LETTER
Cuprospinel from Aogashima Island, Tokyo, Japan
2025 Volume 120 Issue 1 Article ID: 241017b
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2025 Volume 120 Issue 1 Article ID: 241017b
Cuprospinel, which is rarely been found in nature, was identified on the Yuhama coast in Aogashima Island, Tokyo, Japan. Cuprospinel occurred as one of the sublimated minerals in a rim of black spherical aggregates with hematite as the core. In one sample, the cuprospinel was found to have a composition of (Cu0.86Fe2+0.08Mg0.04Mn0.02Ca<0.01)(Fe3+1.98Al0.01Ti<0.01)O4, and exhibited a crystal structure with tetragonal symmetry (I41/amd) and unit cell parameters of a = 5.826(2) Å, c = 8.682(5) Å, and V = 294.7(3) Å3. Another sample had a composition of (Cu0.60Mg0.21Fe2+0.10Mn0.05Ca0.03)(Fe3+1.99Al0.01Ti<0.01)O4, and its crystal structure exhibited cubic symmetry (Fd3m) with unit cell parameters of a = 8.358(3) Å and V = 583.7(6) Å3. Both samples may have undergone growth at high temperature and subsequent annealing at low temperature, so that the difference in crystal structure may be mainly due to the abundance of Cu2+ Jahn-Teller cations. This is the first demonstration of the occurrence of tetragonal cuprospinel in a natural environment.
Cuprospinel, with an ideal composition of Cu2+Fe3+2O4, was described as a new mineral species that occurred in a burnt ore dump at the Rambler mine, Canada, where it was formed as the result of spontaneous combustion of mined copper-zinc ore (Nickel, 1973). Its formation process was therefore semi-artificial. Cuprospinel is also known to occur as an incidental product of ore processing in smelters (e.g., Lanteigne et al., 2012). In nature, the occurrence of cuprospinel has been reported in volcanic material and gold deposits (Mallick et al., 2005; Sholeh et al., 2016); however, no analytical evidence has been presented. Thus, cuprospinel with substantial evidence of natural origin was unknown for a long time. Completely natural cuprospinel was first found as a sublimated mineral in oxide fumaroles associated with the Tolbachik volcano, Kamchatka Peninsula, Russia (Pekov et al., 2018), and its crystal structure was found to be cubic. On the other hand, it has also been reported that cuprospinel can have cubic and tetragonal phases depending on the composition and temperature by synthetic experiments (e.g., Ohbayashi et al., 1966; Singhal et al., 2013).
In 2021-2022, we surveyed Aogashima Island in Tokyo and discovered the occurrence of several sublimated minerals, including cuprospinel. Furthermore, the cuprospinel we identified had a tetragonal crystal structure, which has not previously been found in a naturally occurring sample. Here, we focus on cuprospinel from Aogashima Island and briefly report its mineralogical features.
Aogashima Island is a volcanic island belonging to the Izu Islands, Tokyo, Japan. The survey site was Yuhama coast, located in the southern part of Aogashima Island (32°26′47′′N 139°45′43′′E). Supplementary Figure S1 shows a view of the survey site in 2021 (Supplementary Figs. S1 and S2 are available online from https://doi.org/10.2465/jmps.241017b). Yuhama coast is covered with basalt to andesite blocks that range in size from several meters to tens of meters. These blocks are considered to have been caused by a collapse of the somma. At present, no fumarolic activity has been observed at the somma or the Yuhama coast, although sublimated minerals are present along the cracks and gaps in the blocks (Fig. S1). These minerals are thus considered to have been formed during past volcanic activity. Representative sublimated minerals are shown in Supplementary Figure S2; augite, aegirine-augite, volborthite, tenorite, chlorapatite and cuprorivaite occur in these cracks and gaps (Nishio-Hamane et al., 2022).
Cuprospinel was identified as one of the sublimated minerals and occurs in the rim of black, opaque, spherical aggregates up to 1 mm in size, which often have a hematite core (Fig. 1). The thickness of the cuprospinel rim varies from aggregate to aggregate. Although crystal faces are occasionally observed on the outermost surface of the aggregate, no grain boundaries are observed in the cross section. The occurrence of cuprospinel on hematite is common to the Tolbachik volcano (Pekov et al., 2018). In the Aogashima Island sample, small magnesioferrite particles are occasionally found in the hematite core. No other minerals are found in the assemblage, although pyroxene and/or volborthite are typically scattered around the hematite-cuprospinel aggregates.
The chemical composition of the cuprospinels varied from sample to sample, with the main variation being in the Cu content. Here, a sample with high Cu content (sample #1) and a sample with low Cu content (sample #2) are treated as representative. The chemical compositions of these samples were analyzed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS; IT100, JEOL) at 15 kV and 0.8 nA. Cu metal, Fe metal, MgO, Mn metal, CaSiO3, Al2O3, and KTiPO5 were used as standards for Cu, Fe, Mg, Mn, Ca, Al, and Ti, respectively. The Fe2+ and Fe3+ ratios were estimated based on stoichiometry. The analytical results are summarized in Table 1. Sample #1 has an empirical formula of (Cu0.86Fe2+0.08Mg0.04Mn0.02Ca<0.01)(Fe3+1.98Al0.01Ti<0.01)O4, while that of sample #2 is (Cu0.60Mg0.21Fe2+0.10Mn0.05Ca0.03)(Fe3+1.99Al0.01Ti<0.01)O4, which is in the cuprospinel composition range, but with a lower Cu content than sample #1.
| Sample #1 | Sample #2 | ||||
| wt% (n = 5) | wt% (n = 8) | ||||
| Average. | Range. | Average. | Range. | ||
| CuO | 28.55 | 28.04-29.49 | CuO | 20.68 | 18.88-22.67 |
| MgO | 0.64 | 0.39-0.86 | MgO | 3.74 | 2.77-5.22 |
| MnO | 0.72 | 0.37-1.19 | MnO | 1.65 | 0.43-3.27 |
| CaO | 0.07 | 0-0.17 | CaO | 0.69 | 0.05-3.58 |
| Fe2O3 | 68.83 | 67.70-69.17 | Fe2O3 | 72.13 | 68.51-74.63 |
| Al2O3 | 0.11 | 0-0.25 | Al2O3 | 0.17 | 0.03-0.38 |
| TiO2 | 0.16 | 0-0.46 | TiO2 | 0.03 | 0-0.21 |
| Total | 99.08 | Total | 99.07 | ||
| Fe2O3* | 66.16 | Fe2O3* | 68.62 | ||
| FeO* | 2.40 | FeO* | 3.15 | ||
| Total | 98.81 | Total | 98.72 | ||
| *calc. | *calc. | ||||
| apfu | apfu | ||||
| Cu | 0.86 | Cu | 0.60 | ||
| Fe2+ | 0.08 | Fe2+ | 0.10 | ||
| Mg | 0.04 | Mg | 0.21 | ||
| Mn | 0.02 | Mn | 0.05 | ||
| Ca | 0.00 | Ca | 0.03 | ||
| Fe3+ | 1.98 | Fe3+ | 1.99 | ||
| Al | 0.01 | Al | 0.01 | ||
| Ti | 0.00 | Ti | 0.00 | ||
| O= | 4 | O= | 4 | ||
Powder X-ray diffraction (XRD; Ultrax18, Rigaku) measurements were performed on samples #1 and #2 using a micro-XRD technique with CrKα radiation at 40 kV and 150 mA. The diffractometer incorporated a 100 µm collimator equipped with a curved position-sensitive proportional counter and an oscillating sample stage. Figure 2 shows powder XRD patterns for the cuprospinels. The patterns were clearly different between the samples, and indicated a tetragonal crystal structure for sample #1 with a Cu0.86 content and a cubic crystal structure for sample #2 with a Cu0.60 content. Sample #1 can be indexed as tetragonal with the I41/amd space group and unit cell parameters of a = 5.826(2) Å, c = 8.682(5) Å, and V = 294.7(3) Å3. Sample #2 can be indexed as cubic with the Fd$\bar{3}$m space group and unit cell parameters of a = 8.358(3) Å and V = 583.7(6) Å3. The diffraction data are summarized in Supplementary Table S1 (Supplementary Table S1 is available online from https://doi.org/10.2465/jmps.241017b).
The chemical composition of cuprospinel from sample #1 was closest to the end member of any cuprospinel reported to date (Fig. 3). On the other hand, sample #2, which had almost the same occurrence, had a lower Cu content than sample #1, even though the Fe3+ content was the same. This may be due to differences in the gas composition, temperature, and/or oxygen fugacity during formation. Synthesis experiments have revealed that cuprospinel forms a wide range of solid solutions in the presence of accessory components (e.g., Mounkachi et al., 2012; Singhal et al., 2013). However, these experiments were conducted at temperatures much higher than those experienced by natural samples, and the solid solutions of cuprospinel at temperatures equivalent to those in natural samples are still largely unknown. The conditions for the formation of the Aogashima samples are also not clear. Nevertheless, the discovery of Aogashima sample #1 has demonstrated that cuprospinel close to the end-member composition occurs in nature.
The crystal structure of cuprospinel varied between the samples; tetragonal for sample #1 with high Cu content (Cu0.86) and cubic for sample #2 with low Cu content (Cu0.60) (Fig. 2). Table 2 summarizes the compositions and crystal structures of the cuprospinels reported to date. The sample #1 cuprospinel with the highest Cu and Fe3+ contents is the only sample with a tetragonal crystal structure. Synthesis experiments have shown that cuprospinel with the ideal CuFe2O4 composition is stable in a tetragonal phase at room temperature (Ohbayashi et al., 1966; Balagurov et al., 2013). The sublimated minerals in Aogashima Island contain hydrous minerals such as volborthite, which suggests that even minerals formed at high temperatures were subjected to subsequent low-temperature conditions. Therefore, the tetragonal sample #1 cuprospinel is likely the result of annealing at a low-temperature or crystallization at low temperature. However, cuprospinel does not contain low-temperature minerals within its interior; therefore, formation at high temperature and annealing at low temperature are more likely.
| Cu and Fe3+ (apfu) | Symmetry | a (Å) | c (Å) | Reference |
| Cu0.86 Fe3+1.98 | Tetragonal | 5.826 | 8.682 | Sample #1: This study |
| Cu0.60 Fe3+1.99 | Cubic | 8.358 | = a | Sample #2: This study |
| Cu0.80-0.57 Fe3+2.00-1.89 | Cubic | 8.369 | = a | Nickel (1973) |
| Cu0.83-0.57 Fe3+1.90-1.02 | Cubic | 8.402 | = a | Pekov et al. (2018) |
Sample #2 with a lower copper content (Cu0.60) has a cubic phase. The occurrence is almost the same for samples #1 and #2; therefore, it is assumed that the thermal history is also the same, and the origin of the cubic phase is thus likely due to a solid solution effect. When the amount of Cu2+ Jahn-Teller ions is small, the spinel structure is stable in the cubic phase. For example, for x = 0.6 or less, the CuxMg1−xFe3+2O4 system exhibits a cubic crystal structure (Singhal et al., 2013). Only cubic phase cuprospinels were reported by Nickel (1973) and Pekov et al. (2018) (Table 2), although some have high Cu contents (Fig. 3). These reports did not discuss any correspondence between the composition and the crystal structure determined by XRD measurements; therefore, it is possible that samples with low Cu contents were used. Another possibility is the thermal history. These samples were taken from active fumaroles or from a burned ore dump, and so may have been collected at high temperature and then rapidly cooled. For ideal CuFe2O4, the region of stability for the cubic phase is above 360-440 °C (e.g., Ohbayashi et al., 1966; Balagurov et al., 2013), and thus the cubic phase may be the result of rapid cooling. To understand the origin of the symmetry of cuprospinel, it is necessary to clarify the correspondence between composition, XRD, and thermal history, but these points were unclear in the reports of Nickel (1973) and Pekov et al. (2018).
In conclusion, cuprospinel from Aogashima Island was determined to have two different crystal structures, tetragonal and cubic, mainly due to the influence of composition. Considering the location (Fig. S1), the cuprospinel-bearing blocks were most likely originated from the somma. Although the original location of the blocks is not clear, the composition of the volcanic gases, temperature, and/or oxygen fugacity were likely different at each location, which was the reason the cuprospinels occurred with different compositions. However, the volcanic activity when the cuprospinels occurred has already subsided, so that the specific conditions for the formation of cuprospinel have not yet been determined.
We would like to express our gratitude to all the islanders who warmly welcomed us during our survey on Aogashima Island. We would also like to thank those who provided information to mindat.org, which helped us to begin this study.
Supplementary Figures S1-S2 and Table S1 are available online from https://doi.org/10.2465/jmps.241017b.
