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 CuFe
23+O
4, 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)Al
2O
4, 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 CuAl
2O
4. 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 km
3 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 CO
2, HF and HCl [
17], while in 1976–1977 these gases were significantly more enriched in H
2O, CO
2, SO
2, 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 Fe
3+ 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 Co
Kα 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).
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 Cu
2+→Zn
2+ (
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
xAl
2O
4 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
xAl
2O
4 with full isomorphism [
25]. The solid-solution series between cuprospinel and thermaerogenite, with the main substitution scheme Al
3+→Fe
3+, 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 Fe
3+-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 CuAl
2O
4 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 Fe
2+) were inherited from the primary chrome-spinel phase whereas Cu and Zn were taken from gas and Fe
2+ was partly oxidized to Fe
3+ 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 CuAl
2O
4 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 Cu
2+ 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 CuCo
2S
4, 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+2O
4, 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 Cu
2+, 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) Å3] 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 (Cu0.831Zn0.100Mg0.043Ni0.022)Σ0.996(Fe3+1.725Al0.219Mn3+0.048Ti0.008)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.