Ezochiite, Cu⁺(Rh³⁺Pt⁴⁺)S₄, a new mineral in the thiospinel group from Hokkaido, Japan

Abstract

Ezochiite, a newly-discovered platinum-group mineral in the thiospinel group having the ideal formula Cu⁺(Rh³⁺Pt⁴⁺)S₄, was discovered in samples from the Tomamae coast near Tomamae town, Hokkaido, Japan. Additional specimens were later found in the Shosanbetsu river, Ainusawa river and Obira coast, Hokkaido. Ezochiite crystallized in melt pockets trapped in isoferroplatinum grains, occurring in the form of anhedral grains less than 5 µm in length. Ezochiite is associated with sulfide minerals such as braggite, cooperite, torryweiserite, and chalcopyrite. It 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 by analogy with related thiospinel group minerals and a density of 6.66 g·cm⁻³ was calculated from the empirical formula and powder X-ray diffraction data. The empirical formula, on the basis of 7 apfu was (Cu⁺_0.85Fe³⁺_0.15)_Σ1.00(Rh³⁺_1.09Pt⁴⁺_0.78Ir³⁺_0.08Pt²⁺_0.05)_Σ2.00S_4.00 for a specimen obtained from the Tomamae coast. The powder X-ray diffraction study indicated that the mineral exhibits the spinel structure, space group Fd3m, with lattice parameters a = 9.8559(14) Å and V = 957.4(4) ų (Z = 8). Evidence for a spinel structure was also provided by Raman spectra. Data from samples of ezochiite and cuprorhodsite from samples sourced in Hokkaido showed a compositional relationship based on coupled Fe³⁺_0.5Rh³⁺-Cu⁺_0.5Pt⁴⁺ substitution. Ezochiite is not rare. It is also found various other geological environments, including in ophiolites, Ural-Alaskan intrusions and mafic-ultramafic intrusions.

INTRODUCTION

Spinel supergroup minerals having the general formula AB₂X₄ can be divided into three groups based on the X anion. These comprise the oxyspinel group (where X is O²⁻), the thiospinel group (S²⁻) and the selenospinel group (Se²⁻). The thiospinel group can be further divided into the carrollite subgroup (A⁺B^3.5+₂S₄) and the linnaeite subgroup (A²⁺B³⁺₂S₄) based on the valence state of the B site cation (Bosi et al., 2019). In the current nomenclature, three thiospinel minerals containing copper and platinum-group elements (PGE) are referred to as cuprorhodsite (Cu⁺_0.5Fe³⁺_0.5)Rh³⁺₂S₄, which is in the linnaeite subgroup, and cuproiridsite Cu⁺(Ir³⁺Ir⁴⁺)S₄ and malanite Cu⁺(Ir³⁺Pt⁴⁺)S₄, which are in the carrollite subgroup. On the other hand, prior to the establishment of this nomenclature, the chemical formulae of cuprorhodsite and malanite were reported as CuRh₂S₄ and CuPt₂S₄, respectively, thus producing a compositional mismatch (Fig. 1). Cabri et al. (2023) strongly disagreed with the change in the ideal formulae of these minerals and argued that the original ones should be maintained. However, this was not officially endorsed by Commission on New Minerals, Nomenclature and Classification of the the International Mineralogical Association (IMA-CNMNC). Therefore, we base our study on the officially IMA-approved nomenclature.

Figure 1. Changes in classification of copper- and PGE-bearing thiospinels in the Rh-Ir-Pt system based on the current IMA-approved nomenclature. Cuprorhodsite, malanite, and cuproiridsite were previously divided by dominancy in the Rh-Ir-Pt system (a), while subgroups based on the valence state of the B site are used in the present-day nomenclature (b). Currently, cuprorhodsite is the only known species in the linnaeite subgroup. The Cu⁺B^3.5+₂S₄ minerals 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, and cuproiridsite correspond to the RhPt, IrPt, and 2Ir types, respectively.

In the Rh-Ir-Pt diagram of the carrollite subgroup, which assumes that Pt is in the form of Pt⁴⁺ or Pt²⁺, the cations as B^3.5+₂ are 2Rh (Rh³⁺Rh⁴⁺), 2Ir (Ir³⁺Ir⁴⁺), 2Pt (Pt⁴⁺_1.5Pt²⁺_0.5), RhPt (Rh³⁺Pt⁴⁺), IrPt (Ir³⁺Pt⁴⁺), or RhIr (Ph³⁺Ir⁴⁺) (Fig. 1b). Shortly, the materials previously termed cuprorhodsite should now be reclassified taking into account one of these four possibilities: (Cu⁺_0.5Fe³⁺_0.5)Rh³⁺₂S₄ (the present cuprorhodsite), which is in the linnaeite subgroup, or the 2Rh, RhIr or RhPt combinations noted above, which are in the carrollite subgroup (Fig. 1). Among these, analytical data corresponding to RhPt has been known for a long time. Cabri et al. (1981) previously reported a mineral having the formula (Rh,Ir)PtCuS₄ that was extracted from the Yubdo ultramafic complex in Ethiopia. Assuming that this mineral has a spinel structure, it would currently be considered to have the formula Cu⁺(Rh³⁺Pt⁴⁺)S₄ (= RhPt) and so would be a member of the carrollite subgroup. A review of previously reported thiospinels indicates that RhPt compositions have been found in many locations, including the Manampotsy area in Madagascar (Legendre and Augé, 1992), the San Lorenzo area in Ecuador (Weiser and Schmidt-Thome, 1993), the Fenero-Ojén massif in Spain (Garuti et al., 1995), the Aluchinskii massif in Russia (Gornostayev et al., 1999), the Penikat intrusion in Finland (Barkov et al., 2000), the Bulqiza complex in Albania (Çina et al., 2002), the Kytlym complex in Russia (Garuti et al., 2002), the Zolotaya intrusion in Russia (Shcheka et al., 2004), the Burgastain area in Mongolia (Oyunchinmeg et al., 2009), the Coldwell complex in Canada (Ames et al., 2017), the Bushveld complex in South Africa (Rudashevsky and Rudashevsky, 2020), the Tugidak island in Alaska (Belkin and Grosz, 2021), and the Tomamae coast of Hokkaido, Japan (Nishio-Hamane and Saito, 2022).

With revisions in nomenclatures, it is often necessary to re-examine previously described minerals and update the associated data. On this basis, we conducted a more detailed mineralogical investigation of thiospinel samples obtained from the Tomamae region and discovered a new species having the ideal composition Cu⁺(Rh³⁺Pt⁴⁺)S₄. This new mineral from the Hokkaido area was named ezochiite (IMA no. 2022-101) after the old name for Hokkaido, which was known as ‘Ezochi’ prior to 1869. The mineral and its name have been approved by the CNMNC and a holotype specimen has been deposited in the collection of the National Museum of Nature and Science, Japan (NSM M-49764). The purpose of this study is to describe new minerals and review past minerals using current classification.

OCCURRENCE

The sampling sites employed in the present study are in northwestern Hokkaido, representing the same region previously examined by Nishio-Hamane and Saito (2022). This area included the Moshosanbetsu river, the Shosanbetsu river, the Ainusawa river, the Tomamae coast, the Obira coast, the Kamikinenbetsusawa river, the Obirashibe river, and the Numatapon river. The location of each site is reported by Nishio-Hamane and Saito (2022). During the present research, placer platinum-group minerals (PGM) grains were collected using a specific gravity selection process and were found to consist primarily of an Os-Ir-Ru-based alloy together with a small number of Pt-based alloy grains. The grain sizes of both alloys were less than 5 mm in the rivers and typically less than 0.5 mm on the coast. A much greater number of grains could be collected in a given time span on the coast compared with the rivers.

The Pt-based alloy grains were composed of isoferroplatinum although the rims of these grains had occasionally been transformed into tulameenite or tetraferroplatinum. Nishio-Hamane and Saito (2022) previously demonstrated that such grains can contain a variety of inclusions by SEM observation for a cross section of a sample. The present study also reports the discovery of ezochiite inclusions. The type locality is the placer deposit on the coast of Tomamae town (44°17′09′′N, 141°38′58′′E). Ezochiite was also found in other placer deposits from the Shosanbetsu river in Shosanbetsu village, the Ainusawa river in Haboro town and the Obira coast of Obira town. These placer deposits are all secondary deposits that have been re-deposited after falling out from sedimentary rocks. The original source of the PGM is thought to have been ultramafic rocks that were previously distributed throughout the Teshio mountains (e.g., Suzuki, 1950; Hata, 1961; Nishio-Hamane and Saito, 2022).

APPEARANCE

The appearances of ezochiite and cuprorhodsite are similar and both occur among numerous types of inclusions found in the isoferroplatinum-based grains. Ezochiite in the type specimen is shown in Figure 2. It is present as fine anhedral grains up to 5 µm in length in regions ranging from spherical to irregularly-shaped and with diameters in the range of 100-200 µm within the isoferroplatinum-based grains. These cavities were filled with ezochiite or cuprorhodsite along with smaller amounts of braggite, cooperite and/or isoferroplatinum. Nishio-Hamane and Saito (2022) also reported the coexistence of bowieite with cuprorhodsite, compositionally equivalent to ezochiite, in an isoferroplatinum-based grain acquired from the Tomamae coast. In addition, torryweiserite, braggite, and chalcopyrite are found to coexist with ezochiite in an isoferroplatinum-based grain from the Shosanbetsu river (Figs. 3a and 3b). In the case of the isoferroplatinum-based grain from the Ainusawa river, ezochiite coexists with laurite and chalcopyrite (Figs. 3c and 3d). It should also be noted that the ezochiite fragments in the Obira coast samples occasionally contain mineral fragments that are too small to be analyzed with a conventional equipment (Figs. 3e and 3f). The compositional range of the inclusions within a single cavity is small whereas the mineral composition varies among cavities even in a single isoferroplatinum grain.

Figure 2. Occurrence of ezochiite and associated inclusions in an isoferroplatinum grain from the Tomamae coast. A cross-sectional back-scattered electron image of a grain (a) and magnified views of regions comprising ezochiite (b) and cuprorhodsite (c).

Figure 3. Occurrence of ezochiite and associated inclusions in isoferroplatinum grains from the Shosanbetsu river [(a), (b)], the Ainusawa river [(c), (d)], and the Obira coast [(e), (f)].

Ezochiite is opaque with a metallic luster and a bluish gray color in reflected light. However, mainly due to the small size, the majority of the physical and optical properties of ezochiite could not be assessed. The hardness of this mineral has not been measured, although it is estimated to be about 5 by analogy with related thiospinel group minerals such as malanite (with a value of 5), cuproiridsite (5), and cuprorhodsite (5). A density of 6.66 g·cm⁻³ was calculated from the empirical formula and from powder X-ray diffraction (XRD) data. Ezochiite is likely brittle and the cleavage plane is predicted to be {111} by analogy with other thiospinel group minerals.

RAMAN SPECTROSCOPY

Raman spectra of ezochiite specimens were obtained using a Renishaw inVia Reflex spectrometer equipped with a 532 nm Nd-YAG laser passing through a confocal microscope with 50× objective lens. The laser power at the sample surface was approximately 10 mW and spectra were obtained over the range of 100 to 600 cm⁻¹ using a 532 nm diode laser. Figure 4 shows the resulting spectrum. Although five phonons (A_1g, E_g, and three T_2g) are Raman active in a spinel structure, one of the T_2g phonons usually has the lowest energy and appears at approximately 100 cm⁻¹ (e.g., Brüesch and D’ambrogio, 1972; Watanabe et al., 1986). The spectrometer used in the present work was insufficiently sensitive around 100 cm⁻¹ and so the lowest energy T_2g peak was not detected. However, the other four phonons originating from the spinel structure were clearly observed. Previous studies also identified spinel structure for synthetic copper- and PGE-bearing thiospinel phases by observing four peaks (Ito et al., 2003; Zhang et al., 2010).

Figure 4. The Raman spectrum of ezochiite. Four phonons (A_1g, E_g, and two T_2g) attributed to the spinel structure can be observed.

CHEMICAL COMPOSITION

Chemical analyses were conducted using a scanning electron microscope (JEOL IT-100) equipped with an energy dispersive X-ray spectrometry attachment operating at 15 kV and 0.8 nA with a 1 µm beam diameter. The ZAF method was used for data correction. The standards employed in these analyses comprised pure elements (Fe, Ni, Cu, and PGE) and pyrite (S). Table 1 summarizes the chemical compositions determined for representative ezochiite specimens. Because Pt³⁺ is not typically present in this mineral, as indicated by an assessment of a synthetic Cu⁺₂(Pt²⁺Pt⁴⁺₃)S₈ phase and by spinel nomenclature (Gross and Jansen, 1994; Bosi et al., 2019), the excess charge was balanced between Pt²⁺ and Pt⁴⁺ on the basis of 7 apfu. However, since Pt²⁺ generally prefers square-planar arrangement with S as a ligand, it should be noted that for spinel structures without such a coordination environment, the values are only estimated for charge balance. The empirical formulae determined for these ezochiite specimens following this charge balance were (Cu⁺_0.85Fe³⁺_0.15)_Σ1.00(Rh³⁺_1.09Pt⁴⁺_0.78Ir³⁺_0.08Pt²⁺_0.05)_Σ2.00S_4.00 for the Tomamae coast specimen, (Cu⁺_0.76Fe³⁺_0.26)_Σ1.02(Rh³⁺_1.29Pt⁴⁺_0.59Pt²⁺_0.07Ir³⁺_0.02)_Σ1.97S_4.01 for the Shosanbetsu river specimen, (Cu⁺_0.89Fe³⁺_0.17)_Σ1.06(Rh³⁺_1.15Pt⁴⁺_0.68Ir³⁺_0.08Pt²⁺_0.05)_Σ1.96S_3.99 for the Ainusawa river specimen, and (Cu⁺_0.80Fe³⁺_0.18)_Σ0.98(Rh³⁺_1.21Pt⁴⁺_0.57Pt²⁺_0.17Ir³⁺_0.08)_Σ2.03S_3.98 for the Obira coast specimen. It should be noted that the ideal formula for ezochiite can be written as Cu⁺(Rh³⁺Pt⁴⁺)S₄, which requires 12.97 wt% Cu, 21.01 wt% Rh, 39.83 wt% Pt, and 26.19 wt% S, for a total of 100 wt%.

Table 1. The chemical compositions of ezochiite specimens sourced from Hokkaido

Locality:	Tomamae	Shosanbetsu	Ainusawa	Obira
	wt% (Range of n = 15)	wt%	wt%	wt%
Cu	11.45 (10.37-12.22)	10.52	12.00	10.73
Fe	1.78 (1.24-2.42)	3.23	1.98	2.11
Rh	23.66 (22.43-24.64)	29.17	25.16	26.40
Ir	3.06 (1.64-4.45)	0.92	3.21	3.37
Pt	33.94 (30.83-36.88)	28.23	29.90	30.83
S	27.09 (26.04-27.68)	28.15	27.08	26.94
Total	100.98	100.22	99.33	100.38
Basis of	Σ = 7	Σ = 7	Σ = 7	Σ = 7
Cu+	0.85	0.76	0.89	0.80
Fe3+	0.15	0.26	0.17	0.18
Σ	1.00	1.02	1.06	0.98
Rh3+	1.09	1.29	1.15	1.21
Pt4+	0.78	0.59	0.68	0.57
Ir3+	0.08	0.02	0.08	0.08
Pt2+	0.05	0.07	0.05	0.17
Σ	2.00	1.97	1.96	2.03
S	4.00	4.01	3.99	3.98

CRYSTALLOGRAPHY

Single-crystal XRD analyses could not be carried out due to the small grain sizes. Hence, a micro-XRD technique was employed using a Rigaku Ultrax18 diffractometer with CrKα radiation operating at 40 kV and 200 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 ezochiite was obtained in situ using the sample shown in Figure 2b and is presented in Figure 5. Although isoferroplatinum and braggite peaks appeared in this pattern because these minerals were in close physical proximity to the ezochiite, it was still possible to isolate and identify the peaks related to ezochiite. It should be noted that the diffraction intensities were not completely consistent with the calculated values. This discrepancy can likely be attributed to the insufficient generation of Debye rings based on the small sample and the uneven grain sizes and orientations, even when using the oscillating stage. Even so, the overall diffraction profile was in good agreement with the expected spinel structure, with 12 peaks indexed in the d range of 3.48-1.233 Å according to the space group Fd3m (#227). The resulting data are summarized in Table 2. The seven strongest ezochiite lines in the powder XRD pattern [presented here as d in Å (I/I₀) hkl] were: 2.97 (98) 311, 2.45 (76) 400, 2.26 (44) 331, 1.898 (100) 333+511, 1.503 (22) 533, 1.380 (25) 551+711, and 1.283 (37) 553+731. The unit cell parameters were determined to be a = 9.8559(14) Å and V = 957.4(4) ų (Z = 8).

Figure 5. The X-ray powder diffraction pattern of the ezochiite specimen shown in Figure 1b. The bars below the diffraction profile indicate peak positions with calculated intensities. Bge, braggite; Ifpt, isoferroplatinum.

Table 2. Powder X-ray diffraction data for ezochiite

hkl	Iobs.	Icalc.*	dobs. (Å)	dcalc. (Å)
220	6	10	3.48	3.48
311	98	100	2.97	2.97
222		22		2.85
400	76	48	2.46	2.46
331	44	8	2.26	2.26
422		4		2.01
333	100	6	1.898	1.897
511		34
440	19	64	1.740	1.742
531	7	12	1.666	1.666
442		1<		1.643
620		2		1.558
533	22	16	1.503	1.503
622	20	15	1.485	1.486
444		17		1.423
551	25	8	1.380	1.380
711		3
642		4		1.317
553	37	24	1.283	1.283
731		28
800	20	34	1.233	1.232

^* Diffraction intensities of Cu(RhPt)S₄ composition calculated based on the structural model by Riedel et al. (1981).

A Cu⁺(Rh³⁺Pt⁴⁺)S₄ phase (ezochiite analogue) has not yet been synthesized but the Raman spectra and powder XRD pattern obtained in the present work were fully in agreement with other cubic spinel phases. That is, the structure of this mineral appears to consist of a heteropolyhedral framework composed of CuS₄ tetrahedra isolated from one another and sharing corners with neighboring (Rh_0.5Pt_0.5)S₆ octahedra (Fig. 6).

Figure 6. A crystal structure model for ezochiite having a cubic aristotype spinel structure as generated using the VESTA program (Momma and Izumi, 2011). The structural model is based on Riedel et al. (1981).

RELATIONSHIP TO OTHER SPECIES

Before the nomenclature of the spinel supergroup was established, cuprorhodsite, malanite, and cuproiridsite were simply divided using the cation dominant rule in the Rh-Ir-Pt system as CuRh₂S₄, CuPt₂S₄, and CuIr₂S₄ (Fig. 1a). However, the change in the nomenclature (Bosi et al., 2019) has also modified these ideal chemical compositions. Consequently, based on the current classification system, the mineral compositions must be reexamined because the older cuprorhodsite and malanite compositions overlap with that of ezochiite.

Figure 7 shows the compositional distribution of thiospinels on the basis of their occurrences. Here, each locality in Hokkaido was assigned to an ophiolite and the associated data are plotted in the Rh-Ir-Pt and the Cu-Pt diagrams along with data from the Bulqiza complex and the Fenero-Ojén massif (Fig. 7a). The compositions of the Hokkaido samples are distributed across the linnaeite and carrollite subgroups, with the former being all cuprorhodsite and the latter all ezochiite. These data also exhibit a pronounced Rh-Pt rich trend and are poor in Ir. As a result, the compositions show a continuous distribution along a substitution line connecting cuprorhodsite (Cu⁺_0.5Fe³⁺_0.5)Rh³⁺₂S₄ and ezochiite Cu⁺(Rh³⁺Pt⁴⁺)S₄, indicating the occurrence of the Fe³⁺_0.5Rh³⁺-Cu⁺_0.5Pt⁴⁺ substitution. Minerals from the Bulqiza complex can also be classified as ezochiite and appear near the ideal substitution line. The compositions from the Fenero-Ojén massif are also in the ezochiite area although some are considerably richer in Ir. Those data appear in the Ir- and RhIr-dominant areas in the linnaeite and the carrollite subgroups, respectively. Despite these few exceptions, ezochiite is evidently a common mineral in ophiolites.

Figure 7. Compositional diagrams of thiospinels by occurrence type, showing ophiolites (a), Ural-Alaskan type intrusions (b), and mafic-ultramafic intrusions (c). The black line in the Cu-Pt diagram is a straight line connecting the end-member compositions of cuprorhodsite and ezochiite.

A review of previous reports of cuprorhodsite and malanite specimens from Ural-Alaskan intrusions shows that many of these compositions can be plotted in the regions currently associated with cuprorhodsite or ezochiite (Fig. 7b). Therefore, ezochiite is also common in the Ural-Alaskan intrusions. There is an exception for a sample from the Burgastain area. It is enriched in Pt and some of the associated data points appear in the Pt-rich region related to the linnaeite subgroup. Note that this composition has not yet been established as a mineral species. The compositions from the Kytlym complex are also typically rich in Ir and some can be plotted in the area associated with an RhIr composition in the carrollite subgroup, which also has not yet been established as a discrete mineral species.

Thiospinel samples previously obtained from mafic-ultramafic intrusions within an orthomagmatic deposit are currently also reclassified as cuprorhodsite or ezochiite, with some exceptions. Ezochiite is evidently common even in this type of deposit. Some compositions of the carrollite subgroup are rich in Rh and thus correspond to an as-yet unestablished 2Rh mineral species.

FORMATION OF EZOCHIITE

Isoferroplatinum is thought to be formed in a melt because the palladium-subgroup platinum-group elements (Rh, Pd, and Pt) are incompatible compared with the iridium-subgroup platinum-group elements (Ru, Os, and Ir) (e.g., Leblanc, 1991; Garuti et al., 2002). The composition of ezochiite (cuprorhodsite) in the Hokkaido samples was also rich in Rh and Pt, with little Ir and no Os or Ru (Table 1 and Fig. 7a). The environment in which ezochiite (cuprorhodsite) occurs appears to be limited to small cavities encased in isoferroplatinum grains. These cavities have morphologies ranging from spherical to elliptical and often contain other sulfide minerals, with combination of minerals varying from different cavities even in a single grain (Figs. 2 and 3). These findings suggest that isoferroplatinum trapped melts with different compositions during its own crystallization and growth, and that the trapped melts subsequently became a closed system and crystallized their respective minerals. Consequently, ezochiite (cuprorhodsite) is thought to be a primary mineral that crystallized directly from the trapped melt, such that the resulting assemblies of minerals and their specific compositions likely reflect the composition of the trapped melt. Since each mineral showed little evidence of a unique morphology, it appears that there was little to no difference in the crystallization temperature, with the exception of laurite from the Ainusawa river (Fig. 3b). This laurite had a cubic shape, suggesting that it was originally present as a small crystal in the melt. The melting point of laurite is also typically much higher than that of isoferroplatinum (e.g., Brenan and Andrews, 2001).

CONCLUSION

This work established a new mineral, ezochiite Cu⁺(Rh³⁺Pt⁴⁺)S₄, according to current nomenclature. The new mineral does not seem particularly rare, as it has long been reported to occur in various sources under the names cuprorhodsite and malanite. The ezochiite and cuprorhodsite identified in samples from the Hokkaido area form solid solution based on a coupled Fe³⁺_0.5Rh³⁺-Cu⁺_0.5Pt⁴⁺ substitution. In the case of these specimens, ezochiite (cuprorhodsite) was evidently crystallized from a melt trapped in isoferroplatinum.

ACKNOWLEDGMENTS

The authors thank Hirotada Gotou for providing support during the micro-XRD analyses using a position-sensitive proportional counter. Constructive reviews by anonymous reviewers improved the manuscript. With deep gratitude, Luca Bindi for insightful comment and meticulous editorial work.

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