Chemical composition, crystal structure and spontaneous polarization of swedenborgite

Abstract

The chemical composition of swedenborgite sample obtained from the type locality, Långban, Värmland, Sweden, was determined using scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was observed that swedenbolgite crystals possess both Ca-free and Ca-containing zones. The crystal structures of swedenborgite [space group P6₃mc, a = 5.4402(10) Å c = 8.8690(9) Å, Z = 2] was refined to R1 = 0.012 using 1573 unique reflections. In addition, the threshold energy of the Sb K-edge XANES spectrum of swedenborgite was found to be higher than that of Sb₂O₃, but almost the same as that of Sb₂O₅. These results indicated that the oxidation state of Sb in swedenborgite was almost pentavalent, although the presence of a small amount of trivalent Sb is also suggested. It was therefore assumed that the following substitution relationship exists: 2Na⁺ + Sb⁵⁺ ⇄ 2Ca²⁺ + Sb³⁺, and that the charge balance of Ca occupation is achieved by reduction of some Sb⁵⁺ ions in the Ca-containing zone. The general formula of swedenborgite was therefore expressed as (Na_1−xCa_x)Be₄Sb⁵⁺_1−0.5xSb³⁺_0.5xO₇ (x = 0.0 or 0.05-0.07). The distortions of the Be-O distances along the c-axis and O-Be-O angles of BeO₄ trigonal pyramid in swedenborgite were significantly larger than those in BeO bromellite. Opposite coordinate shifts between cations and anions along the c-axis occur because of the asymmetric arrangement around the NaO₁₂ tetradecahedra with upper face sharing and lower edge sharing. The structure of swedenborgite therefore exhibits a biased arrangement of cations and anions parallel to the c-axis, which induces spontaneous polarization.

INTRODUCTION

Swedenborgite, NaBe₄SbO₇, a sodium beryllium antimonite mineral, was first discovered in Långban, Värmland, Sweden, and was described by Aminoff (1924), and again by Aminoff and Blix (1933). This mineral occurs in association with calcite, bromellite, manganophyllite, hematite, and richterite in skarn of a metamorphosed Fe-Mn ore body (Palache et al., 1951; Huminicki and Hawthorne, 2001). Pauling et al. (1935) clarified the crystal structure of swedenborgite and Povarennykh et al. (1982) reported its infrared absorption spectrum. Huminicki and Hawthorne (2001) refined the crystal structure of swedenborgite and represented its chemical formula as (Na_0.89Ca_0.04□_0.07)Be₄SbO₇, suggesting that a small amount of Ca is incorporated into the structure via a substitution reaction (i.e., 2Na → Ca + □). In their model, a large number of vacancies are generated at the Na site, which correlated with the amount of incorporated Ca. They also reported that its crystal structure consists of SbO₆ octahedra alternately arranged with [Be₄O₁₃] clusters, which are fragments of the BeO bromellite structure. The structure of swedenborgite is therefore closely related to that of bromellite, which exhibits dielectric properties and is widely used industrially because of its ability to acts as both an excellent electrical insulator and a good thermal conductor. Sb exists in four main oxidation states (−3, 0, +3, and +5). The Sb-O bond could be regarded as predominantly covalent as estimated from the difference of Pauling electronegativity between antimony (2.05) and oxygen (3.44). It is necessary to observe the chemical bonding state of Sb in this mineral and compare it with other Sb-bearing minerals.

We herein report reanalysis of the chemical composition of swedenborgite using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS), and confirm the oxidation state of Sb atoms in swedenborgite using Sb K-edge XAFS analysis. Furthermore, we reexamine the detailed structure of swedenborgite and determined its absolute structure using single-crystal X-ray diffraction. Finally, we discuss the atomic arrangement that gives rise to spontaneous polarization observed in swedenborgite.

EXPERIMENTS

Sample and chemical analysis

Single crystals of swedenborgite were obtained from the type locality, Långban, Värmland, Sweden (Kumamoto University, C133-swedenborgite-201201a). As shown in Figures 1 and 2, the swedenborgite sample emits strong fluoresces under short-wave UV light (peak wavelength 365 nm). The crystals were observed to be colorless and transparent, and no color center was detected. The chemical composition was determined using a JEOL scanning electron microscope (SEM, JSM-7001F operated at 15 kV, 1.0 nA) equipped with an Oxford energy-dispersive X-ray spectrometer (EDS, Aztec SYSTEM). The following standard materials were employed: NaAlSi₃O₈ for Na, CaSiO₃ for Ca, and metal antimony for Sb. Since the analysis of beryllium in micron-sized samples is challenging, the residual of the analyzed total value was estimated as the beryllium content. The number of atoms per formula unit (apfu) of swedenborgite samples was estimated taking the total number of oxygen to be seven (Table 1). The results were acceptable as quantitative values with an error of less than 2%. The beryllium content as apfu was approximately 4.0, indicating no deficiency of this element.

Figure 1. Microscope (left) and fluorescence (right) images of swedenborgite in calcite. Scale bar is 1 mm.

Figure 2. SEM image (left) and distribution (right) of zone 1 [(Na_0.94Ca_0.06)Be₄Sb⁵⁺_0.97Sb³⁺_0.03O₇] and zone 2 (NaBe₄SbO₇) in swedenborgite. The blue light emission from this specimen is also shown. Scale bar is 50 µm.

Table 1. Chemical composition of zone 1 [(Na_0.94Ca_0.06)Be₄Sb⁵⁺_0.97Sb³⁺_0.03O₇] and zone 2 (NaBe₄SbO₇) in swedenborgite

	Zone1			Zone2
Zone1	Average	Minimum	Maximum	Average	Minimum	Maximum
Na2O	10.06(23)	9.72	10.49	10.93(46)	10.22	11.61
CaO	1.08(10)	0.86	1.21	0.00	0.00	0.00
Sb2O5	54.42(17)	54.12	54.64	54.07(32)	53.51	54.59
Total	65.55(10)	65.39	65.68	65.00(25)	64.60	65.37
BeO calc	34.45(10)	34.32	34.61	35.00(25)	34.63	35.40
O = 7
Na	0.947(23)	0.913	0.989	1.024(45)	0.956	1.089
Ca	0.056(5)	0.045	0.063	0.000	0.000	0.000
Sb	0.98(3)	0.977	0.985	0.970(6)	0.961	0.978
Be	4.018(8)	4.008	4.029	4.062(19)	4.034	4.092

XAFS measurement

Sb K-edge XANES analysis was performed to determine the valence state of Sb in swedenborgite, using Sb minerals and compounds, such as Sb₂S₃ stibnite, AsSbS₃ getchellite, Pb₃Sn₄FeSb₂S₁₄ cylindrite, welshite, romeite, Sb₂O₃ and Sb₂O₅, as reference materials. The Sb K-edge XANES measurements were performed using a Si (311) double crystal monochromator at the BL-NW10A beamline of the PF-AR ring at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan (Tobase et al., 2015; Hongu et al., 2019). The beam size was 0.3 × 0.2 mm. The spectra near the Sb K-edge were collected in fluorescence mode using a Lytle-type detector. The XANES data were analyzed using the XAFS93 program (Yoshiasa et al., 1999; Kitahara et al., 2021), and the corresponding spectra are shown in Figure 3.

Figure 3. Sb K-edge XANES spectra of swedenbourgite, Sb₂O₃, Sb₂O₅, and antimony minerals as reference.

Crystal structure analysis

The single crystal employed for crystal structure analysis was carefully selected from parts where Ca was not detected (defined as Zone 2). The crystallographic data were collected on a Rigaku R-AXIS RAPID imaging plate diffractometer. Systematic absences were consistent with the space group P6₃mc and no evidence of lower symmetry was detected. The intensities of reflections were measured using graphite-monochromatized MoKα radiation (0.71073 Å). A total of 1704 reflections were collected. After Lorentz and polarization corrections, an absorption correction was performed using the integration method based on the shape of the specimen. Details of data correction method are described in the Supplementary CIF file (extended data file; Supplementary CIF file is available online from https://doi.org/10.2465/jmps.240209). The crystal structure analysis was performed using the SHELXL program (Sheldrick, 2015). The R1 index (R1 = Σ||Fo| − |Fc||/Σ|Fo|) converged to 0.0119 using anisotropic displacement parameters. The Flack parameter (Flack, 1983) was refined to 0.042(23), and it was confirmed that the crystal consisted of a single domain, and the absolute structure was correct. The experimental details and crystallographic data are presented in Table 2. The atomic coordinates and anisotropic atomic displacement parameters for swedenborgite are listed in Table 3. Selected interatomic distances and angles are given in Table 4.

Table 2. Experimental details and crystallographic data of swedenborgite

Mineral name	Swedenborgite
Formula by site occupancy model	NaBe4SbO7
a (Å)	5.4402(10)
c (Å)	8.8690(9)
V (Å3)	227.32
Z	2
Space group	P63mc
Crystal size (mm)	0.0129 × 0.0136 × 0.080
Radiation used (Å)	0.71073
Diffractometer	RAPID
Monochromater	Graphite
2θrange (°)	<60.8
Index ranges	−7 ≤ h ≤ 7, −7 ≤ k ≤ 7, −12 ≤ l ≤ 12
No. of measured reflections	1704
No. of independent reflections with |Fo| ≥ 2σ(|Fo|)	1573
Rint/Rsigma	0.0245/0.0224
Goodness of fit = S	1.204
R1	0.0119
wR2	0.0272
Diff. peak and hole	0.33 and −0.47
Absolute structure	131 of Friedel pairs used in the refinement
Absolute structure parameter, Flack	0.042(23)

Table 3. Atomic coordinates and atomic displacement parameters for swedenborgite

	x	y	z	U11	U22	U33	U23	U13	U12	Ueq
Na	0.33333	0.66667	0.6247(4)	0.16667	0.0135(11)	0.0135(11)	0.0139(15)	0.00000	0.00000	0.0136(7)
Sb	0.33333	0.66667	0.00000(2)	0.16667	0.00449(18)	0.00449(18)	0.00449(19)	0.00000	0.00000	0.00449(17)
Be(1)	0	0	0.0627(15)	0.002(2)	0.002(2)	0.008(3)	0	0	0.0008(12)	0.0037(14)
Be(2)	0.1650(7)	0.8350	0.3129(6)	0.0069(18)	0.0069(18)	0.006(2)	0.0023(11)	−0.0023(11)	0.003(2)	0.0065(10)
O(1)	0	0	0.3730(6)	0.0059(16)	0.0059(16)	0.0023(19)	0	0	0.0030(8)	0.0047(9)
O(2)	0.4950(4)	0.5050	0.3704(4)	0.0055(9)	0.0055(9)	0.0100(10)	0.0006(7)	−0.0006(7)	0.0029(10)	0.0070(5)
O(3)	0.1622(5)	0.8378	0.1270(3)	0.0098(12)	0.0098(12)	0.0055(12)	−0.0019(7)	0.0019(7)	0.0072(13)	0.0073(6)

Table 4. Selected interatomic distances (Å) and angles (°) for swedenborgite

Na-O2	×3	2.714(4)	O2-Na-O2′	×3	62.14(14)
Na-O2	×3	2.722(4)	O2-Na-O2′	×3	57.99(11)
Na-O3	×3	2.7205(5)	O3-Na-O3′	×3	58.24(18)
Na-O3	×3	2.7205(5)	O3-Na-O3′	×3	61.76(18)
			O2-Na-O3	×6	58.84(11)
			O2-Na-O3	×6	61.69(9)
Sb-O2	×3	1.984(4)	O2-Sb-O2	×3	89.81(14)
Sb-O3	×3	1.967(3)	O3-Sb-O3	×3	90.47(14)
			O3-Sb-O2	×6	89.86(9)
Be1-O1		1.683(17)	O1-Be1-O3	×3	110.4(4)
Be1-O3	×3	1.631(6)	O3-Be1-O3′	×3	108.5(5)
Be2-O1		1.644(7)	O1-Be2-O3		108.3(5)
Be2-O2	×2	1.637(4)	O2-Be2-O2′		107.4(4)
Be2-O3		1.649(6)	O1-Be2-O2	×2	112.0(3)
			O2-Be2-O3	×2	108.6(3)

RESULTS AND DISCUSSION

Chemical analysis and determination of the chemical formula of swedenborgite

A histogram of Ca content (apfu) detected in the swedenborgite specimens is presented in Figure 4, wherein it can be seen that the Ca content distribution is clearly separated into two zones. More specifically, the zone containing Ca was defined as Zone 1, while the zone without Ca was defined as Zone 2 (Table 1 and Fig. 2). Zone 1 was found to exhibit stronger blue fluoresce under short-wave UV light than Zone 2.

Figure 4. Histogram of measured number (frequency) of Ca content as atom per formula unit (apfu) in the swedenborgite specimens.

Huminicki and Hawthorne (2001) assumed that the swedenborgite structure had Na site that had been substituted by vacancies and Ca. However, from the chemical analysis of Zone 1, the sum of the cations (Na + Ca) at the Na site was 1.00 apfu (0.95 for Na and 0.05 for Ca), and this value for Ca (i.e., 0.05) was identical to that reported by Huminicki and Hawthorne (2001), thereby indicating the presence of few vacancies at the Na sites.

The threshold photon energies of Sb K-edge spectra for swedenborgite, Sb₂O₅, and Sb₂O₃ were 30.4493, 30.4506, and 30.4463 keV, respectively. The threshold energy of the Sb K-edge XANES spectrum of swedenborgite is higher than that of Sb₂O₃ and is almost the same as that of Sb₂O₅. Although the oxidation state of Sb in swedenborgite was determined to be almost pentavalent, it was considered that a portion of the Sb may be present in the trivalent state. Based on the results of our chemical analyses, it was not considered that this swedenborgite specimen exhibited a large cation deficiency. It was therefore assumed that the following substitution relationship exists: 2Na⁺ + Sb⁵⁺ ⇄ 2Ca²⁺ + Sb³⁺, and that charge balance of Ca occupation is achieved by reduction of some Sb⁵⁺ ions in Zone 1. As outlined in Table 1, the formulae for average chemical compositions of Zone 1 and 2 were determined to be (Na_0.94Ca_0.06)Be₄Sb⁵⁺_0.97Sb³⁺_0.03O₇ and NaBe₄SbO₇, respectively. Furthermore, it was observed that the Ca content had a bimodal distribution in the histogram and did not continuously replace Na ions, thereby giving the general formula of (Na_1−xCa_x)Be₄Sb⁵⁺_1−0.5xSb³⁺_0.5xO₇ (x = 0.0 or 0.05-0.07).

Crystal structure and distortions of the BeO₄ tetrahedra in swedenborgite and bromellite

Our analysis confirmed the structure proposed by Huminicki and Hawthorne (2001), in which the crystal structure of swedenborgite consists of BeO₄ tetrahedra, SbO₆ octahedra, and NaO₁₂ tetradecahedra (Fig. 5). Be1 and Be2 form the Be₄O₁₃ cluster (Huminicki and Hawthorne, 2001). In the Be1O₄ and Be2O₄ tetrahedra, one Be-O bond elongates parallel to the c-axis, as observed in the trigonal pyramid (site symmetry of 3m) in BeO bromellite with wurtzite-type structure. The tetrahedron adopts a trigonal pyramid with a base parallel to the (0001) plane, and the vertices of the Be-O bonds elongating parallel to the c-axis are arranged downward, as shown in Figure 5. At the Be1 trigonal pyramidal site, the Be1-O1 distance of 1.683(17) Å parallel to the c-axis is longer than the basal Be1-O3 distance of 1.631(6) Å (Table 4 and Fig. 6), while at the Be2 trigonal pyramidal site, the Be2-O3 distance of 1.649(6) Å parallel to the c-axis is longer than the basal bond distances of 1.644(7) Å (Be2-O1) and 1.637(4) Å (Be2-O2). As shown in Figure 5, the Be atoms are located above the center of the BeO₄ trigonal pyramidal site in the c-axis direction.

Figure 5. Crystal structure of swedenborgite. Each site is color-coded [NaO₁₂ tetradecahedron (yellow), SbO₆ octahedron (brown), and BeO₄ trigonal pyramid (green)]. The crystal structure was drawn using VESTA (Momma and Izumi, 2011).

Figure 6. Coordination polyhedra (NaO₁₂ tetradecahedra-, SbO₆ octahedral-, BeO₄ trigonal pyramidal-sites) in swedenborgite. The one-sided arrows show the relative atomic displacements along the c-axis direction.

Swedenborgite can be closely related to BeO bromellite, the data obtained herein for the BeO₄ trigonal pyramid of swedenborgite were compared with the previously reported data for bromellite (Hazen and Finger, 1986). More specifically, in bromellite, the apical and basal Be-O distances in the BeO₄ trigonal pyramid were 1.651(3) and 1.647(1) Å, respectively, while the wide and narrow O-Be-O angles were 109.0(1)° and 109.9(1)°, respectively. These differences are significant, but not large, and are common among wurtzite-type compounds (Yoshiasa et al., 2003). On the other hand, the variation in the Be-O bond distances and O-Be-O angles in swedenborgite were found to be greater than those in bromellite. In particular, the Be1-O1 distance [1.683(17) Å], where the O1 ion is located at the center of the Be₄O₁₃ cluster (Huminicki and Hawthorne, 2001), is elongated. O1 is coordinated only to Be ions and forms the OBr₄ tetrahedron with one Be1 and three Be3. The distortion of the BeO₄ tetrahedra in swedenborgite was therefore determined to be significaltly large compared with that observed in bromellite.

Atomic arrangements causing spontaneous polarization in swedenborgite

Figure 6 shows the coordination polyhedron for each cation site along with the positional shift against the oxide ions along the c-axis. For the NaO₁₂ tetradecahedron, the lower three Na-O2 bond distances of 2.722(4) Å is longer than the upper three bond distances of 2.714(4) Å (Table 4), due to the effect of repulsions from Be2 ions in the edge-sharing trigonal pyramidal sites. As a result, the Na site was located above the center of the NaO₁₂ tetradecahedron in the c-axis direction. In addition, the lower three Sb-O2 distances of 1.984(4) Å in the SbO₆ octahedron were found to be longer than the upper three Sb-O3 distances of 1.967(3) Å, and this can be attributed to the effect of repulsions from the Na ion in the face-sharing tetradecahedral sites. As a result, the Sb ion was located above the center of the SbO₆ octahedron.

The basal face of the SbO₆ octahedron shares one triangular face with NaO₁₂ tetradecahedron (Fig. 5). In the NaO₁₂ tetradecahedron, the only face shared between the polyhedra is the O2-O2-O2 triangle, which consists only of O2 oxide ions. The three basal edges of the NaO₁₂ tetradecahedron share one edge with each of the three Be2O₄ trigonal pyramids. The adjacent upper part of the upper triangular face (consisting only of O3) of the SbO₆ octahedron is an empty octahedron, and so, an upward shift of Sb can occur easily. The position of the O3 site also shifted upward (Fig. 7). It was therefore considered that shifts in the atomic position occur along the c-axis to reduce the electrostatic repulsion between Na and Sb and between Na and Be. These electrostatic repulsions and the resulting structural relaxation generate a large positional asymmetry in the c-axis direction. Such polyhedral distortions and relaxation by positional shifts of Na, Sb, and Be along the c-axis direction in swedenborgite must cause chargebias, and as a result, this structural asymmetry should give rise to spontaneous polarization.

Figure 7. Ferroelectric atomic displacements of oxide ions (O1, O2, and O3) along the c-axis direction (arrows). A significant downward displacement of O2 ions was recognized.

Finally, the z coordinate of each atom in swedenborgite was compared with an ideal position without atomic displacement in hexagonal closet packing structure (Table 5). It was found that the z coordinates of oxygen atoms (especially O2 site) greatly deviate from their ideal positions; these z coordinates and deviations agreed with those obtained by Huminicki and Hawthorne (2001) (Fig. 8). A particularly large displacement occurs in O2 owing to the asymmetric arrangement around the NaO₁₂ tetradecahedra with upper face sharing and lower edge sharing along the c-axis. In addition, considering that the Be, Na, and Sb cations shift from the center of each polyhedron in the c-axis direction (see Figs. 6 and 7), swedenborgite exhibits charge bias on the upper and lower sides in the c-axis direction, as observed in BaTiO₃ and PbTiO₃ with spontaneous polarization (Nakatani et al., 2016; Yoshiasa et al., 2016).

Table 5. Comparison of the observed z coordinate and the ideal one, without atomic displacement in hexagonal closet packing structure, of each atomic position in swedenborgite

	Observed z coordinate	Ideal z coordinate	Difference Δz
Na	0.6247(4)	0.6250	−0.0003
Sb	0.0000	0.0000	0.0000
Be1	0.0627(15)	0.0625	0.0002
Be2	0.3129(6)	0.3125	0.0004
O1	0.3730(6)	0.3750	−0.0020
O2	0.3704(4)	0.3750	−0.0046
O3	0.1270(3)	0.1250	0.0020

Figure 8. Deviation of the z coordinate from the ideal position (Δz) against the valence state of each ion [◆, this study; , Huminicki and Hawthorne (2001)]. The upper and lower sections of the plot represent positively and negatively charged, respectively.

CONCLUSIONS

A swedenborgite sample obtained from the type locality, Långban, Värmland, Sweden, was subjected to various analytical techniques to determine its chemical composition. XAFS analysis showed that Sb component is mainly pentavalent, although some trivalent Sb species are also present. Chemical analyses revealed that swedenborgite can be divided into two compositional regions: a pure zone (NaBe₄SbO₇) and a zone containing Ca [i.e., (Na_1−xCa_x)Be₄Sb⁵⁺_1−0.5xSb³⁺_−0.5xO₇ (x = 0.05-0.07)]. As a result, the following substitution relationship was proposed: 2Na⁺ + Sb⁵⁺ ⇄ 2Ca²⁺ + Sb³⁺, where charge balance is achieved by reduction of some Sb⁵⁺ ions. It was also found that the blue fluorescence emitted from the Ca-containing zones appeared to be stronger than that observed for the pure zones. Furthermore, the distortions along the c-axis of the Be-O distances and the O-Be-O angles of the BeO₄ trigonal pyramid in swedenborgite were significantly larger than those in BeO bromellite. Moreover, opposite coordinate shifts between the cations and anions occur along the c-axis due to the asymmetric arrangement around the NaO₁₂ tetradecahedra with upper face sharing and lower edge sharing. As a result, swedenborgite possesses a charge bias on the upper and lower sides along the c-axis, which induces spontaneous polarization.

ACKNOWLEDGMENTS

This study was performed under the auspices of the Photon Factory (PAC No. 2021G505). This research was partially supported by the JSPS KAKENHI (Grant Number JP23K03530).

SUPPLEMENTARY MATERIAL

Supplementary CIF file is available online from https://doi.org/10.2465/jmps.240209.

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