- E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart
Matsubaraite, Sr4 Ti5(Si2 O7)2 O8, is a new member of the perrierite-chevkinite group found in the jadeitite from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P21/a (pseudo C2/m). a = 13.848(7), b = 5.626(2), c = 11.878(6) Å, β = 114.19(4)°, V = 844 Å3 and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are 3.16 (70) (400), 3.09 (95) (−403), 3.01 (90) (−313). 2.96 (95) (311), 2.71 (100) (004). 2.17 (90) (−421). Electron microprobe analysis gave SiO2 22.60, TiO2 39.06, SrO 38.84 total 100.50 wt.%, corresponding to Sr3.92Ti5.11 Si3.93O22 on the basis of O = 22. The crystal structure analysis (R = 0.0372) revealed that the space group is pseudo C2/m, and that matsubaraite is Sr and Ti-analogue of perrierite-(Ce), Ce4Fe2Ti3 (Si2 O7)2O8, or Zr-free analogue of rengeite, Sr4ZrTi4 (Si2 O7)2 O8. It is transparent, grey with blue tint with adamantine luster. Streak is white, cleavage not observed. The hardness is VHN100 681-743 kg mm−2 (Mohs' 5.5). The calculated density is 4.13 g cm−3. It occurs as long prismatic euhedral to subhedral crystal with interstitial natrolite in a boulder of lavender-colored Ti-bearing jadeitite from the bed of the Kotaki-gawa river, Itoigawa City, Niigata Prefecture, central Japan. Matsubaraite is considered to have crystallized during later stage activity of high-P/T metamorphism. The mineral is named after Dr Satoshi Matsubara of department of Geology, National Science Museum in recognition of his works on strontium dominant minerals from Japan.
The Itoigawa-Ohmi district is the most famous locality for jade in Japan (Kawano, 1939; Ohmori, 1939; Iwao, 1953; Chihara, 1971). During mineralogical and petrological research on jade from the Itoigawa-Ohmi district, we have found some Sr-dominant minerals such as two new minerals, itoigawaite SrAl2Si2O7 (OH)2·H2O (Miyajima et al., 1999) and rengeite Sr4 Zr Ti4 (Si2O7)2O8 (Miyajima et al., 2001), and rare minerals such as stronalsite SrNa2Al4 Si4O16, lamprophyllite Na2 (Sr, Ba)2 Ti3 (SiO4)4 (OH, F)2, slawsonite SrAl2 Si2 O8, tausonite SrTiO3 and strontium-apatite (Sr, Ca)5(PO4)3 (OH, F). The new mineral described here, matsubaraite, found as euhedral to subhedral prismatic crystals with high refractive indices within natrolite, which is interfillings of jadeite, is named in honour of Dr Satoshi Matsubara of the National Science Museum, Tokyo, for his significant contributions on mineralogy of strontium dominant minerals. The mineral data and the name have been approved by the Commission on New Mineral and Mineral Names, International Mineralogical Association (#2000-027). The type specimens of matsubaraite are deposited at the National Science Museum, Tokyo, under the registered number NSM-M28084 and at Fossa Magna Museum, Itoigawa, Niigata, under the catalogue number FMM01309.
The Itoigawa-Ohmi district of the Renge belt (Nishimura, 1998) is characterized by a serpentinite melange with high-P/T type schists, jade, albitite, rodingite and metagabbro, and various fragments of Palaeozoic accretionary complexes composed of greenstone, limestone and chert (Nakajima et al., 1992). The Itoigawa-Ohmi district is located in the easternmost part of the Renge belt, which is the oldest of the high-P/T metamor-phic belts in the Japanese Islands (Fig. 1).
Physical and optical properties
The matsubaraite is transparent grey with blue tint with adamantine lustre. No cleavage was observed. The Vickers microhardness is 681-743 kg mm−2 (100g load), corresponding to 5.5 on the Mohs scale. The density could not be measured due to insufficiency of material, but the calculated density is 4.13 g cm−3. There is no fluorescence in the either long- or short-wave ultraviolet radiation. Matsubaraite is optically biaxial positive. The refractive indices are higher than titanite, however, these could not be measured because these are too high for normal liquid. Matsubaraite can be distinguished by the defferent colour and paragenesis from rengeite. Rengeite is transparent greenish brown with adamantine lustre and often shows close association with pinkish zircon.
Chemical analyses of matsubaraite were performed on a JEOL JXA-8800 Superprobe using the wavelength-dispersive mode. A conventional ZAF correction routine was used for data reduction. The accelerating voltage was 15 kV, the beam current was 20 nA, and the beam was focused to diameter 2 to 3 μm. The standards used were wollastonite (Si), TiO2 (Ti), SrF2 (Sr). Backscattered electron images and chemical analysis show that the composition of matsubaraite does not vary grain-by-grain and within individual grains. The average of 6 electron micro-probe measurements of matsubaraite gave SiO2 22.60, TiO2 39.06, SrO 38.84 total 100.50 wt.% (Table 1). It leads to the empirical formula, Sr3.92 Ti5.11 Si3.93 O22 on the basis of O = 22. Consequently, the ideal formula of matsubaraite is Sr4 Ti5 Si4 O22.
The matsubaraite is found in lavender-coloured jade (FMM01309), which was collected in the bed of the Kotaki-gawa river (Fig. 2). It is a boulder ∼ 50 cm in diameter. The mineral occurs as a tiny prismatic crystal ∼ 0.3 mm long in jadeitite (Fig. 3a). Under the microscope, the host jadeitite is composed of essentially pure jadeite, Tibearing jadeite and natrolite with minor constituents of lamprophyllite, titanite, zircon, rutile, tausonite, rengeite and matsubaraite. The specimen is characterized by the existence of nearly euhedral jadeite crystals in the natrolite matrix. Matsubaraite as euhedral prismatic crystals occurs interstitially within jadeite grains (Fig. 3b). The matsubaraite also closely associates with titanite, lamprophyllite, tausonite, rengeite and zircon (Fig. 3c), and often occurs as hollow crystal (Fig. 3d).
The chemical compositions of other members of perrierite-chevkinite group minerals are also given in Table 1.
The previously known mineral with composition close to matsubaraite is rengeite, which is a member of the perrierite-chevkinite group found in the jadeitite from the Itoigawa-Ohmi district. Rengeite is characterized by very high Sr, Ti and Zr, and very low REE and Fe compared to perrierite, chevkinite and strontiochevkinite (Miyajima et al., 2001). Matsubaraite has a very simpler composition than rengeite, with a very high content of Sr and Ti, and no REEs and other components. The polymorphs perrierite and chevkinite have the ideal stoichiometry A43+B2+C23+Ti2Si4O22 (Ito, 1967) where A = REE, B = (Fe, Mg), C = (Fe, Al). In matsubaraite, it is though that the A, B and C sites are occupied by Sr2+, Ti4+ and Ti4+ cations, respectively.
X-ray crystallography and crystal structure
The X-ray powder diffraction pattern for matsubaraite was obtained using a Gandolfi camera of 114.6 mm diameter employing Ni-filtered CuKα radiation. A fragment of the single crystal of matsubaraite was picked up from the thin section, which was analysed for chemical composition, under a binocular microscope with an effort to reduce the contamination of associate minerals such as jadeite and natrolite, and was put on a glass fiber (10 μm in diameter). Because matsubaraite is free from uranium and thorium, it is nonmetamict and gives excellent diffraction profile. The powder X-ray diffraction data of matsubaraite are given in Table 2. The unit cell parameters were refined from the powder X-ray diffraction data with internal Si standard (NBS, #640b) using a computer program by Toraya (1993): a = 13.848(7), b = 5.626(2), c = 11.878(6) Å, β = 114.19(4)° andV = 844 Å3.
The single crystal fragment was investigated with a precession camera and a Rigaku RASA-7R four-circle diffractometer. The intensity data were collected with the diffractometer using graphite monochromatized MoKα radiation (50 kV, 250 mA). Experimental details of the data collection are given in Table 3. Several very weak reflections, which violate the extinction rule for C2/m, h + k = 2n+1 for hkl, were observed in the single crystal investigation on matsubaraite. Therefore the true space group of matsubaraite is P21/a, as for rengeite (Miyajima et al., 2001; Miyawaki et al., 2002) and synthetic material, La4Mg2Ti3(Si2O7)2O8 with the perrierite type structure (Calvo & Faggiani, 1974). Two models with the space groups of P21/a and C2/m were examined in the present calculation of refinements. The result of the crystal structure analysis of perrierite-(Ce) by Gottardi (1960) and that of a related synthetic material by Calvo & Faggiani (1974) were used as the initial parameters of C2/m and P21/a models, respectively. The computer program package for crystal structure analysis ‘teXsan’ (1993) recommended the C-lattice, and the refinement converged with R = 0.025, Rw = 0.028 for 1490 observed reflections with I>3ω(I), whereas the calculation with the P21/a model was unstable. Then, the data reductions to Fo2 with corrections for Lorentz, polarization and absorption (Ψ-scan procedure) were made with a computer program by Dr. Kazumasa Sugiyama of the University of Tokyo (personal communication). The computer program, SHELXL-97 (Sheldrick, 1997), was employed for the further refinement of crystal structure. Scattering factors for neutral atoms and anomalous dispersion factors were taken from the International
Tables for X-ray Crystallography, Volume C (1992). Full-matrix least-squares refinement was performed by refining positional parameters, scale factor, and displacement parameters. The site occupancy factors (sof) were not refined because this specimen does not contain any detectable substituents. The refinement with C2/m model using anisotropic displacements converged successfully. Strong correlations among parameters were found, and many of the equivalent isotropic displacement parameters had non-positive values in the calculation with P21/a model with anisotropic displacement parameters. The calculation with the P21/a model with isotropic displacements converged to R1 [Fo > 4σ(Fo)] = 0.0372, wR2 (all reflections) = 0.1514, Goodness of Fit = 0.816. The result of refinement with the C2/m model is summarized in Table 3. The final positional parameters and equivalent isotropic displacement parameters are given in Table 4. Table 5 shows anisotropic displacement parameters. Selected interatomic distances and bond angles are summarized in Table 6.
The atomic positions in P21/a model with the isotropic displacement parameters are basically consistent with those in C2/m model. The deviations of atomic positions in P21/a model from the individually corresponding positions in the C2/m model are negligible small, except for x of Ti(2) and y of O(7), O(8) and O(9). In the P21/a model with lower symmetry, the imaginary pseudo-mirror planes at y = O and 0.5 are sustained, generally.
The occurrence of jadeite as euhedral to subhedral crystal with interstitial natrolite (Fig. 3b) implies that jadeite have been crystallized in the free space such as in metamorphic fluid. Matsubaraite occurs as isolated long prismatic crystal and fan-shape aggregates with euhedral to subhedral jadeite in the natrolite matrix (Fig. 3a, b). It is clear that matsubaraite and jadeite were formed before natrolite. The matsubaraite may be formed in the same stage of the crystallization of jadeite. The matsubaraite closely associates with titanite, lamprophyllite, tausonite, rengeite and zircon (Fig. 3c). Titanium may be relatively immobile element during metamorphism (e.g. Tatsumi & Kogiso, 1995). Therefore titanite may have been the sources for the Ti of matsubaraite. Matsubaraite never shows a direct contact with zircon. In contrast, rengeite usually shows close association contact with zircon directly. These facts support the idea that matsubaraite could not be crystallized at near side of zircon under the existence of both Ti and Zr. The rengeite may be formed instead of matsubaraite under such a condition.
Some strontium dominant minerals such as matsubaraite, rengeite, lamprophyllite and tausonite have been found in the host jadeitite. These minerals have been crystallized at the same or the later stage of formation of jadeite. Both Sr and Ca are member of alkali-earth elements, however, the chemical affinity of these elements for clinopyroxene is quite different. Matsui et al. (1977) shows trace element partitions between phenocrysts and ground-mass. According to their study, the partition coefficients of Sr and Ca between augite phenocryst and groundmass are 0.1 and 2.0, respectively. This difference is attributed to their different ionic radii, which are 1.21 and 1.08 Å, respectively (Shannon & Prewitt, 1969). Strontium was not detected in the present chemical analysis of jadeite. The ionic radius of Sr might be too large to be accepted in the six-fold coordinated M2 site in jadeite. Therefore Sr is not distributed into jadeite, and Sr content in residual metamorphic fluid is significantly increased after crystallization of jadeite. Consequently, the Sr is distributed into Sr-dominant minerals such as matsubaraite, rengeite, tausonite and lamprophyllite. Matsubaraite is considered to have crystallized by interaction between pre-existing titanite and Sr-rich residual metamorphic fluid during high-P/T metamorphism in the subduction zone.
Structure of matsubaraite
Perrierite and chevkinite are dimorphous. They are distinguished by their different monoclinic β angle (113° and 100° respectively), associated with different cation-oxygen bond length patterns in their structures (Calvo & Faggiani, 1974). Two space groups, C2/m and P21/a are reported for the chevkinite- and perrierite-type materials with general formula of A4BC2Ti2(Si2O7)2O8, where A = Ce3+, La3+, Th4+, Ca2+, Na+ and the other rare earths, B = Fe2+, Mg2+, Ca2+ and the other transition metals, C = Ti4+, Fe3+, Fe2+, Mg2+ (Pen & Pan, 1964; Gottardi, 1960; Calvo & Faggiani, 1974; Yang et al., 1991; Miyawaki et al., 2002). The true space groups for minerals and synthetic materials in the chevkinite-perrierite group are under discussion.
The crystal structure of matsubaraite could be refined with the space group of C2/m with a better result, i.e., positive anisotropic displacement parameters for all the sites, in comparison with P21/a. The atomic coordinates are basically consistent with each other in the two models, C2/m and P21/a. Therefore, the diffractions out of keeping with C2/m are so weak. The space group of rengeite can be regarded as pseudo-C2/m, though the true space group of rengeite is P21/a.
The coodination polyhedra of Sr2+ ions in matsubaraite is as large as those of Sr2+ ions in rengeite, and is larger than those of lanthanides in perrierite and its synthetic La-Mg-analogue (Table 6). On the other hand, the Ti(1)O6-octahedron in matsubaraite is smaller than the corresponding octahedra in rengeite (ZrO6), perrierite (FeO6) and the synthetic La-Mg-analogue (MgO6), respectively. According to Ito & Arem (1971), the perrierite structure is stabilized with larger cations in the A site and smaller cations in the octahedral B and C sites, relative to the chevkinite structure. The crystal structure of matsubaraite may be the most stable among the structures of perrierite-(Ce), rengeite, La4Mg2Ti3(Si2O7)2O8, and matsubaraite.
The Si(1)-O(7)-Si(2) angle, through the bridging oxygen atom of the Si2O7 disilicate group, in the crystal structure of matsubaraite, 163.7°, is similar to that of La4Mg2Ti3(Si2O7)2O8 [perrierite type], 165.6°, and is slightly bended than that of rengeite, 170.0° (Table 6). Calvo & Faggiani (1974) pointed out that the displacement parameter for the bridging O(7) is the largest among the O sites in the perrierite or chevkinite structures. Such a feature was also observed in rengeite. The displacement parameter for the bridging O(7) in matsubaraite is as large as that of O(8), which is one of the vertices of Si(2)-tetrahedron, and is not shared with any Ti-octahedra.
The difference in the ionic radii of the cations can be observed as the difference in the lattice volumes of minerals in perrierite and chevkinite group. The reported unit cell parameters for members of the perrierite-chevkinite group are given in Table 7. The approximate unit cell volumes for chevkinite, perrierite, strontiochevkinite (recalculated as “Sr-bearing perrierite”) and rengeite are 829, 842, 863 and 866 Å3, respectively. The unit cell volume for matsubaraite (V = 844 Å3) is following rengeite and the “Sr-bearing perrierite”.
The authors are much indebted to Dr S. Matsubara of National Science Museum for his discussion and to Dr K. Yokoyama of National Science Museum for electron probe analysis. The authors are grateful to Emeritus Prof. K. Aoki of Tohoku University, Dr K. Kunugiza of Toyama University and Dr T. Oba of Joetsu University of Education for their discussion and encouragement, to Mrs M. Shigeoka of National Science Museum and Miss M. Kubota of Fossa Magna Museum for their assistance with thin and polished sections, and to Mr and Mrs Masuoka for their hospitality time spent at Tokyo. The authors are grateful to Dr. K. Sugiyama of the University of Tokyo, for his suggestions and advice in the processing of diffraction data and in the calculation for structure refinement.
- Received 17 December 2001.
- Modified version received 15 May 2002.
- Accepted 6 June 2002.