Toledoite, TiFeSi, a New Mineral from Inclusions in Corundum Xenocrysts from Mount Carmel, Israel

Abstract

During our nanomineralogical investigation of melt inclusions in corundum xenocrysts from the Mount Carmel area, Israel, seven new oxide and alloy minerals have been discovered since 2021. Herein, we report toledoite (TiFeSi; IMA 2022-036), a new alloy mineral. Toledoite occurs as irregular crystals 2–16 μm in size, with gupeiite (Fe₃Si), jingsuiite (TiB₂), ziroite (ZrO₂), osbornite (TiN), xifengite (Fe₅Si₃), and naquite (FeSi) in corundum Grain WG1124E-1. Toledoite has an empirical formula (Ti_0.83Cr_0.07Mn_0.06V_0.02)(Fe_0.96Mn_0.04)(Si_0.99P_0.04) and an orthorhombic Ima2 TiFeSi-type structure with the following cell parameters: a = 7.00(1) Å, b = 10.83(1) Å, c = 6.29(1) Å, V = 477(1) ų, Z = 12. Toledoite is a high-temperature alloy phase, formed under extremely reduced conditions in melt pockets in corundum xenocrysts derived from the upper mantle beneath Mount Carmel in Israel. The name was given in honor of Vered Toledo, of Shefa Gems Ltd. for her support and for providing corundum xenocrysts from the Mount Carmel region for this investigation of new minerals.

1. Introduction

Mineral assemblages containing super-reduced phases (formed in low oxygen fugacities below the iron–wüstite buffer) are commonly associated with explosive volcanic events. These include kimberlites, alkali basalts, and tholeiitic basalts, as well as in ophiolites linked to deep subduction along continental plate margins [1,2,3,4]. In volcanic localities, super-reduced phases are frequently found as inclusions within xenoliths composed of corundum aggregates. The relationships between these different phases within melt inclusions have been crucial in comprehending the genesis of super-reduced magma-fluid systems [4].

We have studied the mineralogy of crystals down to nanoscale in melt inclusions in corundum xenocrysts coming from volcanic centers and associated alluvial deposits in the Mount Carmel area, Israel. Since 2021, we have identified seven IMA-approved new minerals: griffinite (Al₂TiO₅), magnéliite (Ti³⁺₂Ti⁴⁺₂O₇), ziroite (ZrO₂), sassite (Ti³⁺₂Ti⁴⁺O₅), mizraite-(Ce) (Ce(Al₁₁Mg)O₁₉), toledoite (TiFeSi), and yeite (TiSi) [5]. Reported here is toledoite; we will also provide additional information on the origin and stability of reduced high-temperature minerals from the upper mantle.

Natural iron silicides are found as components of fulgurites, planetary materials, in meterorites and interstellar dust. For example, xifengite (Fe₅Si₃) is found with the reduced-oxide minerals tistarite (Ti₂O₃) and kaitianite (Ti³⁺₂Ti⁴⁺O₅) in the matrix of the Allende meteorite—the largest known carbonaceous chondrite, which formed under reducing conditions in the solar nebula [6].

Toledoite (IMA 2022-036) is a new mineral. Its chemical formula is TiFeSi, and it exhibits an Ima2 TiFeSi-type structure. The name is in honor of Vered Toledo (b. 1968), of Shefa Gems Ltd. for her support and for providing corundum xenocrysts from the Mount Carmel region for this investigation of new minerals. The type of toledoite is included in corundum grain WG1124E-1, which is part of Mount Carmel mount WG1124E. The mount was deposited in the mineralogy collection of the Università degli Studi di Milano, Via Mangiagalli, 34 - 20133 Milano, Italy under registration number MCMGPG-H2021-006.

2. Materials and Methods

The samples of this study are corundum xenoliths found to occur in the pyroclastic ejecta coming from small cretaceous basaltic volcanoes exposed on Mount Carmel (Figure 1) and in placer gemstone deposits in the terraces of the Paleocene to Pleistocene proto-Kishon river. The modern Kishon River and its tributary Mizra river now drain Mount Carmel and the adjacent Yisre’el Valley, and the Kishon River enters the sea near Haifa in northern Israel. U-Pb dating of super-reduced minerals in the corundum aggregates [4] indicates that much of the xenolith material in the paleoterrace placer deposits was derived from Miocene to Paleocene basalts outcropping in the Yisre’el Valley.

The studied grain was separated from a 400-tonne bulk sample of a basal paleoplacer deposit of the Paleocene–Pleistocene paleo-Kishon River (BS1124; coordinates: 35.106E/32.685N). The xenoliths can be described as aggregates of skeletal corundum crystals containing melt pockets in which the reduced mineral assemblages are found [4].

Recently, the genesis of the super-reduced phases has been the subject of discussion, with several authors arguing for an anthropogenic origin [7,8,9,10,11]. Unfortunately, the authors of these papers have chosen to ignore the extensive geological and geochemical evidence for the natural origin of the Mount Carmel xenolith material. This evidence has been summarized in detail in [12,13]. The latter reference presents U-Pb dating of carmeltazite and other super-reduced phases in the corundum aggregates, which shows a pattern of age distribution (Paleocene to Miocene, Cretaceous and older) that is consistent with zircon dating [14] of the volcanic rocks thought to be the immediate sources of the super-reduced phases [5]. It is clear from these data, as well as the geological context, that the Mount Carmel material was not produced by human beings.

For the characterization of the sample, we used an electron probe microanalyzer (EPMA) and a field-emission scanning electron microscope (SEM) equipped with an X-ray energy-dispersive spectrometer (EDS) and a detector for electron backscatter diffraction (EBSD), which allowed us to characterize both the composition and the structure of toledoite and associated phases. The instruments used for these analyses were a ZEISS 1550VP Field-Emission SEM (ZEISS Group, Oberkochen, Germany) equipped with an Oxford X-Max EDS, which was used for back-scatter electron (BSE) imaging and fast elemental analysis. For quantitative WDS elemental microanalyses of toledoite, we used a JEOL 8200 EPMA (JEOL Ltd., Tokyo, Japan) (15 kV and 10 nA, focused beam) interfaced with the Probe for EPMA program from Probe Software, Inc. (Eugene, OR, USA). The focused electron beam was ~150 nm in diameter. Quantitative WDS analyses were processed with the CITZAF matrix correction procedure. Analytical results are given in

Table 1. EPMA analytical results (in wt%, n = 12) for toledoite.

Constituent	Mean	Range	SD	Probe Standard
Fe	40.47	39.92–41.17	0.37	Fe metal
Ti	29.94	29.39–30.84	0.47	Ti metal
Si	20.83	20.29–21.29	0.25	Si metal
Mn	3.96	3.69–4.14	0.11	Mn2SiO4
Cr	2.73	2.54–2.91	0.10	Cr metal
V	0.77	0.71–0.85	0.04	V metal
P	0.82	0.71–1.24	0.17	GaP
Total	99.52

.

Due to the small size of the crystal, conventional X-ray studies could not be carried out. Therefore, EBSD analyses at a submicrometer scale were carried out, as in other previous studies of new minerals [5], by using an HKL EBSD system mounted on the ZEISS 1550VP SEM. Operation conditions were 20 kV and 6 nA in focused-beam mode using a 70° tilted stage, and in variable-pressure mode (25 Pa). A single crystal of silicon was used for calibration of the EBSD system. Structural correspondence was obtained and cell constants were derived by matching the experimental EBSD patterns with those obtained from the structures of synthetic Ti-Fe-Si, Ti-Si, and Fe-Si phases available in the ICSD.

Because of the very small size of the crystals, the determination of most of the physical properties (optical, hardness, fracture, cleavage, habit, density, etc.) was impracticable without risking destruction of the material.

3. Results

Toledoite occurs with gupeiite (Fe₃Si) [15], jingsuiite (TiB₂) [16], ziroite (ZrO₂) [5], osbornite (TiN) [17], xifengite (Fe₅Si₃) [15], and naquite (FeSi) [18] in melt inclusions in corundum grain WG1124E-1 (Figure 2). Ti-bearing xifengite and osbornite are also present in the Allende meteorite [6]. Other inclusions in this corundum grain contain an Allende-like Ti,Al,Zr-oxide [6], khamrabaevite (TiC), spinel, grossmanite-kushiroite-diopside, and Ce-silicate. Toledoite has been found as anhedral crystals (size: 2–16 μm). It is opaque and black in color.

Chemical data (Table 1—electron probe microanalyses) point to the empirical formula (based on three atoms’ pfu) of (Ti_0.83Cr_0.07Mn_0.06V_0.02)(Fe_0.96Mn_0.04)(Si_0.99P_0.04). The simplified formula can be written as (Ti,Cr,Mn)(Fe,Mn)Si. The ideal formula is TiFeSi, which requires Ti 36.32, Fe 42.37, Si 21.31, and a total 100 wt%.

The EBSD patterns obtained for toledoite are best indexed by using the orthorhombic Ima2 TiFeSi-type structure and match the cell known for synthetic TiFeSi [19] (Figure 3), with a mean angular deviation of 0.33–0.36°. The following cell parameters have been obtained: a = 7.00(1) Å, b = 10.83(1) Å, c = 6.29(1) Å, V = 477(1) ų, Z = 12. A calculated density of 5.53 g·cm⁻³ is obtained by using the empirical formula and the unit cell volume estimated from EBSD data. Powder Cell version 2.4 was used to calculate X-ray powder diffraction data (

Table 2. Calculated X-ray powder diffraction data for toledoite (I_rel > 1). The strongest lines in the powder X-ray diffraction pattern are in bold.

h	k	l	d [Å]	Irel
1	1	0	5.8789	1
0	1	1	5.4392	6
0	2	0	5.4150	3
1	2	1	3.5402	1
2	0	0	3.5000	5
0	0	2	3.1450	3
0	3	1	3.1310	5
2	1	1	2.9433	7
2	2	0	2.9394	4
0	2	2	2.7196	3
0	4	0	2.7075	1
1	4	1	2.3434	1
2	0	2	2.3393	44
2	3	1	2.3335	100
3	1	0	2.2810	4
1	3	2	2.2460	3
2	2	2	2.1475	89
2	4	0	2.1415	38
1	5	0	2.0692	4
0	1	3	2.0585	36
0	4	2	2.0519	33
0	5	1	2.0480	34
3	2	1	2.0284	8
3	3	0	1.9596	2
1	2	3	1.8831	2
3	1	2	1.8465	4
0	3	3	1.8131	17
0	6	0	1.8050	8
2	1	3	1.7743	12
2	4	2	1.7701	12
2	5	1	1.7676	11
4	0	0	1.7500	36
3	4	1	1.7016	1
1	6	1	1.6840	1
3	3	2	1.6632	2

, in Å for CuKα₁, Bragg–Brentano geometry) from the unit cell parameters reported above, the crystallographic data of synthetic TiFeSi [19], and the empirical formula.

In the structure of toledoite, each Fe atom forms four bonds with adjacent Si atoms creating layers of two different FeSi₄ distorted tetrahedra (<Fe1-Si> = 2.374 Å, distortion parameter in the sense of [20], D = 0.01342, and <Fe2-Si> = 2.405 Å, D = 0.00628) with Ti atoms present in the empty cavities between them (Figure 4a,b). Ti is five-fold coordinated with Si atoms, forming three different TiSi₅ distorted square pyramids (<Ti1-Si> = 2.606 Å, D = 0.02593, <Ti2-Si> = 2.572 Å, D = 0.00459 and <Ti3-Si> = 2.575 Å, D = 0.00620) that share edges among them and with FeSi₄ tetrahedra. The FeSi₄ tetrahedra share edges and apexes, thus forming (Fe₆Si₁₄)_∞ chains extending along [100] (Figure 4c). The union of these chains by sharing apexes builds a three-dimensional framework. Toledoite has a very different structural topology than the other natural titanium ferro-silicide, zangboite (TiFeSi₂ [21]), found in Luobusha, Tibet, China. In zangboite, iron is bonded to 6 Si atoms in very distorted FeSi₆ octahedron (<Fe1-Si> = 2.3646 Å, distortion parameter D = 0.01009, and <Fe2-Si> = 2.3854 Å, distortion parameter D = 0.02728), while Ti is seven-fold coordinated with Si in distorted TiSi₇ caped octahedra (<Ti1-Si> = 2.691 Å, distortion parameter D = 0.01806, and <Ti2-Si> = 2.6534 Å, distortion parameter D = 0.01397). In zangboite, FeSi₆ octahedra share a face and edges and form three-octahedra-wide ribbons along [001], which share vertices with adjacent ribbons. The spaces among the ribbons are filled by TiSi₇-capped octahedra sharing faces.

Electron backscattering diffraction coupled with electron microprobe analyses allowed us to identify gupeiite—(Fe_2.28Mn_0.28Ti_0.19Cr_0.15V_0.01)Si_1.09, xifengite—(Fe_3.54Ti_0.60Mn_0.45Cr_0.25V_0.03)Si_3.13, and naquite—(Fe_0.64Ti_0.27Mn_0.08Cr_0.07V_0.01)Si_0.94 as associated phases.

Toledoite is natural TiFeSi with an Ima2 TiFeSi-type structure. Synthetic TiFeSi is well known [19,23,24,25]. Toledoite is not related to other minerals to our knowledge. The structure is a superstructure of the hexagonal ordered Fe₂P-type [19]. Synthetic TiFeSi may also show the hexagonal P-62m β-form polymorph (a = 6.257(2), c = 3.497(1) (Å), V = 118.55(9) ų [24]), which is stable at high-T, but inverts to the orthorhombic phase after annealing at 1273 K for seven days.

4. Discussion

The alloy phases studied herein occur as inclusions in aggregates of corundum crystals. They represent a variety of trapped melts, melts and crystals, and subsolidus assemblages. Melts and crystals form upon cooling, both prior to eruption and during quenching upon eruption of the basalts that host the xenoliths [26]. The crystallization of Fe-free phases from silicate melt(s) is due to the immiscible separation of melts from the coexisting silicate melt under highly reducing conditions. Multiple stages of immiscibility, as well as the chemistry and evolution of these melts, are reported in [26]; like yeite (TiSi [5]), toledoite adds more detail to this picture.

Toledoite (TiFeSi) corresponds to the τ₂ phase of [24] in their description of the Fe-Ti-Si system at atmospheric pressures. It melts congruently above 1923 K, but crystallizes from a wide range of melts over a large T range; the temperatures on the Fe-Si side of the ternary are lower than those on the Ti-Si side (Figure 5). This makes toledoite a key phase in the evolution of low-Si Fe-Ti-Si melts. It can coexist with numerous solid and melt phases and controls the initial fractional crystallization of much of the Fe-Ti-Si system; it is responsible for much mineralogical diversity in the examples from Mount Carmel.

Toledoite can occur in two associations (Figure 2): (1) with osbornite (Ti(N,O)), jingsuite (TiB₂), and gupeiite (Fe₃Si); and (2) in spheroidal melt inclusions intimately intergrown with xifengite, naquite, and khamrabaevite (TiC). The first association seems to have formed from a boron-rich silicide melt in equilibrium with an osbornite melt favoring the crystallization of toledoite, gupeiite and jingsuite. Gupeiite does not form directly from a melt in the Fe-Ti-Si ternary but appears in a peritectic reaction (L+Fe₂Ti = τ₂+Fe₃Si; 1433 K) and coexists with toledoite in the subsolidus below 1373 K. This could indicate that the original melt first crystallized Fe₂Ti, and the present assemblage reflects the peritectic assemblage.

The relationships among phases in assemblage (2) suggest that TiC crystallized from a complex Fe-Ti-Si melt, which first crystallized TiC, then broke down into the three silicide phases. However, the assemblage τ₂+Fe₅Si₃+FeSi does not appear in the liquid lines of descent in the Fe-Ti-Si ternary of [24]. In the subsolidus system at 1173–1273 K, toledoite can coexist in equilibrium with either xifengite or naquite, but not with both, and xifengite and naquite do not coexist. The detailed history of this assemblage is therefore not clear.

The wide range of immiscible melt compositions and crystallization conditions captured in the xenoliths from Mount Carmel is shown by these alloy minerals. Therefore, they add some new insights into the processes occurring in a highly reduced magmatic system. Far from being a one-locality oddity, this highly reduced corundum-related assemblage is very similar to other associations that have been already reported from chromitites described in the Luobusa ophiolite in SE Tibet [27,28], and from many other localities in intraplate and subduction-zone tectonic settings [4,26]. A significant role for mantle-derived CH₄+H₂ fluids in magmatic processes is deduced from these occurrences.

Icosahedral quasicrystalline alloys have been described in the Ti-Fe-Si system [29]. The icosahedral phase in this system is stable in a narrow compositional range around Ti₇₀Fe₂₄Si₆. For comparison to the icosahedral quasicrystal, the composition of toledoite based on 100 atoms is Ti₂₈Fe₃₂Si₃₃ with minor Mn, Cr, V and P, making it quite far from the theoretical composition of an icosahedral quasicrystal. Nonetheless, it should be kept in mind that a wide relative variation in the Ti/(Fe+Si) ratio has been observed among the reduced alloy phases in the corundum grains from Mount Carmel [4]. This implies that the Israeli rocks could be the right place to search for the first quasicrystal of terrestrial origin.

5. Conclusions

Toledoite (TiFeSi) is a new alloy mineral that has been discovered in melt inclusions in corundum xenocrysts coming from the Mount Carmel area, Israel. Its chemical composition and crystal structure match those of the synthetic analogue identified by EPMA and EBSD data. Many physical properties cannot be obtained because of its small size, although the data here reported are sufficient to support the correct identification. Toledoite formed under extremely reduced conditions occurring in the upper mantle. It is found in several distinct mineral assemblages and provides more insights into the natural origin of super-reduced mineral phases. The overall mineral associations imply that the low oxygen fugacity of its formation was produced by the interaction of mantle-derived CH₄-H₂ fluids with syenitic melts produced by extreme fraction of mantle-derived mafic melts.