LETTERS Mnsi2o5 with the Titanite Structure: a New High-Pressure Phase in the Mno-Sio2 Binary

American Mineralogist, Volume 83, pages 657±660, 1998 LETTERS MnSi2O5 with the titanite structure: A new high-pressure phase in the MnO-SiO2 binary THILO ARLT,1,* THOMAS ARMBRUSTER,2 PETER ULMER,3 AND TJERK PETERS1 1Mineralogisch-petrographisches Institut, UniversitaÈt Bern, Baltzerstrasse 1, CH 3012 Bern, Switzerland 2Laboratorium fuÈr Chemische und Mineralogische Kristallographie, UniversitaÈt Bern, Freiestrasse 3, CH 3012 Bern, Switzerland 3Institute for Mineralogy and Petrography, ETH-Zentrum, Sonneggstrasse 5, CH 8092 ZuÈrich, Switzerland ABSTRACT Single crystals of MnSiO(SiO4) with the titanite structure together with MnSiO3 clinopyroxene were synthesized from a MnO-SiO2 oxide mixture at 1000 8C and 9.2 GPa in a Ê multi-anvil press. The crystal structure of MnSi2O5 [space group C2/c, a 5 6.332(1) A, b 5 8.161(1) AÊ , c 5 6.583(1) AÊ , b 5 114.459(3)8, and V 5 309.66 AÊ 3] was re®ned at room temperature from single-crystal X-ray data to R1 5 2.23%. The monoclinic MnSi2O5 phase has the titanite aristotype structure and is similar to the monoclinic Ca-analogue CaSi2O5. Si occurs in compressed octahedral coordination, replacing Ti in titanite, and in tetrahedral coordination as an orthosilicate group. Mn has a distorted sevenfold coordination with Mn- O distances between 2.086 and 2.365 AÊ . INTRODUCTION and approximately 0.2 GPa. The structure type of monoclinic CaSi2O5 is that of the titanite aristotype (Angel The binary phases in the system MnO-SiO2 are braun- ite, tephroite, rhodonite, and pyroxmangite: These min- 1997). The A2/a space group (C2/c in standard setting) erals are well known in natural metamorphic manganese of titanite is also exhibited by the mineral malayaite deposits (e.g., Abs-Wurmbach et al. 1983). At high pres- CaSnSiO5 at room conditions (Higgins and Ross 1977; Groth et al. 1996; Kunz et al. 1997) and is found in pure sure, the olivine tephroite (Mn2SiO4) is stable up to 14 titanite CaTiSiO5 at high temperature (Taylor and Brown GPa (Ito et al. 1974) whereas the pyroxenoids (MnSiO3) transform to clinopyroxene at 7 GPa and 1000 8C (Aki- 1976; Kek et al. 1997), high pressure (Kunz et al. 1996), moto et al. 1972). At pressures above 12.5 GPa, a tetrag- and ambient conditions in partially substituted natural titanite (Oberti et al. 1991; Hughes et al. 1997). onal majorite-like garnet Mn3(MnSi)(SiO4)3 is known in which Si occupies both the tetrahedral position and the EXPERIMENTAL TECHNIQUE sixfold-coordinated octahedral Y position (Akimoto et al. 1972; Fujino et al. 1986). No Si-richer phase than Mn The starting material used for the synthesis of MnSi2O5 SiO3 has been reported so far in the MnO-SiO2 system. ``titanite'' consisted of a 1:1.25 mol% mixture of MnO However, in the CaO-SiO2 system, Kanzaki et al. (1991) (Aldrich, 99%) and SiO2 (Johnson Matthey, Specpure). identi®ed b-Ca2SiO4 (larnite) and a phase CaSi2O5 with a Approximately 4 mg of ®nely ground oxide mix and 0.2 titanite-like structure by powder X-ray diffraction. From ml of H2O (added by micro-syringe) were welded into a the shift of 29Si MAS NMR peaks, the authors suggested platinum capsule of 1.6 mm outer diameter. that octahedral Si replaces Ti in titanite-type CaSi2O5. The The synthesis was performed at 9.2 6 0.2 GPa pressure stability ®eld of CaSi2O5 has been found to lie above 9.2 and 1000 8C for 32 h in a Walker-type multi-anvil ap- GPa at 1000 8C, limited by the breakdown to Ca-perov- paratus (Walker et al. 1990). Tungsten-carbide cubes with skite and stishovite at approximately 12 GPa (Gasparik truncated edge lengths of 12 mm were used. The pressure et al. 1994; Wang and Weidner 1994; Kubo et al. 1997). transmitting octahedron and gasket-®ns are fabricated At room conditions, the crystal structure of CaSi2O5 was from MgO-based castable ceramics (Ceramcast 584), and solved by Angel et al. (1996) to be triclinic with Si in were ®red at 1150 8C for at least 10 h. Stepped graphite fourfold-, ®vefold-, and sixfold-coordinated sites. Re- heaters of 3.5 mm outer diameter were used to minimize cently, this triclinic structure has been found to transform thermal gradients. Pressure calibration is based on the fol- to monoclinic (space group A2/a) at room temperature lowing phase transitions: garnet-perovskite in CaGeO3 (6.1 GPa at 1000 8C, Susaki et al. 1985), rutile-b-PbO2 * E-mail: [email protected] structure in TiO2 (7.7 GPa at 1100 8C, Akaogi et al. 0003±004X/98/0506±0657$05.00 657 658 ARLT ET AL.: MnSi2O5 WITH THE TITANITE STRUCTURE TABLE 1. CCD Data collection and re®nement of MnSi2O5 TABLE 2. Atomic coordinates, bond-valence sums, and isotropic displacement parameters for MnSi2O5 Diffractometer Siemens SMART CCD system Ê 3 Site Atom x/ay/bz/c BVS Beq (A ) X-ray radiation sealed tube MoKa, graphite monochromated A Mn 0 0.32050(6) ¼ 1.89 0.74(1) X-ray power 50 kV, 40 mA M Si 0 0 0 4.06 0.45(2) Temperature 293 K T Si 0 0.3234(1) ¾ 4.19 0.44(2) Detector to sample distance 5.21 cm O1 O 0 0.0649(3) ¼ 2.02 0.50(4) Detector 2u angle 278 O2 O 20.1979(3) 0.5518(2) 0.0913(3) 1.98 0.72(3) Resolution 0.77 AÊ O3 O 0.0919(3) 0.2076(2) 0.6018(3) 2.08 0.61(3) Rotation axis and width v/0.38 2 Total number of frames 1271 Note: BVS 5 bond-valence (Brown 1996). Beq 5 8/3p Si[Sj(Uijai*aj*ai´aj)]. Frame size 512 3 512 pixels Data collection time per frame 10 s Collection mode automated hemisphere 2 22 Re¯ections measured 837 (0.0326 P) 1 0.74 P] with P 5 [Max(Foc,0) 1 2F ]/3. Max 2u 54.3; 28 # h $ 7, All atoms were re®ned with anisotropic displacement pa- 210 # k $ 10, 28 # l $ 4 rameters. Bond-valences were calculated according to Unique re¯ections 329 Brown (1996) using the program VALENCE. Re¯ections . 2sl 304 Space group C2/c ESULTS Cell dimensions (AÊ ) a 5 6.332(1), b 5 8.161(1), R c 5 6.583(1), b5114.459(3)8 The single-crystal structure re®nement of MnSi O at Volume (AÊ 3) 309.66 2 5 R(int) 3.17% after empirical 293 K gave cell parameters (Table 1), atom coordinates, absorbance correction the isotropic displacement parameters (Table 2), inter- R(s) 2.99% atomic distances and angles (Table 3), and the anisotropic Number of least squares parameters 41 GooF 1.133 displacement parameters (Table 4). Systematic extinctions R1, Fo . 4s(Fo) 2.23% are in agreement with the space group C2/c. A polyhedral R1, all data 2.38% wR 2 (on F 2) 6.40% representation of the crystal structure of titanite-type MnSiO(SiO4) is shown in Figure 1. 22 22 22 Note: Rint 5 [S|F oo(F )mean|]/(SF o), Rs 5 (SsF 0)/SF,R o1 5 (S\Fo |2| 222 22 1/2 222 Figure 2 shows the Raman microprobe spectrum of the Fc\)/(S|Fo|) wR2 5 {[S(F oc2 F ) ]/[Sw(F o) ]} , Goof 5 {[Sw(F oc2 F ) ]/ (n 2 p)}1/2. new phase. Measurements up to 4000 cm21 did not show peaks characteristic of OH-groups, but this does not pre- clude trace OH-groups in the structure. The weak bands at 585 and 758 cm21 have positions very close to the 1992), and coesite-stishovite in SiO (9.2 GPa at 1200 8C, 2 21 Yagi and Akimoto 1976). The quenched sample consisted bands found in stishovite (588 and 754 cm ) and are of two crystalline phases, one slightly pinkish and the therefore assumed to represent Si-O stretching and O-Si- O bending modes of the SiO octahedra. other yellowish. No silica phase was found by micro- 6 Raman spectroscopy. The crystals of both phases range DISCUSSION in size up to 0.1 mm. Raman spectra were excited by the The crystal structure of MnSi O is similar to that of 632.8 nm line of a HeNe-laser and the Raman signal was 2 5 monoclinic CaSi O recently described by Angel (1997). analyzed with a Dilor Labram II detection system. Spec- 2 5 tra were collected at 293 K and measured from 100 to Both structures are that of the titanite aristotype. The MnSi O structure consists of corner-sharing chains of 4000 cm21. 2 5 SiO octahedra that run parallel to the c axis and chains The cell parameters of the two synthesized phases (sin- 6 gle crystals) were measured on a three-circle X-ray go- niometer equipped with a Siemens CCD-detector. The Ê TABLE 3. Selected interatomic distances (A) in MnSi2O5 and cell dimensions of the pinkish phase are in agreement related phases with those of the MnSiO3-clinopyroxene re®ned by To- MnSi O CaSi O * CaTiSiO ² konami et al. (1979). The monoclinic unit cell of the sec- 2 5 2 5 5 T-O2 32 1.617(2) 1.631(2) 1.644(1) ond phase was unknown in the MnO-SiO2 system. A yel- T-O3 32 1.629(2) 1.629(2) 1.649(1) lowish crystal, approximately 0.08 3 0.08 3 0.08 mm in Average(4) 1.623 1.630 1.647 size, was subsequently employed for the structure re®ne- M-O1 32 1.7289(8) 1.709(1) 1.871(1) ment.

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