Journal of Mineralogical and Petrological Sciences, Volume 109, page 109–117, 2014

Magnesiorowlandite–(Y), Y4(Mg,Fe)(Si2O7)2F2, a new mineral in a at Souri Valley, Komono, Mie Prefecture, central Japan

Satoshi MATSUBARA*, Ritsuro MIYAWAKI*, Kazumi YOKOYAMA*, Masako SHIGEOKA*, Koichi MOMMA* and Sadaoki YAMAMOTO**

*Department of 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 , albite, K–, 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 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 , 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. Matsubara, R. Miyawaki, K. Yokoyama, M. Shigeoka, K. Momma and S. Yamamoto

Table 2. Powder X–ray diffraction data for magnesiorowlandite–(Y) and rowlandite–(Y)

* Souri Valley, central Japan. Present study. ** Kazakhstan. Shipovalov and Stepanov (1971). *** Kazakhstan. Krivokoneva et al. (1974). † Barringer Hill, Texas, USA. Frondel (1961). ‡ Clear Creek, Texas, USA. Crook et al. (1978). Magnesiorowlandite–(Y) from Mie Prefecture, central Japan 113

Table 3. Crystal data, data collection information, and refinement lated from the interatomic distances following the proce- details dure of Brown and Altermatt (1985) using the parameters of Brese and O’Keeffe (1991). The values listed in Table 6 are weighted averages according to the occupancies in the final refinement and to those with the introduction of Nd instead of Yb at the Y1 site.

DISCUSSION

In the chemical composition of magnesiorowlandite–(Y) (Table 1), Mg dominates over Fe, and Y is the predom- inant rare earth element (REE). Therefore, this mineral is aMg–dominant analogue of rowlandite–(Y). Rowlan- dite–(Y) was originally described as a silicate of Y and Fe with U and Ca as minor constituents, associated with yttrialite–(Y) and gadolinite–(Y) in the pegmatite at Bar- ringer Hill, Llano County, Texas, USA (Hidden, 1891). Later, Hess (1908) reported that the mineral contained F, and Frondel (1961) confirmed F in the type specimen. Frondel (1961) conjectured a formula (Y,Fe,Ca,Ce)3 (SiO4)2(F,OH), and later Crook et al. (1978) determined the chemical composition by means of an electron mi- croprobe to confirm the formula. However, Peacor et al. (1982) pointed out some errors in the chemical analysis by Crook et al. (1978) to conclude the formula in ques- tion. On the other hand, Shipovalov and Stepanov (1971) described rowlandite–(Y) from Kazakhstan to suggest an- other formula (Y,Dy)4Fe(Si2O7)2F2. Krivokoneva et al. (1974) reported that chemical composition of metamict rowlandite–(Y) from the Kola Peninsula could be express- ed as Y4Fe(Si2O7)2F2 rather than (Y,Ce,Fe)3(SiO4)2F. Holtstam and Andersson (2007) found a mineral having the chemistry of (Y,Ce,Nd)4MgSi4O14F2 from Malmkärra in south–central Sweden. They described this mineral (i.e., the ‘unnamed mineral D’ in their paper) as a Mg– analogue of rowlandite–(Y) referring the formula Y4Fe (Si2O7)2F2 of Shipovalov and Stepanov (1971). Magne- siorowlandite–(Y) from Souri Valley, central Japan is tallography, Volume C (1992). Since Yb is the predom- chemically identical with the ‘unnamed mineral D’. inant heavy rare earth element (REE) in magnesiorowl- The type specimen of rowlandite–(Y) occurred in andite–(Y), the scattering curve of Yb was used in the the metamict state. Frondel (1961) heated a part of the calculations to represent the lanthanides (Ln; La–Lu). type specimen for 1 h in N2 at 900 °C to recrystallize it The occupancy parameters of Mg and Fe were fixed to for X–ray diffraction examinations. Although the recrys- 0.6 and 0.4, respectively, according to the chemical com- tallized phase could not be identified with any yttrium position. Those of Y and Yb were individually refined at silicates known at that time (Frondel, 1961), it was later the 2 crystallographically independent Y–sites, whereas probed and found to have a diffraction pattern identical the total Y:Ln (Yb) atomic ratio was restrained as with that of non–metamict crystalline rowlandite–(Y) 1.3:0.7 based on the chemical composition. The final po- from Kazakhstan (Shipovalov and Stepanov, 1971; Kri- sitional parameters and anisotropic displacement parame- vokoneva et al.,1974) (Table 2). Shipovalov and Stepa- ters with equivalent isotropic displacement parameters nov (1971) reported lattice parameters in the triclinic sys- are given in Table 4. Selected interatomic distances are tem after their single crystal X–ray diffraction study as a summarized in Table 5. The bond valences were calcu- = 6.59, b = 8.65, c = 5.53 Å, α = 99.03°, β = 104.13°, γ = 114 S. Matsubara, R. Miyawaki, K. Yokoyama, M. Shigeoka, K. Momma and S. Yamamoto

Table 4. Final atom positions and anisotropic displacement parameters (Å2) with equivalent isotropic displacement parameters for magne- siorowlandite–(Y)

* 0.568(3)Y + 0.432Yb. ** 0.732(3)Y + 0.268Yb. *** 0.6Mg + 0.4Fe.

Table 5. Interatomic distances (Å) for magnesiorowlandite–(Y) rowlandite–(Y) from Kazakhstan (Shipovalov and Stepa- nov, 1971). The X–ray diffraction pattern of magnesior- owlandite–(Y) corresponds well to those of rowlandite– (Y) (Table 2), which is indicative of isomorphism be- tween these minerals. In the crystal structure of magnesiorowlandite–(Y) (Fig. 2), REEs occupy two independent sites, the 8–coor- dinated Y1 and 7–coordinated Y2 sites. Magnesium in the Mg site is partially substituted by Fe and forms an octahedron with 4 O and 2 F anions. The (Mg,Fe)O4F2 octahedra connect the diortho Si2O7 groups to form a chain running parallel to the c axis (Fig. 2a). The chains are flattened into a sheet parallel to (110), and are stacked alternately with the other sheet of REE polyhedra to form a layered structure (Fig. 2b). Vlasov (1966) classified rowlandite–(Y) as a fluo- rine variety of thalénite–(Y). Thalénite–(Y) used to be de- scribed as a sorosilicate (pyrosilicate), Y2Si2O7, which is closely related to yttrialite–(Y) [Y2Si2O7; P21/m] (Bata- lieva and Pyatenko, 1972) and keiviite–(Yb) [Yb2Si2 O7; C2/m] (Yakubovich et al., 1986). The crystal struc- 3 91.47°, and V = 301.0 Å . Although Shipovalov and Ste- tures of these minerals consist of the diortho Si2O7 panov (1971) did not report on the crystal structure of groups. Kornev et al. (1972) determined the crystal struc- rowlandite–(Y), they suggested the Si2O7 groupinthe ture of thalénite–(Y) to verify the triortho Si3O10 groups structure after their IR spectroscopic investigation. in the crystal structures. Magnesiorowlandite–(Y), which The present study on the crystal structure of magne- has diortho Si2O7 groups, is classified into sorosilicate siorowlandite–(Y) revealed that this mineral is triclinic, minerals together with yttrialite–(Y) and keiviite–(Yb), P1, and the unit cell parameters are a = 6.527(6), b = rather than in a group related to thalénite–(Y). 8.656(9), c = 5.519(5) Å, α = 99.09(8)°, β = 104.17(7)°, The Si–O–Si angle is 127.5(5)° through the bridging 3 γ = 91.48(8)°, V = 297.9(5) Å , and Z = 1. The cell di- O1 atom between the Si1 and Si2 of diortho Si2O7 group mension is almost congruent with that of non–metamict in the crystal structure of magnesiorowlandite–(Y). This Magnesiorowlandite–(Y) from Mie Prefecture, central Japan 115

Table 6. Bond valence sums weighted on the occupancies for magnesiorowlandite–(Y)

Si–O–Si bending angle is slightly or moderately smaller prediction for the segregation of REEs, which was based than that of yttrialite–(Y) [133.9°] and those of thalénite– on their ion sizes into the different sites, was supported (Y) [133.4 and 141.2°], whereas the angle is much small- by the bond valence sum (BVS) calculations (Table 6). er than that of keiviite–(Yb) with a linear conformation The weighted BVS values are 2.450, 3.049, 1.981, 3.826, for the Si2O7 group [180°]. It is worth noting that inter- and3.942forY1(Y0.57Yb0.43), Y2 (Y0.73Yb0.23), Mg atomic distances from the central Si to bridging O1 are (Mg0.6Fe0.4), Si1, and Si2, respectively, on the basis of longer than those to non–bridging O2, O3, O4, O5, O6, the final occupancy parameters in the structure refinement and O7 in both of the Si–tetrahedra of magnesiorowlan- (Table 6, upper part). The BVS for Y1 (Y0.57Yb0.43)of dite–(Y) (Table 5). 2.450 is remarkably lower than the ideal value of 3 for The combination of 7– and 8–coordinated REE sites REE3+, whereas the others are close to the ideal values. in a crystal structure is a characteristic feature of magne- The replacement of the BVS parameter for Yb with that siorowlandite–(Y) (Figs. 2c and 2d). The 8–coordinated of Nd at the Y1 site, namely Y1 (Y0.57Nd0.43), showed a Y1 polyhedron (the mean Y1–O distance; 2.447 Å) was great improvement for the value, 3.166 (Table 6, lower larger than the 7–coordinated Y2 polyhedron (the mean part). The biased distribution of Nd and Yb into the larger Y2–O distance; 2.318 Å) (Table 5). The refinement of Y1 and smaller Y2 sites, respectively, is reasonable from occupancy parameters of Y sites showed that the smaller the viewpoint of crystal chemistry, although this is still Y cation was biased towards the smaller Y2 site. Al- only a rough and extremely simplified estimation using though the scattering factor for Yb was employed as the BVS parameters of representative REEs, Nd, and Yb. the representative lanthanoids in the present crystal struc- Magnesiorowlandite–(Y) shows a similarity to tha- ture analysis, it is presumed that larger light REE, e.g., lénite–(Y) in the coordination of Y of the crystal struc- Nd, would be concentrated into the larger Y1 site, where- ture. The Y ions are coordinated by 6 O anions in the as the smaller heavy REE, e.g., Yb, would occupy the crystal structures of yttrialite–(Y) (Batalieva and Pyaten- smaller Y2 site with Y, the predominant REE there. The ko, 1972) and keiviite–(Yb) (Yakubovich et al., 1986). 116 S. Matsubara, R. Miyawaki, K. Yokoyama, M. Shigeoka, K. Momma and S. Yamamoto

Figure 3. Chondrite–normalized lan- thanide distribution patterns of magnesiorowlandite–(Y) and row- landite–(Y). Specimens are: mag- nesiorowlandite–(Y), Souri, Japan (this study); rowlandite–(Y), Bar- ringer Hill and Clear Creek, Texas, USA (Crook et al., 1978); rowlan- dite–(Y), Kazakhstan (Shipovalov and Stepanov, 1971); and magne- siorowlandite–(Y), Sweden (Holt- stam and Andersson, 2007).

This is the minimum coordination number for Y3+ and values of La) (Fig. 3). Values for Pm and Eu, the exceed- the comparably smaller heavy REE3+. Non 6–coordinated ingly scarce elements, are not plotted on Figure 3. In fact, 3+ 3+ 3+ light REE , such as larger Ce or Nd , has been ob- the Eu2O3 concentration was under 0.01 wt%, which was served in rare earth minerals. In the crystal structure of the detection limit in the present quantitative determina- magnesiorowlandite–(Y), Y and lanthanoids occupy the tion made by the electron microprobe. A negative anom- independent 2 crystallographic sites with different coor- aly is suggested for Eu. The upward–sloping pattern dis- dination numbers, 8 for the Y1 and 7 for the Y2 (Table plays stagnations in the region of heavy REEs from Tb to 5). The 8–coordinated polyhedra are the most common Yb for magnesiorowlandite–(Y). The two samples of row- for both the larger light REE and the smaller Y as well landite–(Y) from Barringer Hill and Clear Creek, Texas as the heavy REE ions among the overall REE polyhedra (Crook et al., 1978) showed deficiencies for Dy and Ho in the crystal structures of rare earth silicates, whereas the in addition to a shortage of Pr and Nd, whereas rowlan- 7–coordinated polyhedra are less frequently observed for dite–(Y) from Kazakhstan (Shipovalov and Stepanov, the larger light REE ions (Miyawaki and Nakai, 1996). 1971) displays the normal increasing levels from La to The ionic radius of Y is comparable to those of Ho. On the contrary, a distinct trend for the lanthanoid heavy REEs, such as Dy and Yb, and is smaller than distribution pattern was reported for magnesiorowlan- those of light REEs, such as Ce and Nd. The Y–predom- dite–(Y) [mineral D of Holtstam and Andersson (2007)] inant minerals tend to have lanthanoid distribution pat- from south–central Sweden; the trend showed a decreas- terns rich in heavy REEs. However, Nd, a larger light ing slope rich in the light REEs, which is typical for Ce– REE, dominates over Dy and Yb among the lanthanides predominant REE minerals such as monazite–(Ce). Al- in magnesiorowlandite–(Y). Such a specific feature in the though partial chemical analyses were carried out for REE chemistry has been reported for kimuraite–(Y), the type specimen of rowlandite–(Y) (Hidden, 1891; Y1.57Nd0.107Gd0.083Dy0.079Er0.053Sm0.033Ho0.020La0.018 Frondel, 1961), Frondel (1961) reported that Ce and Nd Yb0.017Pr0.015Eu0.013Tb0.012Tm0.005Lu0.002Ce0.001Ca0.99(CO3)4• were in the range of 1 to 5%, whereas La, Er, Lu, Dy, Ho, 6.12H2O (Nagashima et al., 1986). Miyawaki and Nakai Gd, and Pr were present in amounts less than 1%. This (1996) assumed that the crystal structure of kimuraite–(Y) suggests the dominances of the larger light REEs in the would consist of a corrugated layer of 9–coordinated Y– Y–predominant REE mineral. The unusual enrichment of polyhedra and CO3 triangles. However, the second dom- CeandNdintheY–predominant rowlandite–(Y) and inant Nd in kimuraite–(Y) suggests different crystallo- magnesiorowlandite–(Y) can be accounted for by the graphic sites for Nd and Y in the crystal structure of ki- two independent crystallographic sites, the Y1 and Y2, muraite–(Y). with differences in size and coordination number, in the The chondrite–normalized lanthanide distribution crystal structure of magnesiorowlandite–(Y) and the iso- pattern of magnesiorowlandite–(Y) from Souri Valley morphous rowlandite–(Y). (present study), and those of rowlandite–(Y) from Texas (Crook et al., 1978) and from Kazakhstan (Shipovalov ACKNOWLEDGMENTS and Stepanov, 1971), show trends whereby heavy REEs were enriched (e.g., relatively high values of Lu and low The authors are grateful to Dr. Akira Kato and Prof. Igor Magnesiorowlandite–(Y) from Mie Prefecture, central Japan 117

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