2023 年 118 巻 1 号 論文ID: 230605
Kiryuite and gunmaite were found as new minerals from Tsukubara, Kiryu City, Gunma Prefecture, Japan. Kiryuite is a Mn-rich analogue of viitaniemiite with an ideal formula of NaMnAl(PO4)F3 that mainly occurs as a white powder in cracks in triplite aggregate and occasionally forms porous plate-like aggregates up to 5 mm in size, while the grains are several micrometers in size. In most cases, the aggregates are also accompanied by small amounts of other minerals such as goyazite, gorceixite, and fluorite. Kiryuite shows white streaks with a vitreous luster, and its Mohs hardness is estimated to be 5 by analogy with viitaniemiite. The calculated density is 3.32 g·cm−3 based on the empirical formula and unit cell volume refined from powder X-ray diffraction (XRD) data. The empirical formula for kiryuite calculated on the basis of O = 4 and F + OH = 3 is Na0.97(Mn0.56Ca0.38Fe0.04Mg0.02)Σ1.00Al0.98P1.02O4(F2.29OH0.71)Σ3. Kiryuite is monoclinic (P21/m) with a = 5.425(4) Å, b = 7.128(4) Å, c = 6.817(6) Å, β = 109.41(7)°, and V = 248.7(3) Å3 (Z = 2). The parameters [d in Å (I/I0) hkl] for the six strongest lines associated with kiryuite in the powder XRD pattern are 3.123 (57) 002, 2.923 (53) 012 and 120, 2.877 (100) 121, 2.560 (27) 200, 2.263 (43) 103, and 2.155 (76) 221. Gunmaite is a new alunite-related mineral with a new structure type that has an ideal formula of (Na2Sr)Sr2Al10(PO4)4F14(OH)12. Gunmaite mainly occurs as a core in hexagonal tablet crystals that coexist with gorceixite and goyazite-like minerals. Gunmaite is colorless and transparent with a greasy to resinous luster and is non-fluorescent. As a bulk crystal including gunmaite, the Mohs hardness is 5, cleavage is perfect on {001} planes, and the tenacity is brittle. The calculated density based on the empirical formula is 3.38 g·cm−3 using the empirical formula and single-crystal XRD data. The empirical formula for gunmaite calculated on the basis of O = 16 and F + OH = 26 is (Na1.72Sr0.70Mg0.56Ca0.01)Σ2.99(Sr1.32Ba0.68)Σ2(Al9.82Mg0.18)Σ10P3.99O16F16.21(OH)9.79. Gunmaite is trigonal (R3m) with a = 6.9972(2) Å, c = 50.270(2) Å, and V = 2131.51(13) Å3 (Z = 3). The structure consists of two PO4, AlO2(OH)4, AlF6, AlOF3(OH)2, SrO6(OH)6, and NaF8 polyhedra. Kiryuite and gunmaite are products of the final stage of hydrothermal activity in association with greisenization.
Phosphate minerals often occur as minor accessory constituents in a wide range of magmatic and hydrothermal systems, and are thus expected to be an important key to understanding evolution and metasomatic processes. Viitaniemiite, NaCaAl(PO4)F3, is a rare phosphate and was initially discovered in granitic pegmatite in the Eräjärvi district of Finland (Lahti, 1981). Viitaniemiite is considered to be a product of high-F activity and has been found in pegmatite, topaz-zinnwaldite granite, alkali silicocarbonatites, W-Sn-bearing greisen, and hydrothermal quartz vein (e.g., Ramik et al., 1983; Liferovich et al., 1998; Petrík et al., 2011; Shirose and Uehara, 2014; Števko et al., 2015, 2018). Although viitaniemiite from alkali silicocarbonatites is close to the end-member composition, other occurrences occasionally contain significant amounts of Mn close to Ca as a second component. For example, the Mn/(Mn + Ca) ratio for viitaniemiite reaches 0.43 in the hydrothermal quartz vein in association with S-type granite, located about 10 km north-west of Rožňava, Gemeric Unit, Slovak Republic (Števko et al., 2015).
Alunite supergroup minerals are also known to occur in a variety of environments, and several phosphate minerals are members of this group. Goyazite, SrAl3(PO4)(PO3OH)(OH)6, is one of the members in the alunite supergroup and has been found in various occurrences such as in pegmatite, carbonatite, soil, coal deposits, salt deposits, and brazing deposits (e.g., Liferovich et al., 1998, 1999; Matsubara and Kato, 1998; Lykova et al., 2020; Pekov et al., 2020; Yuste et al., 2020; Garate-Olave et al., 2021; Di et al., 2022). Goyazite commonly forms solid solutions with crandallite and gorceixite as Ca and Ba varieties, respectively (e.g., Liferovich et al., 1998). Ohara and Kyono (2021) recently reported the occurrence of goyazite in a hydrothermal quartz vein from Tsukubara, Kiryu City, Gunma Prefecture, Japan. This goyazite contained little Ca and was characterized by Sr-Ba zoning. We have studied the area around this quartz vein in detail and found two new unique quartz veins.
One quartz vein includes phosphate minerals, and viitaniemiite was found in association with triplite. Viitaniemiite in this quartz vein is often rich in Mn, and further investigation confirmed the presence of Mn-dominant viitaniemiite as a new mineral. The other quartz vein also includes phosphate minerals that are similar in composition to goyazite, although it is characterized by high Na and F contents. Further investigation revealed that this is also a new mineral with a new structure and chemical composition related to goyazite. The new Mn-dominant viitaniemiite and goyazite-related new minerals were named kiryuite (2021-041) and gunmaite (2022-080), respectively, after the locality. The mineral data and names for both new minerals have been approved by the International Mineralogical Association (IMA), Commission on New Minerals, Nomenclature, and Classification. A holotype specimen has been deposited in the collection of the National Museum of Nature and Science, Japan (NSM M-48724 and 49762 for kiryuite and gunmaite, respectively). Here, we describe the new minerals, kiryuite and gunmaite.
The Ashio Mountains rise in the northern part of the Kanto region of Japan, from northeastern Gunma Prefecture to southwestern Tochigi Prefecture, and the Jurassic accretionary complex is widely distributed throughout. The tectonostratigraphic classification from the lower to upper levels is the Kurohone-Kiryu Complex, the Omama Complex, the Kuzu Complex, and the Gyodosan Complex (Ito, 2021), and these complexes are composed mainly of mudstone, sandstone, and chert with siliceous claystone. Siliciclastic igneous rocks of Cretaceous to Paleogene age are also distributed throughout. The Kubogahara granodiorite, the Sori granodiorite, and the Ashikaga granodiorite are intruded in the northern part, the northwestern part, and the south part in the Ashio Mountains, respectively. Their influence is recognized as contact metamorphism and quartz veins in the accretionary complex.
The Ashio Mountains are also well known as one of the major manganese deposit areas in Japan, and many manganese deposits have been widely found in the chert layer (e.g., Kawata and Ozawa, 1954; Sudo et al., 1991). With reference to the 1:200000 geological map of this area, there is only one tungsten deposit marked in the location that corresponds to Tsukubara, Kiryu City, Gunma Prefecture (Sudo et al., 1991). There is only a brief mention of this tungsten mine in the course of a survey of uranium deposits; however, no details are available (Hayashi and Igarashi, 1962). Therefore, one of the authors (Y. Ohara) started a survey in 2013 to confirm the authenticity of the mine and found several abandoned tunnels. He noticed that tungsten resources were already only traces, although the tunnels had been developed along the quartz veins in association with small amounts of phosphate minerals (Ohara and Kyono, 2021). A further field survey conducted by two of the authors (I. I. and Y. Ohara) revealed one unique quartz vein (#1 vein) (36°30′41′′N 139°25′26′′E) apart from the abandoned mine. Two other authors (Y. Ohki and H. H.) later joined the survey and found another separate quartz vein (#2 vein) at a distance of approximately 60 m.
The #1 vein was 10-30 cm in width and crossed the sandstone almost horizontally. The #1 vein was exposed for a distance of approximately 10 m, where a white sericite-rich part and a black manganese-rich part were observed. An aggregate of muscovite + quartz with a size of ∼ 10 cm was observed inside the sericite-rich part, and triplite was also found inside the black manganese-rich part. Triplite is irregularly shaped and is typically finely fractured, with fluorite, fluorapatite, kiryuite, viitaniemiite, goyazite, and gorceixite filling the cracks. Fluorapatite, goyazite, gorceixite, and fluorite were also evident as euhedral crystals with sizes of a few hundred micrometers in the spaces between the quartz crystals. Small amounts of hübnerite and arsenopyrite were also found in the vein.
The #2 vein also occurs on the same scale as the #1 vein, is 10-30 cm in width and also crosses the sandstone almost horizontally; the two veins are separated by a distance of about 60 m. This vein is very similar to the sericite-rich part in the #1 vein. However, the #2 vein includes small gray-colored aggregates that consist of topaz and quartz. Hexagonal tablet crystals including gunmaite were found in the cavities between the quartz crystals in this vein.
Kiryuite mainly occurs as a white powder in the cracks of triplite crystals and along the triplite-quartz boundary (Fig. 1). Kiryuite occasionally forms porous plate-like aggregates up to 5 mm in size, while the grain size is several micrometers. Therefore, the aggregates are observed as turbid under an optical microscope due to the grain boundary effect. In most cases, the aggregates are also accompanied by small amounts of other minerals such as goyazite, gorceixite, and fluorite, and kiryuite-only assemblages are rarely found. As a result, fragments suitable for optical measurement and single-crystal structure analysis could not be obtained. Kiryuite has white streaks with a vitreous luster, and its Mohs hardness is estimated to be 5, which is analogous to that for viitaniemiite. The calculated density is 3.32 g·cm−3 based on the empirical formula and unit cell volume refined from powder X-ray diffraction (XRD) data. Although the optical properties were not measured due to the small grain size, the mean refractive index obtained from the Gladstone-Dale relationship (Mandarino, 1981) using the empirical formula and calculated density was 1.578. Kiryuite and viitaniemiite are indistinguishable in terms of occurrence and appearance and are only distinguishable by chemical analysis.
Chemical analyses were conducted using scanning electron microscopy (SEM; JEOL IT-100) equipped with energy dispersive X-ray spectroscopy (EDS; 15 kV, 0.8 nA, 1 µm beam diameter). The ZAF method was used for data correction, and the standards used were CaF2, NaAlSi3O8, MgO, Al2O3, LaP5O14, CaSiO3, MnSiO3, and FeS2 for F, Na, Mg, Al, P, Ca, Mn, and Fe, respectively. The representative chemical composition of kiryuite was obtained as the average of 14 analyses from a single aggregate in the type specimen because all analysis points were within kiryuite (Table 1 and Fig. 2). Since insufficient material was available for direct determination, the H2O wt% was calculated based on the stoichiometry. The empirical formula for kiryuite calculated on the basis of O = 4 and F + OH = 3 is Na0.97(Mn0.56Ca0.38Fe0.04Mg0.02)Σ1.00Al0.98P1.02O4(F2.29OH0.71)Σ3. The empirical formula is Na(Mn,Ca)AlPO4(F,OH)3, and the ideal formula is NaMnAl(PO4)F3, which requires P2O5 27.63, Al2O3 19.85, MnO 27.61, Na2O 12.06, F 22.19, -O=F −9.34, and total 100 wt%. Another aggregate (aggregate #2 in Fig. 2) was also analyzed in a similar way, but the analysis points lay in the viitaniemite region (Fig. 2).
| Aggregate #1(type specimen) | Aggregate #2 | ||||||
| Kiryuite | Viitaniemiite | ||||||
| wt% (n = 14) | wt% (n = 8) | ||||||
| Avg. | Min. | Max | Avg. | Min. | Max | ||
| Na2O | 11.99 | 11.66 | 12.28 | Na2O | 12.14 | 11.52 | 12.59 |
| CaO | 8.54 | 6.51 | 10.44 | CaO | 11.90 | 10.28 | 13.26 |
| MnO | 15.88 | 13.74 | 18.71 | MnO | 12.97 | 11.11 | 14.37 |
| FeO | 1.20 | 0.89 | 1.37 | FeO | 0.47 | nd | 0.87 |
| MgO | 0.25 | 0.03 | 0.83 | MgO | 0.06 | nd | 0.18 |
| Al2O3 | 19.98 | 19.57 | 20.37 | Al2O3 | 20.26 | 19.70 | 21.15 |
| P2O5 | 28.72 | 27.99 | 29.17 | P2O5 | 28.57 | 27.62 | 29.40 |
| F | 17.37 | 16.52 | 18.27 | F | 17.48 | 15.95 | 18.58 |
| H2O* | 2.53 | H2O* | 2.53 | ||||
| -O=F | 7.31 | -O=F | 7.36 | ||||
| Total | 100.13 | Total | 99.02 | ||||
| apfu | apfu | ||||||
| Na | 0.97 | Na | 0.98 | ||||
| Ca | 0.38 | Ca | 0.53 | ||||
| Mn | 0.56 | Mn | 0.46 | ||||
| Fe | 0.04 | Fe | 0.02 | ||||
| Mg | 0.02 | Mg | 0.00 | ||||
| Σ | 1.00 | Σ | 1.01 | ||||
| Al | 0.98 | Al | 0.99 | ||||
| P | 1.02 | P | 1.01 | ||||
| O = | 4 | O = | 4 | ||||
| F | 2.29 | F | 2.30 | ||||
| OH | 0.71 | OH | 0.70 | ||||
| Σ = | 3 | Σ = | 3 | ||||
*Calculated value from stoichiometry
A micro-XRD measurement technique was applied to powder XRD measurements for kiryuite using a diffractometer (Rigaku Ultrax18, CrKα radiation, 40 kV, 200 mA, 100 µm collimator) equipped with a curved position sensitive proportional counter and an oscillating sample stage. Figure 3 shows a powder XRD pattern for kiryuite, which was successfully obtained using the sample shown in Figure 1, and the results are summarized in Table 2. Although goyazite peaks were present in the diffraction pattern due to the close relationship of occurrence, peaks for kiryuite could be uniquely identified. Since kiryuite is a Mn-rich analogue of viitaniemiite, the diffraction peaks were indexed with reference to the crystal structure and space group (P21/m) of viitaniemiite described by Pajunen and Lahti (1984). The unit cell parameters, as refined from the powder XRD data, are a = 5.425(4) Å, b = 7.128(4) Å, c = 6.817(6) Å, β = 109.41(7)°, and V = 248.7(3) Å3 (Z = 2). The parameters [d in Å (I/I0) hkl] for the six strongest lines of kiryuite in the powder XRD pattern were 3.123(57) 002, 2.923(53) 012 and 120, 2.877(100) 121, 2.560(27) 200, 2.263(43) 103, and 2.155(76) 221.
| hkl | I/I0 | Icalc. | dobs. (Å) | dcalc. (Å) |
| 001 | 6 | 6.430 | ||
| 100 | 3 | 5.117 | ||
| 101 | 21 | 4.869 | ||
| 011 | 14 | 4.774 | ||
| 110 | 1 | 4.157 | ||
| 111 | 13 | 8 | 4.008 | 4.021 |
| 020 | 5 | 5 | 3.560 | 3.564 |
| 101 | 9 | 3.480 | ||
| 102 | 2 | 3.252 | ||
| 002 | 57 | 37 | 3.213 | 3.215 |
| 111 | 26 | 16 | 3.124 | 3.127 |
| 021 | 3 | 3.117 | ||
| 112 | 13 | 2.959 | ||
| 012 | 53 | 21 | 2.923 | 2.931 |
| 120 | 53 | 22 | 2.923 | 2.925 |
| 121 | 100 | 100 | 2.877 | 2.876 |
| 201 | 22 | 6 | 2.707 | 2.706 |
| 200 | 57 | 28 | 2.560 | 2.559 |
| 211 | 1< | 2.530 | ||
| 121 | 17 | 20 | 2.493 | 2.490 |
| 202 | 2 | 2.435 | ||
| 210 | 13 | 4 | 2.404 | 2.408 |
| 122 | 1< | 2.402 | ||
| 102 | 22 | 14 | 2.391 | 2.388 |
| 022 | 1< | 2.387 | ||
| 212 | 27 | 10 | 2.305 | 2.304 |
| 112 | 1 | 2.264 | ||
| 103 | 43 | 15 | 2.263 | 2.263 |
| 031 | 13 | 2 | 2.279 | 2.229 |
| 113 | 1< | 2.157 | ||
| 221 | 76 | 29 | 2.155 | 2.155 |
The crystal structure was also investigated using transmission electron microscopy (TEM; JEOL-2100, operated at 200 kV). The selected area electron diffraction patterns (SAED) were consistent with the XRD results and revealed no superlattice reflections or streaks, while 010 reflection appeared due to multiple reflections (Fig. 4). Therefore, kiryuite can be treated as a simple Mn-rich analogue of viitaniemiite.
The effect of Mn substitution is a decrease in the a-, b-, and c-axis lengths and the unit cell volume (Fig. 5). The compression of each axis and the volume reduction are related to the increased amount of Mn at the octahedral sites. On the other hand, the compression is slightly anisotropic, probably because Mn-centered octahedra have both compressible and less compressible bonds. The M-F bond lengths are typically shorter than those of M-O and are therefore considered to be more incompressible. The slight increase in monoclinic distortion with increasing in Mn content may be due to this difference in compressibility.
Kiryuite, NaMnAl(PO4)F3, corresponds to a Mn-dominant analogue of viitaniemiite, NaCaAl(PO4)F3. Both kiryuite and viitaniemiite have a very low content of in divalent cations other than Ca and Mn. Therefore, it is difficult to estimate whether there could be other variants besides these. In terms of anion content, Pajunen and Lahti (1984) reported an ideal ratio of 2F + (OH), not 3F, from the results of a single-crystal structural analysis of viitaniemiite, while the current IMA formula for viitaniemiite is set at 3F. The analytical value for kiryuite is also close to 2F + (OH), which is similar to the results of Pajunen and Lahti (1984); however, we were unable to follow up on this result by structural analysis due to the occurrence of powder-like kiryuite. Further study is thus required to determine the ideal anion ratio for both minerals.
Gunmaite mainly occurs as the core in hexagonal tablet crystals, and gorceixite and goyazite-like mineral with a zonal texture due to the Sr-Ba content are often present at the rim (Fig. 6). The outermost rim in the yellow crystal consists of kintoreite or segnitite. The relationship between the core and rim is occasionally reversed, with gunmaite located at the rim. The diameter of the hexagonal crystals can be up to 500 µm, with a thickness of less than 100 µm in many cases. The gunmaite areas are euhedral to subhedral and up to 200 micrometers in diameter. Gunmaite is colorless and transparent with a greasy to resinous luster and is non-fluorescent. For bulk crystals of gunmaite, the Mohs hardness is 5, cleavage is perfect on {001} planes, and the tenacity is brittle. The calculated density is 3.38 g·cm−3 using the empirical formula and single-crystal XRD data. Although the optical properties were not measured due to complex occurrence, the mean refractive index obtained from the Gladstone-Dale relationship (Mandarino, 1981) using the empirical formula and calculated density is 1.557.
Chemical analyses were conducted using SEM (JEOL IT-100) together with EDS (15 kV, 0.35 nA, 5 µm beam diameter). Gunmaite is sensitive to electron beams and prone to Na migration for high beam currents and small beam diameters. The ZAF method was used for data correction. The following standards were used: CaF2, NaAlSi3O8, MgO, Al2O3, KTiPO5, CaSiO3, SrSO4, and BaSO4 for F, Na, Mg, Al, P, Ca, Sr, and Ba, respectively. Since insufficient material was available for direct determination, the H2O wt% was calculated based on the stoichiometry estimated by structural analysis. The representative chemical composition of gunmaite was obtained as the average of 8 analyses from a single crystal (Table 3). The empirical formula of gunmaite calculated on the basis of O = 16 and F + OH = 26 is (Na1.72Sr0.70Mg0.56Ca0.01)Σ2.99(Sr1.32Ba0.68)Σ2(Al9.82Mg0.18)Σ10P3.99O16F16.21(OH)9.79. The empirical formula is [Na2(Sr,Mg)](Sr,Ba)2Al10(PO4)4F14(OH,F)12, and the ideal formula is (Na2Sr)Sr2Al10(PO4)4F14(OH)12, which requires Na2O 4.34, SrO 21.76, Al2O3 35.69, P2O5 19.87, F 18.62, H2O 7.57, -O=F 7.84, total 100 wt%.
| wt% (n = 8) | apfu | ||||
| Avg. | Min. | Max | Na | 1.72 | |
| Na2O | 3.61 | 2.93 | 4.26 | Sr | 0.70 |
| SrO | 14.19 | 13.41 | 14.81 | Mg | 0.56 |
| BaO | 7.04 | 6.33 | 8.21 | Ca | 0.01 |
| CaO | 0.02 | nd | 0.11 | Σ | 2.99 |
| MgO | 2.05 | 1.59 | 2.34 | ||
| Al2O3 | 33.94 | 32.12 | 35.34 | Sr | 1.32 |
| P2O5 | 19.19 | 18.31 | 19.90 | Ba | 0.68 |
| F | 20.88 | 19.13 | 23.54 | Σ= | 2 |
| H2O* | 5.98 | ||||
| -O=F | 8.79 | Al | 9.82 | ||
| Total | 98.10 | Mg | 0.18 | ||
| Σ= | 10 | ||||
| P | 3.99 | ||||
| O = | 16 | ||||
| F | 16.21 | ||||
| OH | 9.79 | ||||
| Σ = | 26 | ||||
*Calculated value from stoichiometry
A single-crystal XRD analysis was conducted using a Rigaku R-AXIS RAPID microdiffractometer equipped with a curved imaging plate detector and monochromatized MoKα radiation. The Rigaku RAPID AUTO software package was used to process the diffraction data, including the application of a numerical absorption correction. The crystal structure was solved by the direct methods and refined by the full-matrix least squares technique using the Jana2006 program (Petříček et al., 2014). Several crystals were selected and checked for their quality. However, the amount of sample was limited, and even selected crystals contained stacking faults (Fig. 7). As a result, it was not possible to obtain a high-quality crystal that could be analyzed in detail. Therefore, the analysis was conducted with the goal of building a structural model, and in the process, the temperature factors were refined isotropically. The chemical formula is fixed with reference to the value obtained by the chemical analysis, and the presence of Mg at the Al site is not considered due to the close atomic number for these atoms. This analysis resulted in a convergence of R1 to 6.4%, which is sufficient for constructing a structural model. Details of the data collection are listed in Table 4 and the CIF file (Supplementary CIF file available online from https://doi.org/10.2465/jmps.230605). The final atom coordinates with isotropic atomic parameters are summarized in Table 5. The refined unit cell parameters are a = 6.9972(2), c = 50.270(2), V = 2131.51(13) Å3, and Z = 3 with the trigonal R3m (#166) space group.
| Chemical formula | (Na1.74Sr0.69Mg0.57)(Sr1.32Ba0.68)Al10P4O28F14H12 |
| Temperature | 293 K |
| Radiation | MoKα |
| Crystal size | 0.17 × 0.12 × 0.02 mm |
| Space group | R3m (#166) |
| Unit cell dimensions | a = 6.9972(2), c = 50.270(2) Å |
| Volume | V = 2131.51(13) Å3 |
| Z | 3 |
| F(000) | 2077 |
| Absorption coefficient μ (MoKα) | 5.48 mm−1 |
| Diffractometer | R-AXIS RAPID |
| Voltage, Current | 50 kV, 40 mA |
| θ max | 27.5° |
| No. of measured, independent and observed [I > 3σ(I)] reflections |
6945, 354, 293 |
| R1[F2 > 2σ(F2)], wR2(F2), S | 0.0635, 0.0793, 3.88 |
| Δρmax | 8.80 (0.332 Å from the O1 site) |
| Δρmin | −2.56 (0.430 Å from the P2 site) |
| Atom | Site | x | y | z | 100Uiso |
| Na1 | 9e | 0.5 | 0 | 0 | 4.68 (14) |
| Sr1 | 6c | 2/3 | 1/3 | 0.22197 (3) | 1.66 (6) |
| Al1 | 9d | 1/3 | 1/6 | 1/6 | 1.05 (10) |
| Al2 | 3a | 2/3 | 1/3 | 1/3 | 1.62 (18) |
| Al3 | 18h | 0.4986 (3) | 0.5014 (3) | 0.27617 (6) | 1.40 (8) |
| P1 | 6c | 0 | 0 | 0.21147 (10) | 1.20 (10) |
| P2 | 6c | 1/3 | 2/3 | 0.22925 (11) | 1.82 (12) |
| O1 | 6c | 1/3 | 2/3 | 0.1997 (2) | 0.83 (7) |
| O2 | 18h | 0.2425 (11) | 0.1213 (6) | 0.20168 (14) | 0.83 (7) |
| O3 | 18h | 0.4525 (6) | 0.5475 (6) | 0.24048 (14) | 0.83 (7) |
| O4 | 6c | 0 | 0 | 0.2430 (2) | 0.83 (7) |
| O5 | 18h | 0.5430 (7) | 0.0860 (13) | 0.17695 (15) | 0.83 (7) |
| O6 | 18h | 0.3910 (12) | 0.1955 (6) | 0.26750 (14) | 0.83 (7) |
| F1 | 18h | 0.5362 (5) | 0.4638 (5) | 0.31174 (11) | 0.83 (7) |
| F2 | 18h | 0.5708 (9) | 0.7854 (5) | 0.28398 (12) | 0.83 (7) |
| F3 | 6c | 2/3 | 1/3 | 0.01752 (19) | 0.83 (7) |
| H1o5 | 18h | 0.496486 | −0.007028 | 0.188795 | 0.99 |
| H1o6 | 18h | 0.278524 | 0.139262 | 0.258439 | 0.99 |
| Na1, 0.55Na + 0.23Sr + 0.19Mg | |||||
| Sr1, 0.66Sr + 0.34Ba | |||||
In the powder XRD pattern, it was not possible to unequivocally index the gunmaite powder pattern, because of the presence of peaks due to gorceixite and goyazite-like minerals, which have common positions due to structural similarity. Instead, calculation based on single-crystal refinement results is more reliable. Thus, powder XRD patterns with intensities greater than 1 after rounding were calculated using the ReciPro program (Seto and Ohtsuka, 2022) assuming Bragg Brentano optics with a CuKα source. The calculation results are listed in Table 6. The parameters [d in Å (I/I0) hkl] for the seven strongest lines of gunmaite in the powder XRD pattern are 5.458 (62) 104, 3.499 (24) 110, 3.425 (23) 113, 2.965 (100) 119, 2.138 (32) 1022, 1.900 (38) 039, and 1.749 (43) 220. The 110, 119, 033, and 220 diffraction peaks for gunmaite overlap with the 110, 119, 039, and 220 diffraction peaks for gorceixite. The main diffraction peaks for the goyazite-like mineral have the same d values, but indexing has not yet been achieved because the structure is still under investigation.
| hkl | Icalc. | dcalc. (Å) | hkl | Icalc. | dcalc. (Å) |
| 003 | 3 | 16.757 | 033 | 1 | 2.005 |
| 006 | 1 | 8.378 | 303 | 3 | 2.005 |
| 101 | 23 | 6.016 | 0,2,19 | 6 | 1.993 |
| 104 | 4 | 5.891 | 1,1,21 | 1 | 1.976 |
| 009 | 3 | 5.586 | 2,1,13 | 7 | 1.971 |
| 104 | 62 | 5.458 | 036 | 1 | 1.964 |
| 015 | 2 | 5.190 | 306 | 2 | 1.964 |
| 1,0,10 | 5 | 3.869 | 2,0,20 | 2 | 1.934 |
| 110 | 24 | 3.499 | 1,2,14 | 1 | 1.931 |
| 113 | 23 | 3.425 | 039 | 38 | 1.900 |
| 1,0,13 | 2 | 3.260 | 309 | 2 | 1.900 |
| 116 | 13 | 3.228 | 0,0,27 | 1 | 1.862 |
| 0,1,14 | 4 | 3.089 | 0,1,26 | 1 | 1.842 |
| 119 | 100 | 2.965 | 0,2,22 | 3 | 1.824 |
| 024 | 19 | 2.945 | 0,3,12 | 2 | 1.819 |
| 205 | 4 | 2.901 | 3,0,12 | 1 | 1.819 |
| 0,0,18 | 19 | 2.793 | 1,2,17 | 2 | 1.811 |
| 027 | 10 | 2.792 | 1,1,24 | 1 | 1.797 |
| 1,0,16 | 2 | 2.789 | 220 | 43 | 1.749 |
| 208 | 5 | 2.729 | 2,1,19 | 2 | 1.732 |
| 1,1,12 | 11 | 2.685 | 3,0,15 | 1 | 1.730 |
| 0,1,17 | 1 | 2.658 | 0,3,15 | 1 | 1.730 |
| 0,2,10 | 21 | 2.595 | 1,0,28 | 1 | 1.721 |
| 1,0,19 | 12 | 2.425 | 131 | 2 | 1.680 |
| 1,1,15 | 5 | 2.420 | 0,2,25 | 5 | 1.675 |
| 2,0,14 | 6 | 2.316 | 229 | 2 | 1.669 |
| 211 | 2 | 2.288 | 134 | 1 | 1.666 |
| 214 | 6 | 2.253 | 1,1,27 | 4 | 1.644 |
| 125 | 23 | 2.233 | 0,3,18 | 1 | 1.637 |
| 1,1,18 | 3 | 2.183 | 3,0,18 | 1 | 1.637 |
| 0,2,16 | 5 | 2.181 | 2,0,26 | 4 | 1.630 |
| 128 | 5 | 2.152 | 2,1,22 | 2 | 1.618 |
| 1,0,22 | 32 | 2.138 | 1,0,31 | 1 | 1.566 |
| 2,0,17 | 7 | 2.116 | 0,2,28 | 7 | 1.545 |
| 2,1,10 | 2 | 2.084 | 3,0,21 | 1 | 1.544 |
| 0,1,23 | 3 | 2.056 | 3,1,14 | 7 | 1.522 |
| 1,2,11 | 1 | 2.048 | 0,1,32 | 1 | 1.521 |
Figure 8 shows the structure model for gunmaite illustrated using VESTA (Momma and Izumi, 2011). Although the crystal structure is novel, it can be understood starting from goyazite, belonging to the alunite group. The gunmaite structure is obtained by taking two parts from the goyazite structure, rearranging them, replacing some locations with octahedra, and inserting Na sites in the gaps. Conclusively, gunmaite is an alunite-related mineral.
The structure of gunmaite requires a total of 42 anions consisting of 16O2− and 26(F + OH)− based on a cation ratio of (2Na + 3Sr + 10Al + 4P)58+. To verify the appropriateness of the H positions, the H atoms were placed at geometrically idealized positions. The results showed that the F1 to the F3 sites were not preferred for H, because the two H atoms were too close together or the O-H…O arrangement was impossible. Therefore, F was preferentially allocated to these sites, which consumed 14F. Two sites with a multiplicity of 18, such as the O2, the O3, the O5, and the O6 sites, were thus required to consume the remaining 12OH. However, the O2 and the O3 sites are located at two or three corners in a P-centered tetrahedron, and there is no data for PO2(OH)2 or PO(OH)3 tetrahedra in the mineral. Moreover, based on the results of the analysis, a small amount of F still remains that must be accommodated at the OH sites. F placement around P is also not preferential. Therefore, H was eventually assigned to the O5 and the O6 sites around Al and Sr, where 12OH is consumed, for a total allocation of 26(F + OH). The O1 and the O4 sites function as acceptors for O5-H and O6-H, respectively. On the other hand, since gunmaite contains heavy atoms (Sr and Ba) and the crystal quality was poor, it was not possible to estimate the position of H from the residual electron density. Therefore, the tentative H positions presented in Table 5 are geometrically idealized positions. Also, the model predicts a Sr-H distance (2.65 Å) that is slightly shorter than the value of 2.98 Å for goyazite estimated by Kato (1987). This issue needs to be addressed after obtaining high-quality crystals in the future.
The detailed structure of gunmaite is shown in Figure 9, and the bond valence sum calculation results are shown in Table 7. The structure consists of two PO4, AlO2(OH)4, AlF6, AlOF3(OH)2, SrO6(OH)6, and NaF8 polyhedra. The center of the structure is an Al1-centerd octahedron, and the octahedra form layers in the a-b plane direction by sharing O5 between them. The Al3-centered octahedra also form layers in the a-b plane direction, and an Al2-centered octahedron connects the two Al3-centered octahedral layers. The Sr1 site is located between the Al1- and Al3-centered octahedral layers and is 12-coordinated. The Sr1-centered polyhedron shares O2-O5 and O5-O5 edges with the Al1-centered octahedron and shares O3-O6 and O6-O6 edges with the Al3-centered octahedron. There is no bond between Sr1-sentered octahedra, but they are coplanar in the a-b plane. PO4 tetrahedra are also located in the same layer as Sr1-centered polyhedra, with P1-centered tetrahedra linking to Al1-centered octahedra and Sr1-centered polyhedra by sharing O2, and P2-centered tetrahedra linking to Al3-centered octahedra and Sr-centered polyhedra by sharing O3. The gap created by linking the two parts of the goyazite structure contains Na1 and Al2 sites and corresponds to the space between the two Al3-centered octahedral layers. The Na1-centered polyhedron forms a hexagonal bipyramid polyhedron with eight F, which are connected to each other in the a-b in-plane direction by sharing F1-F3 edges, and are connected to the Al2-centered octahedra located in the same layer by sharing F1-F1 edges. The Na-centered polyhedron shares the F1-F3 edge with the Al3-centered octahedron.
| Na1 | Sr1 | Al1 | Al2 | Al3 | P1 | P2 | Sum | Assignment | |
| O1 | 1.43 | 1.43 | O+H | ||||||
| O2 | 0.20 ×3↓ |
0.59 ×2↓ |
1.20 ×3↓ |
2.00 | O | ||||
| O3 | 0.21 ×3↓ |
0.54 | 1.20 ×3↓ |
1.94 | O | ||||
| O4 | 1.09 | 1.09 | O+H | ||||||
| O5 | 0.23 ×3↓ |
0.53 ×4↓×2→ |
1.29 | OH | |||||
| O6 | 0.17 ×3↓ |
0.47 ×2↓→ |
1.11 | OH | |||||
| F1 | 0.12 ×4↓ |
0.36 ×6↓ |
0.44 | 0.93 | F | ||||
| F2 | 0.11 ×2↓ |
0.46 ×2↓→ |
1.03 | F | |||||
| F3 | 0.28 ×2↓×3→ |
0.85 | F | ||||||
| Sum | 1.27 | 2.43 | 3.29 | 2.19 | 2.85 | 4.69 | 5.02 | ||
| Ideal | 1.33 | 2 | 3 | 3 | 3 | 5 | 5 |
Note: O+H-atom functions as accepter for the three hydrogen bond.
Gunmaite is a novel alunite-related mineral with the formula (Na2Sr)Sr2Al10(PO4)4F14(OH)12. There also appears to be Sr-Mg and Sr-Ba substitutions at Na and Sr sites, respectively, to yield [Na2(Sr1−xMgx)](Sr1−yBay)2Al10(PO4)4F14(OH)12. Based on the structural analysis, the Al2 sites have a slightly larger Al-anion distance than that for the other Al sites, which results in a smaller bond valence sum, so that a larger cation than Al can be accommodated; this is probably Mg, based on the compositional analysis. If the Al2 site is occupied by Mg, then the chemical formula is [Na(Sr1−xMgx)2](Sr1−yBay)2(Al9Mg)(PO4)4F14(OH)12. In relation to the P-centered tetrahedron, the O1 and the O4 sites have 6c multiplicity and are not shared with other cations. This has the potential to form a PO3(OH) tetrahedron by the attachment of hydrogen. However, the O1 and the O4 sites already function as acceptors for O5-H and O6-H, respectively; therefore, it is unlikely that a new hydrogen atom will be attached. In conclusion, the following two roots, including two variants, are assumed for the gunmaite group.
Root#1 (gunmaite): [Na2(Sr1−xMgx)](Sr1−yBay)2Al10(PO4)4F14(OH)12 (possible magnesium and/or barium variants).
Root#2: [Na(Sr1−xMgx)2](Sr1−yBay)2(Al9Mg)(PO4)4F14(OH)12 (possible magnesium and/or barium variants).
Among the phosphate minerals in quartz veins that include kiryuite and viitaniemiite, it is clear that the prior mineral is triplite. Triplite formed at high temperatures becomes unstable and is decomposed by cracking after formation. The cracks are subsequently filled by kiryuite (viitaniemiite) as the decomposition product of triplite (Fig. 1). Similar quartz veins containing triplite and viitaniemiite were reported in Gemerská Poloma, Slovakia (Števko et al., 2015). In both localities, fluorite, topaz, and apatite occur, which implies high P and F activity (e.g., Webster et al., 1998). Gunmaite is also a F-bearing phosphate mineral. Another feature of the quartz veins is that they are very low in feldspars and instead contain large amounts of muscovite; they also contain hübnerite. These features suggest that the quartz vein occurred in association with greisenization. The area under study had quartz veins that were once developed for tungsten resources (Hayashi and Igarashi, 1962; Sudo et al., 1991). These unique quartz veins are probably derived from granodiorite such as the Souri body (5 km to the north) or the Ashikaga body (10 km to the south), which were formed in the Late Cretaceous.
The authors thank Go Ito and Shusuke Takahashi for providing bibliographic information regarding the abandoned tungsten mine at Tsukubara. Chemical analyses, single-crystal XRD measurements, and powder XRD measurements were performed at the Institute for Solid State Physics, The University of Tokyo. The authors also thank Dr. Hirotada Goto for assistance with powder XRD measurement.
Supplementary CIF file is available online from https://doi.org/10.2465/jmps.230605.
