Shiranuiite, Cu⁺(Rh³⁺Rh⁴⁺)S₄, a new mineral in the thiospinel group from Kumamoto, Japan

Abstract

Shiranuiite, a newly-discovered platinum-group mineral in the carrollite-type thiospinel group with an ideal formula of Cu⁺(Rh³⁺Rh⁴⁺)S₄, was discovered at Haraigawa, Misato machi, Kumamoto Prefecture, Japan. This mineral was named after the ancient name for Kumamoto Prefecture, ‘Land of Fire’, which appeared in an anecdote about Emperor Keiko’s pilgrimage to Kyushu recorded in the Nihon Shoki: ‘the fire that everyone does not know about (= SHIRANUI, in the Japanese historical kana orthography)’. Shiranuiite occurs as the most abundant mineral in nubs accompanied by isoferroplatinum-based grains and is occasionally associated with bowieite, cuprorhodsite, michitoshiite-(Cu), and oxidized platinum-group minerals. Shiranuiite is opaque and has a metallic luster with a bluish gray color in reflected light. The Mohs hardness of this mineral was estimated to be 5 according to the analogous thiospinel-group minerals, and a density of 5.78 g·cm⁻³ was calculated from the empirical formula and powder X-ray diffraction data. The empirical formula, on the basis of 7 apfu is (Cu⁺_0.95Fe³⁺_0.04Ni²⁺_0.01)(Rh³⁺_1.20Rh⁴⁺_0.77Ir⁴⁺_0.06Pt⁴⁺_<0.01)S_3.99. Powder X-ray diffraction measurements indicated that the mineral has the spinel structure and belongs to the space group Fd3m with lattice parameters of a = 9.757(2) Å and V = 928.9(5) ų (Z = 8). Evidence for a spinel structure was also provided by Raman spectroscopy. Both shiranuiite and cuprorhodsite are altered products of bowieite and are highly immiscible in the Cu-Fe system. There is evidence suggesting that shiranuiite formed at a later stage than cuprorhodsite, i.e., these two minerals may form at different stages.

INTRODUCTION

The chemical composition of cuprorhodsite described by Rudashevsky et al. (1985) was (Cu_0.51Fe_0.41)(Rh_1.66Ir_0.23Pt_0.15)S₄, and the ideal formula was summarized to be CuRh₂S₄ with Cu and Rh predominant over Fe and the platinum-group elements (PGE), respectively. Rudashevsky et al. (1998) also described ferrorhodsite, ideally FeRh₂S₄, as the Fe analogue of cuprorhodsite, with empirical formulae (Fe_0.57Cu_0.42)(Rh_1.72Ir_0.23Pt_0.05)S₄ and (Fe_0.52Cu_0.48Ni_0.03)(Rh_1.67Pt_0.16Ir_0.13)S₄. However, experimental works indicated the valence states of this composition were Cu⁺, Fe³⁺, and Rh³⁺ (Plumier and Lotgering, 1970; Plumier et al., 1992). Bosi et al. (2019) then redefined the ideal formula for cuprorhodsite to (Cu⁺_0.5Fe³⁺_0.5)Rh³⁺₂S₄ in the spinel supergroup nomenclature approved by the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification (CNMNC). As a result, ferrorhodsite discredited as corresponding to cuprorhodsite. Although Cabri et al. (2023) disagreed with a change in the ideal formula of cuprorhodsite, this argument was not endorsed by the CNMNC. Therefore, the present study was conducted based on the nomenclature of Bosi et al. (2019) that was officially approved by the CNMNC, so that past cuprorhodsite should now be reclassified into one of four possibilities (Nishio-Hamane and Saito, 2024; Fig. 1): current cuprorhodsite in the linnaeite subgroup, or shiranuiite (2Rh), ezochiite (RhPt), or RhIr mineral in the carrollite subgroup.

Figure 1. Changes in classification of Cu- and PGE-bearing thiospinels in the Rh-Ir-Pt system on the Cu-Fe system based on nomenclature. Cuprorhodsite, malanite, and cuproiridsite were divided by dominancy in the Rh-Ir-Pt system in the past classification (a), while subgroups based on the valence state of the B cations (also linked to the A cations) are established by nomenclature (b). Cuprorhodsite is the only valid species in the linnaeite subgroup. Cu⁺B^3.5+₂S₄ of the carrollite subgroup can be divided into 2Rh (Rh³⁺Rh⁴⁺), 2Ir (Ir³⁺Ir⁴⁺), 2Pt (Pt⁴⁺_1.5Pt²⁺_0.5), RhPt (Rh³⁺Pt⁴⁺), IrPt (Ir³⁺Pt⁴⁺), and RhIr (Rh³⁺Ir⁴⁺) types. Ezochiite, malanite, shiranuiite, and cuproiridsite correspond to RhPt, IrPt, 2Rh, and 2Ir, respectively.

The nomenclature for the spinel supergroup divided the thiospinel group (AB₂S₄) into two subgroups based on the valence state of the B cations (also linked to the A cations); the linnaeite subgroup (A²⁺B³⁺₂S₄) and the carrollite subgroup (A⁺B^3.5+₂S₄). Current cuprorhodsite, ideally (Cu⁺_0.5Fe³⁺_0.5)Rh³⁺₂S₄, is classified into the linnaeite subgroup. However, the classification of the carrollite subgroup with the Cu⁺B^3.5+₂S₄ composition is subdivided. In the Rh-Ir-Pt diagram, with the assumption that Pt is Pt⁴⁺ and Pt²⁺, the combinations of ‘B^3.5+₂’ are 2Rh (Rh³⁺Rh⁴⁺), 2Ir (Ir³⁺Ir⁴⁺), 2Pt (Pt⁴⁺_1.5Pt²⁺_0.5), RhPt (Rh³⁺Pt⁴⁺), IrPt (Ir³⁺Pt⁴⁺), and RhIr (Rh³⁺Ir⁴⁺) (Fig. 1). The 2Ir, RhPt, and IrPt phases correspond to cuproiridsite, ezochiite, and malanite, respectively, while the 2Rh, 2Pt, and RhIr phases have not yet been defined as mineral species. A compound with the CuRh₂S₄ composition that is identical with the 2Rh phase can be stably synthesized and is well known as a superconductor (e.g., Robbins et al., 1967; Van Maaren et al., 1967), where the valence state is considered to be Cu⁺(Rh³⁺Rh⁴⁺)S₄ (e.g., Ito et al., 2003; Mazumdar, 2018).

A placer deposit involving platinum-group minerals (PGM) was recently discovered at Haraigawa, Misato machi, Kumamoto Prefecture, which was the first deposit based on the palladium-subgroup platinum-group elements (PPGE; Rh, Pd, and Pt) in Japan (Nishio-Hamane et al., 2019). Many PGM have been reported there, and cuprorhodsite has also been found. A detailed study of this cuprorhodsite sample has revealed the presence of thiospinel with the ideal formula Cu⁺(Rh³⁺Rh⁴⁺)S₄, which is different from cuprorhodsite and was thus subsequently established as a new mineral.

This new mineral was named shiranuiite (IMA No. 2023-072a) based on a Japanese legend that leads to the ancient name for Kumamoto Prefecture, Kyushu, Japan. In the Nihon Shoki (Chronicles of Japan), the oldest book that records the history of Japan, Emperor Keiko, the 12th emperor of Japan, lost his bearing at sea during a pilgrimage to Kyushu, but was able to land safely because he fortunately saw fire in the distance. He then asked, ‘Who lit the fire?’ to which everyone reportedly replied that it was ‘The fire that everyone does not know about (= SHIRANUI, in Japanese historical kana orthography)’. From this lore, this area came to be called ‘Land of Fire’, now known as Kumamoto Prefecture. Shiranuiite was named in honor of this interesting episode of the locality. The mineral and its name of shiranuiite have been approved by the CNMNC. The holotype specimen has been deposited in the collection of the National Museum of Nature and Science, Japan (NSM M-50086).

OCCURRENCE

Shiranuiite was found in a PGM placer obtained from Haraigawa, Misato machi, Kumamoto Prefecture, Japan (32°34′50′′N 130°47′59′′E). The area under study was in the clinopyroxenite mass distributed northwest of the Hikawa Dam. The clinopyroxenite mass has a depression near the center, and a small river, approximately 2-3 m wide and 2 km long, passes through this area and joins the Shakain River. PGM can be collected as a placer in this small river. Most of the PGM placer collected was composed of isoferroplatinum-based grains that are irregularly shaped with an average size of less than 1 mm, although grains up to 4 mm were also obtained. The surface layer of the placer is often altered to tulameenite or tetraferroplatinum. The isoferroplatinum-based grains are characterized by many inclusions and nubs. The nubs were often accompanied by very rare PGM, with minakawaite (IMA No. 2019-024) with RhSb and michitoshiite-(Cu) (IMA No. 2019-029a) with Rh(Cu_1−xGe_x) (0 < x ≤ 0.5) being found as new minerals (Nishio-Hamane et al., 2019; Tanaka et al., 2020). Placer that consists of the iridium-subgroup platinum-group elements (IPGE; Ru, Os, and Ir) is quite rare in this river.

APPEARANCE

Isoferroplatinum-based grains are often accompanied by black to gray colored nubs from 20 to 500 µm in size. Shiranuiite and cuprorhodsite occur as dominant minerals of such nubs, although both are indistinguishable based on the occurrence mode and can only be distinguished by chemical analysis. A comparison of both indicates that cuprorhodsite commonly occurs, whereas shiranuiite is very rare. Figure 2 shows a cross section of a nub dominated by shiranuiite. Nubs consisting of shiranuiite often involve oxidized PGM and fragments of tulameenite or tetraferroplatinum (Figs. 2a and 2b). Isoferroplatinum-based grains with the nubs are often rich in inclusions (Fig. 2c), and shiranuiite is occasionally mixed with michitoshiite-(Cu) in nubs (Fig. 2d).

Figure 2. Back-scattered electron images of the cross sections of nubs containing shiranuiite. Oxidized PGM is often accompanied by shiranuiite [(a) and (b)]. Isoferroplatinum-based grain with a nub is often rich in inclusions (c), and shiranuiite is occasionally mixed with michitoshiite-(Cu) in the nub (d).

In some cases, nubs consisting of shiranuiite and/or cuprorhodsite are accompanied by bowieite. Bowieite with the ideal composition of Rh₂S₃ is the second most abundant inclusion after laurite (erlichmanite) in the isoferroplatinum-based grains, although bowieite is rarely observed at the surface. Laurite (erlichmanite) has been known as a very stable PGM, while bowieite is not (e.g., Cabri et al., 2022). When bowieite appears on the surface of a grain, it is occasionally replaced by shiranuiite and/or cuprorhodsite (Fig. 3). The order of formation between shiranuiite and cuprorhodsite would not be the same in all cases, but traces of shiranuiite that formed later than cuprorhodsite have been observed. Shiranuiite is developed along the cracks in cuprorhodsite aggregates (Fig. 4).

Figure 3. Back-scattered electron image of nub consisting of shiranuiite, cuprorhodsite, and bowieite (a), mapping images for Rh (b), Fe (c), Cu (d), and mineral distribution (e). Bow, bowieite; Crh, cuprorhodsite; Sir, shiranuiite.

Figure 4. Shiranuiite occurrence developed around cracks in the cuprorhodsite assemblage as shown by a back-scattered electron image (a) and Cu mapping (b).

Nubs that consist of shiranuiite show a metallic luster with a black color and a bluish gray color in reflected light. Shiranuiite in nubs is present as fine anhedral grains up to 5 µm. The Mohs hardness of shiranuiite has not been measured, although it is estimated to be 5 according to those of analogous thiospinel group minerals malanite (5), cuproiridsite (5), and cuprorhodsite (5) (Rudashevsky et al., 1985; Zuxiang, 1996). The calculated density is 5.78 g·cm⁻³ from the empirical formula and unit-cell parameters refined from powder X-ray diffraction (XRD) data. The tenacity of shiranuiite is considered to be probably brittle, and cleavage is estimated to be along the {111} planes according to the isotypic malanite (e.g., Zuxiang, 1996).

RAMAN SPECTROSCOPY

A Raman spectrum of shiranuiite 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 and a spectrum was obtained over the range of 100-600 cm⁻¹ (Fig. 5) Although five phonons (A_1g, E_g, and three T_2g) are Raman active in the spinel structure, one of the T_2g phonons typically has the lowest energy and appears at approximately 100 cm⁻¹ (e.g., Brüesch and D’ambrogio, 1972; Watanabe et al., 1986). However, the spectrometer used in the present work was insufficiently sensitive around 100 cm⁻¹, so that the lowest energy T_2g peak was not detected. The other four phonon peaks that originate from the spinel structure were clearly observed. Previous studies also identified the spinel structure for a synthetic CuRh₂S₄ phase with four peaks observed (Ito et al., 2003; Zhang et al., 2010).

Figure 5. Raman spectrum of shiranuiite. Four phonon peaks (A_1g, E_g, and two T_2g) attributed to the spinel structure were obtained, as for the synthetic CuRh₂S₄ phase.

CHEMICAL COMPOSITION

To select the analytical method, we tried both WDS and EDS, but WDS did not work properly at various currents and voltages. This is probably because grain size was too small for WDS, and another factor seems to be that since the sample was always located at the edge of the placer grain, it became slightly slanted during polishing. Therefore, EDS, which is not as rigorous as WDS with respect to sample size and inclination, was chosen.

Chemical analyses were conducted using a scanning electron microscope (JEOL IT-100) equipped with an energy dispersive X-ray spectroscopy attachment operated at 15 kV and 0.8 nA with a 1 µm beam diameter. Spectra were acquired in 40 s of live time. The ZAF method was used for data correction. The standards employed in these analyses were comprised of pure elements (Fe, Ni, Cu, Rh, Ir, and Pt) and pyrite (S). Shiranuiite is mainly composed of Rh, with trace amounts of Ir and Pt for platinum-group elements. Since the M lines of Ir and Pt overlap in EDS, they were analyzed using the nonoverlapping L line. As a result, the amount of Pt was negligible.

According to experimental and theoretical investigations and the spinel nomenclature (e.g., Hagino et al., 1995; Oda et al., 1995; Ito et al., 2003; Bosi et al., 2019), the valence states of Cu and Fe in thiospinel are considered to be Cu⁺ and Fe³⁺. Experimental and theoretical works also support the Cu⁺(Rh³⁺Rh⁴⁺)S₄ valence distribution (e.g., Ito et al., 2003; Mazumdar, 2018). Therefore, the cation charge of shiranuiite is balanced between Rh³⁺ and Rh⁴⁺ on the basis of 7 apfu. Shiranuiite occasionally coexists with Ir-bearing oxidized PGM, where the most stable iridium oxide is commonly IrO₂; therefore, the Ir charge was fixed at Ir⁴⁺ for the calculation. The analytical results are given in Table 1. The empirical formula determined for shiranuiite following this charge balance was (Cu⁺_0.95Fe³⁺_0.04Ni²⁺_0.01)(Rh³⁺_1.20Rh⁴⁺_0.77Ir⁴⁺_0.06Pt⁴⁺_<0.01)S_3.99. It should be noted that the ideal formula for shiranuiite is Cu⁺(Rh³⁺Rh⁴⁺)S₄, which requires Cu 15.98, Rh 51.76, S 32.26, for a total of 100 wt%.

Table 1. Chemical composition of shiranuiite

	wt% (n = 16)
	Average	Minimum	Maximum		apfu
Cu	14.96	13.41	15.91	Cu	0.95
Fe	0.49	0.13	1.06	Fe	0.04
Ni	0.12	n.d.	0.37	Ni	0.01
Rh	50.28	48.80	53.37
Ir	2.67	1.08	4.45	Rh3+	1.20
Pt	0.17	n.d.	0.67	Rh4+	0.77
S	31.78	30.89	32.64	Ir4+	0.06
Total	100.47			Pt4+	<0.01
				S	3.99

CRYSTALLOGRAPHY

Single-crystal XRD analyses could not be performed due to the small grain sizes as shown in Figure 2. Therefore, a micro-XRD technique was employed using a Rigaku Ultrax18 diffractometer with CrKα radiation operated at 40 kV and 150 mA. This instrument incorporated a 100 µm collimator equipped with a curved position sensitive proportional counter and an oscillating sample stage. The powder XRD pattern for shiranuiite is presented in Figure 6. As shown in Figure 2, shiranuiite is often closely accompanied by tulameenite, and both appear in the X-ray profile. The two unindexed peaks may also originate from oxidized PGM, which is often associated with shiranuiite (Fig. 2) because these peaks cannot be explained by symmetry lowering of the spinel phase. Although the diffraction intensities were not completely consistent with the calculated values due to incomplete Debye rings, the overall diffraction profile is very compatible with the spinel structure, with 15 peaks indexed in the d = 3.45-1.222 Å range with the space group Fd3m (#227). Table 2 summarizes the diffraction data for shiranuiite and the synthetic CuRh₂S₄ phase. The seven strongest shiranuiite lines in the powder XRD pattern [presented here as d in Å (I/I₀) hkl] are: 2.95 (55) 311, 2.44 (36) 400, 1.879 (61) 333+511, 1.725 (100) 440, 1.487 (29) 533, 1.270 (46) 731+533, and 1.220 (44) 800. The unit cell parameters were calculated by the least squares method using CellCalc program (Miura, 2003) and determined to be a = 9.757(2) Å and V = 928.9(5) ų (Z = 8).

Figure 6. Powder XRD pattern for shiranuiite by micro-XRD method. The bars below the diffraction pattern show the peak positions and simulated intensities for shiranuiite (orange) and tulameenite (purple) calculated as the ideal compositions. The two small arrows (pink) shown above the diffraction pattern are diffraction positions that could not be indexed as shiranuiite or tulameenite.

Table 2. Powder XRD data for shiranuiite and synthetic CuRh₂S₄

Shiranuiite				Synthetic CuRh2S4
(This study)				Riedel et al. (1981)
Imeas	dmeas. (Å)	dcalc. (Å)	hkl	Icalc.*	dcalc. (Å)	hkl
-	-**	5.633	111	38	5.653	111
6	3.45	3.450	220	17	3.462	220
55	2.95	2.942	311	100	2.952	311
7	2.82	2.817	222	9	2.826	222
36	2.44	2.439	400	45	2.448	400
16	2.23	2.238	331	3	2.246	331
8	1.990	1.992	422	6	1.999	422
61	1.879	1.878	333+511	35	1.884	333+511
100	1.725	1.725	440	69	1.731	440
15	1.649	1.649	531	7	1.655	531
-	-	1.626	442	1<	1.632	442
-	-	1.543	620	3	1.548	620
29	1.487	1.488	533	15	1.493	533
16	1.471	1.471	622	6	1.476	622
18	1.409	1.408	444	15	1.413	444
-	-	1.366	551+711	6	1.371	551+711
9	1.304	1.304	642	7	1.308	642
46	1.270	1.270	731+533	24	1.275	731+533
44	1.220	1.220	800	38	1.224	800
a = 9.757(2) Å				a = 9.791 Å
V = 928.9(5) Å3				V = 938.6 Å3

^* Recalculated with CrKα for comparison with experimental observations.

^** The 111 reflection cannot be observed due to geometrical constraints of micro-XRD.

The Cu⁺(Rh³⁺Rh⁴⁺)S₄ phase (shiranuiite analogue) has been well known as a cubic aristotype spinel phase (e.g., Riedel et al., 1981). That is, the structure consists of a heteropolyhedral framework composed of CuS₄ tetrahedra isolated from one another with corners shared by adjacent RhS₆ octahedra. Although several modifications of spinel structures are known, all Cu- and PGE-containing thiospinel minerals have a cubic aristotype structure at room temperature (Table 3). These structures have slightly different lattice parameters, with shiranuiite and its corresponding compound having the smallest lattice, which is consistent with the small ionic radius of Rh⁴⁺ compared to that of other PGE.

Table 3. List of cubic Cu- and Rh-Ir-Pt PGE-bearing thiospinel minerals

Mineral (Phase)	Ideal formula	a (Å)	Reference
Linnaeite subgroup
Cuprorhodsite (Rh)	(Cu+0.5Fe3+0.5)Rh3+2S4	9.88	1
		9.85*	2
		9.832**	3
Unidentified (Ir)	(Cu+0.5Fe3+0.5)Ir3+2S4	-
Unidentified (Pt)	(Cu+0.5Fe3+0.5)(Pt4+Pt2+)S4	-
Carrollite subgroup
Shiranuiite (2Rh)	Cu+(Rh3+Rh4+)S4	9.791*	4
		9.757	3
Cuproiridsite (2Ir)	Cu+(Ir3+Ir4+)S4	9.92***	1
		9.847*	5
Unidentified (2Pt)	Cu+(Pt4+1.5Pt2+0.5)S4	-
Ezochiite (RhPt)	Cu+(Rh3+Pt4+)S4	9.856	6
Malanite (IrPt)	Cu+(Ir3+Pt4+)S4	9.91	7
Unidentified (RhIr)	Cu+(Rh3+Ir4+)S4	9.82†	8

^* Ideal compound.

^** XRD pattern and data are shown in Supplementary Figure S1 and Table S1, respectively (Supplementary Fig. S1 and Table S1 are available online from https://doi.org/10.2465/jmps.240529).

^*** Data of type mineral, although the composition is rather classified as an unidentified mineral (Ir) of the linnaeite subgroup according to current nomenclature because it is Ir dominant with the Cu/(Cu + Fe) < 0.75 composition.

^† Intermediate value between CuRh₂S₄ and CuIr₂S₄ by synthesis experiment.

1, Rudashevsky et al. (1985); 2, Plumier and Lotgering (1970); 3, This study; 4, Riedel et al. (1981); 5, Furubayashi et al. (1994); 6, Nishio-Hamane and Saito (2024); 7, Zuxiang (1996); 8, Matsumoto et al. (1999).

DISCUSSION

Minerals previously termed cuprorhodsite should now be reclassified into one of the following four possibilities: current cuprorhodsite in the linnaeite subgroup, or shiranuiite (2Rh), ezochiite (RhPt), or RhIr mineral in the carrollite subgroup (Fig. 1). Therefore, Nishio-Hamane and Saito (2024) reclassified past cuprorhodsite into current subgroups based on the nomenclature and examined their contents. Although most have been reclassified into cuprorhodsite for the linnaeite subgroup and ezochiite for the carrollite subgroup, only one composition from the Bushveld complex reported by Rudashevsky and Rudashevsky (2020) corresponds to shiranuiite. As this example shows, the occurrence of shiranuiite has been extremely limited worldwide to date. This could suggest that shiranuiite formation at Haraigawa occurred by a unique formation process.

Figure 7 shows the compositional distribution of thiospinel from Haraigawa. The compositions of thiospinel are clearly divided into Cu/(Cu + Fe) around 0.5 and 1.0, which correspond to the linnaeite subgroup and carrollite subgroup, respectively (Fig. 7a). The compositions in the linnaeite subgroup of the Rh-Ir-Pt diagram show a Rh-Ir trend, and all of them were classified into cuprorhodsite (Fig. 7b). The compositions in the carrollite subgroup also show an almost common Rh-Ir trend, and they are all shiranuiite (Fig. 7c). Both conclusively show almost the same distribution of Rh/(Rh + Ir + Pt) content; therefore, they have a common origin for the Rh/(Rh + Ir + Pt) content, which is bowieite. When bowieite is on the surface of the placer particles, it is accompanied by cuprorhodsite and/or shiranuiite (Fig. 3). Of the three Rh-S minerals from Haraigawa, only bowieite has a consistent Rh/(Rh + Ir + Pt) content with cuprorhodsite and shiranuiite (Fig. 7c). On the other hand, the immiscibility between cuprorhodsite and shiranuiite in the Cu-Fe system may be caused by a difference of the formation stage. Shiranuiite occasionally occurs near cracks in cuprorhodsite aggregate (Fig. 4), which suggests that the formation of shiranuiite occurs later than cuprorhodsite. At that stage, shiranuiite is probably more stable than cuprorhodsite and formation involves the dissolution of Fe and/or the addition of Cu with simultaneous valence change for Rh. The occurrence of oxidized PGM and michitoshiite-(Cu) in shiranuiite aggregate is probably a further alteration that occurs later (Fig. 2) because the shiranuiite assemblage has been further eroded.

Figure 7. Compositional diagrams of thiospinels from Haraigawa. The compositions are plotted against the Cu/(Cu + Fe) ratio to distinguish between the linnaeite and carrollite subgroups (a). Mineral species were considered in the Rh-Ir-Pt system within each subgroup [(b) and (c)]. The compositions of Rh-S minerals (bowieite, kingstonite, and miassite) are also shown for comparison (c).

However, due to the paucity of occurrence, there is currently limited information on the formation and further alteration conditions of shiranuiite. Primary PGM including bowieite at Haraigawa may have originated in a magma chamber (Nishio-Hamane et al., 2019), but the subsequent alteration processes and conditions that involve the formation of new minerals such as shiranuiite, minakawaite, and michitoshiite-(Cu) are still not fully understood. Synthesis experiments have shown that the CuRh₂S₄ phase that is identical with shiranuiite is not stable below 500 °C, even in the Cu-Rh-S system (Karup-Møller and Makovicky, 2007). Alteration beyond shiranuiite may occur below this temperature, although shiranuiite still remains in small quantities. PGM formation, especially during alteration processes, requires further study to reveal the details of this unique Haraigawa deposit.

CONCLUSION

Cuprorhodsite described before the establishment of the nomenclature for the spinel supergroup has now been reclassified into one of four minerals: current cuprorhodsite in the linnaeite subgroup, or shiranuiite (2Rh), ezochiite (RhPt), or an unidentified mineral (RhIr) in the carrollite subgroup. This work has established a new mineral, shiranuiite Cu⁺(Rh³⁺Rh⁴⁺)S₄, according to current nomenclature. Shiranuiite occurs at Haraigawa, Misato machi, Kumamoto Prefecture Japan as a nub associated with isoferroplatinum-based placer grains. The origin of the nub with shiranuiite is bowieite, which underwent alteration and transformed into shiranuiite. Cuprorhodsite has a similar origin, and shiranuiite may have been formed at a later stage than cuprorhodsite. Shiranuiite may undergo further alteration, and further research is required to clarify such conditions.

ACKNOWLEDGMENTS

The authors thank Hirotada Gotou for support during the micro-XRD analyses using a position-sensitive proportional counter. We are grateful to Luca Bindi, who handled our manuscript, and to two anonymous reviewers for their insightful comments greatly improving the manuscript.

SUPPLEMENTARY MATERIALS

Supplementary Figure S1 and Table S1 are available online from https://doi.org/10.2465/jmps.240529.

REFERENCES

•  Bosi,  F.,  Biagioni,  C. and  Pasero,  M. (2019) Nomenclature and classification of the spinel supergroup. European Journal of Mineralogy, 31, 183-192.
•  Brüesch,  P. and  D’ambrogio,  P. (1972) Lattice dynamics and magnetic ordering in the chalcogenide spinels CdCr₂S₄ and CdCr₂Se₄. Physica Status Solidi B, 50, 513-526.
•  Cabri,  L.J.,  Oberthür,  T. and  Keays,  R.R. (2022) Origin and depositional history of platinum-group minerals in placers - a critical review of fact and fiction. Ore Geology Reviews, 144, 104733.
•  Cabri,  L.J.,  Mcdonald,  A.M.,  Oberthür,  T. and  Vymazalová,  A. (2023) An examination of platinum-group element thiospinels. The Canadian Journal of Mineralogy and Petrology, 61, 1109-1121.
•  Furubayashi,  T.,  Matsumoto,  T.,  Hagino,  T. and  Nagata,  S. (1994) Structure and magnetic studies of Metal-Insulator transition in thiospinel CuIr₂S₄. Journal of Physical Society of Japan, 63, 3333-3339.
•  Hagino,  T.,  Seki,  Y.,  Wada,  N.,  Tsuji,  S., et al (1995) Superconductivity in spinel-type compounds CuRh₂S₄ and CuRh₂Se₄. Physical Review B, 51, 12673.
•  Ito,  M.,  Hori,  J.,  Kurisaki,  H.,  Okada,  H., et al (2003) Pressure-induced superconductor-insulator transition in the spinel compound CuRh₂S₄. Physical Review Letters, 91, 077001.
•  Karup-Møller,  S. and  Makovicky,  E. (2007) The system Cu-Rh-S at 900°, 700°, and 500°C. The Canadian Mineralogist, 45, 1535-1542.
•  Matsumoto,  N.,  Endoh,  R.,  Nagata,  S.,  Furubayashi,  T. and  Matsumoto,  T. (1999) Metal-insulator transition and superconductivity in the spinel-type Cu(Ir_1−xRh_x)₂S₄ system. Physical Review B, 60, 5258-5265.
•  Mazumdar,  S. (2018) Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides. Physical Review B, 98, 205153.
•  Miura,  H. (2003) A unit cell parameter refinement program on windows computer. Nihon Kessho Gakkaishi, 45, 145-147 (in Japanese).
•  Nishio-Hamane,  D.,  Tanaka,  T. and  Shinmachi,  T. (2019) Minakawaite and platinum-group minerals in the placer from the clinopyroxenite area in serpentinite mélange of Kurosegawa belt, Kumamoto Prefecture, Japan. Journal of Mineralogical and Petrological Sciences, 114, 252-262.
•  Nishio-Hamane,  D. and  Saito,  K. (2024) Ezochiite, Cu⁺(Rh³⁺Pt⁴⁺)S₄, a new mineral in the thiospinel group from Hokkaido, Japan. Journal of Mineralogical and Petrological Sciences, 119, 008.
•  Oda,  T.,  Shirai,  M.,  Suzuki,  N. and  Motizuki,  K. (1995) Electronic band structure of sulphide spinels CuM₂S₄ (M=Co, Rh,Ir). Journal of Physics: Condensed Matter, 7, 4433.
•  Plumier,  R. and  Lotgering,  F.K. (1970) Antiferromagnetic interactions between Fe³⁺ ions at a large distance in Fe_1/2Cu_1/2Rh₂S₄. Solid State Communications, 8, 477-480.
•  Plumier,  R.,  Sougi,  M. and  Soubeyroux,  J.L. (1992) Neutron diffraction reinvestigation of Fe_1/2Cu_1/2Rh₂S₄. Journal of Alloys and Compounds, 178, 51-56.
•  Riedel,  E.,  Karl,  R. and  Rackwitz,  R. (1981) Mossbauer studies of thiospinels. V. The systems Cu_1−xFe_xMe₂S₄ (Me=Cr,Rh) and, Cu_1-xFe_xCr₂(S_.7Se_.3)₄. Journal of Solid State Chemistry, 40, 255-265.
•  Robbins,  M.,  Willens,  R.H. and  Miller,  R.C. (1967) Superconductivity in spinel CuRh₂S₄ and CuRh₃Se₄. Solid State Communication, 5, 933-934.
•  Rudashevsky,  N.S.,  Men’shikov,  Y.P.,  Mochalov,  A.G.,  Trubkin,  N.V., et al (1985) Cuprorhodsite CuRh₂S₄ and cuproiridsite CuIr₂S₄ - new natural thiospinels of platinum-group elements. Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, 114, 187-195.
•  Rudashevsky,  N.S.,  Mochalov,  A.G.,  Men’shikov,  Yu.P. and  Shumskaya,  N.I. (1998) Ferrorhodsite (Fe,Cu)(Rh,Pt,Ir)₂S₄ - a new mineral. Zapiski Vserossiyskogo Mineralogicheskogo Obshchestva, 127, 37-41.
•  Rudashevsky,  N.S. and  Rudashevsky,  V.N. (2020) New data for malanite and cuprorhodsite from chromitites of the Bushveld complex, South Africa. Geology of Ore Deposits, 62, 796-802.
•  Tanaka,  T.,  Shinmachi,  T.,  Kataoka,  K. and  Nishio-Hamane,  D. (2020) Michitoshiite-(Cu), IMA 2019-029a. CNMNC Newsletter No. 53; CNMNC Newsletter 53. European Journal of Mineralogy, 32, 209-213.
•  Van Maaren,  N.H.,  Schaeffer,  G.M. and  Lotgering,  F.K. (1967) Superconductivity in sulpho- and selenospinels. Physical Letter A, 25, 238-239.
•  Watanabe,  J.,  Udagawa,  M.,  Kamigaichi,  T. and  Ohbayashi,  K. (1986) Lattice vibrational Raman spectra of Cd(In_1−xCr_x)₂S₄ mixed crystal. Journal of Physics C, 19, 2351-2362.
•  Zhang,  L.,  Ling,  L.S.,  Qu,  Z.,  Tong,  W., et al (2010) Study of lattice dynamics in the CuIr₂S₄ system. The European Physical Journal B, 77, 83-86.
•  Zuxiang,  Y. (1996) Malanite - a new cupric platinum (Pt³⁺) and iridium (Ir³⁺) sulfide. Acta Geologica Sinica, 70, 309-314.