During the study of the mineral assemblage of rock fragments recovered from volcanic tuffs and associated placer deposits in the drainage of the Kishon River, near Haifa (northern Israel), several exotic phases have been identified as accessory minerals (e.g., [1
] and references therein). In that area, a series of small volcanoes produced mafic to ultramafic pyroclastic rocks (vent breccias, tuffs) in upper Cretaceous time [2
]. These rocks contain a wide variety of xenocrysts, including megacrysts of clinopyroxene, ilmenite, zircon and corundum. Among them, aggregates of corundum crystals (Carmel Sapphire TM
) are common in pyroclastic ejecta and in associated alluvial deposits. Many of these aggregates contain crystals of an unidentified Zr-Al-Ti-bearing phase, up to 80 μm in length. Chemical analysis and X-ray single-crystal diffraction studies allowed the characterization of the new Zr-Al-Ti phase, with the simplified formula ZrAl2
. This new mineral was named carmeltazite from Mt Carmel (“CARMEL”) and from the 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, under the number 2018-103. The holotype specimen of carmeltazite is deposited in the mineralogical collections of the Museo di Storia Naturale, Università degli Studi di Firenze, Via G. La Pira 4, Florence, Italy, under catalogue number 3293/I.
The mineralogical description of carmeltazite, as well as its crystal structure, are given in this paper.
2. Occurrence of Carmeltazite
The new mineral described here, carmeltazite, occurs in pockets of trapped melt interstitial to, or included in, skeletal corundum crystals (Figure 1
, Figure 2
and Figure 3
). The earliest parageneses consist of tistarite (Ti2
) ± carmeltazite ± Mg-Al spinel in a matrix of Ca-Mg-Al-Si-O glass.
The silicate melts (probably basaltic) parental to this assemblage had previously been progressively desilicated by the exsolution of immiscible Fe-Ti oxide melts and Fe-Ti-Zr-silicide melts (found also as inclusions in carmeltazite; Figure 2
), and the crystallization of moissanite and khamrabaevite (TiC), at f
= ΔIW-6 or less. This process continued, producing progressively lower f
, witnessed especially by the appearance of Ti2+
-bearing phases (osbornite, khamrabaevite, unnamed TiB2
, and unnamed TiO).
3. Mineral Description and Physical Properties
Carmeltazite (Figure 1
) occurs as black crystals, up to 80 μm in length and a few μm thick. The streak is reddish brown and the luster is metallic. The calculated density is 4.122 g·cm−3
based on the ideal formula and single-crystal data (see below). Density was not measured because of the small amount of available material.
In plane-polarized incident 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.
The reflectance was measured in air by means of a MPM-200 microphotometer (CRAIC Technologies, San Dimas, CA, USA) equipped with a MSP-20 system processor on a Zeiss Axioplan ore microscope (Zeiss, Oberkochen, Germany). Filament temperature was approximately 3350 K. Readings were taken for specimen and standard (SiC) under the same focus conditions. The diameter of the circular measuring area was 0.05 mm. Reflectance percentages in the form (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).
4. Chemical Data
Quantitative chemical analyses were carried out using a CAMECA-100X electron-microprobe (CAMECA Instruments, Gennevilliers, France), operating in WDS (Wavelength Dispersive Spectrometry) mode. The experimental conditions were: accelerating voltage 20 kV, beam current 20 nA, and beam size 1 μm. Counting times are 15 s for peak and 20 s for background. Standards are (element, emission line): wollastonite (Si K
α, Ca K
α), zircon (Zr K
α), Hf wire (Hf L
α), synthetic UO2
α), synthetic ThO2
α), kyanite (Al K
α), Cr metal (Cr K
α), synthetic TiO2
α), synthetic ScPO4
α), synthetic YPO4
α), and synthetic MgO (Mg K
α). Carmeltazite is chemically homogeneous within the analytical uncertainties of our measurements. Table 1
gives analytical data (average of eight spot analyses).
The empirical formula (based on 11 oxygen atoms pfu, and assuming all Ti and Sc as trivalent) is (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 analytical total is excellent; the calculated relative error on the valence equilibrium Ev (defined as Ev (%) = (Ev (+) − Ev (−)) × 100/Ev (−)) indicates a very small excess of positive charges.
5. X-ray Crystallography
A small carmeltazite fragment was extracted from the polished section shown in Figure 1
and mounted on a 5 μm diameter carbon fiber, which was, in turn, attached to a glass rod. X-ray single-crystal intensity data were collected using an Oxford Diffraction Xcalibur 3 diffractometer (Oxford Diffraction Ltd., Abingdon, UK), equipped with a Sapphire 2 CCD area detector, with Mo K
α radiation. The detector to crystal working distance was 6 cm. The refined unit-cell parameters are: a
= 14.0951 (9), b
= 5.8123 (4), c
= 10.0848 (7) Å, and V
= 826.2 (1) Å3
The collected data were integrated and corrected for standard Lorentz polarization factors with the CrysAlis
RED package [4
]. The program ABSPACK in CrysAlis
] was used for the absorption correction. In total, 1546 unique reflections were collected. The statistical tests (|E2
−1| = 0.980) and the reflection conditions indicated the space group Pnma
. The positions of most of the atoms were determined by means of direct methods. A least-squares refinement on F2
using heavy-atom positions and isotropic temperature factors gave an R
factor of 0.156. Three-dimensional difference-Fourier synthesis yielded the position of the remaining atoms. The program Shelxl
] was used for the refinement of the structure. Crystal data and details of the intensity data collection and refinement are reported in Table 2
. We note here that the wR
value is rather high, although we tried different absorption correction options.
The site occupancy factor at the cation sites was allowed to vary (Ti vs. Al and Zr vs. Ti for the octahedral sites and Si vs. structural vacancy for the tetrahedral site) using scattering curves for neutral atoms taken from the International Tables for Crystallography
The tetrahedral site showed a mean electron number of 12.6 and was thought to be occupied by Al and the available minor Si (i.e., Al0.87
). Indeed, although the site scattering was <13 and the mean bond distance could indicate that minor Mg could substitute for Al, we thought that partitioning the minor Si in the tetrahedron would be the right choice. The M
1 site, a site that shows a peculiar geometry with a 1 + 4 coordination with a refined site scattering of 14.3, was thought to be occupied by Al with minor Mg, Sc, Ca, Y and Hf (i.e., Al0.68
). The composition of M1 was refined simply as mixed Al + Ti site (see Supplementary Material
: carmeltazite.cif). The mean electron numbers at the four octahedral M
sites were the following: 37.3 (M
2 site), 22.0 (M
3 site), 20.7 (M
4 site), and 20.8 (M
5 site) corresponding to Zr0.85
, and Ti0.87
, respectively. Altogether, taking into account the different multiplicity of the structural sites, the refined X-ray formula can be written as (Ti3+3.75
. Such a formula is in excellent agreement with that obtained from electron microprobe: (Ti3+3.60
Final atomic coordinates and equivalent isotropic displacement parameters are given in Table 3
, whereas selected bond distances are presented in Table 4
. Bond valence sums calculated using the parameters by Brese and O’Keeffe [7
] and the following cation distributions are shown in Table 5
M1 = Al0.68Mg0.22Sc3+0.04Ca0.03Y0.02Hf0.01
Taking into account the refined mean electron numbers at the different sites, the cation-site preferences, and the polyhedral environments, we arrived to the site distributions reported above. Although we realize that some of the bond-valence sums (e.g., M1) are very far from the ideal values, we were not able to identify another site distribution that matches the refined site scattering values.
The diffraction rings (Table 6
) from the same fragment used for the single-crystal study were obtained with an Oxford Diffraction Xcalibur PX Ultra diffractometer (Oxford Diffraction Ltd., Abingdon, UK) fitted with a 165 mm diagonal Onyx CCD detector (Oxford Diffraction, Abingdon, UK) and using copper radiation (CuK
α, λ = 1.54138 Å). The working conditions were: 50 kV, 50 mA, and 3 h of exposure; the detector-to-sample distance was 7 cm. The program Crysalis
] was used to convert the observed diffraction rings to a conventional powder diffraction pattern. The least squares refinement gave the following values: a
= 14.076 (2), b
= 5.8124 (8), c
= 10.0924 (9) Å, and V
= 825.7 (1) Å3