Carmeltazite, ZrAl2Ti4O11, a New Mineral Trapped in Corundum from Volcanic Rocks of Mt Carmel, Northern Israel

Abstract

The new mineral species carmeltazite, ideally ZrAl2Ti4O11, was discovered in pockets of trapped melt interstitial to, or included in, corundum xenocrysts from the Cretaceous Mt Carmel volcanics of northern Israel, associated with corundum, tistarite, anorthite, osbornite, an unnamed REE (Rare Earth Element) phase, in a Ca-Mg-Al-Si-O glass. In reflected light, carmeltazite is weakly to moderately bireflectant and weakly pleochroic from dark brown to dark green. Internal reflections are absent. Under crossed polars, the mineral is anisotropic, without characteristic rotation tints. Reflectance values for the four COM wavelengths (Rmin, Rmax (%) (λ in nm)) are: 21.8, 22.9 (471.1); 21.0, 21.6 (548.3), 19.9, 20.7 (586.6); and 18.5, 19.8 (652.3). Electron microprobe analysis (average of eight spot analyses) gave, on the basis of 11 oxygen atoms per formula unit and assuming all Ti and Sc as trivalent, the chemical formula (Ti3+3.60Al1.89Zr1.04Mg0.24Si0.13Sc0.06Ca0.05Y0.02Hf0.01)Σ=7.04O11. The simplified formula is ZrAl2Ti4O11, which requires ZrO2 24.03, Al2O3 19.88, and Ti2O3 56.09, totaling 100.00 wt %. The main diffraction lines, corresponding to multiple hkl indices, are (d in Å (relative visual intensity)): 5.04 (65), 4.09 (60), 2.961 (100), 2.885 (40), and 2.047 (60). The crystal structure study revealed carmeltazite to be orthorhombic, space group Pnma, with unit-cell parameters a = 14.0951 (9), b = 5.8123 (4), c = 10.0848 (7) Å, V = 826.2 (1) Å3, and Z = 4. The crystal structure was refined to a final R1 = 0.0216 for 1165 observed reflections with Fo > 4σ(Fo). Carmeltazite exhibits a structural arrangement similar to that observed in a defective spinel structure. The name carmeltazite derives from Mt Carmel (“CARMEL”) and from the dominant metals present in the mineral, i.e., Titanium, Aluminum and Zirconium (“TAZ”). The mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (2018-103).

Ti3+ in corundum traces crystal growth in a highly reduced magma

“…This f O 2 also is close to the stability limit of the assemblage SiO 2 + C, which is consistent with the appearance of moissanite (SiC) as inclusions in the corundum. The tistarite is typically resorbed and overgrown by carmeltazite (ZrAl 2 Ti 4 O 11 26 ; Fig. 2 ).…”

Cr2O3 in corundum: Ultrahigh contents under reducing conditions

“…Petrographic observations show that the earliest phenocrysts in these pockets are Mg-Al-Ti spinel, followed by tistarite (Ti 2 O 3 ) and then carmeltazite (ZrAl 2 Ti 3+ 4 O 11 ; Griffin et al 2018b). More evolved melts precipitated REE-rich hibonite (CaAl 12 O 19 ), moissanite (SiC) and Zr-REE phases.…”

“…xenocrysts formed at ca 1 GPa, close to the crust-mantle boundary, and crystallized over a T range of approximately 1450°C to 1200°C. Dating of large xenocrystic zircons from the pyroclastic rocks (Griffin et al 2018b) suggests that the crust was underplated by mafic magmas from ca 285-100…”

International Mineralogical Association

“…As can be seen from its formula, the structure of carmeltazite is cation-and anion-deficient relative to spinels, while its symmetry is reduced to orthorhombic. Perhaps even more remarkable than its public image or structure is the association of carmeltazite with other Ti 3+ and carbide minerals, which indicates very unusual geological conditions in their volcanic cradle and promises new exciting discoveries in the future (Griffin et al 2018).…”