Osbornite (TiN) is extremely rare in nature (commonly found in enstatite meteorites) and has not yet been identified correctly to form naturally in terrestrial settings. Due to its thermodynamic stability and thermal shock resistance, TiN has wide industrial applications, mainly as coatings. However, as the melting temperature of TiN is very high (~3000°С), coatings are produced at much lower temperatures via physical or chemical vapor deposition. Also, anthropogenic analogues of osbornite are often observed in pyrometallurgical slags. Therefore, it is critical to distinguish between anthropogenic and naturally occurring osbornite. A detailed petrographic study was undertaken on in situ osbornite found within unusual gehlenite-bearing breccias from wadi Zohar, Negev Desert of the pyrometamorphic Hatrurim Complex. The Hatrurim Complex, which extends through Israel, Palestine, and Jordan within the Dead Sea Rift zone, mainly comprises larnite, gehlenite, and spurrite rocks. Osbornite, in close association with iron phosphides, barringerite, and schreibersite, occurs at contacts between gehlenite, paralava, and calcinated clasts of host sedimentary rocks. Based on investigation of pseudowollastonite and Fe-P series phases, osbornite is formed at low pressure, extremely high temperatures (~1200-1500°С), and reduced conditions, following pyrolysis of organic matter contained in the sedimentary protolith. This is the first identification of in situ osbornite in terrestrial rocks and indicates that high-temperature and highly reduced conditions, which are common for meteorites, may occur at/near the Earth’s surface as a result of sustained pyrometamorphism in particular settings. Our findings also provide relevant data and criteria for comparing osbornite occurrences elsewhere and ultimately evaluating their origins.

Nitrogen is a typical component of organic minerals and inorganic N-bearing minerals, in which it is usually present as ammonium or nitrate complexes. In contrast, nitrides are exceptionally rare in nature and generally have extraterrestrial origins. The following nitrides have been documented in meteorites: nierite, Si3N4 [1]; sinoite, Si2N2O [2]; roaldite, γ-Fe4N [3]; carlsbergite, CrN [4]; uakitite, VN [5]; and osbornite, TiN [6]. The few exceptional terrestrial findings of nitrides include siderazot, Fe3N1.33 in volcanic materials of Mt. Etna, Sicily [7] and oreillyite, Cr2N [8] and qingsongite, BN [9] reported in heavy mineral separates from Mt. Carmel, Israel and Loubousa chromitites, Tibet, China, respectively. There have also been the discovery of low-temperature nitride-bearing mercury minerals such as gianellaite, (Hg2N)2SO4 [10]; kleinite, Hg2N((SO4)0.25Cl0.5)·0.5H2O [11]; mosesite, Hg2N(Cl,SO4,MoO4,CO3)·H2O [12]; and comancheite, Hg55N24(OH,NH2)4(Cl,Br)34 [13] from Terlingua, Brewster County, Texas, USA.

Natural titanium nitride, osbornite, (TiN) was first described in 1852 from a meteorite (aubrite) which fell near Bustree, India [6]. Since then, osbornite has been routinely identified in enstatite meteorites [1416] and was also found in the 81P/Wild 2 comet dust [17].

TiN is implemented in a range of industries owing to its thermodynamic stability, thermal resistance, and robustness under mechanical stress [1820]. Extensive applications of TiN coatings include cutting tools, extrusion castings, automotive and aerospace parts, computer disk drives, precision and surgical/medical instruments, and artificial human organs. These coatings are produced using physical or chemical vapor deposition processes [20]. Anthropogenic analogues of osbornite are common in slags from metal and special ceramic production processes [2124]. Mass production and high stability of titanium nitride may cause environmental pollution by artificial TiN grains, similarly to technogenic diamond, silicon carbide (moissanite), corundum, etc. [2527]. In contrast to these other anthropogenic analogues, the bulk of anthropogenic TiN particles from industrial coatings is nanosized and is thus unlikely to be detected in natural objects. However, larger grains that form intergrowths or inclusions in slag products [2124], such as iron or corundum, can often survive in natural settings. In fact, osbornite on Earth has exclusively been found in mineral separates from bulk rock samples, in which its micrometre- and nanometre-sized inclusions are mainly hosted in corundum [2844] and are thus unsupportive of natural origins [27, 45].

Osbornite has recently been discovered in situ within gehlenite-bearing breccias of the Hatrurim Complex, Israel, in association with “meteorite”-type phosphides, such as schreibersite, barringerite, allabogdanite, and andreyivanovite [46]. Here, we investigate the terrestrial origins of in situ osbornite by studying its mineral associations, composition, structure, and Raman shift. In addition, we discuss the physicochemical conditions and formation mechanisms for osbornite and associated phosphides, with implications for the petrogenesis of the Hatrurim Complex.

2.1. Hatrurim Complex

The pyrometamorphic Hatrurim Complex (Mottled Zone) comprises larnite-, spurrite- and gehlenite-bearing rocks which form large outcrops (up to 100 s of km2) along the Dead Sea Rift zone in the territories of Israel, Jordan, and Palestine [4751]. These lithologies formed under oxidizing, sanidinite facies metamorphic conditions (700-1400°C and low pressure) and therefore contain minerals with predominantly trivalent iron [52]. Pyrometamorphism of the Hatrurim Complex was initially proposed to occur from the burning of bitumen-rich protoliths [48]; recently, proposed hypothesis of the “mud volcanoes” has shown that fire activation and protolith transformation was the result of methane delivery via supply channels from gaseous traps within the tectonically active zone of the Dead Sea Rift [50, 53]. Moreover, this transformation process involved the combustion of by-products (gases, fluids, and melts) at high temperatures which reacted with minerals of the early “clinker association” and significantly increased the mineral diversity within the complex, including the reduced phosphide associations [46, 54, 55].

Phosphide-bearing rocks were found at three localities within the Hatrurim Complex: Daba Siwaqa, Jordan (N31.367°, E36.187°), the Hatrurim Basin, Negev Desert, Israel: wadi Halamish (N31.161°, E35.301°) and wadi Zohar (N31.183°, E36.291°) [46, 54, 56]. In Jordan, phosphides were observed in diopside paralava within bedrock [54, 57, 58], whereas Israeli findings of phosphide-bearing rocks were found ex situ at the wadi Halamish (diopside paralava) and wadi Zohar (gehlenite breccia) [54, 56, 57, 59]. In 2019, we were able to find a bedrock source of phosphide-bearing gehlenite breccia, which contains numerous osbornite grains [46].

2.2. Samples and Methods of Investigation

More than 300 samples of phosphide-bearing breccia were collected during field trips in 2019 and 2021, and in the 13 samples, osbornite was revealed. Unambiguously natural terrestrial origin of osbornite is carefully monitored by observing and sampling the mineral grains at different levels from the outcrop to hand specimen and thin sections (Figure 1).

The morphology and chemical composition of osbornite and associated minerals were investigated using Philips XL30, Phenom XL, and Quanta 250 EDS-equipped scanning electron microscopes (Institute of Earth Sciences, University of Silesia, Poland). The chemical composition of osbornite was measured with a CAMECA SX100 electron microprobe analyzer (EMPA, Micro-Area Analysis Laboratory, Polish Geological Institute—National Research Institute, Warsaw, Poland): WDS, accelerating voltage=15kV, beam current=40nA, beam diameter~2 μm. The following standards and lines were used: apatite = Ca; rutile = Ti; hematite = Fe; V metal = V; Cr2O3 = Cr; BN = N; sanidine = Al; and diopside = Si.

Raman spectra of osbornite and associated minerals were recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Earth Sciences, University of Silesia, Poland) equipped with an air-cooled solid laser (532 nm) and a CCD camera operating at –61°C. An air Zeiss LD EC Epiplan-Neofluar DIC-100/0.75NA objective was used. Raman-scattered light was focused through a broad band single mode fibre with an effective pinhole size about 30 μm and a monochromator with a 600 mm-1 grating. The power of the laser at the sample position was ~10-15 mW. Integration times of 3 s with accumulation of 30 scans were chosen, and the resolution is 3 cm-1. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm-1).

Single-crystal X-ray study of osbornite crystal was carried out using a SuperNova diffractometer with a mirror monochromator (MoKα, λ=0.71073Å) and an Atlas CCD detector (Agilent Technologies) at the Institute of Physics, University of Silesia, Poland. The osbornite structure was refined using the SHELX-97 program [60].

Osbornite grains up to 40 μm in size were found in phosphide-bearing explosive breccia forming vertical zone 4-5 m in width in layered pinkish low-temperature Hatrurim Complex rocks at the Arad-Dead Sea road, wadi Zohar, Negev Desert, Israel [46]. The breccia comprises hydrogrossular-bearing porous clasts of the altered host rock cemented by a gehlenite-bearing paralava matrix, which also have large (up to 1.5 cm) polymineral Fe-(±C)-phosphide aggregates along clast boundaries [46]. These rounded aggregates consist of barringerite, Fe2Phex; schreibersite, Fe3P; native iron, α-Fe; and eutectic schreibersite-iron (± cohenite), in which thin murashkoite, FeP, zones are sporadically observed (Figure 2(a); Table S1). Moreover, V-Cr-bearing allabogdanite, Fe2Prhomb, and V-bearing andreyivanovite, FeCrP were found in narrow tongues of paralava enriched in pseudowollastonite, together with barringerite [46].

The host rocks are hydrated and porous and composed primarily of hydrogrossular, hydrosilicates (tacharanite, Ca12Al2Si18O33(OH)36; tobermorite, Ca4Si6O17(H2O)2·(Ca·3H2O); afwillite, Ca3[SiO3(OH)]2·2H2O)), and calcite. They were formed as a result of low-temperature alteration of pyrometamorphic clinker-like rocks of the Hatrurim Complex, which were originally sedimentary rocks of the Gareb Formation (Maastrichtian) [48, 49, 61, 62].

Gehlenite-bearing paralava is extremely heterogeneous with respect to mineral composition and texture, and amygdaloidal types of paralava are often observed. Gehlenite-flamite paralava represent the least altered type [46]. Progressive alteration leads to the replacement of flamite (α-Ca2SiO4 stabilized by the P, Na, and K impurities) by rankinite (Ca3Si2O7) within central fragments of paralava (Figure 2(b); Figure S1), which is then completely replaced by pseudowollastonite and cuspidine (which also crystallizes along walls of the gaseous channels) at the contact with the host rock. At clast boundaries within the breccia, gehlenite in paralava is completely replaced by hydrogrossular. Accessory minerals of gehlenite-bearing paralava include fluorapatite, Si-bearing perovskite, Cr-Fe-spinel, pyrrhotite, native iron and occasional sphalerite, and wüstite. Phosphides rarely occur within the paralava and are generally restricted to the porous fragments of the rock. Unlike paralava, all host rock clasts have been recrystallized into fine-grained, porous rocks consisting of hydrogrossular, tacharanite, and tobermorite-like hydrosilicates (Figure 2(c)). Case-like pseudomorphs within these clasts are rectangular or square, which may indicate a possible precursor gehlenite, while the presence of small grains (<10 μm) of iron phosphides (usually, barringerite), native iron, baghdadite, zirconolite, cuspidine, and pseudowollastonite may represent potential relics of high-temperature metamorphism (Figure 2(c); [46]). The sedimentary protoliths of the host rocks were most likely altered to clinker-like rock prior to entrainment within gehlenite paralava. At the contact zone between paralava and breccia, small (20-30 μm) rounded grains with compositions close to hydrogrossular are dispersed throughout the calcite-hydrosilicate matrix (Figure 2(d)). The morphology of these grains supports formation via melting of clinker-like fragments at the contact with paralava, which were subsequently replaced by secondary minerals.

Geochemical data indicate that gehlenite paralava and hydrogrossular-bearing host rock fragments were derived from the same protolith [46].

Relatively large (up to 30 μm), rare osbornite grains are found exclusively at the contact between paralava and small host rock clasts, where they form intergrowths with phosphides (Figures 1(b), 1(c), and 3). Osbornite has not been observed within larger host rock clasts or gehlenite paralava. In the narrow tongue of the paralava enriched in pseudowollastonite, many small grains of osbornite were found, which are clearly distinguishable due to their golden color under the optical microscope (Figures 4(a) and 4(b)).

In rare cases, osbornite is partially replaced by rutile, and it is observed as inclusions in perovskite (Figures 4(d)–4(f)). It should be underlined that osbornite from the all known “terrestrial” findings does not carry features of secondary alterations [2933, 41, 42, 44].

Grains of osbornite have also been observed within central parts of small clasts of host rock, where they form intergrowths with perovskite, murashkoite, and barringerite (Figure 5). Osbornite crystal morphology and mineral associations vary from xenomorphic (Figure 1(c)) to skeletal grains (Figures 3(e) and 5(d)); intergrowths with barringerite, schereibersite, and Cr-bearing pyrrhotite (Figures 3(c), 3(d), and 5(e)); and rims around barringerite (Figure 4(c)). Osbornite is also found in association with graphite (Figure 5(e)) [46] and paqueite; the latter of which only occurs at the narrow contact zone of hydrogrossular fragments (Figures 5(b) and 5(f)). Paqueite is a new mineral discovered within refractive inclusions of the Allende meteorite [63, 64] and has the stoichiometric formula, Ca3Ti4+1.00Si2.00(Al2.00Ti4+0.75Si0.25)∑3O14, in our samples. In rare cases, osbornite is partially replaced by rutile and may form inclusions in perovskite (Figures 4(d)–4(f)).

Although there is some variability between grains, osbornite composition is close to stoichiometric TiN (Table 1). It contains impurities of V (up to 2.56 wt.%), Са (up to 2.41 wt.%), Cr (up to 1.09 wt.%), and Fe (up to 0.89 wt.%) (Table 1). The main bands in the Raman spectrum of osbornite are as follows (Figure 6): 569 cm-1 (TO-transverse optical mode), 319 cm-1 (LA-longitudinal acoustic), and 221 cm-1 (TA-first-order transverse acoustic) and shoulder about 452 cm-1 (2A-second-order acoustic) [65]. The position of band 221 cm-1 is related to stoichiometric cubic TiN (Fm-3 m, a=4.24Å); in synthetic phases with composition TiN0.55 (Fm-3 m, a=4.22Å), its band shifts to 250 cm-1 [66].

Single-crystal X-ray diffraction was performed on the grain shown in Figure 1(c) and confirms the crystal structure of osbornite presented by stoichiometric phase TiN (Fm-3 m, a=4.2429519; Tables 24).

The origin of naturally occurring terrestrial osbornite, and its association with highly reduced conditions [41], needs to be diligently assessed due to the significant potential for contamination of geological samples by anthropogenic analogues. In most technological processes, titanium nitride coating is produced at temperature above 1000°C and under reducing conditions in a nitrogen atmosphere using physical or chemical vapor deposition [20]. In contrast, crystallization of extraterrestrial osbornite occurs at high temperatures (>1500°C), super-reduced conditions, i.e., fO2< iron-wȕstite buffer ΔIW6 [35, 38] and in the presence of fluid (gas, plasma) [67].

Our observations of in situ osbornite forming in association with typical meteoritic phosphides in explosive breccias of the Hatrurim Complex (e.g., Figures 1(c), 2(c)–2(e), 4(c), 5(c), and 5(e)) thus imply high-temperature and super-reduced physicochemical conditions of crystallization, which are rarely achieved in the Earth’s crust. Such reduced conditions are also unusual for other occurrences of gehlenite paralava in the Hatrurim Basin, which contain Fe3+-bearing minerals [52]. As the majority of osbornite crystals were found in pseudowollastonite-rich zones together with phosphides at the rock contact (Figures 3(c), 3(d), 4(c), and 5(b)–5(e)), we can use osbornite as a tracer mineral for petrogenesis. A likely source of the reducing agent (graphite-like carbon) is the pyrolytic decomposition of bitumen in the sedimentary protolith of the Hatrurim Complex, which also played a significant role in the formation of phosphides and became a source of nitrogen. Osbornite formation here can be tentatively assigned to the following reaction involving carbothermic reduction and nitridation of titanium oxide: TiO2+N+C=TiN+CO2g [68, 69]. In addition, the presence of graphite intimately associated with osbornite (Figure 5(e)) is evidence of the highly reduced formations conditions.

A similar process has been proposed for the recent discovery of V-Cr-bearing allabogdanite and V-bearing andreyivanovite associating with barringerite and schreibersite in the pseudowollastonite-enriched zone in the studied wadi Zohar breccia, with phosphide forming via a high-temperature carbothermal reduction process involving gaseous products from decomposition of organic matter at low pressures and high temperatures 1200-1500°С [46]. In this case, geochemical data suggest that the gehlenite paralava and altered rock clasts in breccia were likely derived from the same sedimentary protolith [46]. The latter is speculated to contain layers or lenses enriched in phosphorite-like materials, namely, the P-Si-Fe and P-Si-C series with a carbonate-argillaceous cement, analogous to the lithological units at the unconformable boundaries between the Campanian Mishash and the Maastrichtian bituminous Ghareb formations (major protolith unit of the Hatrurim Complex) [46, 70]. The sedimentary protolith was initially calcinated and transformed to a clinker-like rock; therefore, we propose that in our case, the main reducing agents during phosphide formation were not bitumen materials but products of its pyrolytic decomposition, i.e., graphite-like substances with admixtures of sulfur and nitrogen. Moreover, the formation temperature of gehlenite paralava was potentially as high as 1500°С and was likely maintained by gaseous fluxes which were instrumental in melting of the contact sedimentary rocks and accumulation of liquid droplets of native Fe, which subsequently formed complex aggregates with phosphides (Figure 2(a)) [46]. The process of iron and phosphorus reduction and phosphide formation can be described by the following reactions:
where n=1,2,3 [46].

In conclusion, we document the first unquestionable evidence for terrestrial origins of natural osbornite from phosphide-bearing breccia in the Hatrurim Complex, Israel. The conditions of formation and mineral association of osbornite, in contrast to other osbornite findings, are similar to those in meteorites.

All data generated during this study are included in this published article and its supplementary materials. Original data are available on request from the corresponding author.

The authors declare that they have no conflicts of interest.

Investigations were supported by the National Science Center of Poland Grant (grant number 2021/41/B/ST10/00130).

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