High-symmetry sulfate-rich imayoshiite from the Shijiangshan mine, Inner Mongolia, China, and its crystal structure

Abstract

Imayoshiite, Ca₃Al(CO₃)[B(OH)₄](OH)₆·12H₂O, is a rare mineral belonging to the ettringite group. The imayoshiite discussed in this paper was discovered in hydrothermally altered pockets at the Shijiangshan mine, Hexigten Banner, Chifeng City, Inner Mongolia, China. Before its discovery at the Shijiangshan mine, the only known locality for this mineral was Suisho-dani, Ise City, Mie Prefecture, Japan. The formation of Shijiangshan imayoshiite relates to the low-temperature hydrothermal processes during the late skarn stage. Its euhedral crystals exhibit a combination of hexagonal dipyramid {10-12} and hexagonal prism {10-10}. The empirical chemical formula of the sample is Ca₃(Al_0.78Si_0.11Fe_0.01)_∑0.90{[B(OH)₄]_0.91(CO₃)_0.60(SO₄)_0.45}_∑1.96(OH)_5.80·12.39H₂O, with Z = 2 (based on Ca_apfu = 3). Due to its disordered structure and specific crystal-chemical features, the crystal’s structure was solved and refined with a higher symmetry space group (P6₃/mcm) compared to the previously reported (P6₃). Using this space group and unit-cell parameters of a = 11.0663(6) Å, c = 10.6387(6) Å, and V = 1128.30(14) ų, the refinement converged to R = 0.0492. X-ray photoelectron spectroscopy (XPS) showed that all S atoms are present as SO₄²⁻. This study presents new data on the high-symmetry variety of imayoshiite, i.e., imayoshiite-P6₃/mcm, and summarizes the complex ligand morphology resulting from disorder in the ettringite group.

INTRODUCTION

Imayoshiite is a complex, hydrated calcium carbonate-borate mineral belonging to the ettringite group. Due to varied isomorphous substitutions, several independent species within this group have been described and classified. To date, fourteen members (Table 1) have been officially approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA), while two unnamed species have been described (Kusachi et al., 2008; Chukanov et al., 2019).

Table 1. Members of ettringite group

Cell parameters of c-axis & space group	Mineral	Formula
c ≈ 20 Å P3 1c	Bentorite	Ca6Cr2(SO4)3(OH)12·26H2O
	Buryatite	Ca3(Si,Fe3+,Al)(SO4)B(OH)4(OH,O)6·12H2O
	Ettringite	Ca6Al2(SO4)3(OH)12·26H2O
	Siwaqaite	Ca6Al2(CrO4)3(OH)12·26H2O
	Sturmanite	Ca6Fe3+2(SO4)2.5[B(OH)4](OH)12·25H2O
c ≈ 10 Å P63	Micheelsenite	(Ca2Y)Al(PO3OH)(CO3)(OH)6·12H2O
	Tatarinovite	Ca3Al(SO4)[B(OH)4](OH)6·12H2O
	Thaumasite	Ca3(SO4)[Si(OH)6](CO3)·12H2O
	Hielscherite	Ca3Si(SO4)(SO3)(OH)6·11H2O
	Imayoshiite	Ca3Al(CO3)[B(OH)4](OH)6·12H2O
	Jouravskite	Ca3Mn4+(SO4)(CO3)(OH)6·12H2O
	Chiyokoite	Ca3Si(CO3)[B(OH)4]O(OH)5·12H2O
c ≈ 10 Å P63/m	Kottenheimite	Ca3Si(SO4)2(OH)6·12H2O
	Carraraite	Ca3(SO4)[Ge(OH)6](CO3)·12H2O
Possible c ≈ 20 Å P3 1c Single crystal diffraction data are lacking	Charlesite	Ca6(Al,Si)2(SO4)2[B(OH)4](OH,O)12·26H2O

The simplified general formula for ettringite group minerals is Ca₃M(OH)₁₂R_1.5-2·nH₂O (based on Ca_apfu = 3), where M = Al³⁺, Cr³⁺, Fe³⁺, Si⁴⁺, Mn⁴⁺, or Ge⁴⁺, R = (SO₄)²⁻, (CrO₄)²⁻, (SeO₄)²⁻, (CO₃)²⁻, (SO₃)²⁻, (PO₃OH)²⁻, or B(OH)⁴⁻, and n = 11-13 (Moore and Taylor, 1968; Pekov et al., 2012; Chukanov et al., 2016, 2019). The crystal structure is defined by positively charged {Ca₃[M(OH)₆]·12H₂O} columns, with channels between them containing R anion groups or H₂O.

The majority of ettringite group mineral structures have been determined (Gatta et al., 2019). As previously reported, the c parameter and symmetry of ettringite group minerals are governed by the occupancy of the crystallographically distinct sites by R anion groups or H₂O in the channels. These minerals can be roughly divided into three types (Table 1): one trigonal (P3₁c) and two hexagonal (centrosymmetric P6₃/m and non-centrosymmetric P6₃). Unlike the hexagonal members, the trigonal phase shows a doubling of the c parameter. Each R-site is located on the threefold axes and can be ordered or disordered, depending on the orientation and composition of the anionic groups occupying the site.

Imayoshiite, with the ideal formula Ca₃Al(CO₃)[B(OH)₄](OH)₆·12H₂O, was officially approved as a mineral species in 2013. It was originally discovered in the cavities of xenoliths composed of gabbro and its pegmatite at Suisho-dani, Ise City, Mie Prefecture, Japan (Nishio-Hamane et al., 2015). Since then, no other localities have reported this mineral. The structure has a hexagonal unit cell with a non-centrosymmetric space group P6₃, and the structure refinement results show orientational disorders of B(OH)₄⁻ and CO₃²⁻ in the channels. In 2015, the sulfate analogue of imayoshiite, tatarinovite, was found in the Bazhenovskoe chrysotile asbestos deposit, Middle Urals, Russia. Its ideal formula is Ca₃Al(SO₄)[B(OH)₄](OH)₆·12H₂O. Anionic disorders were also observed in the channels (Chukanov et al., 2016).

We report here a new occurrence of imayoshiite from the Shijiangshan mine in Linxi County, Chifeng City, Inner Mongolia. New spectroscopic, thermal, and crystal structure data have provided new information for this mineral. Chemical analysis confirmed that the tested mineral samples belong to the imayoshiite-tatarinovite solid-solution series, with the composition being closer to imayoshiite, as indicated by the apfu of C slightly higher than that of S. Additionally, a previously unreported centrosymmetric space group P6₃/mcm was determined. This space group, distinct from previously reported imayoshiite, retains the same end-member formula and bonding structure, classifying it as a polymorph of imayoshiite (Hatert et al., 2023). For clarity, this polymorph will be referred to as imayoshiite-P6₃/mcm throughout this text. We also discuss the crystal structure of ettringite-group minerals and the types of polyhedra to explain the observed structural changes.

OCCURRENCE AND GENERAL APPEARANCE

The imayoshiite-P6₃/mcm samples were collected from the Shijiangshan mine, Linxi County, Hexigten Banner, Inner Mongolia, China, at geographical coordinates 43°43′15′′N, 117°50′32′′E. The deposit is a hydrothermal system rich in Pb, Zn, and Ag. Over the last decade, the Shijiangshan mine has become a popular site for collecting rare, well-formed boron minerals, including olshanskyite, pentahydroborite, roweite, and borcarite. These mineral associations were also reported in the Fuka mine, Bicchu-cho, Takahashi City, Okayama Prefecture, Japan (Omae et al., 2002), where other B-bearing ettringite group minerals occur (Kusachi et al., 2008; Lykova et al., 2020). Notably, the Shijiangshan mine recently yielded two new mineral species: shijiangshanite (Sun et al., 2022) and shinichengite (Sun et al., 2023).

Imayoshiite-P6₃/mcm crystals from the Shijiangshan mine are colorless, transparent, with a vitreous luster, and occur as granular aggregates. The crystals were found in hydrothermally altered pockets (Fig. 1a). Figure 1b shows that single grains mainly occur as euhedral crystals between 0.1 and 0.5 mm in size, which the combined appearance of a hexagonal dipyramid {10-12} and a hexagonal prism {10-10}. Based on the Laue group 6/mmm with a:b:c = 1:1:1.0402, the ideal crystal form and crystal face symbols for imayoshiite-P6₃/mcm were generated using SHAPE software and are shown in Figure 2.

Figure 1. Appearance of imayoshiite-P6₃/mcm. (a) Imayoshiite-P6₃/mcm and associated minerals (datolite, andradite, bultfonteinite, and wiluite). (b) The secondary electron scanning electron microscope (SEM) image of imayoshiite-P6₃/mcm shows the appearance of a hexagonal dipyramid {10-12}, a hexagonal prism {10-10}, and perfect cleavage in {10-10}.

Figure 2. Drawing of an idealized crystal of imayoshiite-P6₃/mcm from the Shijiangshan mine.

The formation of imayoshiite-P6₃/mcm is associated with low-temperature hydrothermal activity during the late skarn stage. Minerals associated with imayoshiite-P6₃/mcm include andradite, wiluite, datolite, and bultfonteinite. Wiluite and andradite are typical early skarn-stage minerals. As magma evolved and hydrothermal fluid separated, volatiles such as B and CO₂ gradually enriched (Meinert et al., 2005; Romer et al., 2005). Highly hydrated B-bearing imayoshiite-P6₃/mcm formed under water-saturated conditions (Aleksandrov and Troneva, 2012).

EXPERIMENTS AND RESULTS

Fourier transform infrared (FTIR) spectroscopy

The FTIR spectrum of imayoshiite-P6₃/mcm was obtained using the KBr pellet method on a JTENSOR 37 FTIR spectrometer (Fig. 3). The spectrum was recorded in the 4000-400 cm⁻¹ region by averaging 16 scans with a resolution of 4 cm⁻¹. The spectrum displays characteristics of both imayoshiite (Nishio-Hamane et al., 2015) and tatarinovite (Chukanov et al., 2016). The sharp band at 3613 cm⁻¹ and the broad band at 3422 cm⁻¹ are attributed to O-H stretching vibrations (Galliski et al., 2010). The two weak broad bands at 2485 and 2225 cm⁻¹ are also associated with O-H stretching vibrations, particularly O-H in HSO₄⁻. The reversible equilibrium of SO₄²⁻ + H₂O ↔ HSO₄⁻ + OH⁻ is indicated by the weak bands, suggesting a leftward shift (Chukanov et al., 2016). The H₂O bending mode occurs at 1679 cm⁻¹ (Xue et al., 2017).

Figure 3. Infrared spectra of (1) imayoshiite, Ca₃Al(CO₃)[B(OH)₄](OH)₆·12H₂O [data from Nishio-Hamane et al. (2015)], (2) tatarinovite, Ca₃Al(SO₄)[B(OH)₄](OH)₆·12H₂O [data from Chukanov et al. (2016)], and (3) imayoshiite-P6₃/mcm from the Shijiangshan mine (this study).

A series of peaks appear in the spectrum within the 1500-560 cm⁻¹ region, primarily associated with anion groups in the channel. The peaks at 1413 and 879 cm⁻¹ correspond to the stretching vibrations of the CO₃²⁻ anion (David et al., 2014). In highly hydrous media, there is a dynamic equilibrium B(OH)₄ + H⁺ ↔ B(OH)₃ + H₂O (Ward and Millero, 1974). Two weak peaks at 1238 and 1197 cm⁻¹ may be related to a small admixture of B(OH)₃ (Pushcharovsky et al., 2004). Peaks at 987 and 956 cm⁻¹ are attributed to B-O stretching and O-B-O bending vibration in the B(OH)₄⁻ anion (Lykova et al., 2020). Consistent with the XPS results, the peaks at 1120, 659, and 580 cm⁻¹ correspond to stretching vibrations of the SO₄²⁻ anion (Pekov et al., 2012).

An independent peak at 715 cm⁻¹ corresponds to the Si-O stretching vibration in Si(OH)₆ octahedra, with its weak intensity attributed to the low Si content (Malinko et al., 2001). Additional peaks at 551 and 422 cm⁻¹ are ascribed to the Al-O stretching and O-Al-O bending vibrations, respectively (Chukanov et al., 2016).

Thermal analysis

Thermogravimetry (TG) and differential thermogravimetry (DTG) analyses were performed using a Discovery TGA 55 analyser at the Beijing University of Science and Technology. The heating atmosphere was air, with heating and gas flow rates set at 10 °C/min and 100.0 mL/min, respectively. The temperature for the first heating experiment ranged from 22 to 700 °C, with an initial sample mass of 5.01929 mg. The TG and DTG curves are presented in Figure 4. Two weight loss steps are observed in the TG curve. The DTG curve shows a strong peak at 128 °C, indicating the rapid loss of H₂O molecules. No significant peak is observed in the DTG curve between 200-600 °C, while the TG curve gradually decreases, indicating the loss of OH⁻. By 600 °C, the mass loss was 49.42%, representing the total water content of the sample. The peak at 653 °C on the DTG curve indicates the loss of CO₂ (Kusachi et al., 2008). The TG curve indicates that 2.01% of the mass was lost between 600 and 700 °C, but it is unclear whether all CO₂ was completely lost (Table 2).

Figure 4. TG and DTG curves for imayoshiite-P6₃/mcm. (a) The testing temperature range is from room temperature to 700 °C. (b) The testing temperature range is from room temperature to 900 °C.

Table 2. TG and TGA results for imayoshiite-P6₃/mcm

	Temperature (°C)	Peak	Mass loss (%)	Corrected mass loss (%)	Assignment
Room temperature to 700 °C	-600	128	49.42		H2O, OH
	600-700	652	2.01		CO2
Room temperature to 900 °C	-600	116	63.76	49.42	Adsorbed water, H2O, OH
	600-750	664	3.66	4.27	CO2
	750-900	862	3.19	3.72	SO3

Two TG and TGA experiments were performed at temperature ranges of room temperature to 700 and 900 °C.

To determine the CO₂ content, a second heating experiment was conducted, ranging from 33 to 900 °C, with a sample mass of 2.25900 mg (Fig. 5). The total water loss was 63.76%, involving some absorbed water. The DTG peak at 664 °C corresponds to the loss of CO₂ (Kusachi et al., 2008). The total CO₂ content is indicated by the weight loss between 600 and 750 °C. Due to adsorbed water, the CO₂ content was adjusted for the chemical formula calculation based on H₂O content obtained from the first experiment (Table 3). The DTA peak at 862 °C is partially due to the release of SO₃ (Table 3) (Sytle et al., 2002; Grier et al., 2002).

Figure 5. XPS spectra of imayoshiite-P6₃/mcm. The location of the S peak featured in the red box indicates that all S atoms were in the form of SO₄²⁻.

Table 3. XPS spectral features of imayoshiite-P6₃/mcm from Shijiangshan mine

Element	Peak energy (eV)	Peak assignment
O 1s	532.08	O2−, OH−, H2O
Ca 2s	439.08	Ca2+
Ca 2p	347.39	Ca2+
C 1s	284.88	CO32−
B (Reference Material B2O3)	192.07	B(OH)4−
S (Reference Material Na2SO4)	169.08	SO42−
Al 2p	74.08	[Al(OH)6]

X-ray photoelectron spectroscopy (XPS)

In S-bearing ettringite group minerals, SO₃²⁻ and SO₄²⁻ are possible species-defining anion groups (Pekov et al., 2012). However, FTIR spectroscopy cannot differentiate between these two anion groups. Additionally, due to structural refinement, the anion groups are disordered in the channel, making it difficult to determine the number of O coordinated with S. To determine the form of S, a Thermo Scientific K-alpha XPS system was used under a vacuum of approximately 5 × 10⁻⁹ mbar. The X-ray source was mono AlKα with an energy of 1486.6 eV, a voltage of 15 kV, and a beam current of 15 mA. The analyzer operated in constant analyzer energy (CAE) mode.

The peak associated with S element occurs at 169.08 eV, close to the peak observed in Na₂SO₄ (Taylor et al., 1999). No visible peak related to SO₃²⁻ was observed, indicating that all S atoms are in the form of SO₄²⁻ (Fig. 6). Atomic forms, peak positions, and peak assignments are listed in Table 3.

Figure 6. Unwarping sing-crystal diffraction image to align with the origin vector (000). (a) The observed diffraction pattern of imayoshiite-P6₃/mcm, showcasing the absence of diffraction points for l = 2n + 1 (where n = 0, ±1, ±2, …), reveals the symmetry elements consistent with a -c- glide plane. (b) The simulated diffraction pattern of imayoshiite-P6₃/mcm which matches well with the observed pattern. (c) The simulated diffraction pattern of imayoshiite (Nishio-Hamane et al., 2015) which do not exhibit the extinction characteristics of a -c- glide plane.

Inductively coupled plasma-mass spectrometry (ICP-MS)

Ettringite group minerals easily lose water when exposed to electron beams and vacuum conditions. The use of electron probe microanalysis (EPMA) is unsuitable because water loss leads to an overestimation of certain elements. Instead, samples were analyzed using a Thermo Scientific X series II ICP-MS. Carefully selected 500 mg powder samples were dissolved in a mixture of HCl and HF acids and then analyzed by ICP-MS to determine the major components: Ca, Al, Si, Fe, B, and S (Table 4). Combining this information with the TG data, the empirical formula was determined to be Ca₃(Al_0.78Si_0.11Fe_0.01)_∑0.90{[B(OH)₄]_0.91(CO₃)_0.60(SO₄)_0.45}_∑1.96(OH)_5.80·12.39H₂O, Z = 2 (based on Ca_apfu = 3). The empirical formula confirms that the mineral belongs to the imayoshiite-tatarinovite solid-solution series and is closer to imayoshiite due to C_apfu > S_apfu. The simplified formula is Ca₃Al(CO₃)[B(OH)₄](OH)₆·12H₂O.

Table 4. Chemical composition of imayoshiite and tatarinovite solid-solution series

Constituent	Present work	Imayoshiite**	Tatarinovite***	Atom	Present work
CaOwt%	26.95	28.20	27.40	Caapfu	3.00
Al2O3	6.41	7.60	6.34	Al	0.78
SiO2	1.07	1.17	2.43	Si	0.11
Fe2O3	0.29		0.03	Fe	0.01
B2O3	5.26	5.47	4.06	B	0.91
SO3	5.74	0.84	8.48	S	0.45
CO2	4.21*	7.49	4.2	C	0.59
H2O	49.42*	50.02	46.1	H	34.23
Total	99.35	100.80	99.04

^* from TG data. ^** Nishio-Hamane et al., 2015. ^*** Chukanov et al., 2016.

Single crystal X-ray diffraction (SC-XRD)

The single-crystal structure was determined using a Rigaku Oxford Diffraction XtaLAB PRO-007HF microfocus rotating anode X-ray source (1.2 kW, MoKα, λ = 0.71073 Å) coupled with a hybrid pixel array detector diffractometer. The refined unit-cell parameters are a = 11.0663(6) Å, c = 10.6387(6) Å, and V = 1128.30(14) ų based on 2107 reflections. The a:b:c ratio calculated from the unit-cell parameters is 1:1:1.0402. The statistical tests on the distribution of |E| values (|E² − 1| = 0.917, expected 0.968 centro-symmetry, 0.763 non-centro-symmetry) suggest centric symmetry for the tested sample. Examination of the systematic absences indicated the symmetry elements 6₃ (without exception), -c- [with 136 exceptional diffraction peaks where I > 3σ(I); I/σ(I) = 3.0 compared to a total I/σ(I) = 47.6, indicating relatively low intensity], leading to the determination of the central space group P6₃/mcm. Refinements were performed using OLEX2-1.3 (Dolomanov et al., 2009). The structure was solved with the SHELXT (Sheldrick, 2015) structure solution program using Intrinsic Phasing and refined with the SHELXL (Sheldrick, 2015) refinement package using least squares minimization. The structure was refined to R = 0.0492 based on 500 independent reflections with I > 2σ(I) (Tables 5-7).

Table 5. Data collection information and structure-refinement parameters for the imayoshiite-P6₃/mcm from Shijiangshan mine

Crystal data
Mr	586.22
Crystal system, space group	Hexagonal, P63/mcm
Temperature (K)	293
a, c (Å)	11.0663 (6), 10.6387 (6)
V (Å3)	1128.30 (14)
Z	2
Radiation type	MoKα
μ (mm−1)	0.92
Crystal size (mm)	0.03 × 0.03 × 0.03
Data collection
Diffractometer	XtaLAB PRO-007HF
Absorption correction	Multi-scan
Tmin, Tmax	0.383, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	4173, 554, 500
Rint	0.030
I/σ(I)	47.6
(sin θ/λ)max (Å−1)	0.686
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.049, 0.172, 1.18
No. of reflections	554
No. of parameters	44
Δρmax, Δρmin (e Å−3)	0.81, −0.72

Table 6. Coordinates and displacement parameters for imayoshiite-P6₃/mcm from Shijiangshan mine

Site	Wyk.	s.o.f.	x/a	y/b	z/c	Ueq
Ca	6g	Ca1.00	1	0.80522(8)	3/4	0.0161(4)
Al(Si)	2b	Al0.87Si0.13	1	1	1	0.0107(6)
B(C,S)	4d	B0.46C0.31S0.23	2/3	1/3	1/2	0.0262(9)
O1=OH	12k	O1.00	0.1339(2)	1	0.8900(2)	0.0163(6)
O2=H2O	12j	O1.00	0.7419(4)	0.5901(4)	3/4	0.0372(8)
O3=H2O	12k	O1.00	1	0.6565(3)	0.5826(3)	0.0378(9)
OC = O	12i	O0.31	0.7341(4)	0.4682(7)	1/2	0.0158(16)
OB1=(OH,O)	24l	O0.345	0.6209(8)	0.4293(8)	0.5425(7)	0.0393(17)
OB2=(OH,O)	8h	O0.345	2/3	1/3	0.6362(14)	0.055(4)
Site	U11	U22	U33	U23	U13	U12
Ca	0.0204(6)	0.0158(5)	0.0136(6)	0	0	0.0102(3)
Al(Si)	0.0121(7)	0.0121(7)	0.0079(10)	0	0	0.0061(4)
B(C,S)	0.0237(13)	0.0237(13)	0.031(2)	0	0	0.0119(6)
O1=OH	0.0179(9)	0.0185(12)	0.0125(11)	0	−0.0003(8)	0.0093(6)
O2=H2O	0.0422(19)	0.0298(17)	0.0404(19)	0	0	0.0185(15)
O3=H2O	0.0277(16)	0.0331(13)	0.0509(19)	−0.0193(14)	0	0.0139(8)
OC = O	0.012(3)	0.007(3)	0.026(4)	0	−0.005(3)	0.0037(17)
OB1=(OH,O)	0.049(4)	0.038(4)	0.040(4)	0.006(3)	0.006(3)	0.028(4)
OB2=(OH,O)	0.064(5)	0.064(5)	0.037(7)	0	0	0.032(3)

Table 7. Selected interatomic distances (Å) for imayoshiite-P6₃/mcm from Shijiangshan mine

Ca-O1	2.4223(16)	×4	B(S)-OB1	1.459(7)	×3
Ca-O2	2.651(4)	×2	B(S)-OB2	1.449(14)	×1
Ca-O3	2.425(3)	×2	<B(S)-O>	1.457
<Ca-O>	2.480
Al-O1	1.888(3)	×6	C-OC	1.293(7)	×3

The chain structure consists of Al(OH)₆ octahedra and triple edge-sharing Ca(H₂O)₄(OH)₄ polyhedra extending along the c-axis (Fig. 7). Si replaces a small numbers of Al atoms. Si(OH)₆ octahedra are common in ettringite group minerals, but this is an unusual example of Si in six-fold coordination at low pressure and temperature (Tschauner et al., 2020). The actual restriction on the occurrence of ^[6]Si in crystal structures is the crowding of cations around oxygen atoms. The channels are occupied by B(OH)₄⁻, CO₃²⁻, and SO₄²⁻ anion. There should be two distinct sites in the channel, but intense disorder merges these two sites, explaining the determination of the higher space group P6₃/mcm. The original two sites can be considered equivalent to one site in our sample (Fig. 8). Based on the pfu of B(OH)₄⁻ + CO₃²⁻ + SO₄²⁻ = 2, there are 0.92 B(OH)₄⁻, 0.62 CO₃²⁻, and 0.46 SO₄²⁻. The C atom coordinates with three oxygen atoms (OC), forming a planar triangle. S and B atoms form tetrahedral ligands sharing the same oxygen atoms (OB1 and OB2). The tetrahedra have two distinct central symmetry orientations, with their distribution disordered, each occupying half the sites. The two orientations are referred to as forward tetrahedron and reverse tetrahedron.

Figure 7. Crystal struc of imayoshiite-P6₃/mcm (H not given). B(OH)₄⁻, CO₃²⁻, and SO₄²⁻ are disordered in the channel.

Figure 8. Arrangement of tetrahedral and triangular groups in the crystal structure of disordered imayoshiite-P6₃/mcm. C coordinates with oxygen atoms OC, forming a planar triangle (in blue); S and B share the same oxygen atoms OB1 and OB2, forming tetrahedral ligands with two different central symmetry directions (in green).

DISCUSSION AND SUMMARY

The structure type and change in space group of ettringite group minerals

Ettringite group minerals are primarily classified into two types based on unit-cell dimensions and space group. The first type is characterized by c ≈ 10 Å with a P6₃ space group, while the second type has c ≈ 20 Å with a P3₁c space group. The difference between these two types is related to the arrangement of anion groups. Our tested sample corresponds more closely to the first type but with a higher symmetry central space group (P6₃/mcm). The observed change in the space group may be due to disorder. However, while disorder is common in this mineral group, few cases of disorder result in changes to the space group (e.g., Carraraite. Merlino and Orlandi, 2001). This higher symmetry space group P6₃/mcm may not represent the true space group of this mineral, but rather reflects anion disorder leading to centric symmetry in statistical tests and weak systematic absences. Using a higher symmetry space group for structural analysis offers a more accurate representation of the disordered characteristics of imayoshiite-P6₃/mcm from the Shijiangshan mine.

Summary of anions polyhedra geometry in channels

The channels of ettringite group minerals contain three fundamental geometric ligands: planar triangle (CO₃²⁻), trigonal pyramidal (AsO₃²⁻ and SO₃²⁻), and tetrahedra [SO₄²⁻, PO₃(OH)²⁻, CrO₄²⁻, B(OH)₄⁻, and SeO₄²⁻]. Due to the disorder, these ligands may coexist at the same site with rotations, combinations, and centrosymmetry, resulting in complex geometric forms. The morphology of the these ligands is summarized in Figure 9, which is crucial for understanding anion types in the channels of ettringite group minerals and for accurately analyzing their structures.

Figure 9. Different kinds of ligands in the channels of ettringite group minerals: ①Tetrahedron [e.g., Jouravskite (Chukanov et al., 2019), Kottenheimite (Chukanov et al., 2012), Ettringite (Gatta et al., 2019)]. ②Planar triangle [e.g., Jouravskite (Chukanov et al., 2019)]. ③Trigonal pyramidal. ④Combination of tetrahedron and planar triangle [e.g., Carraraite (Merlino and Orlandi, 2001)]. ⑤Combination of trigonal pyramidal and planar triangle [e.g., Hielscherite (Pekov et al., 2012)]. ⑥Combination of tetrahedrons with rotation [e.g., Siwaqaite (Juroszek et al., 2020a)]. ⑦Combination of tetrahedrons with centrosymmetry [e.g., Sturmanite (Pushcharovsky et al., 2004), imayoshiite (Nishio-Hamane et al., 2015)]. ⑧Combination of planar triangles with rotation [e.g., Bentorite (Juroszek et al., 2020b), imayoshiite (Nishio-Hamane et al., 2015)]. ⑨Combination of trigonal pyramidal with rotation [e.g., Siwaqaite (Juroszek et al., 2020a)]. ⑩Combination of planar triangle and tetrahedrons with centrosymmetry [e.g., Tatarinovite (Chukanov et al., 2016)].

ACKNOWLEDGMENTS

We express our gratitude to Wei Peng for providing the samples. We thank the editors and reviewers who have provided suggestions for revisions to the manuscript. Special thanks to Dr. Stuart Mills and Dr. Koichi Momma for handling our manuscript, Prof. Ferdinando Bosi for his invaluable advice during the revision process, and Prof. Igor Pekov and another anonymous reviewer for their insightful and constructive feedback. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41672043 and 92262303) and Fundamental Research Funds for the Central Universities (2-9-2022-030).

SUPPLEMENTARY MATERIAL

Supplementary CIF file is available online from https://doi.org/10.2465/jmps.231013.

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