Journal of Mineralogical and Petrological Sciences, Volume 109, page 109–117, 2014 Magnesiorowlandite–(Y), Y4(Mg,Fe)(Si2O7)2F2, a new mineral in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan Satoshi MATSUBARA*, Ritsuro MIYAWAKI*, Kazumi YOKOYAMA*, Masako SHIGEOKA*, Koichi MOMMA* and Sadaoki YAMAMOTO** *Department of Geology and Paleontology, National Museum of Nature and Science, 4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan **Obiracho, Yokkaichi, Mie 512–0921, Japan Magnesiorowlandite, Y4(Mg,Fe)(Si2O7)2F2,aMg–analogue of rowlandite–(Y), was found in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan. The mineral occurs as aggregates composed of gray massive and white powdery parts. The aggregates are up to 1 cm in diameter scattered in the pegmatite. The mineral is associated with quartz, albite, K–feldspar, muscovite, allanite–(Ce), gadolinite–(Y), and ‘yftisite–(Y)’. It is transparent and gray to white in color with a vitreous to oily luster. The streak is white and cleavage is not observed. The Mohs hardness is 5 to 5½. The calculated density is 4.82 g/cm3. It is biaxial negative and re- fractive indices are α = 1.755 (5) and γ = 1.760 (5) with non–pleochroism. Electron microprobe (WDS) analysis gave SiO2 28.61, FeO 2.94, MnO 0.35, MgO 2.77, CaO 0.03, Y2O3 36.02, La2O3 0.29, Ce2O3 2.64, Pr2O3 0.64, Nd2O3 4.72, Sm2O3 2.82, Gd2O3 4.45, Tb2O3 0.69, Dy2O3 4.87, Ho2O3 0.50, Er2O3 1.64, Tm2O3 0.34, Yb2O3 2.02, Lu2O3 0.69, ThO2 0.24, F 4.56, –F2=O 1.92, total 99.91 wt% (average of 16 analyses), and led to the empirical formula, (Y2.71Nd0.24Dy0.22Gd0.21Ce0.14Sm0.14Yb0.09Er0.07Pr0.03Tb0.03Lu0.03Ho0.02Tm0.02La0.01Ca0.01 Th0.01)∑3.98(Mg0.58Fe0.35Mn0.04)∑0.97Si4.00O13.97F2.03 on the basis of O + F = 16. The mineral is triclinic, P1, a = 6.527(6), b = 8.656(9), c = 5.519(5) Å, α = 99.09(8), β = 104.17(7), γ = 91.48(8)°, V = 297.9(5) Å3,Z=1.It is non–metamict, and the strongest lines in the powder XRD pattern [d(Å) (I/I0) hkl] are 4.95 (33) 110; 3.64 (37) 021 ; 3.54 (38) 111 ; 3.08 (100) 201 , 021; 2.92 (26) 211, 210; 2.68 (32) 112; 2.65 (26) 130 , 012 , 002; 2.63 (28) 220. The crystal structure was determined and refined to R1 = 0.0736 for 1645 reflections with I >2σ(I)of single crystal XRD data. In the crystal structure, the Si2O7 group, together with (Mg,Fe)O4F2 octahedron, connects the 7–coordinated and 8–coordinated REEs polyhedra to form the 3–dimensional structure. Keywords: Magnesiorowlandite–(Y), New mineral, Pegmatite, Souri Valley, Komono INTRODUCTION in metamict state. Later, Shipovalov and Stepanov (1971) described crystalline rowlandite–(Y) from Kazakhstan The pegmatites at Souri Valley and in the surrounding with a triclinic cell dimension, but they did not report area are one of the famous gadolinite–(Y) localities in any structure data. As the Mg–analogue from Souri Val- Japan. During mineralogical surveys by the last author ley is non–metamict giving sharp reflections in X–ray dif- (S.Y.), an unfamiliar mineral in a pegmatite was noted. fraction studies, we were able to determine the crystal The chemical and X–ray diffraction studies indicated that structure without any heating treatments. the mineral is a Mg–analogue of rowlandite–(Y). The The Mg–analogue of rowlandite–(Y) from Souri original rowlandite–(Y), which was described from Bar- Valley is named as magnesiorowlandite–(Y) for its chem- ringer Hill, Texas, USA (Hidden, 1891), and the mineral ical relation to rowlandite–(Y), Y4FeSi4O14F2. The min- subsequently found from the Kola Peninsula Russia, are eral data and the name have been approved by the Com- mission on New Minerals, Nomenclature and Classi- doi:10.2465/jmps.131126 fication of the International Mineralogical Association R. Miyawaki, [email protected] Corresponding author (no. 2012–010). The type specimen is deposited at the 110 S. Matsubara, R. Miyawaki, K. Yokoyama, M. Shigeoka, K. Momma and S. Yamamoto National Museum of Nature and Science, Japan, under Table 1. Chemical composition of magnesiorowlandite–(Y) the registered number NSM–M43624. OCCURRENCE Magnesiorowlandite–(Y) was found in a pegmatite block that was a part of debris washed out from talus in Souri Valley located in Komono, Mie Prefecture, central Japan (Lat. 35°0′35′′ N, Long. 136°27′33′′ E). The upper zone of the valley is developed by the Cretaceous Suzuka granite, and includes many pegmatites (Harayama et al., 1989). The pegmatite minerals are mainly composed of quartz, albite, K–feldspar, and muscovite, together with accessory minerals such as allanite–(Ce), gadolinite–(Y), and ‘yftisite–(Y)’. A large crystal of thalénite–(Y) was also found in another pegmatite block in this valley. Magnesiorowlandite–(Y) occurs as aggregates com- posed of massive gray and powdery white components. The aggregates are up to 1 cm in diameter and are scat- tered in the pegmatite (Fig. 1). The massive gray part resembles thalénite–(Y). PHYSICAL AND OPTICAL PROPERTIES Magnesiorowlandite–(Y)isgraytowhitewithwhite streak. It is transparent and the luster is vitreous to oily. Cleavage is not observed and fracture is uneven. The te- nacity is brittle. The density could not be measured di- robe analysis. The powder X–ray diffraction pattern for rectly because of the small grain size. The calculated den- non–heated magnesiorowlandite–(Y) was obtained using sity is 4.82 g/cm3 on the basis of the empirical formula a Gandolfi camera, 114.6 mm in diameter, employing Ni– and refined unit cell dimensions. The Mohs hardness is 5 filtered CuKα radiation. The data were recorded on an to 5½. The mineral is biaxial negative and refractive in- imaging plate (IP), and were processed with a Fuji BAS– dices are α = 1.755 (5) and γ = 1.760 (5) with non–pleo- 2500 bio–image analyzer using a computer program writ- chroism. ten by Nakamuta (1999). The X–ray diffraction pattern (Table 2) is basically identical with those of raw and heat- CHEMICAL COMPOSITION ed samples of rowlandite–(Y). The reflections of powder X–ray diffraction pattern were indexed by reference to the Chemical analyses were carried out with a JEOL JXA– single crystal X–ray diffraction data. The unit cell param- 8800M WDS electron microprobe analyzer (15 kV, 20 eters of the triclinic system were refined with an internal nA, beam diameter 1 µm). The averaged values for 16 Si–standard reference material (NBS #640b) using a com- analyses and standard materials are shown in Table 1. puter program by Toraya (1993); a = 6.555(12), b = The empirical formula is (Y2.71Nd0.24Dy0.22Gd0.21Ce0.14 8.65(2), c = 5.530(14) Å, α = 99.3(3), β = 104.14(19), 3 Sm0.14Yb0.09Er0.07Pr0.03Tb0.03Lu0.03Ho0.02Tm0.02La0.01Ca0.01 γ = 91.4(2)°, and V = 299.4(12) Å . These values are Th0.01)∑3.98(Mg0.58Fe0.35Mn0.04)∑0.97Si4.00O13.97F2.03 on comparable to those refined from the single crystal X– the basis of O + F = 16. The simplified formula is ray diffraction data (Table 3). Y4(Mg,Fe)Si4O14F2. The single crystal X–ray diffraction data were ob- tained with the same fragment on a Rigaku AFC–7R dif- X–RAY CRYSTALLOGRAPHY fractometer using graphite–monochromatized MoKα ra- diation. Experimental details of the data collection pro- X–ray diffraction investigations were carried out with a cedure are given in Table 3. The triclinic space group fragment of small size (0.07 × 0.03 × 0.01 mm) that was symmetry of P1 was suggested by the single crystal X– 2 picked from the thin section used for the electron microp- ray diffraction data. Data reduction to Fo with Lorentz Magnesiorowlandite–(Y) from Mie Prefecture, central Japan 111 Figure 1. Photographs of the type specimen of magnesiorowlandite– (Y) in a pegmatite. (a) Shows the whole specimen with a white rec- tangle marking the area where a photomicrograph was taken. The positions of aggregate of magne- siorowlandite–(Y) are indicated by red arrows. (b) Shows a photomi- crograph of the lamella texture of massive gray and powdery white components. (a) (b) (c) (d) Figure 2. VESTA (Momma and Izumi, 2011) illustrations of the crystal structure of magnesiorowlandite–(Y). Sites are indicated by colors as follows: green for Mg, blue for Si, yellow for Y1, pink for Y2, red for O, and purple for F. (a) Chains of (Mg,Fe)O4F2 octahedra and diortho Si2O7 groups flattened into a sheet parallel to (110). (b) Stacking sequence of the alternating sheets of (Mg,Fe)O4F2 octahedra–diortho Si2O7 and of two different Y polyhedra. (c) Coordination of the larger Y1 cation with 6 O and 2 F anions. (d) Coordination of the smaller Y2 cation with 7 O anions. and polarization corrections and correction for absorption positions of Y, Mg, Si, and some O and F atoms were (φ–scan procedure) were carried out with a computer pro- determined, and those of the other O and F atoms were gram by Dr. Kazumasa Sugiyama of Tohoku University found in the difference Fourier map after the refinements (pers. communication, 2000). The crystal structure was (SHELXL–97: Sheldrick, 2008). The scattering factors analyzed using the direct method with the Patterson cal- for the neutral atoms and anomalous dispersion factors culation by means of SHELXS–97 (Sheldrick, 2008). The were taken from the International Tables for X–ray Crys- 112 S.