Miyawakiite-(Y), □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃, a new mineral from Suishoyama, Kawamata Town, Fukushima Prefecture, Japan

Abstract

Miyawakiite-(Y), a newly discovered mineral having the ideal formula □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃ of a new structure type, was found in a granitic pegmatite from Suishoyama, Kawamata Town, Fukushima Prefecture, Japan. It is composed of tiny crystals of granular, plate-like, or columnar shape up to 0.8 mm with a pale-yellow color. It is transparent with a vitreous luster, occurring as a secondary mineral in a cavity in an aggregate composed of allanite-(Y) and britholite-(Y). The Mohs hardness is 3-4. Its tenacity is brittle, and its calculated density is 2.95 g·cm⁻³. Under plane-polarized light, the mineral is pleochroic with O = colorless and E = pale yellow. It is uniaxial (+), with refractive indices of ω = 1.593(3) and ε = 1.654(2) (white light). Dispersion is weak. The empirical formula, calculated on the basis of 4 C and 8 Si atoms per formula unit, is (□_0.70K_0.30)_Σ1.00(Y_2.95Dy_0.21Yb_0.12Er_0.10Gd_0.08Sm_0.02Tb_0.01Ca_0.33)_Σ3.82(Fe²⁺_1.14Mg_0.48Mn_0.24)_Σ1.86Si₈C₄O_31.42[(H₂O)_2.70(OH)_0.30] after electron microprobe, Raman spectroscopic, and crystal structure analyses. The refined unit-cell parameters determined by single-crystal X-ray diffraction are a = 17.53637(9), c = 9.55702(8) Å, V = 2939.02(4) ų, and Z = 4 in conjunction with the I4/mcm (#140) tetragonal space group. The structure is quite unique: the Y- and Fe-centered polyhedral arrangement with CO₃ triangles forms a prismatic framework, with channels leading in the c-axis direction. The SiO₄ tetrahedral networks are developed in this channel, forming a zeolite-like framework with K sites inside. The correlation between the OH/H₂O and K contents results in a solid-solution series of □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃-KY₄Fe₂(Si₈O₂₀)(CO₃)₄[(H₂O)₂(OH)].

INTRODUCTION

On the western edge of the Abukuma Mountains in north-eastern Japan, granitic pegmatites are distributed in 11 areas from north to south, and minerals containing rare-earth elements (REEs) and actinide elements have long been noted (e.g., Sanbonsugi, 1953, 1958). The granite at Suishoyama contains the most abundant pegmatite in the Kawamata area and is known to comprise many rare minerals. In particular, britholite-(Y) (originally reported as ‘abukumalite’), iimoriite-(Y), proto-ferro-suenoite, and iwashiroite-(Y) have been discovered as new minerals from Suishoyama (e.g., Hata, 1938; Kato and Nagashima, 1970; Sueno et al., 2002; Hori et al., 2006; Williams et al., 2013). Tengerite-(Y) has also been redefined using specimens from Suishoyama (Miyawaki et al., 1993). Among these minerals, iimoriite-(Y) has the unusual feature of being a REE-bearing silicate mineral but also having a carbonate group. The occurrence of caysichite-(Y) with similar characteristics was also later reported (Yamada et al., 2004; Kobayashi, 2012). One of the authors of the present study (T.K.) investigated further and found a yellow secondary mineral associated with caysichite-(Y). It was subsequently established as a new mineral composed of a Y-bearing silicate with a carbonate group.

This new mineral was named miyawakiite-(Y) in honor of Dr. Ritsuro Miyawaki (b. 1959)—mineralogist, crystallographer, and director of the Department of Geology and Paleontology at the National Museum of Nature and Science, Japan—for his outstanding contributions to descriptive mineralogy and crystallography. He has contributed to the description of more than forty new mineral species, with REE-bearing minerals making up one-third of them. Dr. Miyawaki served as the chair of the Commission on New Minerals, Nomenclature and Classification for the International Mineralogical Association (IMA-CNMNC) from 2018 to 2022. Over the course of his tenure, he carefully considered and evaluated more than 500 proposals, always aiming to improve the quality of new mineral proposals. After his term as chairman ended, we submitted a proposal for miyawakiite-(Y); under the new chairman, Dr. Ferdinando Bosi, it was officially approved as a new mineral (IMA 2024-003). The holotype specimen is deposited in the collections of the National Museum of Nature and Science, Tsukuba, Japan with specimen number NSM-M51975. We here describe the new mineral miyawakiite-(Y).

OCCURRENCE

The Kawamata area is mainly composed of granite and contains five pegmatite deposits: the Suishoyama No. 1, Suishoyama No. 2, Kanayama, Kojima, and Kawamata deposits (e.g., Omori and Kikuchi, 1961). Suishoyama is a small mountain with an altitude of 262 m, located at Iizaka, Kawamata Town, Fukushima Prefecture, Japan (37°41′01′′'N 140°36′55′′E) and corresponds to the Suishoyama No. 1 deposit. There, an upright cylindrical pegmatite with a maximum length of ∼ 60 m in the NE-SW direction and at least 70 m in the vertical direction developed, and many vertical basalt dykes penetrate the pegmatite. They were mined for feldspar and quartz from 1916 until the 1980s.

Biotites that occur here are accompanied by REE-containing minerals, and the primary minerals tend to be rich in Y: aeschynite-(Y), allanite-(Y), britholite-(Y), clinofergusonite-(Y), fergusonite-(Y), gadolinite-(Y), hingganite-(Y), iwashiroite-(Y), thalénite-(Y), xenotime-(Y), and yttrialite-(Y). Subsequently occurring secondary minerals are also rich in Y: caysichite-(Y), iimoriite-(Y), kamphaugite-(Y), lokkaite-(Y), synchysite-(Y), and tengerite-(Y). Such Y-containing minerals have long been attracting attention, and many of these are collected from the dump (Iimori, 1938; Hata, 1938; Omori and Hasegawa, 1953; Kato and Nagashima, 1970; Kato, 1973; Yamada et al., 2004; Hori et al., 2006; Miyawaki et al., 2010; Kobayashi, 2010a, 2010b, 2011, 2012, 2013, 2014, 2015, 2019, 2021). A sample containing miyawakiite-(Y), about 7 cm in length and 4 cm in width, was collected at the dump by T.K. in 2020. Miyawakiite-(Y) is closely accompanied by allanite-(Y) and britholite-(Y), which occurs in small cavities and crevices in aggregates composed of them. The occurrence and hydrous carbonate nature of the mineral are common to the secondary minerals previously reported from Suishoyama. On the other hand, it is not clear where in the pegmatite body the aggregates containing miyawakiite-(Y) occur and how they coexist with biotite, quartz, feldspar, etc.

PHYSICAL AND OPTICAL PROPERTIES

Miyawakiite-(Y) often exhibits the appearance of yellow clay that fills the cavity in the aggregate; however, crystals occasionally occur. The crystals can be granular, plate-like, or columnar; they are pale yellow in color and transparent, with a vitreous luster (Fig. 1a) and a maximum size of 0.8 mm. The streak is white. Colorless and transparent crystals of caysichite-(Y) occasionally accompany miyawakiite-(Y) in the same cavities. Miyawakiite-(Y) is nonfluorescent. Its Mohs hardness, as measured by rubbing a reference mineral powder against a sample fixed onto a glass slide, is 3-4. Its tenacity is brittle, whereas its cleavage and parting could not be determined. Its density is 2.95 g·cm⁻³, as calculated from the empirical formula and single-crystal X-ray diffraction (XRD) data. It is uniaxial (+), with refractive indices of ω = 1.593(3) and ε = 1.654(2) (white light). Under plane-polarized light, the mineral is pleochroic with O = colorless and E = pale yellow. It exhibits weak dispersion.

Figure 1. Representative occurrence of miyawakiite-(Y) (a) and its back-scattered electron image (b). The crystals are pale-yellow and vary in shape from columnar to plate-like; they were found in cavities and cracks in an aggregate composed of allanite-(Y) and britholite-(Y) (a). The crystals were cracked and slight zoning was observed (b).

RAMAN SPECTROSCOPY

The Raman spectrum of miyawakiite-(Y) was obtained using a Renishaw inVia Reflex spectrometer equipped with a 532 nm Nd-YAG laser passing through a confocal microscope with a 50× objective lens. The laser power at the sample surface was approximately 10 mW. The spectrum was obtained over the wavenumber range 100-4000 cm⁻¹. Figure 2 shows the resultant spectrum. Overall, strong fluorescence occurs, which causes a large background and weak peaks. The major Raman bands are tentatively assigned as follows. The bands from 2850 to 3350 cm⁻¹ are due to the O-H stretching vibrations in H₂O and OH groups. The weak band at 1607 cm⁻¹ originates from H-O-H bending vibrations. The bands from 1110 to 1452 cm⁻¹ are attributable to C-O stretching vibrations of the CO₃ group. Bands at wavenumbers less than 1000 cm⁻¹ are buried in strong fluorescence and are rarely detected; however, the region is generally associated with vibrations of M-O (M = metal) polyhedral bonds. The bands from 2350-2500 cm⁻¹ have not yet been even tentatively assigned.

Figure 2. Raman spectrum of miyawakiite-(Y).

CHEMICAL COMPOSITION

Chemical analyses were carried out using a JEOL JXA-8105 electron microprobe analyzer (WDS mode, 15 kV, 2 nA, 3 µm beam diameter). The ZAF method was used for data correction. The standards employed in these analyses were quartz, hematite, bustamite, periclase, wollastonite, sanidine, REEP₅O₁₄, and Tb metal for Si, Fe, Mn, Mg, Ca, K, REE, and Tb, respectively. Although insufficient material was available for the direct determination, single-crystal X-ray refinement indicated the presence of CO₃, OH, and H₂O in the structure (confirmed by Raman spectroscopy). Thus, the CO₂ and H₂O concentrations were estimated on the basis of the empirical formula calculated along with the □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃-KY₄Fe₂(Si₈O₂₀)(CO₃)₄[(H₂O)₂(OH)] solid-solution series, as discussed later. The total wt% obtained from these calculations was slightly less than 100, possibly because of electron-beam-induced damage. Analytical data are given in Table 1. The empirical formula, calculated on the basis of 4 C and 8 Si atoms per formula unit (apfu), is (□_0.70K_0.30)_Σ1.00(Y_2.95Dy_0.21Yb_0.12Er_0.10Gd_0.08Sm_0.02Tb_0.01Ca_0.33)_Σ3.82(Fe²⁺_1.14Mg_0.48Mn_0.24)_Σ1.86Si₈C₄O_31.42[(H₂O)_2.70(OH)_0.30]. The simplified formula is (□,K)(Y,Dy,Yb,Er,Gd,Ca)₄(Fe,Mg,Mn)₂Si₈C₄O₃₂[(H₂O),(OH)]₃. The ideal formula is □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃, which requires Y₂O₃ 34.58 wt%, FeO 11.00 wt%, SiO₂ 36.80 wt%, CO₂ 13.48 wt%, and H₂O 4.14 wt%, totaling 100 wt%.

Table 1. Chemical composition of miyawakiite-(Y)

	wt% (n = 32)				apfu
	Avg.	Min.	Max	□	0.70
K2O	1.05	0.07	2.41	K	0.30
CaO	1.38	0.28	2.69	Σ =	1
Y2O3	24.93	21.21	26.88
Sm2O3	0.24	n.d.	1.10	Y	2.95
Gd2O3	1.13	0.09	2.10	Dy	0.21
Tb2O3	0.09	n.d.	1.28	Yb	0.12
Dy2O3	2.97	1.73	3.91	Er	0.10
Er2O3	1.41	0.65	2.02	Gd	0.08
Yb2O3	1.70	0.72	2.65	Sm	0.02
MgO	1.46	0.91	2.12	Tb	0.01
MnO	1.25	0.23	2.27	Ca	0.33
FeO	6.13	4.66	7.16	Σ	3.82
SiO2	35.92	34.91	37.07
CO2*	13.16			Fe	1.14
H2O*	3.84			Mg	0.48
Total	96.65			Mn	0.24
				Σ	1.86
				Si =	8
				C =	4
				O	31.42
				H2O	2.70
				OH	0.30
				Σ =	3

^* calculated on the basis of C = 4, Si = 8, and H₂O + OH (= K) = 3 apfu.

The crystals show slight zoning (Fig. 1b), which may be a combined effect of K content, REE-Ca substitution, and Fe-Mn-Mg substitution. The K content is charge balanced by H₂O-OH as described below, but the charge balance mechanism by REE-Ca substitution has not yet been identified. In this case, it is suggested that the charge is balanced by Fe³⁺ (e.g., epidote-group mineral) and/or O-OH content (e.g., agardite-series mineral). Nevertheless, since the Ca/(Ca + REE) content is extremely small, the relationship with Ca substitution is not discussed in this paper. This is a topic for future work.

X-RAY CRYSTALLOGRAPHY

The single-crystal XRD experiment was performed using a Rigaku Synergy-Custom diffractometer equipped with a HyPix-6000He detector and a monochromatized MoKα radiation source. The CrysAlis^Pro software package was used to process the diffraction data, including applying a numerical absorption correction. The crystal structure was solved by the charge-flipping method using the Superflip software (Palatinus and Chapuis, 2007), by which the new structure type was derived. The SHELXL-2019/2 software (Sheldrick, 2015) was used for refinement of the crystal structure with neutral atom-scattering factors. A full-matrix least-squares refinement with anisotropic atomic displacement parameters was conducted. During the refinement process, the occupancies of K, Y, Fe, and OW2 sites were considered. This analysis resulted in a convergence of R1 to 3.86% based on 2121 unique reflections, which is sufficient for constructing a structural model. The S value is 1.379. This slightly large value may be attributed to small chemical inhomogeneity within the crystal. Details of the sample, data collection, and structure refinement are provided in Table 2 and the CIF file (Supplementary CIF file is available online from https://doi.org/10.2465/jmps.240722). The final atom coordinates and equivalent isotropic atomic displacement parameters are summarized in Table 3. Selected interatomic distances are shown in Table 4, and bond valences are shown in Table 5. The refined unit-cell parameters were a = 17.53637(9), c = 9.55702(8) Å, V = 2939.02(4) ų, and Z = 4, in conjunction with the I4/mcm (#140) tetragonal space group.

Table 2. Experimental details of the single-crystal X-ray diffraction analysis of miyawakiite-(Y)

Crystal size (mm)		0.10 × 0.07 × 0.07
Space group		Tetragonal, I4/mcm
Unit-cell dimensions	a (Å)	17.53637(9)
	c (Å)	9.55702(8)
	V (Å3)	2939.02(4)
Z		4
Radiation		MoKα (λ = 0.71073 Å)
Diffractometer		Rigaku Synergy-Custom
Absorption coefficient (mm−1)		9.40
θmin-θmax(°)		1.6-38.0
Collected reflections		150142
Unique reflections		2121
Rint (%)		3.37
Index ranges	h	−30 → 30
	k	−30 → 30
	l	−16 → 16
Refinement on F2 using		SHELXL-2019/2 (Sheldrick, 2015)
R1 (%)		3.86
wR2 (%)		7.00
No. of parameters		92
Goodness of fit (S)		1.379
Weighting scheme*		w = 1/[σ2(Fo2) + (0.0152P)2 + 18.9278P]
Δρmax (e Å−3)		0.81
Δρmin (e Å−3)		−0.86

^* The function of the weighting scheme is w = 1/(σ²(F_o²) + (a·P)² + b·P), where P = (Max(F_o²) + 2F_c²)/3, and the parameters a and b are chosen to minimize the differences in the variances for reflections in different ranges of intensity and diffraction angle.

Table 3. Fractional atomic coordinates and isotropic* or equivalent isotropic displacement parameters (Ų); below, atomic displacement parameters (Ų)

	Occupancy	x	y	z	Uiso*/Ueq
K1	K0.332(10)	0.5	0	0.25	0.0363(19)
K2	K0.114(7)	0.3937(14)	0.1337(17)	0	0.114(13)
Y1	Y0.860(3)Dy0.140	0.11484(2)	0.11484(2)	0.25	0.00740(6)
Fe1b	Fe0.265(4)Mg0.138	0.25539(5)	0.24461(5)	0.19068(19)	0.0114(4)
Fe1a	Fe0.193(5)	0.25	0.25	0.25	0.0109(9)*
Si1	Si1.0	0.69728(3)	0.07685(3)	0.16096(6)	0.00720(11)
C1	C1.0	0.1354(2)	0.03167(17)	0	0.0136(5)
O1	O1.0	0.15124(14)	0.10303(13)	0	0.0130(4)
O2	O1.0	0.75931(9)	0.13719(9)	0.2159(2)	0.0151(3)
O3	O1.0	0.29996(15)	0	0.25	0.0199(5)
O4	O1.0	0.12526(13)	−0.00143(11)	0.11859(17)	0.0234(4)
O5	O1.0	0.71280(16)	0.05353(17)	0	0.0204(5)
O6	O1.0	0.61164(9)	0.11164(9)	0.1738(3)	0.0134(4)
OW1	O1.0	0.2704(3)	0.2296(3)	0	0.0586(19)
OW2	O0.47(2)	0.5	0	0.543(4)	0.146(19)
	U11	U22	U33	U12	U13	U23
K1	0.0223(17)	0.0223(17)	0.064(5)	0	0	0
K2	0.083(15)	0.18(3)	0.081(13)	−0.038(17)	0	0
Y1	0.00737(7)	0.00737(7)	0.00745(8)	−0.00008(6)	−0.00045(5)	0.00045(5)
Fe1b	0.0063(4)	0.0063(4)	0.0217(9)	0.0002(4)	−0.0037(3)	0.0037(3)
Si1	0.0071(2)	0.0058(2)	0.0087(2)	−0.00042(15)	−0.00109(17)	0.00122(17)
C1	0.0239(14)	0.0098(11)	0.0071(9)	0.0004(10)	0	0
O1	0.0183(10)	0.0101(8)	0.0105(8)	−0.0005(7)	0	0
O2	0.0088(6)	0.0104(6)	0.0262(8)	−0.0017(5)	−0.0028(5)	−0.0026(6)
O3	0.0232(11)	0.0129(9)	0.0235(11)	0	0	−0.0114(9)
O4	0.0516(12)	0.0109(6)	0.0076(5)	−0.0008(8)	0.0001(7)	0.0011(6)
O5	0.0243(12)	0.0277(13)	0.0092(8)	0.0083(10)	0	0
O6	0.0083(5)	0.0083(5)	0.0238(11)	0.0014(6)	−0.0018(5)	−0.0018(5)
OW1	0.0339(18)	0.0339(18)	0.108(6)	−0.001(2)	0	0
OW2	0.147(18)	0.147(18)	0.15(5)	0.104(19)	0	0

Table 4. Selected interatomic distances (Å)

K1-O3	3.508(3) ×4	Y1-O1	2.4817(7) ×2	Si1-O2	1.6058(18)
K1-O6	2.8629(17) ×4	Y1-O2	2.2651(17) ×2	Si1-O3	1.5946(6)
K1-OW2	1.98(4) ×2	Y1-O4	2.3592(19) ×2	Si1-O5	1.6148(11)
	2.80(4) ×2		2.4017(19) ×2
<K1-O>	2.92	<Y1-O>	2.3769	Si1-O6	1.6256(17)
				<Si1-O>	1.6102
K2-O3	3.246(18) ×2	Fe1a-O2	2.0116(16) ×4
K2-O5	3.13(3)	Fe1a-OW1	2.4422(11) ×2	C1-O1	1.282(4)
K2-O6	3.143(5) ×2	<Fe1a-O>	2.377	C1-O4	1.286(3) ×2
K2-OW1	2.74(3)			<C1-O>	1.2844
K2-OW2	3.02(3)	Fe1b-O2	2.0879(19) ×2
			2.101(2) ×2
<K2-O>	3.0864	Fe1b-OW1	1.860(3)
		<Fe1b-O>	2.047

Table 5. Bond valence analysis for miyawakiite-(Y)

	K1	K2	Y1	Fe1b	Fe1a	Si1	C1	BVS	Assignment
O1			0.29				1.34	1.92	O
			×2↓×2→				×1↓×1→
O2		0.00	0.50	0.15	0.15	1.05		1.92	O
		×2↓×1→	×2↓×1→	×4↓×1→	×4↓×1→	×1↓×1→
				0.14
				×4↓×1→
O3	0.01	0.01				1.08		2.17	O
	×4↓×1→	×2↓×2→				×1↓×2→
O4			0.39				1.33	2.07	O
			×2↓×1→				×2↓×1→
			0.35
			×2↓×1→
O5		0.01				1.02		2.05	O
		×1↓×1→				×1↓×2→
O6	0.04	0.01				0.99		2.04	O
	×4↓×1→	×2↓×2→				×1↓×2→
OW1		0.02		0.25	0.03			0.60	H2O/OH
		×1↓×2→		×2↓×2→	×1↓×2→
OW2	0.05	0.01						0.14	H2O
	×2↓×2→	×1↓×4→
BVS	0.30	0.07	3.07	2.05		4.13	3.99

Powder XRD data were not obtained because a sufficient amount of material could not be obtained. Thus, powder XRD patterns with intensities greater than 1 after rounding were calculated using the VESTA program (Momma and Izumi, 2011) assuming Bragg Brentano optics with a CuKα source. The calculation results are listed in Supplementary Table S1 (Supplementary Table S1 is available online from https://doi.org/10.2465/jmps.240722). The parameters [d in Å (I_calc.) hkl] for the seven strongest lines of miyawakiite-(Y) in the powder XRD pattern are 12.400 (100) 110, 6.063 (21) 211, 4.779 (47) 002, 4.384 (23) 400, 3.439 (40) 510, 3.230 (17) 402, and 2.791 (21) 512.

Figure 3a shows the overall view of the crystal structure of miyawakiite-(Y). The structure is unique. The Y- and Fe-centered polyhedral arrangement with CO₃ triangles forms a prismatic framework, with channels leading in the c-axis direction. The SiO₄ tetrahedral networks are developed in this channel, forming a zeolite-like framework with the K sites inside.

Figure 3. Overall view of crystal structure (a) and array of polyhedra with CO₃ (b) of miyawakiite-(Y) illustrated using VESTA (Momma and Izumi, 2011).

DISCUSSION

Figure 3b shows the framework composed of polyhedra. This framework has only one yttrium-dominant site, Y1, which is also occupied by other REEs. A total of eight oxygen atoms are coordinated around the Y1 site: two O1, two O2, and four O4. The four YO₈ polyhedra are arranged squarely in the a-b plane by sharing O4-O4 edges and are continuous in the c-axis direction by sharing O1. The O1 site and the two O4 sites are approximately equilaterally triangular, and the C site for carbon is in the center of the oxygen triangle. The O2-O2 edge of each YO₈ polyhedron is shared with an Fe-centered octahedron, which is connected in the c-axis direction by sharing the OW1 site.

Figure 4 shows the configuration of the Fe-centered polyhedra. The Fe site is located between two OW1 and was found to be split into multiple disordered positions because of variation of chemical species in the OW1 site (Fig. 4a). On the basis of the bond valence sum (BVS) of the OW1 site (Table 5), which is close to 0.5 valence units, OW1 is interpreted as being occupied by both H₂O and OH. When the adjacent OW1 sites are occupied by H₂O, Fe (i.e., Fe1a) is located at the middle of two OW1 sites, forming an elongated FeO₆ octahedron (Fig. 4b). When one of the OW1 is OH, Fe shifts to the Fe1b site, which is closer to the OH (Figs. 4c and 4d). The Fe1b site is far from the other OW1 site, which must be H₂O; thus, the Fe1b site forms an FeO₅ square pyramid. In this structure, the Fe1 site and the OW1 site are alternately repeated along the c-axis, and one OH is locally connected to two Fe atoms. Therefore, OH cannot repeatedly occupy the OW1 site and more than one-half of the OW1 sites must be occupied by H₂O. As a result, the Fe site is disordered and the atomic displacement parameter for the constituents at the OW1 site is large. However, the specific H position is not determined. For example, if the composition of the OW1 site is (H₂O)_0.5(OH)_0.5 and hydrogen bonds are assumed to exist between the OW1 and the O1 sites, the BVSs for the O1 and the OW1 sites are 2.01 and 0.51, respectively. Compared with the values in Table 5, these values are numerically closer to the ideal values. However, the O1-OW1-O1 angle is 176.5 degrees; thus, the H₂O molecule in the OW1 site is unlikely to form hydrogen bonds with both sides of the O1 site. Therefore, the hydrogen bonding networks are disordered and could not be determined.

Figure 4. Configuration of Fe-centered polyhedra. The refined average structure around the Fe1a and the Fe1b sites (a). The Fe1a site is situated halfway between the two OW1 sites, forming an extended FeO₆ octahedron when the neighboring OW1 sites are occupied by H₂O (b). When one OW1 is occupied by OH, Fe splits to the Fe1b site and is biased toward the OH side, resulting in two different polyhedral configurations [(c) and (d)]. In both cases, a combination of pairs of tetrahedral pyramids shares the OW1 occupied by OH.

The SiO₄ tetrahedra share corners and develop a framework along the channels in the polyhedral prism. The framework constitutes a continuous zeolite-like channel along the c-axis, formed by an 8-membered ring composed of SiO₄ tetrahedra; an 8-membered ring and a 4-membered ring are formed on the wall side of the channel (Fig. 5a). The zeolite-like channels are interconnected along the a-b plane via YO₈ and FeO₆/FeO₅ polyhedra. Guest sites inside the channel (the K1 site) and on the sidewalls of the channel (the K2 site) are partially occupied by small amounts of K (Fig. 5b) and possibly by small amounts of H₂O or H₃O⁺. The OW2 site is for a zeolitic water molecule and has large atomic displacement parameters. Moreover, the OW1 site is also weakly bonded to the zeolitic K1 and K2 guest sites. On the basis of such a local structure around OW1 and total charge balance throughout the structure, the positive charge of the zeolitic guest sites should be equivalent to the negative charge of the OW1 site. As a result, the general composition of K_xY₄Fe₂(Si₈O₂₀)(CO₃)₄[(H₂O)_3−x(OH)_x] is derived, where x can vary from 0 to 1, from the viewpoint of the crystal structure as a □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃-KY₄Fe₂(Si₈O₂₀)(CO₃)₄[(H₂O)₂(OH)] solid-solution series. Applying the results of the WDS analysis and the 50% rule, we define the composition range for miyawakiite-(Y) to be 0 ≤ x ≤ 0.5.

Figure 5. Zeolite-like channels of tetrahedra (a) and guest sites (b).

CONCLUSION

An unknown mineral was discovered in a granitic pegmatite from Suishoyama, where various new minerals have been found in the past. It was established as a new mineral and has been named miyawakiite-(Y) in honor of Dr. Ritsuro Miyawaki (b. 1959) under the approval of the CNMNC. Miyawakiite-(Y) is a secondary mineral occurring in an aggregate composed of allanite-(Y) and britholite-(Y). The ideal formula of miyawakiite-(Y) is □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃, whereas the correlation between the OH/H₂O and K contents results in the solid-solution series □Y₄Fe₂(Si₈O₂₀)(CO₃)₄(H₂O)₃-KY₄Fe₂(Si₈O₂₀)(CO₃)₄[(H₂O)₂(OH)]. It is a new structure type, consisting of a prismatic framework of YO₉ polyhedra and FeO₆ octahedra with CO₃ triangles, and a zeolitic framework of SiO₄.

ACKNOWLEDGMENTS

Raman spectroscopic analyses and preliminary chemical analyses were carried out at the Institute for Solid State Physics, University of Tokyo. Chemical analyses were performed by the Department of Geology and Mineralogy, Kyoto University. Single-crystal XRD data were acquired at the Department of Geology and Paleontology, National Museum of Nature and Science. This research accessed budgetary support from the Director General, National Museum of Nature and Science.

SUPPLEMENTARY MATERIALS

Supplementary CIF file and Table S1 are available online from https://doi.org/10.2465/jmps.240722.

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