First Terrestrial Occurrence of Kaitianite (Ti³⁺₂Ti⁴⁺O₅) from the Upper Mantle beneath Mount Carmel, Israel

Abstract

Our nanomineralogical investigation of melt inclusions in corundum xenoliths from the Mount Carmel area, Israel, has revealed seven IMA-approved new minerals since 2021. We report here the first terrestrial occurrence of kaitianite (Ti³⁺₂Ti⁴⁺O₅). Kaitianite occurs as exsolution lamellae in tistarite (Ti₂O₃), in a melt inclusions together with a Ti,Al,Zr-oxide, a MgTi³⁺₂Al₄SiO₁₂ phase, spinel, sapphirine, Ti-sulfide, alabandite, and Si-rich glass in a corundum grain (Grain 1125C2). The chemical composition of kaitianite using electron probe microanalysis is (wt%) Ti₂O₃ 58.04, TiO₂ 37.82, Al₂O₃ 2.87, MgO 0.85, ZrO₂ 0.10, CaO 0.02, SiO₂ 0.02, sum 99.73, yielding an empirical formula of (Ti³⁺_1.78Al_0.12Ti⁴⁺_0.05Mg_0.05)(Ti⁴⁺_1.00)O₅, with the Ti³⁺ and Ti⁴⁺ partitioned, assuming a stoichiometry of three cations and five oxygen anions pfu. Electron back-scatter diffraction reveals that kaitianite has the monoclinic C2/c γ-Ti₃O₅-type structure with cell parameters: a = 10.12 Å, b = 5.07 Å, c = 7.18 Å, β = 112°, V = 342 ų, and Z = 4. Kaitianite is a high-temperature oxide phase, formed in melt pockets under reduced conditions in corundum-aggregate xenoliths derived from the upper mantle beneath Mount Carmel, Israel.

1. Introduction

Super-reduced mineral assemblages are found worldwide in association with explosive volcanism (kimberlites, alkali basalts, tholeiitic basalts) and in ophiolites associated with deep subduction at continental plate margins [1,2,3]. Their origin has been controversial, with claims that they are of anthropogenic origin. The well-documented material found as xenoliths and xenocrysts in small Cretaceous volcanoes and the associated Plio–Pleistocene gem placer deposits at Mount Carmel, northern Israel, are important in this regard because the geological setting, geochemical data, and geochronology effectively rule out any realistic suggestion of human intervention [4,5,6]. Many of the super-reduced phases are found as inclusions in xenoliths of corundum aggregates, and the relationships of different phases in melt inclusions have been critical to understanding the genesis of the super-reduced magma-fluid systems.

During our ongoing nanomineralogical investigation of melt inclusions in the corundum xenocrysts, we have identified seven IMA-approved new minerals since 2021: griffinite (Al₂TiO₅), magnéliite (Ti³⁺₂Ti⁴⁺₂O₇), ziroite (ZrO₂), sassite (Ti³⁺₂Ti⁴⁺O₅), mizraite-(Ce) (Ce(Al₁₁Mg)O₁₉), toledoite (TiFeSi), and yeite (TiSi) [7,8,9,10,11,12,13]. Reported here are the first terrestrial, and second natural, occurrences of kaitianite (Ti³⁺₂Ti⁴⁺O₅) in a corundum xenocryst from Mount Carmel, providing more insights into the origin of reduced high-temperature minerals from the upper mantle. Kaitianite was first discovered in the Allende CV3 meteorite as a refractory mineral formed in the solar nebula [14].

2. Materials and Methods

The studied corundum xenoliths were found to occur in the pyroclastic ejecta of small Cretaceous basaltic volcanoes exposed on Mount Carmel and in placer gemstone deposits in the terraces of the Paleocene to Pleistocene proto-Kishon River, which now drains Mount Carmel and enters the sea near Haifa in northern Israel [2] as well as the tributary Mizra river. Some of the xenolithic material in the paleoterrace deposits may also have derived from Miocene to Paleocene basalts outcropping in the drainage of the Kishon River. The xenoliths are aggregates of skeletal corundum crystals with melt pockets containing reduced mineral assemblages [1,4,15,16,17]. The kaitianite material described here is found in corundum Grain 1125C2 in polished Mount Carmel mount Corundum-18-1.

An electron probe microanalyzer (EPMA), a high-resolution scanning electron microscope (SEM) with X-ray energy dispersive spectrometer (EDS), and electron backscatter diffraction (EBSD) were used to characterize the composition and structure of kaitianite and associated phases. A ZEISS 1550VP Field-Emission SEM (ZEISS Group, Oberkochen, Germany) with an Oxford X-Max EDS (Oxford Instruments, Abingdon, UK) was used for back-scatter electron (BSE) imaging and fast elemental analysis. Quantitative WDS elemental microanalyses of kaitianite and associated phases were carried out using a JEOL 8200 EPMA (JEOL Ltd., Tokyo, Japan) (15 kV and 10 nA, focused beam) interfaced with the probe for EPMA program. Standards for analysis were anorthite (SiKa, AlKa, CaKa), forsterite (MgKa), TiO₂ (TiKa), and zircon (ZrLa). Quantitative elemental microanalyses were processed with the CITZAF correction procedure [18]. The focused electron beam was ~150 nm in diameter. Analytical results of kaitianite and associated phases are given in

Table 1. EPMA analytical results (in wt%) for kaitianite, associated tistarite, Ti,Al,Zr-oxide, MgTi³⁺₂Al₄SiO₁₂ phase, and sapphirine.

Oxide	Kaitianite	SD b	Tistarite	SD	Ti,Al,Zr-Oxide	SD	MgTi3+2Al4SiO12	SD	Sapphirine	SD
	n = 6 a		n = 5		n = 7		n = 11		n = 5
SiO2	0.02	0.01			2.72	0.21	11.95	0.19	11.55	0.08
Ti2O3	58.04	0.15	89.53	0.18	58.04	0.66	28.36	0.70	5.86	0.17
TiO2	37.82	0.10	6.77	0.01
Al2O3	2.87	0.03	0.88	0.04	21.27	0.37	45.73	0.81	63.21	0.19
MgO	0.85	0.01	3.37	0.03	1.86	0.13	9.67	0.08	18.78	0.07
CaO	0.02	0.01	0.02	0.01	0.02	0.01
ZrO2	0.10	0.24			13.53	0.74	3.88	0.37
Total	99.72		100.57		97.44		99.64		99.40
No. O atoms	5		3		3		12		20
Si	0.00				0.06		0.90		1.36
Ti3+	1.78		1.74		1.10		1.79		0.58
Ti4+	1.05		0.12
Al	0.12		0.02		0.57		4.08		8.75
Mg	0.05		0.12		0.06		1.09		3.29
Ca	0.00		0.00		0.00
Zr	0.00				0.15		0.14
Cation sum	3.00		2.00		1.95		20.01		33.98

Note: ^a n = number of analyses. ^b SD = one standard deviation of the mean based on all of the analyses.

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EBSD analyses at a submicrometer scale were performed using methods described in [19,20] for mineral identifications. An HKL EBSD system (Oxford Instruments, UK) on the ZEISS 1550VP Field-Emission SEM was operated at 20 kV and 6 nA in focused beam mode with a 70° tilted stage and in a variable pressure mode (25 Pa). The EBSD system was calibrated using a single-crystal silicon standard. Structural information was obtained and cell constants were derived by matching the experimental EBSD patterns with structures of synthetic phases from the ICSD [21,22].

3. Results

Kaitianite occurs as exsolution lamellae in tistarite, in a melt inclusions together with a Ti,Al,Zr-oxide, a MgTi³⁺₂Al₄SiO₁₂ phase, spinel, sapphirine, Ti-sulfide, alabandite, and Si-rich glass from the corundum xenocryst Grain 1125C2 (Figure 1). The grain was retrieved from a bulk sample. Other inclusions in this corundum grain contain krotite (CaAl₂O₄), grossite (CaAl₄O₇), osbornite (TiN), and Fe metal. Kaitianite is transparent, occurring as thin lamellae, ~0.1 to 2.5 μm thick, ~3 to 40 μm long.

The chemical composition of kaitianite using EPMA (Table 1) shows an empirical formula (based on 5 O pfu) of (Ti³⁺_1.78Al_0.12Ti⁴⁺_0.05Mg_0.05)_Σ2(Ti⁴⁺_1.00)O₅, where Ti³⁺ and Ti⁴⁺ are partitioned, assuming a stoichiometry with three cations and five oxygen anions pfu. The simplified formula is (Ti³⁺,Al)₂Ti⁴⁺O₅, and the ideal formula is Ti³⁺₂Ti⁴⁺O₅. The EBSD patterns can be indexed only by the monoclinic C2/c γ-Ti₃O₅-type structure and match the synthetic γ-Ti₃O₅ cell from [21] (Figure 2) with a mean angular deviation of ~0.38°, revealing the following cell parameters: a = 10.12 Å, b = 5.07 Å, c = 7.18 Å, β = 112°, V = 342 ų, and Z = 4.

Associated tistarite has an empirical formula of (Ti³⁺_1.74Ti⁴⁺_0.12Mg_0.12Al_0.02)_Σ2O₃. The Ti,Al,Zr-oxide is similar to the Allende Ti,Al,Zr-oxide [14]. The MgTi³⁺₂Al₄SiO₁₂ phase has an empirical formula of Mg_1.09Zr_0.14Ti³⁺_1.79Al_4.08Si_0.90O₁₂ and probably is a new mineral. Sapphirine has an empirical formula of (Al_4.11Mg_3.29Ti³⁺_0.58)(Al_4.64Si_1.36)O₂₀.

The Ti-sulfide is a new mineral with an empirical formula of (Ti_1.10Fe_0.21Cr_0.17Mn_0.07V_0.01)_Σ1.56S_2.00 and the P-3m1 TlCdS₂-type. The general formula for this nonstoichiometric Ti-sulfide is Ti_1+xS₂.

4. Discussion

Kaitianite (Ti³⁺₂Ti⁴⁺O₅) is γ-Ti₃O₅ [21]. The newly approved mineral sassite (Ti³⁺₂Ti⁴⁺O₅; IMA 2022-014) [10] is β-Ti₃O₅ [23,24], a member of the pseudobrookite group, joining pseudobrookite (Fe₂TiO₅), armalcolite ((Mg,Fe²⁺)Ti₂O₅), and griffinite (Al₂TiO₅; IMA 2021-110; [7]. Several polymorphs of Ti₃O₅ have been described so far: α–Ti₃O₅ having monoclinic symmetry, C2/m, with a = 9.752(1), b = 3.802(1), c = 9.442(1) Å and β = 91.55(1)° [25]; β-Ti₃O₅ having orthorhombic symmetry, Cmcm, a = 3.798(2), b = 9.846(3), c = 9.988(4) Å [24], which corresponds to sassite; γ-Ti₃O₅, having the monoclinic V₃O₅-type structure, and a = 10.115, b = 5.074, c = 7.182 Å, β = 112° and C2/c space group [21], which corresponds to kaitianite; δ–Ti₃O₅ that shows monoclinic structure and P2/a space group, with lattice parameters a = 9.9651(7), b = 5.0604(4), c = 7.2114(5) Å, and β = 109.3324(9)°, related to γ-Ti₃O₅ by decreasing temperature [22,26,27]; and λ–Ti₃O₅ which is monoclinic C2/m with a = 9.8357(11), b = 3.7935(2), c = 9.9863(7) Å, β = 90.976(6)° and related to α–Ti₃O₅ by a second-order phase transition [28]. The type kaitianite has been described as associated with tistarite and rutile, together with corundum, xifengite, mullite, osbornite, and a new Ti,Al,Zr-oxide mineral in the Allende CV3 carbonaceous chondrite [14]; most of these minerals are also present in the melt pockets in corundum aggregates from Mount Carmel.

Ti³⁺-rich phases are common in melt inclusions in those corundum xenocrysts, including tistarite (Ti₂O₃), magnéliite (Ti³⁺₂Ti⁴⁺₂O₇), sassite (Ti³⁺₂Ti⁴⁺O₅), and kaitianite (Ti³⁺₂Ti⁴⁺O₅) (Figure 3). While low temperature α–Ti₃O₅ is highly ordered (three octahedral Ti sites, with <Ti–O> = 2.015 Å, 2.033 Å, and 2.033 Å), sassite and kaitianite show higher degrees of disorder (two crystallographically independent Ti sites, labeled Ti1 and Ti2, octahedrally surrounded by oxygen atoms, with <Ti1–O> = 2.0271 Å and <Ti2–O> = 2.0385 Å for sassite and <Ti1–O> = 2.0334 Å and <Ti2–O> = 2.0331 Å for kaitianite).

Kaitianite and the associated minerals described here are high-temperature (>1300–ca. 1150 °C) phases, crystallized from melts trapped in intracrystalline and interstitial voids in aggregates of corundum crystals [6,15]. The whole suite is characterized by oxygen fugacity (fO₂) below the levels normally encountered in Earth’s upper mantle or crust (IW to IW-9; [29]). The most reduced parageneses, including hibonite + grossite + V⁰, require a hydrogen-dominated environment, as confirmed by the occurrence of natural hydrides and spectroscopic evidence of hydrogen in vacancies in hibonite [17,29,30,31].

The super-reduced mineral assemblages evolved within a thick Triassic–Miocene gabbroic underplate and are ascribed to the progressive reduction and desilication of differentiated syenitic melts, through interaction with mantle-derived CH₄ + H₂ fluids [3,5,32]. The presence of these fluids is documented by the extremely low fO₂ recorded in associated parageneses, the abundance of carbon-bearing phases including SiC, TiC, Ti(N,C,O), and explosion breccias with amorphous-carbon matrices [2,5,17,29,32,33,34]. Voids bounded by crystal faces and sometimes populated by phenocryst phases can comprise >2% of some samples, implying the presence of a fluid phase, while some melt pockets contain bubbles, presumably gas-filled. Gasses released by ignition are typically ca. 95 at.% H₂ [5]. Desilication of the melts accompanied the separation of Fe–Ti–Si melts and Si⁰ melts, driving saturation or supersaturation of Al₂O₃ in the silicate melts and leading to the crystallization of abundant corundum.

In the corundum aggregates, three broad paragenetic types have been recognized [3]:

Crn-A: hopper to skeletal crystals with strong zoning in Ti due to uptake of Ti³⁺ during rapid crystal growth [35]. Trapped melts are Ca–Mg–Al silicates with high contents of S and incompatible elements. Phase assemblages reflect fO₂ ≤ IW-6, with all Ti as Ti³⁺ (e.g., tistarite, carmeltazite) or Ti²⁺ (TiC, TiN, TiB₂, Ti-Fe alloys);

Crn-B: large homogeneous (unzoned) corundum crystals, typically with Ti contents > 1 wt%. Interstitial pockets contain small amounts of Ca–Al–Mg silicate glass, typically high in REE, Zr, Th, U, and other incompatible elements. Ti is present in phenocrysts as both Ti³⁺ and Ti⁴⁺. Kaitianite is a member of this paragenesis and its occurrence as exsolution lamellae suggests an increase in fO₂ following the crystallization of tistarite, in which all Ti is Ti³⁺;

Crn-C: texturally similar to Crn-B but Ti contents in corundum typically are low (<0.5 wt% Ti). Rare Ca–Al–Na–K silicate glasses are rich in LREE and Ba. The presence of more Ti⁴⁺ phases (rutile, griffinite) suggests a higher mean fO₂ than in Crn-A and Crn-B. Hibonite occurs in all three parageneses; in Crn-A and Crn-B, it contains high levels of Ti³⁺, while in Crn-C the Ti³⁺ contents are very low.

The identification of mixed-valence phases in paragenesis Crn-B, and possibly in Crn-C, adds new information relevant to the interpretation of the origins of the Mount Carmel corundum-aggregate xenoliths. Textural evidence indicates that Crn-B and Crn-C are cumulates from immiscible Fe-rich melts (subsequently depleted in Fe by the separation of Fe-Ti silicide melts; [5]) and Si–Al–Na–K melts, respectively. It seems probable that these melts, with similar but divergent histories, separated into volumes that were affected to different extents by the interaction with the reducing (CH₄ + H₂) fluids before being entrained in the host basalt. Crn-A, in contrast, appears to represent the very rapid crystallization of corundum in the presence of highly reducing (CH₄ + H₂) fluids and residual melts, possibly in melt-escape channels developed above magma chambers.

The details of the evolution of these remarkable parageneses are becoming steadily clearer through these nanomineralogical studies of the melt inclusions in the different corundum parageneses, and related types of xenoliths. Further surprises can be expected.