Guite, the spinel-structured Co²⁺Co³⁺₂O₄, a new mineral from the Sicomines copper–cobalt mine, Democratic Republic of Congo

Abstract

Guite (IMA2017-080), Co₃O₄, is a new mineral species and an important economic mineral found in the Sicomines copper-cobalt mine, located ~11 km southwest of Kolwezi City, Democratic Republic of Congo. The mineral occurs as a granular agglomerate, 50 to 500 μm in size, and is associated closely with heterogenite in a quartz matrix. Guite is opaque, has a dark grey colour with metallic lustre and a black streak. In reflected light microscopy, it is white with no internal reflections. The reflectance values (in air, R in %) are: 27.0 (470 nm); 25.6 (546 nm); 25.2 (589 nm), and 24.6 (650 nm). The average of 20 electron-microprobe analyses is Co 71.53, Cu 0.58, Mn 0.67, Si 0.25, O 26.78, total 99.82 wt.%, corresponding to the empirical formula calculated on the basis of 4 O apfu: (Co²⁺_0.92Cu²⁺_0.02Si⁴⁺_0.02)_Σ0.96(Co³⁺_1.98Mn³⁺_0.03)_Σ2.01O_4.00, with Co²⁺ and Co³⁺ partitioned using charge balance. The ideal formula is Co²⁺Co³⁺₂O₄. Guite is cubic with space group Fd$\bar{3}$m. The unit cell parameters refined from the single crystal X-ray diffraction data are: a = 8.0898(1) Å, V = 529.436(11) ų and Z = 8. The calculated density of guite is 6.003 g/cm³. The eight strongest observed powder X-ray diffraction lines [d in Å (I/I0) (hkl)] are: 4.6714 (16.7) (111), 2.8620 (18.4) (220), 2.4399 (100) (311), 2.3348 (10.4) (222), 2.0230 (24.8) (400), 1.5556 (26.3) (511, 333), 1.4296 (37.7) (440) and 1.0524 (10.1) (731, 553). The crystal structure of guite was determined by single-crystal X-ray diffraction and refined to R = 0.0132 for 3748 (69 unique) reflections. Guite has a typical spinel-type structure with Co²⁺ in tetrahedral coordination with a Co²⁺–O bonding length of 1.941(1) Å, and Co³⁺ in octahedral coordination with a Co³⁺–O bonding length of 1.919(1) Å. The structure is composed of cross-linked framework of chains of Co³⁺–O₆ octahedra sharing the equilateral triangle edges (2.550 Å) in three directions [0 1 1], [1 1 0], [1 0 1] with Co²⁺ filling the tetrahedral interstices among the chains. Guite is named in honour of Prof. Xiangping Gu (1964–).

Introduction

Cobalt is a relatively rare but important metal widely used in production of super alloys, special steel, carbides, diamond tools, magnets, rechargeable batteries and many others (USGS, 2022). It also plays a vital catalytic role in life evolution and biochemical synthesis (He et al., 2021; Russell, 2022). In 2020, the worldwide mine production of cobalt amounted to 140,000 metric tons, with the biggest producer being the Democratic Republic of Congo (~95,000 tons), followed by Russia, Australia, Philippines, Cuba and Canada (Statista, 2021). In Nature, over 100 Co-bearing minerals have been identified, among which 66 mineral species have been approved by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC) (Hazen et al., 2017). Natural cobalt oxide minerals are very rare (Hey, 1962). The spinel-structured Co²⁺-oxide mineral cochromite, (Co,Ni,Fe)(Cr,Al)₂O₄, is one such, found in the Bon Accord nickel deposit, South Africa (De Waal, 1978). The Co³⁺ minerals such as linnaeite (Co²⁺Co³⁺₂S₄) and heterogenite (Co³⁺O(OH)) also occur in some metal sulfide mines (Hey, 1962; Deliens and Goethals, 1973). Although synthetic Co₃O₄ has been studied widely (e.g. Natta and Schmidt, 1926; Hendriks and Albrecht, 1928; Osaki, 2018), and the existence of natural Co₃O₄ predicted by Hazen et al. (2017), its occurrence in Nature has not been reported until now.

During a mineralogical investigation on the cobalt ores from the Sicomines copper–cobalt mine, Democratic Republic of Congo, a mineral with the composition of Co₃O₄ was found, characterised, submitted to the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC) and approved under IMA2017-080 (Lei et al., 2017). The mineral name ‘guite’ (symbol Gui) is in honour of Prof. Xiangping Gu (1964–) of Central South University, Changsha, Hunan province, China. Prof. Gu obtained a BS degree in 1983 and a MS degree in 1986, both from the Central South University, and a DSc degree in 2003 from Hiroshima University in Japan. In the past 40 years he has made significant contributions to mineralogical research and teaching in China; in particular to new mineral discoveries, and is the leading author of over 12, and the co-author of over 22 new minerals to date. The type material of guite is deposited in Geological Museum of China in Beijing with the catalogue number M13711 and a cotype sample is deposited at the RRUFF Project (deposition # R180022) (http://rruff.info).

Here, we present detailed descriptions of the morphology, composition, physical property, and crystallography of this mineral using optical microscopy, electron probe microanalysis (EPMA) and X-ray diffraction (XRD).

Occurrence and mineral association

Guite was found in the Sicomines copper-cobalt mine at 10°44'17.4"S, 25°22'50.4"E, ~11 km southwest of Kolwezi City, Democratic Republic of Congo. The Sicomines Cu–Co deposit is located at the northwest end of the world famous Katanga Cu–Co belt extending from Congo (Kinshasa) to Zambia (Fig. 1a). The Cu–Co ores are hosted in the Proterozoic Roan Formation, which is composed of red and grey–green sandstone and mudstone (R1 group), dolostone and dolomitic sandstone (R2 group) and dolomitic siltstone and mudstone (R3 group) (Fig. 1b). The Co-bearing dolomite in the R2 group is considered to be the source of cobalt (Chen et al., 2012). The samples were taken from the drilling cores in the R2 group (Fig. 1b). Guite is supposed to be a supergene cobalt oxide formed from the precipitation of a cobalt-bearing solution at weakly alkaline and oxidising conditions:

$$3{\rm Co(HC}{\rm O}_3{\rm )}_2 + {\rm }{\textstyle{{1}\over{2}}}{\rm O}_2 + {\rm }12{\rm O}{\rm H}^\ndash \to {\rm C}{\rm o}_3{\rm O}_4 + {\rm }6{\rm C}{\rm O}_3^{2\ndash } + {\rm }9{\rm H}_2{\rm O}.$$

Fig. 1. (a) Simplified regional geological map of the Katanga Cu–Co belt and the location of the Sicomines mine (after Zhao, 2016). Legend: 1. Neozoic cover; 2. Basement strata; 3. Nguba and Kundelungu Formations; 4. Roan Formation; 5. Cu–Co deposits. (b) Plan geological map of the Sicomines Cu–Co deposit (after Chen et al., 2012). Legend; 1: R3 Group of Roan Formation: dolomitic siltstone; 2: R2 Group of Roan Formation: talc-bearing argillaceous dolostone, silicified dolostone, dolomitic shale and siltstone; 3: R3 Group of Roan Formation: coarse-grained arkoses and conglomerate; 4: faults; 5: sampling locations.

Appearance and physical properties

Guite occurs as granular aggregates up to 500 μm in size associated closely with heterogenite in quartz, in which anhedral to subhedral single crystals of guite are estimated to be several to tens of μm (Fig. 2). Guite has a dark grey colour with metallic opaque lustre and black streak. The Mohs hardness is estimated to be 6–6.5. The mineral is brittle with uneven fracture. The calculated density is of 6.003 g/cm³ according to the empirical formula and unit cell volume from powder X-ray diffraction. Guite is non-magnetic as tested by a magnet needle.

Fig. 2. Photomicrographs in reflected light (a–c) and back-scattered electron image (d) of guite. (a) Guite filling the interstitial space between quartz; (b) guite replaced by heterogenite in the interstitial space of quartz; (c) compositional zonation of guite – the core of almost pure Co₃O₄ surrounded by Mn-, Cu-bearing Co₃O₄, further replaced by heterogenite; and (d) back-scattered electron image of the area framed in (c).

In reflected light microscopy, guite has a white colour with no internal reflections. Zonar variation of reflectance and colour, from bright white (Fig. 2a,b) to brownish grey (Fig. 2c), is observed due to compositional zonation. The reflectance values (R, %) at different wavelength (λ, nm) measured using TIDAS MSP400 attached to LEICA DM2500p with SiC standard are shown in Table 1. The colour index is calculated relative to an equienergy lamp source. The data indicate that the reflectance of guite is ~25.4% for pure Co₃O₄ and apparently decreases to 22.9% when Cu, Mn and Si are incorporated (Table 1).

Table 1. Reflectance data for guite.

Colour index:

[1] x = 0.3298, y = 0.3325, Y = 25.4, λ_d = 480 nm, Pe = 1.44%

[2] x = 0.3320, y = 0.3335, Y = 22.9, λ_d = 481 nm, Pe = 0.55%

Chemical composition

The chemical composition of guite was determined on a Shimadzu EPMA-1720 microprobe by wavelength dispersive spectroscopy with operating conditions of accelerating voltage = 15 kV, beam current = 10 nA and beam size = 1 μm. Detectable elements include Co, Cu, Mn, Si and O; the contents of Fe, Mg, V and Ni are below the detection limits (~200 ppm) according to qualitative scans at specific wavelengths. Quantitative analysis was carried out using the following standards (and lines): pure Co (CoKα); pure Cu (CuKα); pure Mn (MnKα); pure Si (SiKα), and pure Fe₃O₄ (OKα). The ZAF4 correction program supplied with the instrument was used for the correction calculation. Compositional zoning is often observed (e.g. Fig 2c), mainly due to the variation of the contents of Cu (up to 2.50 wt.%) and Mn (up to 1.78 wt.%). The average of 20 electron-microprobe analyses is Co 71.53, Cu 0.58, Mn 0.67, Si 0.25, O 26.78, total 99.82 wt.% (Table 2). The empirical formula calculated on the basis of 4 oxygen atoms per formula unit and with partitioning of Co²⁺ and Co³⁺ to achieve charge balance is: (Co²⁺_0.92Cu²⁺_0.02Si⁴⁺_0.02)_Σ0.96(Co³⁺_1.98Mn³⁺_0.03)_Σ2.01O_4.00. The simplified formula is (Co²⁺,Cu²⁺,Si⁴⁺)(Co³⁺,Mn³⁺)₂O₄ and the ideal formula is Co²⁺Co³⁺₂O₄.

Table 2. Chemical data for guite (wt.%, N = 20).

S.D. – standard deviation

Raman spectroscopy

Raman spectra of randomly oriented crystals of guite were measured at two laser wavelengths (532.1 and 632.8 nm) with a Horiba LabRam ARAMIS instrument (laser power = 5 mW, resolution = 2 cm^–1 and scan time = 10 min). All the five theoretical Raman-active modes, calculated from the factor-group analysis on synthetic Co₃O₄ (Hadjiev et al., 1988), are observed respectively at 197 (F_2g), 487 (E_g), 530 (F_2g), 625–630 (F_2g) and 693–697 (A_1g) cm^–1 (Fig. 3). The Raman shifts may be attributed to the Co–O stretching vibration modes (500–700 cm^–1) and the O–Co–O bending vibration modes (100–500 cm^–1) in CoO₄ tetrahedra and CoO₆ octahedra. The relative intensities of peaks vary for different laser wavelengths. The spectrum of 532.1 nm has the strongest peak at 693–697 (A_1g) cm^–1 and the spectrum of 632.8 nm has the strongest peak at 197 (F_2g) cm^–1.

Fig. 3. Raman spectra of guite activated at two laser wavelengths.

Crystallography and crystal structure

Powder X-Kray diffraction data were obtained respectively using a Rigaku D/Max Rapid IIR diffractometer (CuKα; 40 kV; 250 mA; 0.1 mm beam diameter; and exposure time of 1 hour) and a Rigaku Synergy single-crystal diffractometer in Gandolfi powder mode (MoKα; 50 kV; 1 mA; 0.2 mm beam diameter; and exposure time of 20 min). The reflection data are given in Table 3. The eight strongest lines [d in Å (I/I₀) (hkl)] are: 4.6714 (16.7) (111), 2.8620 (18.4) (220), 2.4399 (100) (311), 2.3348 (10.4) (222), 2.0230 (24.8) (400), 1.5556 (26.3) (511, 333), 1.4296 (37.7) (440) and 1.0524 (10.1) (731, 553). The unit cell parameters refined from the powder X-ray diffraction data are: a =8.0848(1) Å, V = 528.45(2) Å³ and Z = 8. The calculated density of guite is 6.003 g/cm³ according to the empirical formula.

Table 3. X-ray powder diffraction data (I in %, d in Å) for guite.*

* The strongest lines are given in bold.

The single-crystal X-ray diffraction data for a guite crystal, ~10 μm in size, were collected on Rigaku XtaLAB Synergy-DS diffractometer with microfocus sealed Mo anode tube at 50 kV, 1 mA and 25 s of frame exposure time. The diffraction data were processed with the Rigaku program CrysAlisPro. The crystal structure was determined and refined using SHELX (Sheldrick, 2015a,b) included in the software Olex2 (Dolomanov et al., 2009). The crystallographic data and refinement statistics are given in Table 4. The structure was solved in space group Fd $\bar{3}$m and refined with anisotropic displacement for all sites. The occupancies of Co, Cu, Mn and Si at the Co1 and Co2 sites were fixed manually according to empirical formula from the chemical compositions. The final anisotropic full-matrix least-squares refinement on F ² for 7 parameters was completed with R ₁ = 1.32% and wR ₂ = 3.52% for all 2007 (69 unique) reflections. The atomic coordinates and displacement parameters are listed in Table 5, and selected bond lengths and angles in Table 6. The bond-valence sums of atoms, calculated according to the actual composition using the parameters given by Brese and O'Keeffe (1991), are presented in Table 7. The structure is illustrated in Fig. 4. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Fig. 4. (a) Crystal structure of guite viewed in [0 1 1], showing the chains of CoO₆ octahedra (green) along the directions [0 1 1], [1 0 1] and [1 1 0] with isolated CoO₄ tetrahedra (blue) occupying the interstitial space of the chains. (b) The shape and edge lengths of CoO₆ octahedra (twisted triprism) and CoO₄ tetrahedra.

Table 4. Information on crystal and structural refinement for guite.

Table 5. Atomic coordinates and displacement parameters (in Å²) for guite.*

* Occupancies: Co1: Co_0.92Si_0.02Cu_0.02Mn_0.01; Co2 Co_0.99Mn_0.01; O1: O_1.00

Table 6. Selected bond distances and angles for guite.

Table 7. Calculated bond-valence sums (in valence units) for atoms in guite.*

* Bond-valence parameters were taken from Brese and O'Keeffe (1991)

Guite has the spinel-type structure in which the Co1 site is in tetrahedral coordination and occupied by Co²⁺ with a Co–O distance of 1.942 Å, and the Co2 site is in octahedral coordination and occupied by Co³⁺ with a Co–O distance of 1.919 Å (Table 6). In compliance with the site symmetry $\bar{3}$m, the six faces of Co³⁺–O₆ octahedra are composed of two equilateral triangles with the O–O distance of 2.550 Å and four isosceles triangles comprising two O–O of 2.689 Å and one O–O of 2.550 Å, which may be described more precisely by a twisted triprism (Fig. 4b). The Co³⁺–O₆ octahedra share the edges at 2.550 Å to form a framework of chains of octahedra in three directions of [0 1 1], [1 1 0] and [1 0 1] with Co²⁺ occupying the interstitial spaces in tetrahedral coordination among the chains (Fig. 4a). The valence states of Co²⁺ at the Co1 site and Co³⁺ at the Co2 site are confirmed by the bond valence sums of the two sites (Table 7), and the different Co–O bonding lengths of the two sites (1.942 vs. 1.919 Å) suggest a high spin electronic structure for Co²⁺ and a low spin electronic structure for Co³⁺ according to the ionic radii of Shannon (1976).

Discussions and implications

In literature, numerous structural and physico-chemical data of synthetic Co₃O₄ have been reported (e.g. Natta and Schmidt, 1926; Hendriks and Albrecht, 1928; Liu and Prewitt, 1990; Douin et al., 2009; Osaki, 2018). All of them have the spinel structure with highly ordered occupation of Co²⁺ in the tetrahedral site and Co³⁺ in the octahedral site at room temperature and ambient atmosphere, with the unit cell a ranging from 8.065 Å to 8.086 Å, the Co–O_tet bonding lengths in CoO₄ tetrahedra from 1.908 Å to 1.988 Å and the Co–O_oct bonding lengths in CoO₆ octahedra from 1.893 Å to 1.933 Å. A linear regression between Co–O_tet and Co–O_oct yields:

$${\rm Co} - {\rm O}_{{\rm oct}} = - 0.4774\times {\rm Co} - {\rm O}_{{\rm tet}} + 2.8431\ ( {R^2 = 0.9667, \;N = 9} ).$$

The negative correlation between Co–O_tet and Co–O_oct seems to indicate a variation of Co²⁺–Co³⁺ distribution between the tetrahedral site and the octahedral site. On the other hand, the high-temperature structural determinations of synthetic Co₃O₄ indicated a tendency of disorder of Co²⁺–Co³⁺ distribution between the tetrahedral site and the octahedral site at high temperatures (e.g. Liu and Prewitt, 1990; Hazen and Yang, 1999; Douin et al., 2009).

Guite is a member of the oxyspinel group in the spinel supergroup (Bosi et al., 2019), which may be classified into Al-, V-, Cr-, Fe-, Co-subgroups according to the elements in the octahedral site, with various end-members of Mg, Si, V, Mn, Fe, Co, Cu, Zn, Ge and Sb occupying the tetrahedral site. The spinel structure is of special significance for probing the material states in the deep mantle (e.g. Hazen and Yang, 1999). As summarised and compared in Table 8, guite is outstanding in the group as it has the smallest unit cell, the biggest density and the highest reflectance. The presence of Si (up to 0.02 apfu), Mn (up to 0.08 apfu) and Cu (up to 0.09 apfu) in guite may predict the natural existence of end members SiCo²⁺₂O₄, Mn²⁺Co³⁺₂O₄ and CuCo³⁺₂O₄, which have been synthesised in the laboratory (e.g. Morimoto et al., 1974; Gautier et al., 1982; Petrov et al., 1989). It is interesting to note that hausmannite (Mn²⁺Mn³⁺₂O₄) and hetaerolite (ZnMn³⁺₂O₄) adopt a tetragonal structure, topologically similar to the spinel-type structure (Yamamoto et al., 1983; Jarosch, 1987). The high-pressure transformation of guite, such as that of chromite to xieite and chenmingite (Chen et al., 2008; Ma et al., 2019) and magnesioferrite to maohokite (Chen et al., 2018), is still unknown though it has been shown to be stable up to 1201 K and 8.7 GPa (e.g. Liu and Prewitt, 1990; Golosova et al., 2020).

Table 8. Unit cell data, calculated density, bonding lengths in tetrahedra and octahedra, and reflectance of the minerals and high pressure phases in the oxyspinel group.

*high-pressure phase. 1. This work; 2. Knop et al. (1968); 3. Fregola et al. (2012); 4. Cámara et al. (2019); 5. Nair et al. (2014); 6. Palache et al. (1944), Hill (1984); 7. Cooley and Reed (1972); 8. Verger et al. (2016); 9. Rao (pers. Comm.); 10. Reznitskii et al. (1995); 11. Plumier (1962); 12. Reuter et al. (1969); 13. O'Neill et al. (1994); 14. Hastings and Corliss (1962); 15. Palache et al. (1944), Kyono et al. (2012); 16. Ma et al. (2019); 17. Ishii et al. (2014); Chen et al. (2008); 18. De Waal (1978), Hirota et al. (1990); 19. Ueno et al. (1999); 20. Nesterov and Rumyantseva (1987), Bosi et al. (2011); 21. Palache et al. (1944), Gaudon et al. (2009); 22. Chen et al. (2018); 23. Rashmi et al. (2017); 24. Palache et al. (1944); 25. Kiselev et al. (2007); 26. Ahmed et al. (2012); 27. Waerenborgh et al. (1994); 28. Sasaki et al. (1982); 29. Yagi et al. (1974); 30. Forster and Hall (1965); 31. Ottemann and Nuber (1972), Welch et al. (2001); 32. Bonazzi et al. (2013), Dunn et al. (1988); 33. Bosi et al. (2014); 34. Solano et al. (2014); Bozi et al. (2019); 35. Collyer et al. (1988); Bozi et al. (2019); 36. Jarosch (1987); and 37. Yamamoto et al. (1983).

In the Li-ion batteries industry, synthetic Co₃O₄ has attracted wide attention as an anode material for its low cost and high theoretical capacity (890 mA h/g) (e.g. Zhang et al., 2017; Xiao et al., 2018). In the Sicomines mine, guite is an important economic mineral and accounts for about one-fifth of the cobalt resource; this discovery of guite originated from the work to increase the recovery rate of cobalt from the ores.