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., Reference Chen, Liu, Yang, Wang, He and Li2012). 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, Reference Zhao2016). 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., Reference Chen, Liu, Yang, Wang, He and Li2012). 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., Reference Hadjiev, Ilievl and Vergilov1988), 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, Reference Sheldrick2015a,Reference Sheldrickb) included in the software Olex2 (Dolomanov et al., Reference Dolomanov, Bourhis, Gildea, Howard and Puschmann2009). 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 (Reference Brese and O'Keeffe1991), 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 (Reference Brese and O'Keeffe1991)

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 (Reference Shannon1976).