Copper in Natural Oxide Spinels: The New Mineral Thermaerogenite CuAl₂O₄, Cuprospinel and Cu-Enriched Varieties of Other Spinel-Group Members from Fumaroles of the Tolbachik Volcano, Kamchatka, Russia

Abstract

This paper is the first description of natural copper-rich oxide spinels. They were found in deposits of oxidizing-type fumaroles related to the Tolbachik volcano, Kamchatka, Russia. This mineralization is represented by nine species with the following maximum contents of CuO (wt.%, given in parentheses): a new mineral thermaerogenite, ideally CuAl₂O₄ (26.9), cuprospinel, ideally CuFe³⁺₂O₄ (28.6), gahnite (21.4), magnesioferrite (14.7), spinel (10.9), magnesiochromite (9.0), franklinite (7.9), chromite (5.9), and zincochromite (4.8). Cuprospinel, formerly known only as a phase of anthropogenic origin, turned out to be the Cu-richest natural spinel-type oxide [sample with the composition (Cu_0.831Zn_0.100Mg_0.043Ni_0.022)_Σ0.996(Fe³⁺_1.725Al_0.219Mn³⁺_0.048Ti_0.008)_Σ2.000O₄ from Tolbachik]. Aluminum and Fe³⁺-dominant spinels (thermaerogenite, gahnite, spinel, cuprospinel, franklinite, and magnesioferrite) were deposited directly from hot gas as volcanic sublimates. The most probable temperature interval of their crystallization is 600–800 °C. They are associated with each other and with tenorite, hematite, orthoclase, fluorophlogopite, langbeinite, calciolangbeinite, aphthitalite, anhydrite, fluoborite, sylvite, halite, pseudobrookite, urusovite, johillerite, ericlaxmanite, tilasite, etc. Cu-bearing spinels are among the latest minerals of this assemblage: they occur in cavities and overgrow even alkaline sulfates. Cu-enriched varieties of chrome-spinels (magnesiochromite, chromite, and zincochromite) were likely formed in the course of the metasomatic replacement of a magmatic chrome-spinel in micro-xenoliths of ultrabasic rock under the influence of volcanic gases. The new mineral thermaerogenite, ideally CuAl₂O₄, was found in the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption. It forms octahedral crystals up to 0.02 mm typically combined in open-work clusters up to 1 mm across. Thermaerogenite is semitransparent to transparent, with a strong vitreous lustre. Its colour is brown, yellow-brown, red-brown, brown-yellow or brown-red. The mineral is brittle, with the conchoidal fracture, cleavage is none observed. D(calc.) is 4.87 g/cm³. The chemical composition of the holotype (wt.%, electron microprobe) is: CuO 25.01, ZnO 17.45, Al₂O₃ 39.43, Cr₂O₃ 0.27, Fe₂O₃ 17.96, total 100.12 wt.%. The empirical formula calculated on the basis of 4 O apfu is: (Cu_0.619Zn_0.422)_Σ1.041(Al_1.523Fe³⁺_0.443Cr_0.007)_Σ1.973O₄. The mineral is cubic, Fd-3m, a = 8.093(9) Å, V = 530.1(10) ų. Thermaerogenite forms a continuous isomorphous series with gahnite. The strongest lines of the powder X-ray diffraction pattern of thermaerogenite [d, Å (I, %) (hkl)] are: 2.873 (65) (220), 2.451 (100) (311), 2.033 (10) (400), 1.660 (16) (422), 1.565 (28) (511) and 1.438 (30) (440).

1. Introduction

Oxide spinels compose one of the most studied mineral families and have numerous implications in the geosciences, chemistry and materials science [1]. In spite of the apparent simplicity, the spinel-type crystal structure exhibits a remarkable flexibility towards cation and anion substitutions, which results in the appearance of minerals accommodating more than two dozens of chemical elements [2]. Copper–bearing oxide spinels, being well studied as synthetic phases in materials science, are however virtually unknown in nature. The only reported Cu-rich spinel-type oxide mineral, cuprospinel, ideally CuFe₂³⁺O₄, is in fact an anthropogenic phase as it has never been found in the natural environments unaffected by anthropogenic influence factors. Cuprospinel was described as a new mineral species from burnt dumps on the property of Consolidated Rambler Mines Limited near Baie Verte, Newfoundland, Canada, where, together with hematite and a Cu-rich (13.9 wt.% CuO) variety of magnesioferrite, it was formed in the result of a spontaneous fire of the mined copper-zinc ore [3]. Thus, these oxides have definite anthropogenic origin, as well as cuprospinel formed as an incidental product of ore processing in ancient and modern smelters [4,5]. Cuprospinel was mentioned in a volcanic material from Mahanadi, Orissa, India [6], and in ores of the Chahnaly gold deposit in SE Iran [7], however, no analytical evidences were given and the identification of this mineral from both localities seems doubtful.

In the course of ongoing research of oxidizing-type fumaroles related to the Tolbachik volcano at Kamchatka, Russia, we have encountered more than twenty oxide minerals. Among those, ten mineral species belonging to the spinel supergroup [2] were identified, including the new mineral deltalumite, (Al_0.67□_0.33)Al₂O₄, the delta-alumina dimorphous with corundum [8]. A remarkable chemical feature of Tolbachik spinels is the common presence of a significant amount of copper. CuO contents ranging from 1 to 18 wt.% are typical for spinel, magnesioferrite, gahnite, franklinite, magnesiochromite, chromite and zincochromite. Besides these Cu-rich mineral varieties, two minerals with species-defining copper were therein discovered: the genuine natural cuprospinel and a new mineral first described in the present paper, thermaerogenite, ideally CuAl₂O₄. The latter was recently approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA no. 2018-021). The name thermaerogenite (Cyrillic: термаэрoгенит) is constructed based on the combination of Greek words θερμός, hot, αέριον, gas, and γενής that means “born by”. Thus, in whole it means born by hot gas, that reflects the fumarolic origin of the mineral. This name also contains the allusion that Cu-rich spinel-type oxides form in volcanic fumaroles and their anthropogenic counterparts, unlike Cu-free and Cu-poor oxide spinels, are numerous and widespread in other geological formations. The type specimen of thermaerogenite is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5192/1.

The Mg-Al-Fe oxide spinels, namely magnesioferrite, magnetite and spinel were previously reported from fumaroles of several active volcanoes [9,10,11,12,13]. However, no information on Cu contents in these minerals was published. Moreover, we did not find any references on Cu-enriched (with CuO content higher than 1 wt.%) natural oxide spinels. Thus, this work is the first report on natural copper-rich oxide members of the spinel supergroup.

2. Occurrence and Mineral Associations

The studied material was collected by us during fieldwork in the period of 2012–2018. The majority of studied samples originate from the Arsenatnaya fumarole, one of the brightest in the mineralogical aspect examples in the world of fumaroles belonging to the oxidizing type (in fumaroles of this type, the increase of oxygen fugacity is a result of the mixing of volcanic gases with the atmospheric air [14,15]). Arsenatnaya is located at the apical part of the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption (below–NB GTFE), Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia (55°41′ N 160°14′ E, 1200 m asl). This scoria cone, formed in 1975, is a monogenetic volcano about 300 m high and approximately 0.1 km³ in volume [16]. Now, more than 40 years after the eruption, its fumarole fields remain active: numerous gas vents with temperatures up to 490 °C were observed by us in 2012–2018. Now the fumarolic gases at the Second scoria cone are compositionally close to atmospheric air, with the contents of <1 vol.% water vapour and <0.1 vol.% acid species, mainly CO₂, HF and HCl [17], while in 1976–1977 these gases were significantly more enriched in H₂O, CO₂, SO₂, HCl and, in some fumaroles, HF [14].

The Arsenatnaya fumarole was uncovered and first studied by us during fieldworks in July 2012. This active fumarole described in papers [18,19] is a near-meridional linear system of mineralized pockets (up to 10–15 cm wide) and cracks situated between blocks of basalt scoria and volcanic bombs in the near-surface part of the scoria cone. The length of the hot area belonging to Arsenatnaya is about 15 m and its width varies from 1–1.5 m in the southern end to 3–4 m in the northern part. Numerous strongly mineralized pockets occur at depths from 0.3 to 4 m. The sublimate minerals form incrustations in the open space of the pockets, fill cracks and pores or replace basalt. Arsenatnaya is one of the hottest fumaroles at the Second scoria cone of the NB GTFE: the temperature measured by us using chromel-alumel thermocouple in 2012–2018 in different pockets immediately after their partial uncovering varies from 360 to 490 °C and, in general, increases with depth. About 160 valid minerals (including 46 new species first discovered here) and >30 insufficiently studied mineral phases have been identified in this unique mineralogical site.

The Cu-bearing oxide spinels in the Arsenatnaya fumarole were found in two mineral assemblages.

The minerals with Al or Fe³⁺ as species-defining components (spinel, gahnite, thermaerogenite, magnesioferrite, franklinite, and cuprospinel) occur in the intermediate in depth, in the polymineralic zone of the fumarole [19] and are associated with tenorite, hematite, orthoclase (As-bearing variety), fluorophlogopite, langbeinite, calciolangbeinite, aphthitalite-type sulfates, anhydrite, krasheninnikovite, vanthoffite, fluoborite, sylvite, halite, pseudobrookite, rutile, corundum and various arsenates: urusovite, johillerite, ericlaxmanite, kozyrevskite, popovite, lammerite, lammerite-β, tilasite, svabite, nickenichite, bradaczekite, dmisokolovite, shchurovskyite, etc. Cu-bearing spinels are among the latest minerals of this assemblage: they occur in cavities and overgrow not only earlier oxides (hematite, tenorite) and silicates but also arsenates and even “saline” sulfates, such as langbeinite-calciolangbeinite series and aphthitalite group minerals (Figure 1a–c and Figure 2a,b). Overgrowing of hematite by Cu-rich oxide spinels is very common; these minerals form epitactic intergrowths (the crystal face {111} of a spinel-group member is coplanar to the face {0001} of hematite crystal) or clusters of randomly oriented crystals (Figure 3a–c). Sometimes, spinels cover hematite as massive crusts (Figure 4a,b). Some associations contain two or more spinel-group minerals. In such cases, the Cu-richest species are typically the latest (Figure 4b).

Cu-enriched chrome-spinels were found in Arsenatnaya in several micro-xenoliths (up to 1 mm across) of ultrabasic rock mainly consisting of olivine (Fo_83-85). These micro-xenoliths embedded in basalt scoria were strongly altered by fumarolic gases. As a result of gas influence, the peripheral part of olivine grains was significantly replaced by hematite (Figure 5a) and primary, magmatic chrome-spinel (crystals up to 0.06 mm) was altered to Zn- and Cu-bearing species. Figure 5b demonstrates such pseudomorphosed crystal with a core represented by twinned lattice-like aggregates of the newly formed Cu-enriched chromite and a rim consisting of Cu-enriched zincochromite.

Besides Arsenatnaya, Cu-bearing varieties of gahnite, magnesioferrite and spinel were found in deposits of extinct fumaroles belonging to the Western paleo-fumarole field at Mountain 1004, a scoria cone located 2 km south of the Second scoria cone of the NB GTFE. Mountain 1004 was formed as a result of an ancient eruption of Tolbachik; the age of this monogenetic volcano and its fumarole fields is evaluated as ca. 2000 years [16]. Spinels occur here in cavities of basalt scoria altered by fumarolic gas. The associated sublimate minerals are diopside, fluorophlogopite, potassic feldspar, indialite, hematite, tenorite, fluorite, sellaite, anglesite, and baryte.

3. Methods

Reflectance spectra for thermaerogenite were obtained in air using a MSF-21 micro-spectrophotometer (LOMO company, St. Petersburg, Russia) with the monochromator slit width of 0.4 mm and beam diameter of 0.1 mm; SiC (Reflectionstandard 474251, No. 545, Germany) was used as a standard.

The Raman spectrum of thermaerogenite was obtained using an EnSpectr R532 Raman microscope (Department of Mineralogy, Moscow State University, Moscow, Russia) with a solid-state laser diode with green radiation (532 nm) at room temperature. The spectrum was processed in the range from 100 to 4000 cm⁻¹ with the use of a holographic diffraction grating with 1800 mm⁻¹ and a resolution equal to 6 cm⁻¹. The microscope was focused onto the sample using a PLNC 40X objective (NA = 0.65). Backscattered Raman signals were collected at 1 s exposure time with 400 spectra accumulations. The spectrum was obtained for a randomly oriented crystal with the diameter of the focal spot on the sample about 1 μm. The output laser power was about 12 mW. The power of laser radiation on the surface of the mineral was significantly less. No thermal damage of the sample was observed. Before conducting the experiments, the instrument was calibrated with Raman line of crystalline silicon (520 cm⁻¹).

Scanning electron microscopic (SEM) studies in secondary electron (SE) and back-scattered electron (BSE) modes were carried out and chemical composition was determined for all studied samples using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Department of Petrology, Moscow State University, Moscow, Russia), with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter. The standards used are: MgAl₂O₄ (Mg,Al), Ni (Ni), Cu (Cu), ZnS (Zn), V (V), FeCr₂O₄ (Cr), FeS₂ (Fe), and MnTiO₃ (Mn,Ti).

Powder X-ray diffraction data were collected using a Rigaku RAXIS Rapid II diffractometer with a curved image plate detector, a rotating anode with VariMAX microfocus optics, using CoKα radiation, in Debye-Scherrer geometry, at an accelerating voltage of 40 kV, a current of 15 mA and an exposure time 15 min. The distance between the sample and the detector was 127.4 mm. Data processing was carried out using osc2xrd software [20].

Single-crystal X-ray diffraction studies were carried out using an Xcalibur S diffractometer equipped with a CCD detector (MoKα radiation).

4. Results

4.1. General Appearance and Physical Properties of Thermaerogenite and Other Cu-Rich Oxide Spinels from Tolbachik

Thermaerogenite forms octahedral crystals up to 0.02 mm across, sometimes skeletal, typically combined in open-work clusters (Figure 6a) up to 1 mm across. Areas “sprinkled” by crystals of the new mineral (Figure 1b) are up to 0.5 cm × 0.5 cm. Besides the major form {111}, narrow {110} faces were observed on some crystals of thermaerogenite. Being visually perfect (Figure 6a), crystals of thermaerogenite have blocky inner structure and these micro-blocks are slightly misoriented, which hampers the single-crystal X-ray diffraction study of the mineral.

Other Cu-rich Al- and Fe³⁺-dominant spinels in Tolbachik fumaroles occur as octahedral crystals (Figure 2a, Figure 3 and Figure 6a–c), sometimes with minor {110} or/and {100} faces, up to 0.1 mm across, only magnesioferrite was observed as larger (up to 1 mm) crystals, typically rhombic dodecahedra (Figure 6d). Gahnite was also found as flattened spinel-law twins on {111} additionally combined in a complex lattice-like aggregates (Figure 2b). A similar lattice-like shape was observed for Cu-bearing chromite (Figure 5b). Spinel, magnesioferrite and gahnite sometimes form crusts up to 1 cm across and up to 0.5 mm thick. Solid crusts of cuprospinel (up to 2 mm across) were observed on hematite.

Thermaerogenite is semitransparent to transparent, with a yellowish streak and strong vitreous lustre. Its colour is brown, yellow-brown, red-brown, brown-yellow or brown-red (Figure 1).

Colour, transparency and lustre of other Cu-rich Al- and Fe³⁺-dominant spinels from Tolbachik are in general correlated with the Al:Fe ratio. The Fe-poor varieties are very light (some samples of spinel and gahnite from Mountain 1004 are colourless or greyish-white) with vitreous lustre. Spinel, gahnite and Al-enriched cuprospinel in typical samples from the Arsenatnaya fumarole are visually indistinguishable from the above-described thermaerogenite. Fe-rich cuprospinel is dark reddish-brown with strong vitreous to adamantine lustre. Magnesioferrite has dark brown to iron-black colour and submetallic lustre.

Thermaerogenite is brittle, with conchoidal fracture (observed under the scanning electron microscope); cleavage or parting is not observed. Its Mohs hardness is ca. 7. Density calculated using the empirical formula of the holotype specimen is 4.870 g/cm³.

4.2. Optical Data for Thermaerogenite

In reflected light, thermaerogenite is optically isotropic, grey, with yellowish internal reflections. The reflectance values are given in

Table 1. Reflectance data for thermaerogenite. Reflectance values for four wavelengths recommended by the IMA Commission on Ore Microscopy are given in bold type.

λ (nm)	R	λ (nm)	R
400	16.4	560	14.0
420	16.0	580	13.7
440	15.7	589	13.6
460	15.4	600	13.4
470	15.2	620	13.2
480	15.1	640	13.0
500	14.8	650	12.9
520	14.5	660	12.8
540	14.2	680	12.5
546	14.2	700	12.3

.

4.3. Raman Spectroscopy of Thermaerogenite

The Raman spectrum of thermaerogenite (Figure 7) is typical for spinel-type oxides [21]. It contains four distinct bands with following wavenumbers of maxima (cm⁻¹, s—strong band): 762s, 590, 284, and 125s. They could be assigned based on the data reported for different oxide spinels in [21]. The strongest band at 762 cm⁻¹ could correspond to A_1g mode, stretching vibrations of O atoms surrounding Al in tetrahedral coordination. The presence of part of Al in the tetrahedral position (8a) is typical for synthetic Cu-rich members of the series Zn_1−xCu_xAl₂O₄ [22]. Broad band with maximum at 590 cm⁻¹ probably corresponds to F_2g(2) or F_2g(3) mode involving divalent cations, (Cu,Zn)–O, whereas weak band at 284 cm⁻¹ corresponds to F_2g(1) mode. Strong band at 125 cm⁻¹ can be assigned as corresponding to lattice modes.

4.4. Chemical Composition of Cu-Rich Oxide Spinels from Tolbachik

Representative electron-microprobe analyses for the studied minerals are given in

Table 2. Chemical composition of Cu-bearing Al- and Fe³⁺-dominant spinel-group oxides from Tolbachik fumaroles: 1–4: thermaerogenite, 5–11: gahnite, 12–16: spinel, 17–20: magnesioferrite, 21: franklinite, and 22–28: cuprospinel.

No.	1 *	2	3	4	5	6	7
wt.%
MgO	-	-	-	0.44	-	5.50	-
CuO	25.01 (23.64–26.86)/1.46	25.47	25.67	20.25	20.07	15.00	17.55
ZnO	17.45 (14.46–18.71)/2.00	18.33	17.43	18.99	23.79	18.29	21.71
Al2O3	39.43 (34.59–45.43)/4.60	45.43	28.30	22.79	49.05	53.90	56.08
Cr2O3	0.27 (0.17–0.33)/0.07	0.17	0.86	-	0.15	0.03	-
Mn2O3	-	-	-	0.36	-	0.10	-
Fe2O3	17.96 (11.47–22.21)/4.76	11.47	24.48	34.29	7.72	5.99	4.63
TiO2	-	-	2.59	2.61	-	0.22	0.24
Total	100.12	100.87	99.33	99.73	100.78	99.03	100.21
formula calculated on the basis of 4 O apfu
Mg	-	-	-	0.023	-	0.242	-
Cu	0.619	0.609	0.673	0.540	0.472	0.334	0.396
Zn	0.422	0.428	0.447	0.496	0.547	0.398	0.479
Al	1.523	1.695	1.156	0.949	1.801	1.874	1.973
Cr	0.007	0.004	0.017	-	0.003	0.001	-
Mn	-	-	-	0.011	-	0.002	-
Fe3+	0.443	0.273	0.639	0.911	0.181	0.133	0.104
Ti	-	-	0.068	0.069	-	0.005	0.006
ΣA2+	1.041	1.037	1.119	1.059	1.019	0.975	0.874
ΣB3+,4+	1.973	1.972	1.881	1.940	1.984	2.015	2.082
No.	8	9	10	11	12	13	14
wt.%
MgO	0.42	-	0.59	2.66	13.27	17.54	19.61
NiO	-	-	-	-	0.87	-	-
CuO	13.72	13.06	13.02	2.94	10.89	6.86	4.83
ZnO	30.66	31.91	29.25	35.66	5.98	8.42	9.37
Al2O3	44.70	54.11	44.30	55.81	32.93	61.12	48.44
Cr2O3	0.19	-	0.18	-	-	-	-
Mn2O3	0.16	-	0.10	-	1.88	0.37	0.93
Fe2O3	8.82	1.75	11.40	2.11	34.29	4.77	13.88
TiO2	1.06	0.11	0.80	-	-	0.41	2.02
Total	99.73	100.94	99.64	99.18	100.11	99.49	99.08
formula calculated on the basis of 4 O apfu
Mg	0.020	-	0.028	0.119	0.601	0.688	0.807
Ni	-	-	-	-	0.021	-	-
Cu	0.333	0.301	0.314	0.066	0.250	0.136	0.100
Zn	0.726	0.718	0.690	0.789	0.134	0.164	0.191
Al	1.690	1.944	1.669	1.970	1.180	1.897	1.576
Cr	0.004	-	0.004	-	-	-	-
Mn	0.004	-	0.003	-	0.048	0.008	0.022
Fe3+	0.213	0.040	0.274	0.048	0.784	0.094	0.288
Ti	0.025	0.003	0.019	-	-	0.008	0.042
ΣA2+	1.079	1.019	1.033	0.974	1.006	0.989	1.098
ΣB3+,4+	1.935	1.987	1.969	2.018	2.012	2.008	1.928
No.	15	16	17	18	19	20	21
wt.%
MgO	21.94	16.04	17.43	17.65	10.56	20.81	8.54
NiO	-	-	-	-	0.35	-	-
CuO	2.18	4.34	5.90	3.22	14.73	1.26	7.91
ZnO	5.24	12.28	3.09	0.60	0.94	0.60	17.99
Al2O3	59.10	58.36	5.30	0.66	5.51	5.72	20.24
Cr2O3	-	0.09	0.33	-	-	-	0.17
Mn2O3	1.56	0.26	0.24	1.32	1.01	1.24	2.29
Fe2O3	8.76	8.10	66.24	77.29	66.64	69.46	36.51
TiO2	0.48	0.24	0.92	-	-	-	5.50
Total	99.26	99.70	99.45	100.74	99.74	99.09	99.15
formula calculated on the basis of 4 O apfu
Mg	0.843	0.640	0.873	0.884	0.552	1.014	0.424
Ni	-	-	-	-	0.010	-	-
Cu	0.042	0.088	0.150	0.082	0.391	0.031	0.199
Zn	0.100	0.243	0.077	0.015	0.024	0.015	0.443
Al	1.796	1.842	0.210	0.026	0.228	0.220	0.795
Cr	-	0.002	0.006	-	-	-	0.005
Mn	0.031	0.005	0.007	0.034	0.027	0.031	0.058
Fe3+	0.170	0.163	1.675	1.953	1.760	1.709	0.915
Ti	0.009	0.005	0.023	-	-	-	0.138
ΣA2+	0.986	0.971	1.099	0.980	0.977	1.060	1.066
ΣB3+,4+	2.006	2.018	1.922	2.013	2.015	1.960	1.910
No.	22	23	24	25	26	27	28
wt.%
MgO	0.75	0.63	4.31	3.68	2.32	0.48	5.05
NiO	0.70	-	-	-	-	-	-
CuO	28.55	27.13	24.48	24.36	25.92	20.81	23.44
ZnO	3.51	7.76	3.39	2.48	4.53	18.82	1.94
Al2O3	4.82	7.52	3.44	1.69	4.12	18.69	3.46
Cr2O3	-	0.22	-	0.23	-	0.24	0.27
Mn2O3	1.65	0.94	2.15	0.85	1.38	0.33	1.20
Fe2O3	59.49	53.49	59.77	65.82	61.51	37.06	63.40
TiO2	0.28	1.75	1.54	-	0.30	2.63	0.41
Total	99.75	99.44	99.08	99.11	100.08	99.06	99.17
formula calculated on the basis of 4 O apfu
Mg	0.043	0.036	0.244	0.211	0.133	0.026	0.283
Ni	0.022	-	-	-	-	-	-
Cu	0.831	0.781	0.704	0.707	0.748	0.573	0.665
Zn	0.100	0.219	0.096	0.071	0.128	0.506	0.054
Al	0.219	0.338	0.154	0.076	0.185	0.802	0.153
Cr	-	0.005	-	0.006	-	0.005	0.006
Mn	0.048	0.030	0.069	0.027	0.045	0.010	0.038
Fe3+	1.725	1.535	1.711	1.902	1.768	1.016	1.793
Ti	0.008	0.050	0.044	-	0.008	0.072	0.011
ΣA2+	0.996	1.036	1.044	0.988	1.008	1.105	1.002
ΣB3+,4+	2.000	1.958	1.979	2.012	2.006	1.904	2.002

* The holotype specimen of thermaerogenite: averaged from four spot analyses value (range)/standard deviation; dash means the content below detection limit.

(Cu-bearing Al- and Fe³⁺-dominant spinels) and

Table 3. Chemical composition of Cu-bearing chrome-spinels from the Arsenatnaya fumarole, Tolbachik: 1–2: magnesiochromite, 3: chromite, 4: zincochromite.

No.	1	2	3	4
wt.%
MgO	10.48	7.28	7.52	2.60
FeO	2.30	6.19	16.38	1.07
CuO	8.97	6.25	5.86	3.96
ZnO	8.54	12.83	2.16	27.88
Al2O3	11.32	11.52	11.06	10.44
V2O3	0.19	0.14	0.16	0.19
Cr2O3	47.45	46.06	48.20	43.42
Fe2O3	10.73	9.70	7.56	10.61
TiO2	0.83	0.89	0.88	0.81
Total	100.82	100.86	99.78	100.98
formula calculated on the basis of 4 O apfu with A + B = 3.00 apfu *
Mg	0.524	0.374	0.388	0.140
Fe2+	0.058	0.160	0.427	0.029
Cu	0.227	0.163	0.153	0.108
Zn	0.212	0.326	0.056	0.745
Al	0.448	0.467	0.451	0.445
V	0.005	0.004	0.004	0.006
Cr	1.259	1.254	1.319	1.243
Fe3+	0.246	0.228	0.179	0.263
Ti	0.021	0.023	0.023	0.022
ΣA2+	1.021	1.023	1.023	1.022
ΣB3+,4+	1.979	1.977	1.977	1.978

* Fe²⁺:Fe³⁺ ratio is calculated by charge balance.

(Cu-bearing chrome-spinels). There are 32 spot analyses of their different chemical varieties which demonstrate chemical variability of Cu-enriched oxide spinels from Tolbachik fumaroles.

Valent states of cations were not determined by direct methods. For properly sublimate minerals (thermaerogenite, cuprospinel, gahnite, spinel, magnesioferrite and franklinite) Fe and Mn are considered as Fe³⁺ and Mn³⁺, respectively, taking into account extremely oxidizing conditions of mineral deposition in Arsenatnaya and similar fumaroles [18]. This is clearly confirmed by the calculation of the empirical formulae using the scheme common for oxide spinels, on the basis of four O atoms per formula unit (apfu): Sums of divalent and trivalent cations are close to 2.00 and 3.00 apfu, respectively (Table 2 and Table 3) that is typical for “2-3 spinels” with the general formula A²⁺B³⁺₂O₄ [2,23]. For the Cu-bearing chrome-spinels formed in the result of the alteration of a primary, magmatic chrome-spinel, we assume the presence of part of iron in a divalent state (while manganese content is below detection limit in these minerals). Their formulae were calculated on the basis of four O apfu with fixing of the cation sum A²⁺ + B^3+,4+ = 3.00 apfu and the Fe²⁺:Fe³⁺ ratio was calculated by charge balance.

Maximum contents of copper detected in different spinel-group minerals from Tolbachik fumaroles are presented in

Table 4. Maximum contents of copper detected in different spinel-group members from Tolbachik.

Mineral Species	Ideal Formula	CuO, wt.%	Cu, apfu
Cuprospinel	CuFe3+2O4	28.6	0.83
Thermaerogenite	CuAl2O4	26.9	0.69
Gahnite	ZnAl2O4	21.4	0.51
Magnesioferrite	MgFe3+2O4	14.7	0.39
Spinel	MgAl2O4	10.9	0.25
Magnesiochromite	MgCr2O4	9.0	0.23
Franklinite	ZnFe3+2O4	7.9	0.20
Chromite	Fe2+Cr2O4	5.9	0.15
Zincochromite	ZnCr2O4	4.8	0.13

and atomic ratios of the major bivalent (A) and trivalent (B) cations are shown in Figure 8a,b (based on 124 spot electron-microprobe analyses).

4.5. X-ray Diffraction Data of Cu-Rich Oxide Spinels from Tolbachik

Powder X-ray diffraction (XRD) data of thermaerogenite in comparison with the calculated data for synthetic CuAl₂O₄ are given in

Table 5. Powder X-ray diffraction data (d in Å) of thermaerogenite and its synthetic end-member analogue.

Thermaerogenite			Synthetic CuAl2O4 *		h k l
Imeas	dmeas	dcalc	Icalc	dcalc
3	4.659	4.694	2	4.664	111
65	2.873	2.875	51	2.856	220
100	2.451	2.452	100	2.436	311
1	2.329	2.347	0.2	2.332	222
10	2.033	2.033	17	2.020	400
6	1.865	1.865	1	1.853	331
16	1.660	1.660	12	1.649	422
28	1.565	1.565	31	1.555	511
30	1.438	1.437	38	1.428	440
4	1.286	1.286	3	1.277	620
6	1.240	1.240	6	1.232	533
1	1.226	1.226	1	1.218	622
1	1.174	1.174	1	1.166	444

* Calculated from the structure data from [24].

. All diffraction reflections are well-indexed in the cubic unit cell, by analogy with common spinel-group minerals [2,23]. Unit-cell dimensions refined from the powder XRD data are presented in

Table 6. Unit-cell dimension a (Å) and volume (V, ų) of Cu-rich spinel-group oxides from Tolbachik, cuprospinel from its type locality and synthetic CuAl₂O₄.

Mineral/Compound	a	V	Method *	Source
Thermaerogenite	8.093(9)	530.1(10)	SCXRD	this work
Thermaerogenite	8.131(1)	537.6(2)	PXRD	this work
Synthetic CuAl2O4	8.079(3)	527(3)	SCXRD	[24]
Cuprospinel	8.402(11)	593(1)	SCXRD	this work
Cuprospinel **	8.369	586	PXRD	[3]
Gahnite	8.124(4)	536.2(5)	SCXRD	this work
Gahnite	8.1327(6)	537.9(1)	PXRD	this work
Spinel	8.149(1)	541.2(2)	PXRD	this work
Magnesioferrite	8.344(13)	581(1)	SCXRD	this work

* Abbreviations SCXRD and PXRD mean single-crystal and powder X-ray diffraction methods, respectively; ** type specimen from Consolidated Rambler Mines Limited, Baie Verte, Newfoundland, Canada.

.

Both single-crystal and powder XRD data for all other studied spinel-group oxides from Tolbachik fumaroles demonstrate their cubic symmetry and systematic absences typical for space group Fd-3m characteristic for “2-3 spinels” [2,23]. The obtained unit-cell parameters a and V are in agreement with the chemical composition of the minerals (Table 6). The major factor which influences these parameters of the studied spinels is the Al:Fe³⁺ ratio. In particular, unit-cell dimensions and the majority of d-spacings of thermaerogenite are slightly higher in comparison with its synthetic end-member analogue CuAl₂O₄ (Table 5 and Table 6) that is mainly caused by admixture of Fe³⁺ which partially substitutes Al in the mineral (Table 2).

Single-crystal XRD data of thermaerogenite allowed to obtain the a parameter of the cubic unit cell (Table 6), however, the crystal structure of the new mineral was not studied due to the low quality of single-crystal diffraction patterns caused by the imperfectness of all tested crystals which consist of slightly misoriented micro-blocks and in fact can be considered as crystal intergrowths.

Chemical composition of thermaerogenite varies from crystal to crystal in both Cu:Zn and Al:Fe ratios (Figure 8). The latter is a cause of slight discrepancy between unit-cell parameters of the new mineral obtained from single-crystal and powder X-ray diffraction data (Table 6).

5. Discussion

All spinel-group oxides found in the middle zones of the Arsenatnaya fumarole are copper-bearing and the majority of the studied samples contain >1 wt.% CuO (Table 2 and Table 3). The maximum contents of copper in these minerals are reported in Table 4. For Tolbachik cuprospinel, the range 20.8–28.6 wt.% CuO (=0.57–0.83 apfu Cu) is found and for thermaerogenite 16.7–26.9 wt.% CuO (=0.41–0.69 apfu Cu).

Zinc is the most typical admixture in thermaerogenite. Gahnite and this mineral form here the continuous solid-solution series with the main substitution scheme Cu²⁺→Zn²⁺ (Figure 8a). Even the Zn-poorest specimen of thermaerogenite contains 14.5 wt.% ZnO (=0.36 apfu Zn), and in the Cu-poorest sample of gahnite from Arsenatnaya, 2.9 wt.% CuO (=0.07 apfu Cu) was detected. The continuous isomorphous series with the general formula Zn_1–xCu_xAl₂O₄ was reported for synthetic spinels [22].

Some crystals of thermaerogenite contain significant Mg admixture (up to 5.4 wt.% MgO), however, a continuous solid solution between spinel and the new mineral is not observed (Figure 8a), unlike the synthetic system Mg_1–xCu_xAl₂O₄ with full isomorphism [25]. The solid-solution series between cuprospinel and thermaerogenite, with the main substitution scheme Al³⁺→Fe³⁺, also demonstrates a significant gap (Figure 8b). Cuprospinel shows the most stable chemical composition among all studied “2-3 spinels” from Tolbachik fumaroles. It is typically characterized by not very high contents of admixtures, including Mg, Zn and Al, and does not form continuous solid-solution series with other oxide spinels (Table 2, Figure 8).

Cuprospinel was described as a new mineral species from burning dumps [3] and its other finds were related to ore smelters [4,5]. These anthropogenic spinel-forming systems are close in conditions to volcanic fumaroles of the oxidizing type in which copper-rich spinel-type oxides crystallize in a completely natural environment at Tolbachik. The similarity of products formed in both systems, as well as data on the synthesis and thermodynamic stability of these phases [26,27,28] and the absence of data on such minerals in other geological formations, demonstrate that the physical and chemical conditions in fumaroles are optimal for the formation of Cu-enriched oxide spinels: there is the combination of high temperature, atmospheric pressure and high oxygen fugacity.

It is doubtless that Cu-rich Al- and Fe³⁺-dominant oxide spinels in fumarole systems of Tolbachik where deposited directly from hot gas as volcanic sublimates. The most convincing evidence of their exhalation origin is the location over crusts of very typical sublimate minerals, especially over alkaline sulfates (Figure 1) in the open space of pockets within a recently formed and still active fumarole at the summit part of a volcanic scoria cone. No sign of occurrence of any other process that could result in the formation of spinels (crystallization from melt or solution, solid-state transformation, etc.) is observed there. Hot volcanic gas was a carrier of “ore” constituents, foremost Cu, Zn and Fe. Basalt scoria which composes the walls of fumarole chambers can be a source of elements having low volatilities in such post-volcanic systems, namely Mg, Al and Ti [29,30]. Our temperature measurements in fumaroles of the Second scoria cone of the NB GTFE and fumaroles born by the Ploskiy Tolbachik eruption of 2012–2013 [31] together with data based on the halite–sylvite solid-solution thermometry [19] show that the mineral assemblages with Cu-bearing oxide spinels were formed at temperatures definitely not lower than 500 °C and probably not higher than 800 °C. This assumption is in agreement with data on synthetic CuAl₂O₄ prepared under the atmospheric pressure in the temperature range 600–1100 °C [28]. Thus, the most probable temperature interval of crystallization of Cu-rich oxide spinels in Tolbachik fumaroles seems to be 600–800 °C.

For the Cu- and Zn-enriched chrome-spinels found in altered micro-xenoliths of ultrabasic rock (Figure 5), we assume the same physical conditions but rather another mechanism of formation. They could be formed due to gas metasomatism, in terminology by Naboko and Glavatskikh [32], i.e., in the result of the replacement of a primary, magmatic chrome-spinel by chemically different spinel-group species under the influence of hot volcanic gas enriched by “ore” components. Chromium, Mg, Al, V and probably part of Fe (which remained as Fe²⁺) were inherited from the primary chrome-spinel phase whereas Cu and Zn were taken from gas and Fe²⁺ was partly oxidized to Fe³⁺ in this process.

The distribution of cations between tetrahedral and octahedral sites in Cu-rich oxide spinels from Tolbachik fumaroles was not examined by us. We only can consider, based on the Raman spectrum (see above), that thermaerogenite contains part of Al in tetrahedral coordination. The cation distribution is well-studied for synthetic Cu-bearing spinel-type oxides including the end-member CuAl₂O₄ and it was shown that they are “largely normal” spinels (in terminology by [33], i.e., with bivalent cations occupying the tetrahedral site for 2/3 or more) but part of Cu²⁺ typically occurs in the octahedral site and the Cu-B³⁺ disorder in general increases with the temperature increase [22,25,27,28,34]. It should be noted that the distribution of bivalent and trivalent cations between tetrahedral and octahedral sites is not a species-defining sign in the light of the IMA-accepted nomenclature of spinel-supergroup minerals: The classification is based only on chemical formulae as resulting from chemical data only [2].

Copper-rich oxide spinels are extremely rare minerals reliably known in Nature only in Tolbachik fumaroles. Unlike them, the chalcogenide members of the spinel supergroup are not so rare. Nine valid minerals with species-defining Cu belong to the thiospinel and the selenospinel groups [2] and one of them, carrollite CuCo₂S₄, is a common sulfide in many ore deposits. Such strong difference in diversity and distribution in nature between oxide and chalcogenide Cu-rich spinels is probably caused by the strong chalcophile character (it is worthy to note that the term chalcophile originates from Greek word χαλκός, copper) of copper.

6. Conclusions

Natural copper-rich oxide spinels are characterized for the first time. They were found in the deposits of active and extinct fumaroles of the oxidizing type related to the Tolbachik volcano, Kamchatka, Russia. This mineralization is represented by nine species belonging to the “2-3 spinels” with the general chemical formula A²⁺B³⁺₂O₄, i.e., to the spinel subgroup of the oxyspinel group within the spinel supergroup [2]. There are (content of CuO in wt.% is given in parentheses for each mineral) two minerals with species-defining Cu²⁺, namely cuprospinel (20.8–28.6) and the new species thermaerogenite (16.7–26.9), and Cu-enriched varieties of gahnite (up to 21.4), magnesioferrite (up to 14.7), spinel (up to 10.9), magnesiochromite (up to 9.0), franklinite (up to 7.9), chromite (up to 5.9), and zincochromite (up to 4.8).

The new mineral species thermaerogenite [ideally CuAl₂O₄, cubic, space group Fd-3m, a = 8.093(9) Å, V = 530.1(10) ų] forms a continuous isomorphous series with gahnite.

Cuprospinel, ideally CuFe³⁺₂O₄, earlier reliably known only as a phase of the anthropogenic origin or as a synthetic compound, is a typical oxide mineral in the Arsenatnaya fumarole at Tolbachik. Its sample with composition (Cu_0.831Zn_0.100Mg_0.043Ni_0.022)_Σ0.996(Fe³⁺_1.725Al_0.219Mn³⁺_0.048Ti_0.008)_Σ2.000O₄ represents the Cu-richest natural spinel-type oxide so far described.

Cu-bearing oxide spinels at Tolbachik have a properly fumarolic origin. Aluminum- and Fe³⁺-dominant species (thermaerogenite, gahnite, spinel, cuprospinel, franklinite, and magnesioferrite) were deposited directly from hot gas as volcanic sublimates. The most probable temperature interval of their crystallization seems to be 600–800 °C. Copper-enriched varieties of chrome-spinels (magnesiochromite, chromite and zincochromite) were formed probably under the same physical conditions but in the result of the metasomatic replacement of a magmatic chrome-spinel in micro-xenoliths of ultrabasic rock under the influence of volcanic gas.

The rarity of Cu-rich oxide spinels in nature is probably caused by the very specific character of conditions optimal for their formation: there is the combination of high temperature, atmospheric pressure, high oxygen fugacity, rich source of copper and hot gas as effective carrier of this element. Such combination is mostly realized in volcanic fumaroles of the oxidizing type enriched by “ore” components: there are really rare, exotic geological objects. The strong affinity of Cu-rich oxide spinels to such environments is confirmed by earlier reported [3,4,5] findings of their anthropogenic counterparts in burning copper mine dumps and ore smelters.