with garnet, ilvaite and epidote (e.g., Huber et al. (2004a). Nakano et al. 1994; Capitani and Mellini 2000). An in- The initial material for pure petedunnite was pre- tercrystalline exchange of Fe and Zn occurs between pared like oxide mix (I) but without Fe and Fe2O3. coexisting hedenbergite and sphalerite, resulting in A hydrous experiment (hd8b4_1a) at 825°C and chemical inhomogeneities within the rims of coexisting 0.5 GPa was conducted using an internally heated gas phases (Fehr and Heuss–Assbichler, 1994; Huber et al. media apparatus (Yoder 1950; Huckenholz et al. 1975) 2004b). The corresponding intercrystalline exchange and a conventional cold-seal pressure vessel (Luth and reaction can be described by the model reaction: Tuttle 1963). Experiments at higher pressures were performed ZnS þ CaFeSi2O6ðhedenbergiteÞ using a piston-cylinder solid media apparatus. Most ¼ FeS þ CaZnSi2O6ðpetedunniteÞ intermediate solid solutions (hd3gb31, hd4gb31, hd5gb21, hd7gb21; see Table 1) were synthesized at the Zinc contents in hedenbergite and coexisting sphalerite Bayerisches Geoinstitut using an end-loaded piston- upto 600 and 9000 ppm, respectively, were observed by cylinder type apparatus. Run conditions for all synthesis Nakano et al. (1994) and Fehr and Heuss–Assbichler experiments are given in Table 1 and all experimental (1994). In order to describe the equilibrium conditions details are given in Huber et al. (2004a). and kinetics of the intercrystalline Zn–Fe exchange For the data collection on a conventional four-circle equilibrium between zincian hedenbergite and ferroan diffractometer (P4, Bruker), operating with monochro- sphalerite, the thermodynamic mixing properties of the matic Mo Ka radiation (tube power 50 kV/30 mA), 1–3 solid solution series hedenbergite–petedunnite have to be crystals of each sample were checked for reflection known. Experimental determinations of thermodynamic intensity, reflection profile shape and twinning to select mixing properties and diffusion constants require the ones assuring the highest data quality. If available, homogeneous material with defined crystal-chemical crystals with well-developed faces were used to realize a properties. Therefore, the aim of this study was to syn- face indexed absorption correction, which was carried thesize intermediate members of the solid solution series out with the program XPREP (Bruker 1994). Otherwise hedenbergite–petedunnite at defined oxygen fugacities preferably isometrical crystal fragments were measured and to determine their structure by X-ray diffraction and to correct the absorption empirically using w-scans. The composition by electron microprobe analysis (EMPA). structural refinements were carried out using SHELXL- Furthermore, these data should be correlated with pre- 97 (Sheldrick 1997) providing full-matrix least-squares vious results of 57Fe Mo¨ ssbauer spectroscopy creating a on F2 . Further experimental details for the individual cross reference of two independent methods, which can hkl measurements and refinements are listed in Table 2. give information on the M1-site. The results presented After the experiments, the composition of the selected here now complete an earlier study of synthetic, crystal- crystals was determined using an electron microprobe line powders of the same material and delivers exact (Camebax SX100) operated at 20 keV acceleration structural data which were not possible to obtain by Ri- voltage and 20 nA beam current. Synthetic wollastonite etveld refinements on powder data (Huber et al. 2004a). (Ca, Si), sphalerite (Zn) and hematite (Fe) were used as Experimental standards and matrix correction was performed by PAP procedure (Pouchou and Pichoir 1984). The reproduc- Syntheses were conducted using pure crystalline phases ibility of standard analyses was less then 1% for each element routinely analyzed. prepared from sources of reagent grade SiO2 (99.995%), Table 1 Experimental conditions for syntheses Sample Initiala T (°C) Pressure (GPa) Time (h) Buffer hd10hk1a III 900 1.2 22 No buffer hd8b41a I 825 0.5 153 fb hd7gb21 II 1,000 1.0 72 C hd5gb21 II 850 1.8 65 C a hd4gb31 II 970 2.0 72 C The initial oxide mixes are hd3gb31 II 850 1.8 64 C described in the text hd2dg21 II 1,200 2.5 72 No buffer fb furnace buffer, C graphite pd91b I 1,000 1.9 72 fb capsule Table 2 Individual measurement conditions and general refinement parameters; measurement conditions and the refinement parameters of the samples hd2dg21 and hd5gb21 are 554 already published elsewhere (Heuer et al. 2002a, b) hd10hk1a hd8b41a hd7gb21 hd5gb21 hd4gb31 hd3gb31 hd2dg21 pd91b Measured/ Ca1.01(2)Fe0.99(2) Ca1.02(4) Fe0.69(4) Ca0.98(1) Fe0.86(7) Ca0.98(2)Fe0.59(3) Ca1.01(1) Ca0.99(1) Fe0.27(9) Ca1.00(1)Fe0.21(2) Ca1.00(2)Zn0.99(3) refined Si1.98(1)O6/ Zn0.27(6)Si2.02(5)O6/ Zn0.16(8)Si2.00(2)O6/ Zn0.46(3)Si1.98(2)O6/ Fe0.40(8) Zn0.60(7) Zn0.75(9)Si1.98(2)O6/ Zn0.85(3)Si1.97(1)O6/ Si2.01(1)O6/