Large titanium nitride (osbornite) crystals in the metallurgical slag from Kladno steel-works (Czech Republic)
Authors: Žáček Vladimír , Škoda Radek 
Published: October 2024

Large crystals of titanium nitride (mineral osbornite) from metallurgical slag were studied by EMPA, XRD and Raman spectroscopy. The sample was collected at metallurgical dump in Kladno in the early 1970s and briefly described by Žáček and Ondruš (2003). The sample has a size of about 40 × 30 × 30 mm and is notable for its high density and conspicuous cubes of copper-like red to gold colour with metallic lustre (Fig. 1–2). The material consists of dominant gehlenite (~ 65% of the sample, all vol. %), osbornite (~ 20%), metallic iron (~ 8%), oldhamite (~ 5%), spinel (~1%) and anorthite (~1%), see Fig 3. All these phases were analyzed with the Cameca SX100 electron microprobe, and osbornite by Raman spectroscopy. Gehlenite and osbornite were also confirmed by XRD (Žáček – Ondruš 2003). Gehlenite contains a low concentration of MgO (0.8–1.1 wt.%) and traces of Na2O (0.06–0.16 wt.%) and Fe2O3 (0.4 wt.%). Aggregates of metallic iron embedded in gehlenite contain low titanium (up to 0.06 wt.% Ti) and elevated phosphorus concentrations (0.53–1.59 wt.% P), whereas tiny iron inclusions embedded in osbornite are richer in titanium (1.31 wt.%) and poorer in P (0.37 wt.%). Oldhamite, a cubic CaS, forms rounded inclusions 20–150 µm in size embedded in massive gehlenite. It is characterized by elevated concentration of Mn (2.7 wt.%) and low Fe (0.3 wt.%) and Mg (0.15 wt.%). Spinel has empirical formula Mg0.98Al1.99O4, and plagioclase is pure anorthite. Compositional data of osbornite are given in Table 1, those of associated minerals in Table 2 and 3. Osbornite forms aggregates of cubic crystals in a small slag cavity. Individual and crystal aggregates, usually up to 5 mm in size, are embedded in massive gehlenite. The largest aggregate of osbornite crystals reaching about 15 × 8 mm in size (Fig. 2). Osbornite contains 76.30–78.21 wt.% Ti, 21.31–23.82 wt.% N, and 0.02–0.46 wt. % Fe and displays stoichiometric composition Ti0.97–1.03N0.96–1.03 with only 0.005 apfu Fe (see Table 1). Lattice parameter is: a = 4.2536(3) A (Žáček – Ondruš 2003). The Raman spectrum of osbornite (Fig. 4) measured on the unpolished surface of cubic crystals showed the following vibrational bands: 1498, 201, 226, 271, 316, 345, 395, 439, 520, 559, 609, 649, 765, 836, and 1075 (all in cm–1). Raman spectra of TiN and other Ti nitrides have been studied by a number of authors (e.g. Spengler et al. 1978, Subramanian et al. 2011, Judek et al. 2021). The spectra can be divided into acoustic (A) and optical (O) part and further to the combination A O, 2A and 2O modes (Spengler et al. 1978; see Fig. 4). In addition to the vibrational bands typical of osbornite (201, 271, 316, 345, 395, 520, 559, 649, 765, 836 and 1075 cm–1), there are also vibrational bands corresponding to rutile and to the TiON phase (149, 226, 395, 439, 520, 609 and 649 cm–1, Spengler et al. 1978, Judek et al. 2021). The position of the vibrational band of osbornite at 201 cm–1 indicates a stoichiometric Ti : N ratio of 1 : 1, which is in agreement with the results of WDS chemical analysis (Spengler et al. 1978). The detected presence of TiON and TiO2 phases on the surface of osbornite crystals indicates oxidation of osbornite, probably at elevated temperatures. A similar effect was achieved during laboratory annealing of TiN layers at temperatures above 250 °C (Glaser et al. 2007) or above 500 °C (Chen et al. 2005). Osbornite had long been considered as an exclusively extraterrestrial mineral (Ramdohr 1973, Casanova 1992), and only a few natural terrestrial localities have been identified so far. It was first detected in heavy mineral concentrates from volcanic breccias at the contact of the Priazov Massif with the Donbas Basin in Ukraine (Tataritsev et al. 1987).