New Minerals from Inclusions in Corundum Xenocrysts from Mt. Carmel, Israel: Magnéliite, Ziroite, Sassite, Mizraite-(Ce) and Yeite

Abstract

Our nanomineralogical investigation of melt inclusions in corundum xenocrysts from the Mt. Carmel area, Israel has revealed seven IMA-approved new minerals since 2021. We report here four new oxide minerals and one new alloy mineral. Magnéliite (Ti³⁺₂Ti⁴⁺₂O₇; IMA 2021-111) occurs as subhedral crystals, ~4 μm in size, with alabandite, zirconolite, Ti,Al,Zr-oxide, and hibonite in corundum Grain 767-1. Magnéliite has an empirical formula (Ti³⁺_1.66Al_0.13Ti⁴⁺_0.15Mg_0.10Ca_0.01Sc_0.01)_Σ2.06 (Ti⁴⁺_1.93Zr_0.08)_Σ2.01O₇ and the triclinic P$\overline{1}$ Ti₄O₇-type structure with the cell parameters: a = 5.60(1) Å, b = 7.13(1) Å, c = 12.47(1) Å, α = 95.1(1)°, β = 95.2(1)°, γ = 108.7(1)°, V = 466(2) ų, Z = 4. Ziroite (ZrO₂; IMA 2022-013) occurs as irregular crystals, ~1–4 μm in size, with baddeleyite, hibonite, and Ti,Al,Zr-oxide in corundum Grain 479-1a. Ziroite has an empirical formula (Zr_0.72Ti⁴⁺_0.26Mg_0.02Al_0.02Hf_0.01)_Σ1.03O₂ and the tetragonal P4₂/nmc zirconia(HT)-type structure with the cell parameters: a = 3.60(1) Å, c = 5.18(1) Å, V = 67.1(3) ų, Z = 2. Sassite (Ti³⁺₂Ti⁴⁺O₅; IMA 2022-014) occurs as subhedral-euhedral crystals, ~4–16 μm in size, with Ti,Al,Zr-oxide, mullite, osbornite, baddeleyite, alabandite, and glass in corundum Grain 1125C1. Sassite has an empirical formula (Ti³⁺_1.35Al_0.49Ti⁴⁺_0.08Mg_0.07)_Σ1.99(Ti⁴⁺_0.93Zr_0.06Si_0.01)_Σ1.00O₅ and the orthorhombic Cmcm pseudobrookite-type structure with the cell parameters: a = 3.80(1) Å, b = 9.85(1) Å, c = 9.99(1) Å, V = 374(1) ų, Z = 4. Mizraite-(Ce) (Ce(Al₁₁Mg)O₁₉; IMA 2022-027) occurs as euhedral crystals, <1–14 μm in size, with Ce-silicate, Ti-sulfide, Ti,Al,Zr-oxide, ziroite, and thorianite in corundum Grain 198-8. Mizraite-(Ce) has an empirical formula (Ce_0.76Ca_0.10La_0.07Nd_0.01)_Σ0.94(Al_10.43Mg_0.84Ti³⁺_0.60Si_0.09Zr_0.04)_Σ12.00O₁₉ and the hexagonal P6₃/mmc magnetoplumbite-type structure with the cell parameters: a = 5.61(1) Å, c = 22.29(1) Å, V = 608(2) ų, Z = 2. Yeite (TiSi; IMA 2022-079) occurs as irregular-subhedral crystals, 1.2–3.5 μm in size, along with wenjiite (Ti₅Si₃) and zhiqinite (TiSi₂) in Ti-Si alloy inclusions in corundum Grain 198c. Yeite has an empirical formula (Ti_0.995Mn_0.003V_0.001Cr_0.001)(Si_0.996P_0.004) and the orthorhombic Pnma FeB-type structure with the cell parameters: a = 6.55(1) Å, b = 3.64(1) Å, c = 4.99(1) Å, V = 119.0(4) ų, Z = 4. The five minerals are high-temperature oxide or alloy phases, formed in melt pockets in corundum xenocrysts derived from the upper mantle beneath Mt. Carmel.

1. Introduction

Worldwide, super-reduced mineral assemblages are commonly associated with explosive volcanic events such as kimberlites, alkali basalts, and tholeiitic basalts, as well as in ophiolites linked to deep subduction along continental plate margins [1,2,3]. The origins of these assemblages have sparked debate, with some attributing them to human activities [4]. However, the extensively documented xenoliths and xenocrysts discovered in small Cretaceous volcanoes and Plio-Pleistocene gem placer deposits at Mt. Carmel, Israel, play a crucial role in this discussion. The geological context, along with thorough geochemical analysis and precise geochronological data, effectively refute any plausible notion of human interference [3,5,6]. Many super-reduced minerals are identified as inclusions within xenoliths composed of corundum aggregates. The relationships between these different phases within melt inclusions have been crucial in interpreting the genesis of super-reduced magma-fluid systems.

A nanomineralogical investigation of melt inclusions found in corundum xenocrysts coming from volcanic centers and associated alluvial deposits in the Mt. Carmel area, Israel, has allowed us to discover 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]. All of them have been approved by the IMA-CNMNC (Commission on New Minerals, Nomenclature and Classification) as requested before naming and reporting the finding of a new mineral phase. Griffinite has been already published in detail [14]. Toledoite will be published in a separate paper. Reported here are another five new minerals, adding more information on the origin of reduced high-temperature minerals from the upper mantle.

Magnéliite (IMA 2021-111), Ti³⁺₂Ti⁴⁺₂O₇ (simply Ti₄O₇), is a new Ti-oxide mineral that corresponds to the first member of the homologous series of Ti-oxides (with Ti_nO_2n−1), known also as Magnéli phases [15]. The name is in honor of Arne Magnéli (1914–1996), for his pioneering work on the structural chemistry of transition-metal oxides.

Ziroite (IMA 2022-013), ZrO₂, is a new Zr-oxide mineral with the P4₂/nmc zirconia(HT)-type structure. The name is derived from its composition.

Sassite (IMA 2022-014), Ti³⁺₂Ti⁴⁺O₅ (simply Ti₃O₅), is another new Ti-oxide mineral with the Cmcm pseudobrookite-type structure. The name is in honor of Eytan Sass (b. 1932), a geologist at the Freddy and Nadine Herrmann Institute of Earth Sciences, Hebrew University of Jerusalem. He performed the excellent mapping work on Mt. Carmel that identified the various volcanic centers.

Mizraite-(Ce) (IMA 2022-027), Ce(Al₁₁Mg)O₁₉, is a new Ce-rich oxide mineral belonging to the magnetoplumbite-group [16], with the P6₃/mmc magnetoplumbite-type structure. The name is after the Mizra river in the Mt. Carmel region, where some corundum xenocrysts investigated in this study (including Grain 198-8) come from alluvial deposits. The tributary Mizra river flows into the Kishon River.

Yeite (IMA 2022-079) is a new alloy mineral, TiSi, with the Pnma FeB-type structure. The name is in honor of Danian Ye (b. 1939), a mineralogist at the Institute of Geology and Geophysics, Chinese Academy of Sciences, for his many contributions to mineralogy and crystal chemistry.

2. Materials and Methods

The corundum xenoliths hosting the new minerals as inclusions occur in the pyroclastic ejecta from small Cretaceous basaltic volcanoes on Mt. Carmel and from placer gemstone deposits found in the terraces of the Paleocene to Pleistocene proto-Kishon river; the modern Kishon River drains Mt. Carmel and the tributary Mizra river and enters the sea near Haifa in northern Israel [2]. Much of the xenolith material in the paleoterrace deposits probably also is derived from Miocene and Pliocene basalt outcroppings in the drainage area of the Kishon River. The xenoliths occur as aggregates of skeletal corundum crystals that enclose melt pockets containing reduced mineral assemblages [1,2,17,18].

All the type materials are deposited in the mineralogy collection of the Università degli Studi di Milano, Via Mangiagalli, 34-20133 Milano, Italy.

The type magnéliite in corundum Grain 767-1 from Mt. Carmel mount Corundum-SY is under the registration number MCMGPG-H2022-001.

The type ziroite in corundum Grain 479-1a from Mt. Carmel mount Corundum-18-1 is under the registration number MCMGPG-H2021-003.

The type sassite in corundum Grain 1125C1 from Mt. Carmel mount Corundum-18-1 is under the registration number MCMGPG-H2021-004.

The type mizraite-(Ce) in corundum Grain 198-8 from Mt. Carmel mount Corundum-18-1 is under the registration number MCMGPG-H2022-005.

The type yeite in corundum Grain 198c from Mt. Carmel mount Corundum-18-1 is under the registration number MCMGPG-H2021-002. This corundum grain also hosts the type griffinite (Al₂TiO₅; IMA 2021-110; [14]).

In order to characterize the composition and structure of the new minerals and associated phases, we used an electron probe microanalyzer (EPMA) and a high-resolution scanning electron microscope (SEM) with an X-ray energy dispersive spectrometer (EDS) and electron backscatter diffraction (EBSD). A ZEISS 1550VP Field-Emission SEM (ZEISS Group, Oberkochen, Germany) with an Oxford X-Max EDS was used for backscatter electron (BSE) imaging and fast elemental analysis. Quantitative WDS elemental microanalyses of the new minerals were carried out using a JEOL 8200 EPMA (JEOL Ltd., Tokyo, Japan) (15 kV and 10 nA, focused beam) and processed with the CITZAF correction procedure [19]. The focused electron beam is ~150 nm in diameter.

EBSD analyses at a submicrometer scale were performed using methods described by [20,21] for studies of micron-sized new minerals. An HKL EBSD system 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. Experimental EBSD patterns allowed the collection of structural information and cell constants that were derived by matching with those of the structures of synthetic phases from the ICSD (Inorganic Crystal Structure Database).

Due to the small size of the samples, most of the physical properties (optical, hardness, fracture, cleavage, habit, density, etc.) were impossible to obtain.

3. Results

3.1. Magnéliite

Magnéliite occurs with alabandite, zirconolite, Ti,Al,Zr-oxide, and hibonite in one inclusion from corundum Grain 767-1 (Figure 1). Other inclusions in this corundum grain contain hibonite and osbornite. It is transparent, occurring as subhedral crystals ~4 μm in size. The Gladstone–Dale relationship [22] gives n = 2.423, obtained from the chemical composition and calculated density.

The chemical composition of magnéliite using EPMA (

Table 1. EPMA analytical results (in wt%, n = 7) for magnéliite.

Constituent	Mean	Range	SD (σ)	Probe Standard
TiO2 *	53.54	53.28–53.93	0.17	TiO2
Ti2O3 *	38.63	38.44–38.84	0.12	TiO2
ZrO2	3.02	2.86–3.20	0.11	zircon
Al2O3	2.20	2.03–2.26	0.08	anorthite
MgO	1.36	1.26–1.51	0.09	forsterite
CaO	0.19	0.17–0.20	0.01	anorthite
Sc2O3	0.22	0.20–0.23	0.01	ScPO4
Total	99.16

* Total titanium has been partitioned between Ti³⁺ and Ti⁴⁺ for charge balance to make ideal stoichiometry.

) shows an empirical formula (based on 7 O pfu) of (Ti³⁺_1.66Al_0.13Ti⁴⁺_0.15Mg_0.10Ca_0.01Sc_0.01)_Σ2.06(Ti⁴⁺_1.93Zr_0.08)_Σ2.01O₇. The simplified formula is (Ti³⁺,Al)₂Ti⁴⁺₂O₇. The ideal formula is Ti³⁺₂Ti⁴⁺₂O₇, which requires Ti₂O₃ 47.36, TiO₂ 52.64, total 100 wt%.

The EBSD patterns can be indexed only by the P$\overline{1}$ Ti₄O₇-type structure and match the synthetic Ti₄O₇ cell from [23] (Figure 2), with a mean angular deviation of 0.32°–0.35°, revealing the following cell parameters: a = 5.60(1) Å, b = 7.13(1) Å, c = 12.47(1) Å, α = 95.1(1)°, β = 95.2(1)°, γ = 108.7(1)°, V = 466(2) ų, and Z = 4. The calculated density is 4.30 g·cm⁻³ using the empirical formula and the unit-cell volume estimated from the EBSD data.

Magnéliite (Ti³⁺₂Ti⁴⁺₂O₇) is a new Ti-oxide mineral. It belongs to the so-called Magnéli phases, i.e., a series of Ti-oxides homologous with Ti_nO_2n−1 (with n = from 4 to 10). The first member of the series, synthetic Ti₄O₇, is well known (e.g., [23,24,25]). The crystal structure of magnéliite can be considered to be derived from the structure of rutile TiO₂ by crystallographic shear of the (121)_rutile plane with a 1/2[0–11]_rutile vector every four octahedra of rutile [26]. The resulting structure has chains of edge-sharing TiO₆ octahedra truncated every four octahedra by the crystallographic shear planes (Figure 3). At room-T, Ti³⁺ and Ti⁴⁺ are disordered among the eight symmetrically independent positions, while at T < 120 K, Ti³⁺ and Ti⁴⁺ are arranged in an ordered fashion to form a Ti³⁺-Ti⁴⁺ pair (bipolarons) and the material becomes a nonmagnetic insulator. Recent data by [26] show that even at room-T some local ordering of Ti³⁺–Ti³⁺ and Ti⁴⁺–Ti⁴⁺ pairs exists.

3.2. Ziroite

Ziroite occurs with baddeleyite, Ce-rich hibonite, and Ti,Al,Zr-oxide in inclusions in corundum Grain 479-1a (Figure 4). Other inclusions in this corundum grain are MgAl-spinel, fluorbritholite-(Ce), osbornite, and hapkeite (Fe₂Si). It occurs as irregular crystals ~1–4 μm in size. It is transparent and shows a brownish black tint. The Gladstone–Dale relationship gives n = 2.342.

The chemical composition of ziroite (

Table 2. EPMA analytical results (in wt%, n = 8) for ziroite.

Constituent	Mean	Range	SD (σ)	Probe Standard
ZrO2	78.52	76.06–79.92	1.18	zircon
TiO2	18.21	16.68–20.47	1.18	TiO2
HfO2	1.43	1.35–1.54	0.08	Hf metal
MgO	0.70	0.62–0.75	0.05	forsterite
Al2O3	0.87	0.81–0.95	0.05	anorthite
Total	99.73

) gives rise to an empirical formula (based on 2 O pfu) of (Zr_0.72Ti⁴⁺_0.26Mg_0.02Al_0.02Hf_0.01)_Σ1.03O₂. The simplified formula is (Zr,Ti)O₂. The ideal formula is ZrO₂.

The EBSD patterns can be indexed only by the tetragonal P4₂/nmc zirconia (HT)-type and match the synthetic ZrO₂ cell values of [28] (Figure 5), with a mean angular deviation of 0.30–0.36°, revealing the following cell parameters: a = 3.60(1) Å, c = 5.18(1) Å, V = 67.1(3) ų, and Z = 2. The calculated density is 5.53 g·cm⁻³ using the empirical formula and the unit-cell volume estimated from the EBSD data.

Ziroite is a tetragonal polymorph of baddeleyite (monoclinic ZrO₂). Synthetic ZrO₂ with the P4₂/nmc zirconia (HT)-type structure is well known (Figure 6) (e.g., [28,29,30]). Reported here is the first natural occurrence of tetragonal ZrO₂.

3.3. Sassite

Sassite occurs with Ti,Al,Zr-oxide, mullite, osbornite, baddeleyite, alabandite, and Si-rich glass in melt pockets trapped in corundum Grain 1125C1 (Figure 7). The mineral occurs as subhedral-euhedral crystals ~4–16 μm in size. It is transparent with a brown color. The Gladstone–Dale relationship gives n = 2.16.

Sassite (

Table 3. EPMA analytical results (in wt%, n = 13) for sassite.

Constituent	Mean	Range	SD (σ)	Probe Standard
Ti2O3 *	45.32	44.81–45.94	0.30	TiO2
TiO2 *	37.32	36.90–37.83	0.24	TiO2
Al2O3	11.73	10.22–13.02	0.83	anorthite
ZrO2	3.70	3.05–5.04	0.68	zircon
MgO	1.29	0.97–1.66	0.24	forsterite
SiO2	0.24	0.15–0.46	0.08	anorthite
MnO	0.15	0.10–0.20	0.04	Mn2SiO4
CaO	0.07	0.05–0.12	0.02	ScPO4
Total	99.82

* Total titanium has been partitioned between Ti³⁺ and Ti⁴⁺ for charge balance to achieve ideal stoichiometry.

) shows an empirical formula (based on 5 O pfu) of (Ti³⁺_1.35Al_0.49Ti⁴⁺_0.08Mg_0.07)_Σ1.99(Ti⁴⁺_0.93Zr_0.06Si_0.01)_Σ1.00O₅. The simplified formula is (Ti³⁺, Al)₂Ti⁴⁺O₅. The ideal formula is Ti³⁺₂Ti⁴⁺O₅, which requires Ti₂O₃ 64.29, TiO₂ 35.71, total 100 wt%.

The EBSD patterns can be indexed only by the orthorhombic Cmcm pseudobrookite-type structure and match the synthetic β-Ti₃O₅ cell from [31] (Figure 8), with a mean angular deviation of 0.31°–0.35°, revealing the following cell parameters: a = 3.80(1) Å, b = 9.85(1) Å, c = 9.99(1) Å, V = 374(1) ų, and Z = 4. The calculated density is 3.81 g·cm⁻³ using the empirical formula and the unit-cell volume estimated from the EBSD data.

Sassite (Ti³⁺₂Ti⁴⁺O₅) is a new member of the pseudobrookite group, joining pseudobrookite (Fe₂TiO₅), armalcolite [(Mg,Fe²⁺)Ti₂O₅], and griffinite (Al₂TiO₅; IMA 2021-110) [14]. Synthetic Ti₃O₅-pseudobrookite is well known (e.g., [31,32]). It is also known as β-Ti₃O₅ because several polymorphs of Ti₃O₅ have been described so far. α–Ti₃O₅ has monoclinic symmetry, C2/m, with a = 9.752(1), b = 3.802(1), c = 9.442(1) Å, and β = 91.55(1)° [33]. β-Ti₃O₅ has orthorhombic symmetry, Cmcm, a =3.798(2), b = 9.846(3), c = 9.988(4) Å [31], which corresponds to sassite. γ-Ti₃O₅ has the monoclinic V₃O₅-type structure, with a = 10.115, b = 5.074, c = 7.182 Å, β = 112°, and C2/c space group [34], which corresponds to the mineral kaitianite, recently described in Allende CV3 carbonaceous chondrite [35]. δ–Ti₃O₅ has a 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)°, and is related to γ-Ti₃O₅ by decreasing temperature [36,37]. λ–Ti₃O₅ is monoclinic C2/m with a = 9.8357(11), b = 3.7935(2), c = 9.9863(7) Å, β = 90.976(6)°, and is related to α–Ti₃O₅ by a second-order phase transition [38].

Kaitianite was first discovered in association with tistarite and rutile together with other refractory phases of corundum, xifengite, mullite, osbornite, and a new Ti,Al,Zr-oxide mineral in the Allende meteorite [35]; most of these minerals are also found in the melt pockets in corundum from Mt. 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 grossmanite. Sc-bearing sassite was identified in the SaU 290 CH3 chondrite as an ultrarefractory phase, labeled as “anosovite,” among the first solids formed in the solar system [39]. While low-temperature α–Ti₃O₅ is highly ordered (three octahedrally Ti sites, with <Ti–O> = 2.015 Å, 2.033 Å, and 2.033 Å), both sassite and kaitianite show higher degrees of disorder (two crystallographically independent Ti sites, labeled Ti1 and Ti2, are octahedrally surrounded by oxygen atoms, having <Ti1–O> = 2.0271 Å and <Ti2–O> = 2.0385 Å in sassite [31] and <Ti1–O> = 2.0334 Å and <Ti2–O> = 2.0331 Å in kaitianite [34]). In sassite, the TiO₆ octahedra are linked by sharing edges and corners, building up a characteristic row extending along the c axis, which is joined to an adjacent row along [100] by sharing edges (Figure 9).

3.4. Mizraite-(Ce)

Mizraite-(Ce) occurs with Ce-silicate and Ti-sulfide in melt pockets between corundum and spinel within Grain 198-8 (Figure 10). Other inclusions in this corundum grain contain Ti,Al,Zr-oxide, ziroite, baddeleyite, thorianite, osbornite, zangboite (TiFeSi₂), wenjiite (Ti₅Si₃), and a [(Mn,Fe,Ti,V,Cr)₄Ti₂]Si₅ alloy. The mineral occurs as euhedral crystals < 1–14 μm in size. It is transparent with a light bluish-green color. The Gladstone–Dale relationship gives n = 1.828.

Mizraite-(Ce) (

Table 4. EPMA analytical results (in wt%, n = 8) for mizraite-(Ce).

Constituent	Mean	Range	SD (σ)	Probe Standard
Al2O3	69.75	69.41–70.07	0.23	Al2O3
Ce2O3	16.30	16.13–16.50	0.14	CePO4
* Ti2O3	5.67	5.64–5.75	0.04	TiO2
MgO	4.45	4.37–4.51	0.04	forsterite
La2O3	1.47	1.40–1.56	0.06	LaPO4
SiO2	0.72	0.69–0.79	0.03	anorthite
CaO	0.71	0.69–0.73	0.02	anorthite
ZrO2	0.67	0.43–0.81	0.12	zircon
Nd2O3	0.29	0.25–0.35	0.04	NdPO4
Total	100.02

* Titanium has been assigned to be Ti³⁺ for charge balance to achieve best stoichiometry.

) exhibits an empirical formula (based on 19 O pfu) of (Ce_0.76Ca_0.10La_0.07Nd_0.01)_Σ0.94(Al_10.43Mg_0.84Ti³⁺_0.60Si_0.09Zr_0.04)_Σ12.00O₁₉. The simplified formula is (Ce,Ca,La)(Al,Mg,Ti³⁺)₁₂O₁₉. The ideal formula is Ce(Al₁₁Mg)O₁₉, which requires Ce₂O₃ 21.45, Al₂O₃ 73.28, MgO 5.27, total 100 wt%.

The EBSD patterns can be indexed only by the hexagonal P6₃/mmc magnetoplumbite structure and match the Ce-bearing hibonite cell of [40] (Figure 11), with a mean angular deviation of 0.32°–0.37°, revealing the following cell parameters: a = 5.61(1) Å, c = 22.29(1) Å, V = 608(2) ų, and Z = 2. The calculated density is 4.16 g·cm⁻³ using the empirical formula and the unit-cell volume estimated from the EBSD data.

Mizraite-(Ce) is the Ce-analog of hibonite, and is a new member of the magnetoplumbite group (A[B₁₂]O₁₉; [16]); it is the first member presenting the heterovalent substitution A²⁺ + B³⁺ → A³⁺ + B²⁺ (Ca²⁺ + Al³⁺ → REE³⁺ + Mg²⁺) as the dominant species-defining exchange. Whenever another magnetoplumbite REE-dominant mineral is described, it would represent a new subgroup along with the magnetoplumbite (A = Pb), hawthorneite (A = Ba), and hibonite (A = Ca) subgroups. Hibonite has a general formula of (Ca,Ce)(Al,Ti,Mg)₁₂O₁₉ and an ideal formula of CaAl₁₂O₁₉. Synthetic Ce(Al₁₁Mg)O₁₉ is not reported, whereas La(Al₁₁Mg)O₁₉, La(Al₁₁Mn)O₁₉, and La(Al₁₁Ni)O₁₉ with the hibonite structure have been synthesized [41,42,43]. Terrestrial hibonite often contains minor Ce and other REEs and has a general formula of (Ca,Ce)(Al,Ti,Mg)₁₂O₁₉ [40,44]. Reported here is the first natural occurrence of Ce(Al₁₁Mg)O₁₉, although zoned “hibonite” grains with REE-rich cores (ΣREE > 0.6 atoms per formula unit) have been described by [45] where kalsilite, leucite, and hibonite occur together with spinel, corundum, sphene, perovskite, Ti-phlogopite, and K-feldspar in a granulite-facies gneiss in the Punalur district in Kerala, southern India. The structure of mizraite-(Ce) has the topology of the magnetoplumbite group minerals with Ln³⁺(Al₁₁M²⁺)O₁₉ stoichiometry and is made of two structural layers: the hexagonal close-packed R-block, containing the Ln³⁺ site, the trigonal bipyramidal M2 site, and the octahedral face-sharing M4 site; and the cubic close-packed S-block, containing layers of M5 octahedra interspaced by the M3 tetrahedra and the M1 octahedra (Figure 12). The spinel blocks contain most of the Al³⁺ in the M1 and M5 sites, and M²⁺ cations are distributed among the octahedral and tetrahedral sites. The Ln³⁺ and remaining Al³⁺ cations are localized in mirror planes, whereas M4 octahedra containing high-charge small cations lie on both sides of the mirror plane (Figure 12). The separation between the two Ln³⁺ sites of the same mirror plane is equal to the a unit cell parameter (ca. 5.6 Å), whereas between two different mirror planes it is approximately 11 Å.

3.5. Yeite

Yeite occurs with wenjiite (Ti₅Si₃) and zhiqinite (TiSi₂) in Ti-Si alloy inclusions in corundum Grain 198c (Figure 13). Other inclusions in this corundum grain contain type griffinite (Al₂TiO₅) [14], rutile, baddeleyite, hibonite, osbornite, khamrabaevite, Ti,Al,Zr-oxide, zirconolite, and jingsuiite. Yeite occurs as irregular-subhedral crystals 1.2–3.5 μm in size. It is opaque and shows a black color.

The chemical composition of yeite (

Table 5. EPMA analytical results (in wt%, n = 7) for yeite.

Constituent	Mean	Range	SD (σ)	Probe Standard
Ti	62.34	62.11–62.71	0.21	Ti metal
Si	36.59	36.47–36.78	0.10	Si metal
P	0.18	0.13–0.25	0.05	GaP
Mn	0.18	0.14–0.31	0.06	Mn2SiO4
Cr	0.08	0.00–0.13	0.04	Cr metal
V	0.06	0.00–0.17	0.06	V metal
Fe	0.00	0.00	0.00	Fe metal
Total	99.43

) gives rise to an empirical formula (based on 2 atoms pfu) of (Ti_0.995Mn_0.003V_0.001Cr_0.001)(Si_0.996P_0.004). The simplified formula is TiSi. The ideal formula is TiSi, which requires Ti 63.04, Si 36.96, total 100 wt%. Associated wenjiite has an empirical formula (Ti_3.70Mn_0.43Cr_0.07V_0.02)Si_3.68. Zhiqinite has an empirical formula Ti_0.99Si_2.01.

The EBSD patterns of yeite can be indexed only by the orthorhombic Pnma FeB-type structure and match the synthetic TiSi cells of [46,47] (Figure 14), with a mean angular deviation of 0.23°–0.32°, revealing the following cell parameters: a = 6.55(1) Å, b = 3.64(1) Å, c = 4.99(1) Å, V = 119.0(4) ų, and Z = 4. The calculated density is 4.24 g·cm⁻³ using the empirical formula and the unit-cell volume estimated from the EBSD data.

In the structure of yeite, each Si atom is coordinated with seven Ti atoms, forming SiTi₇ polyhedra (capped trigonal prism, <Si-Ti> = 2.638 Å, distortion index = 0.01259) that share edges, building up a three-dimensional framework (Figure 15). The structure can be also described as TiSi₇ polyhedra (<Si-Ti> = 2.636 Å, distortion index = 0.01245), with an odder shape, sharing edges.

Yeite is natural TiSi with the Pnma FeB-type structure. Synthetic TiSi with the Pnma FeB-type structure is well known [46,47]. To our knowledge, yeite is not related to other minerals. Other Ti-Si minerals include zhiqinite with an orthorhombic Fddd TiSi₂-type structure [48], kangjinlaite (Ti₁₁Si₁₀) with a tetragonal I4/mmm Ho₁₁Ge₁₀-type structure [49], and a special wenjiite (Ti₅Si₃) with a hexagonal P6₃/mcm Mn₅Si₃-type structure identified in this study.

4. Discussion

The oxide minerals described here are high-temperature phases. They crystallized from melts that were trapped in intracrystalline and interstitial voids in aggregates of corundum crystals [2]. The whole suite of corundum xenoliths is characterized by oxygen fugacity (fO₂) below the levels normally encountered in Earth’s upper mantle or crust (IW to IW-9; [50]). We recognize three broad paragenetic types.

Crn-A: these are hopper to skeletal crystals showing strong zoning in Ti due to the uptake of Ti³⁺ during rapid crystal growth [51]. The composition of the trapped melts is Ca-Mg-Al silicates showing high contents of S as well as incompatible elements. Phase assemblages reflect low fO₂, with all Ti as Ti³⁺ (e.g., tistarite).

Crn-B: these are large homogeneous (unzoned) corundum crystals, which typically show Ti contents > 1 wt%. In these crystals, interstitial pockets contain small amounts of glass, which are typically high in REE, Zr, and other incompatible elements. In phenocrysts, Ti is present as both Ti³⁺ and Ti⁴⁺.

Crn-C: these are texturally similar to Crn-B; however, the Ti contents in corundum are typically low (<0.5 wt% Ti). Rare glasses are rich in LREE and Ba. The presence of more Ti⁴⁺ phases (rutile, griffinite) suggests 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⁺, whereas in Crn-C, the Ti³⁺ contents are very low.

Magnéliite, sassite, and ziroite are members of a large population of Ti-Al-Zr phases, which include carmeltazite, griffinite, tistarite, rutile, “Allende-like” Ti-Zr-Al oxide [35], kaitianite [52], and many as yet undescribed minerals (Figure 16). Part of this variety is due to the presence of Ti as both Ti³⁺ and Ti⁴⁺, reflecting the differences in fO₂ among the three parageneses. Individual phases may show large ranges in solid solution, reflecting substitutions of trivalent (Ti³⁺, Al) and quadrivalent (Ti⁴⁺, Zr) ions.

Magnéliite shows some solid solution of both ZrO₂ and Al₂O₃ (Table 1); it has crystallized from a glass, residual after the crystallization of large hibonite crystals. The type magnéliite is associated with alabandite, which suggests that both crystallized during the ascent of the xenoliths as decreasing pressure led to lower solubility of sulfur in the melt. This assemblage and the low Ti in corundum (0.4 wt% Ti) are characteristic of paragenesis Crn-C.

Sassite is clearly a liquidus phase (Figure 4) together with a Ti-Al-Zr oxide and corundum; the reconstructed melt in these interstitial pockets is low in Si and Ca and very high in Ti, while the residual melt is Al, Si-rich. Sassite shows a very wide range of solid solution toward griffinite (Al₂TiO₅ [14]; Figure 16). The presence of alabandite and baddeleyite suggests quench crystallization during ascent of the xenolith, which is consistent with the quench crystallization of mullite at low P. This is a typical Crn-B paragenesis.

Ziroite can have a significant solid solution of TiO₂ (Table 3; Figure 16). The ability of ziroite to take up Ti can explain the coexistence of ziroite and baddeleyite (Figure 7), as the latter does not appear to take up much Ti. Like sassite, the type ziroite has crystallized from a Ca-Mg-Al-silicate glass, residual after the crystallization of hibonite and a Ti-Al-Zr oxide, in a typical Crn-B paragenesis.

Mizraite-(Ce) is also clearly a liquidus phase, crystallizing from a residual melt high in LREE and S. Its occurrence as interstitial to large exsolved spinel grains suggests that it belongs to paragenesis Crn-B, although the low Ti content of the adjacent corundum (0.4 wt%) is more characteristic of Crn-C.

The study of mixed-valence phases in paragenesis Crn-B, and possibly in Crn-C, provides new information on the interpretation of the origins of the Mt. Carmel corundum-aggregate xenoliths. While the different parageneses share many common features, it has proven difficult to establish common lines of descent between them.

The alloy phases, including yeite as described here, appear as inclusions in aggregates of corundum crystals; they represent trapped melts, melts + crystals, and subsolidus assemblages that formed from the melts on cooling, both prior to eruption and during quenching upon eruption of the host basalts [6]. The immiscible separation of these melts from the coexisting silicate melt under highly reducing conditions allowed the crystallization of Fe-free phases from the silicate melt(s). The chemistry and evolution of these melts through multiple stages of immiscibility have been described in [6]; yeite adds more detail to this picture.

Yeite occurs in spheroidal balls (Figure 13) interpreted as immiscible melts coexisting with the silicate melt from which the enclosing corundum was crystallizing. The smooth, straight, or irregular boundaries between yeite, wenjiite, and zhiqinite suggest that the original melt may have decomposed into mutually immiscible melts or crystallized into the three coexisting phases. However, examination of the phase diagram for the Ti-Si Si binary [53] suggests that the situation was more complex (Figure 17).

This binary is separated into two subsystems by a thermal divide at Ti₃Si₂; the assemblage TiSi+TiSi₂ appears (crystallizes) at a eutectic point (1743 K) on the Si side of the divide, while wenjiite crystallizes from melts on the Ti side of the divide from 2400 K to a eutectic (L → Ti + Ti₅Si₃) at 1613 K. There is no point at which TiSi coexists with Ti₅Si₃. However, the average reconstructed composition of the melts in Figure 13 lies near several cotectics (1773–1673 K) in the Fe-Ti-Si ternary system (Figure 18) [54], making it probable that three phases may have crystallized from the melt over a very short T range in the high-temperature part of this ternary system. As noted by [6], the temperatures in the natural system beneath Mt. Carmel probably were lower than those in the synthetic systems due to the coexistence of a fluid phase rich in H₂, which can lower temperatures in metallic systems by up to several hundred degrees [55].

These alloy minerals thus illustrate the wide range of immiscible-melt compositions and crystallization conditions captured in the xenoliths from Mt. Carmel and give some new insights into processes in this highly reduced magmatic system. This highly reduced corundum-related assemblage is not simply a one-locality oddity; very similar associations have been reported from the Luobusa ophiolite in SE Tibet [48,49] and from many other localities in intraplate and subduction-zone tectonic settings [2,6]. These occurrences imply a significant role for mantle-derived CH₄+H₂ fluids in magmatic processes.

5. Conclusions

Reported here is the discovery of five new minerals, magnéliite (Ti³⁺₂Ti⁴⁺₂O₇), ziroite (ZrO₂), sassite (Ti³⁺₂Ti⁴⁺O₅), mizraite-(Ce) (Ce(Al₁₁Mg)O₁₉), and yeite (TiSi), in melt inclusions in corundum xenocrysts from the Mt. Carmel area, Israel. The description of their chemical composition and the crystal structures of the synthetic analogues that match the EBSD data is provided. Many physical properties cannot be obtained because of the extremely reduced dimensions of the grains (nano scale), but the data are sufficient to support their correct identification. These minerals are high-temperature oxide or alloy phases formed under extremely reduced conditions in the upper mantle, and provide new, important insights into the natural origin of super-reduced mineral assemblages.