Nataliakulikite, Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁, a New Perovskite-Supergroup Mineral from Hatrurim Basin, Negev Desert, Israel

Abstract

Nataliakulikite, Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁, is a mineral intermediate between perovskite CaTiO₃ and brownmillerite Ca₂(Fe,Al)₂O₅. It was discovered as a minor mineral in a high-temperature pyrometamorphic larnite-gehlenite rock at the Nahal Morag Canyon of the Hatrurim Basin, Israel. Nataliakulikite is associated with larnite, flamite, gehlenite, magnesioferrite, Fe³⁺-rich perovskite, fluorapatite, barite, Hashemite, and retrograde phases (afwillite, hillebrandite, portlandite, calcite, ettringite, hydrogarnet, and other hydrated Ca-silicates). The mineral forms brown subhedral or prismatic grains (up to 20 µm) and their intergrowths (up to 50 μm). Its empirical formula (n = 47) is (Ca_3.992Sr_0.014U_0.004)(Ti_1.933Zr_0.030Nb_0.002) (Fe³⁺_0.610Fe²⁺_0.405Cr_0.005Mn_0.005)(Si_0.447Fe³⁺_0.337Al_0.216)O₁₁ and shows Si predominance in tetrahedral site. The unit-cell parameters (HRTEM data) and space group are: a = 5.254, b = 30.302, c = 5.488 Å, V = 873.7 ų, Pnma, Z = 4. These dimensions and Electron backscatter diffraction (EBSD) data strongly support the structural identity between nataliakulikite and synthetic Ca₄Ti₂Fe³⁺₂O₁₁ (2CaTiO₃∙Ca₂Fe³⁺₂O₅), an intermediate compound in the system CaTiO₃-Ca₂Fe³⁺₂O₅. In general, this mineral is a Si-Fe²⁺-rich natural analog of synthetic Ca₄Ti₂Fe³⁺₂O₁₁. The X-ray powder diffraction data (CuKα -radiation), calculated from unit-cell dimensions, show the strongest lines {d [Å], (I_calc)} at: 2.681(100), 1.898(30), 2.627(26), 2.744(23), 1.894(22), 15.151(19), 1.572(14), 3.795(8). The calculated density is 4.006 g/cm³. The crystal structure of nataliakulikite has not been refined because of small sizes of grains. The Raman spectrum shows strong bands at 128, 223, 274, 562, and 790 cm⁻¹. Nataliakulikite from the Hatrurim Basin crystallized under the conditions of combustion metamorphism at high temperatures (1160–1200 °C) and low pressures (HT-region of the spurrite-merwinite facies).

1. Introduction

The natural Si-Fe²⁺-rich analog of the orthorhombic synthetic phase Ca₄Ti₂Fe³⁺₂O₁₁, an intermediate member between perovskite CaTiO₃ and brownmillerite Ca₂FeAlO₅, named nataliakulikite, was discovered in a larnite-gehlenite rock at the Hatrurim Basin, the largest combustion metamorphism complex of the Hatrurim Formation on the territory of Israel [1,2,3,4,5].

Synthetic Fe-rich compounds based on the perovskite and brownmillerite structures (“layered perovskites”) have been intensively studied in material sciences due to their superconductivity and ion-conductivity properties [6,7]. The phase, Ca₄Ti₂Fe³⁺₂O₁₁, was firstly synthesized in the 1980s [8]. At present, three synthetic ordered phases, intermediate between brownmillerite Ca₂FeAlO₅ and perovskite CaTiO₃, are known: Ca₄Ti₂Fe³⁺₂O₁₁ [8,9], Ca₃TiFe³⁺₂O₈ [10,11,12], and Ca₅TiFe³⁺₂Al₂O₁₃ [13].

Nataliakulikite Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁ was approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) as a new mineral species in September 2018 (IMA 2018-061). The mineral is named in honor of Natalia Artyemovna Kulik (b. 1933) from Novosibirsk, Russia, a well-known Russian mineralogist, a reputed expert in mineralogical descriptions of granitic pegmatites [14], minerals of radioactive and rare-earth elements and archaeometry [15,16,17]. She is Emeritus Professor of Mineralogy at the Novosibirsk State University, who during the last 40 years has been teaching hundreds geologists and mineralogists, including some of the authors of this paper. Nataliakulikite is an essential mineral in high-temperature larnite-gehlenite rock found in the Nahal Morag Canyon of the Hatrurim Basin, Israel (Figure 1). The holotype sample of a larnite-gehlenite rock containing abundant nataliakulikite (author number W11-3, see [18,19]) is deposited in the Central Siberian Geological Museum of the V.S. Sobolev Institute of Geology and Mineralogy, Novosibirsk (number VII-101/1). Previously this mineral was described in the Hatrurim larnite rocks as a Si-Fe²⁺-rich natural analog of synthetic Ca₄Ti₂Fe³⁺₂O₁₁ [18]. In general, nataliakulikite is a last natural analog for the important key phases on the phase diagram CaTiO₃-CaFe₂O₅ [20,21,22]. According to the recent nomenclature of the perovskite supergroup [23], it belongs to the anion deficient perovskites (brownmillerite subgroup, non-stoichiometric perovskites group). Brownmillerite Ca₂Fe³⁺AlO₅, srebrodolskite Ca₂Fe³⁺₂O₅, shulamitite Ca₃TiFe³⁺AlO₈ and sharyginite Ca₃TiFe³⁺₂O₈ are other members of this subgroup [24,25,26,27,28,29,30,31].

The occurrences of the Hatrurim Formation (and Hatrurim Basin in first order) are becoming a “mineralogical Mecca” due to numerous findings of new minerals in unique pyrometamorphic rocks [1,32]. Hatrurim Basin is type locality for bayerite, bentorite, ye’elimite, grossite, hatrurite, and nagelschmidtite [1,33,34,35,36]. In last decade more than 20 new minerals were found within the Hatrurim Basin: barioferrite, shulamitite, murashkoite, zadovite, gurimite, hexacelsian, aradite, gazeevite, stracherite, ariegilatite, fluorkyuygenite, negevite, halamishite, zuktamrurite, transjordanite, silicocarnotite, khesinite, flamite, polekhovskyite, zoharite, gmalimite, nazarovite, and others [30,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. Most of them were identified in larnite and spurrite rocks and Ca-rich paralavas; and some of them may be used as indicator minerals to estimate the history of rock formation. For example, the shulamitite/sharyginite association with Fe³⁺-perovskite from the Hatrurim Basin larnite rocks records crystallization under high temperatures (1150–1170 °C) and low pressures (high-T-region of the spurrite-merwinite facies). Coexistence of flamite and larnite in the Hatrurim Ca-Al-rich paralavas reveals specific cooling history (quenching of melt) during their solidification [52].

In this paper, we provide a detailed description of nataliakulikite. Some data on nataliakulikite-bearing rocks from the Hatrurim Basin were reported in a few previous publications [5,18,19].

2. Brief Geological Background for Haturim Basin

The Hatrurim Basin is one of the largest complexes in the Hatrurim Formation (also known as “Mottled Zone”, MZ) in the territories of Israel and Jordan. All Hatrurim Fm. occurrences are localized along the Jordanian-Dead Sea transform fault and are composed of brecciated chalky and marly sediments and metamorphic rocks of the spurrite-merwinite facies, locally altered under low-temperature hydrous conditions [1,4,5].

The specific non-stratigraphic unit of the MZ rocks lies at the top of the Upper Cretaceous–Low Tertiary section and reaches depths from 30 to 120 m below the surface [1]. The term MZ sequence refers to highly diverse sedimentary rocks which underwent post-depositional alteration under highly contrasting conditions. The Hatrurim Basin territory abounds in conical hills composed of spurrite marbles, medium-temperature (T ~ 700–800 °C) combustion metamorphic (CM) rocks, in their lower and middle parts. The rocks, as well as their hydrated varieties, up to 4 m thick, commonly cover large areas (Figure 1). Their upper part consists of coarse-clastic breccias with lumps of marl, limestone, phosphorite, and flint from the strata below. Local spots of the so-called “olive unit” [1,2,59] are restricted to the hill top parts. The highly porous rocks of the olive unit are mainly composed of calcite and zeolite, which cement quartz sand particles and clasts of the underlying sediments. Other rocks of the unit are high-temperature clinopyroxene-anorthite hornfels and paralavas which form specific swells and “ploughed land” structures [59,60].

High-temperature larnite-bearing rocks (T = 1000–1400 °C) were found at several levels of the Hatrurim Fm. section, mainly in the northern and central parts of the Hatrurim Basin (Figure 1). In the lower section within 20 m above the basement, they are from a few tens of centimeters to a few meters thick and neighbour gehlenite hornfels bodies. Both larnite and gehlenite CM rocks can easily transform into pseudoconglomerates with remnant “pebbles” or “cobbles” (from 1–2 to 15–20 cm in diameter) by retrograde hydration and/or carbonation. The secondary products are mainly calcite, aragonite, gypsum, ettringite, and minor Ca silicate-hydrates. Larnite rocks occur as isolated mottles (to 10 m across) among strongly altered varieties. At hilltops, monolithic larnite rocks make up separate isometric massive blocks, plates or cliff scarps, up to 50 m across and 10 m thick [1,2,5,26,30]. Descriptions of individual bodies of these rocks is given in [1,2,3,4,5,26,30,59,60,61,62,63,64].

Many authors interpreted the Hatrurim Fm. complexes as products of in situ combustion of low-calorific fuel, specifically disseminated bituminous matter of marine chalk [2,3,65,66,67,68]. Other authors attributed CM events within the Hatrurim Fm. complexes to local breakthrough and ignition of high-calorific hydrocarbon gases, mainly methane, which may result from mud volcanism [4,5,26,30,52,59,60,63,64,69,70]. Emissions of hydrocarbon gases in the geological past left imprint in abundant foci of high- (800–1100 °С) and ultrahigh-temperature (1200–1500 °С) combustion metamorphism of sediments in the Hatrurim Basin and other MZ localities [26,30,59,60,63,64]. Burning of high-calorific fossil fuel released enough heat to maintain CM alteration and local melting of marly sediments. The burnt carbonate rocks transformed into diverse calcium-rich metamorphic rocks with typical clinker mineralogy [26], whereas marly sediments melted at the same temperature and formed clinopyroxene-bearing paralavas [59].

Despite the controversy about their genesis, all metamorphic rocks of the Hatrurim Basin and other MZ localities are considered to be products of high-temperature (700–1200 °C) solid-state reactions during organic matter combustion, sometimes with further local melting events at 1250–1500 °C [5,26,30,32,41,44,52,59,60,61,62,63,64,65,70,71,72,73].

3. Analytical Methods

Double-polished rock sections (~50 µm in thickness) were used for transmitted and reflected light microscope examination of the Hatrurim larnite-gehlenite rock with nataliakulikite (Figure 3). During the preparation of polished samples, no water-bearing abrasives were used. Identification of minerals was based on energy-dispersive spectra (EDS), back-scattered electron (BSE) images and elemental mapping (EDS system), using a TESCAN MIRA 3MLU scanning electron microscope equipped with an INCA Energy 450 XMax 80 microanalysis system (Oxford Instruments Ltd., Abingdon, UK) at the V.S. Sobolev Institute of Geology and Mineralogy (IGM), Novosibirsk, Russia. EDS analyses of minerals were done in high-vacuum modes at an accelerating voltage of 20 kV, a probe current of 1 nA, and accumulation time of 20 s. The following synthetic compounds, minerals, and pure metals were used as reference standards for most of the elements: SiO₂ (Si and O), Al₂O₃ (Al), diopside (Mg and Ca), albite (Na), orthoclase (K), Ca₂P₂O₇ (P), BaF₂ (Ba and F), Cr₂O₃ (Cr), CsRe₂Cl₆ (Cl), LaPO₄ (La), CePO₄ (Ce), SrF₂ (Sr), metallic Ti, Fe, Mn, Zn, Ni, V and Cu. Correction for matrix effects was done using the XPP algorithm, implemented in the software of the microanalysis system. Metallic Co served for quantitative optimization (normalization to probe current and energy calibration of the spectrometer).

Electron microprobe analyses (EMPA) in wavelength-dispersive (WDS) mode were performed for nataliakulikite and related minerals from the Hatrurim larnite-gehlenite rocks using a JXA-8100 microprobe (Jeol Ltd., Tokyo, Japan) at IGM. Grains (sizes > 5 μm), which were previously analyzed by EDS, were selected for this purpose. The operating conditions were as follows: Beam diameter of 1–2 μm, accelerating voltage of 20 kV, beam current of 30 nA, and counting time of 10 (5 + 5) s. The following standards were used for nataliakulikite and related opaque minerals: Natural wollastonite (Ca and Si) and ilmenite (Ti), synthetic MgAl₂O₄ (Al and Mg), MnO (Mn), Fe₂O₃ (Fe), NiFe₂O₄ (Ni), ZnFe₂O₄ (Zn), Cr₂O₃ (Cr), zircon (Zr), LiNbO₃ (Nb), UO₂ (U), Sr-silicate glass (Sr), and Cu₂O (Cu). Correction for matrix effects was done using a PAP routine [74]. The precision of analysis for major elements was better than 2% relative. The detection limits for elements are (in ppm): Si—207; Ti—116; Cr—128; Al—170; Fe—107; Mn—140; Mg—147; Ca—159; Cu—132; U—251; Sr—163; Ni—132; Zn—266; Zr—254; Nb—227.

The Raman spectra were recorded on a LabRAM HR 800 mm (HORIBA Scientific Ltd., Lat Krabang, Thailand) spectrometer equipped with a 1024 pixel LN/CCD detector and coupled to an Olympus BX40 confocal microscope (Objective X100, Tokyo, Japan) at IGM. A semiconductor laser emitting at 514.5 nm with a nominal power output of 50 mW was used for excitation. In each case, 20 spectra were recorded for 20 s each at a hole diameter of 100 μm and integrated. The spectra were recorded between 100 and 1200 cm^–1, and the monochromator was calibrated using the 520.7 cm^–1 Raman line of elemental Si.

High-resolution transmission electron microscopy (HRTEM) study for FIB-milled foils with nataliakulikite was performed with a TECNAI F20 X-Twin transmission electron microscope (FEI Company, Hillsboro, OR, USA) with a field emission gun FEG as electron source at Helmholtz Centre Potsdam, GFZ, Germany [75,76]. The point resolution is 0.25 nm and the lattice resolution is 0.102 nm with an information limit of 0.14 nm. The TEM is equipped with a GATAN Tridiem imaging filter GIF™ and all HRTEM images were acquired as energy-filtered images applying a 10 eV window to the zero-loss peak. A careful correction of astigmatism of the objective lens was performed using the Digital Micrograph software package. Electron diffraction patterns were recorded on image plates and calculated from high-resolution images using Fast Fourier Transform (FFT) software packages. Phase identification was made by measuring the lengths of the different vectors in the diffraction patterns and calculating the respective d_hkl-spacing. Additionally, the angles between adjacent vectors were measured in the diffraction patterns. The d_hkl-spacing is precise to within three decimals of a nanometer. The angles between the vectors were determined with an accuracy of 0.5°.

Quantitative reflectance measurements for nataliakulikite from the Hatrurim Basin were provided in air relative to a SiC standard using a UMSP 50D Opton microscope-spectrophotometer (Carl Zeiss, Jena, Germany) at the A.E. Fersman Mineralogical Museum RAS, Moscow, Russia.

Electron backscatter diffraction studies were provided for two grains of nataliakulikite. Thin sections intended for EBSD studies were subjected to polishing by Buehler MasterMet2 non-crystallizing colloidal silica suspension (0.02 μm). EBSD measurements were carried out by means of a FE-SEM ZEISS SIGMA VP scanning electron microscope equipped with an Oxford Instruments Nordlys HKL EBSD detector, operated at 20 kV and 1.4 nA in focused beam mode with a 70° tilted stage at Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia. Structural identification of nataliakulikite was performed by matching its EBSD patterns with the reference structural models using program FLAMENCO.

4. Sample Description

Nataliakulikite was discovered in the rocks at the Nahal Morag Canyon, in the central part of the Hatrurim Basin, the Negev desert (approximately 5 km to southeast from Arad, Israel). Latitude—31°13′20.41″ N, longitude—35°17′43.81″ E (Figure 1). In general, it is located near “fair foci” described by Burg et al. [2].

This mineral was found in a larnite-gehlenite fine-grained rock occurring as a brown-gray pebble in “pseudoconglomerates” in the southern part of the Nahal Morag Canyon at Hatrurim Basin (Figure 2 and Figure 3). The pebble is characterized by specific microbending due to alternation of microlayers enriched in red-colored oxides and silicates (Figure 3).

The high-temperature mineral assemblage of the holotype rock sample (W11-3) includes larnite, flamite, gehlenite, magnesioferrite, nataliakulikite, Fe³⁺-rich perovskite, fluorapatite, barite, and hashemite [18]. In addition unidentified K-Ca-Ba-chromate (up to 5 µm) has been found (EDS, n = 4; K₂O—12.6, BaO—28.7, SrO—0.4, CaO—9.1, FeO—2.0, Cr₂O₃—26.3, SiO₂—5.9, SO₃—3.5 wt.%, possible solid solution between tarapacáite K₂CrO₄ and hashemite BaCrO₄). Retrograde phases are afwillite, hillebrandite, portlandite, calcite, ettringite, hydrogarnet, and hydrated P-S-rich Ca-silicates after larnite and flamite. Altered areas of the rock (Figure 3) have a white color and are mainly confined to the outer parts of the “pebble”. In general, the first signs of retrograde transformation (whitening of rock) are related to the replacement of larnite and flamite by hydrated P-S-rich Ca-silicates.

Magnesioferrite is a main opaque mineral in the rock. Nataliakulikite is minor with respect to magnesioferrite, but it is a predominant member of the perovskite-brownmillerite series in the holotype sample, with its bulk content of ~5 vol.% (Figure 4 and Figure 5). Major and trace element composition of the W11-3 holotype sample were previously reported [5]. The data on the chemistry of the main rock-forming minerals are summarized in

Table 1. Chemical composition (WDS-EDS, wt.%) of principal minerals in gehlenite-larnite rock with nataliakulikite (sample W11-3), Nahal Morag Canyon, Hatrurim Basin, Israel.

Mineral	Gehlenite		Larnite	Flamite	Fluorapatite		Magnesioferrite	Perovskite	Barite	Hashemite
Position	core	rim
n	36	7	26	18	3	2	45	6	7	6
SiO2	22.17	21.60	33.03	29.14	3.29	6.28	n.a.	3.81	n.a.	n.a.
TiO2	0.04	0.00	n.d.	n.d.	n.a.	n.a.	0.01	33.36	n.a.	n.a.
ZrO2	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	0.74	n.a.	n.a.
Nb2O5	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	0.06	n.a.	n.a.
V2O5	n.a.	n.a.	n.a.	n.a.	0.52	0.50	n.a.	n.a.	n.a.	n.a.
Cr2O3	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	0.41	0.07
Al2O3	32.91	28.05	0.11	0.14	n.a.	n.a.	3.55	1.92	n.a.	n.a.
Fe2O3	4.25	10.48					74.01	17.66
FeO	0.28	0.18	0.10	0.24	0.19	0.33	1.30		n.d.	n.d.
NiO	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	0.81	n.a.	n.a.	n.a.
ZnO	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	0.66	n.a.	n.a.	n.a.
CuO	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	0.44	n.a.	n.a.	n.a.
MnO	n.d.	n.d.	n.a.	n.a.	n.a.	n.a.	0.27	0.03	n.a.	n.a.
MgO	0.28	0.35	n.d.	n.d.	n.a.	n.a.	17.25	n.d.	n.a.	n.a.
CaO	40.26	39.17	63.22	59.54	55.23	55.91	1.50	41.98	0.67	0.53
BaO	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	62.76	61.45
SrO	n.d.	n.d.	0.27	0.37	0.83	n.a.	n.a.	0.27	0.75	0.44
Na2O	n.d.	n.d.	0.65	0.94	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.
K2O	n.d.	n.d.	0.65	2.49	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.
P2O5	n.d.	n.d.	2.26	6.98	37.53	29.31	n.a.	n.a.	n.a.	n.a.
SO3	n.a.	n.a.	n.d.	n.d.	0.82	6.27	n.a.	n.a.	18.91	9.40
CrO3									16.77	28.20
F	n.a.	n.a.	n.a.	n.a.	2.39	3.16	n.a.	n.a.	n.a.	n.a.
Sum	100.20	98.79	100.31	99.85	100.82	101.75	100.23	99.76	99.86	100.03
Formula based on	5 cations				10 cat in Ca site		3 cat	2 cat
	7 oxygens		4 oxy	4 oxy					4 oxy	4 oxy
Si	1.03	1.03	0.94	0.83	0.55	1.04		0.08
Al	1.80	1.57	0.00	0.00			0.14	0.05
Ti	0.00	0.00					0.00	0.56
Zr+Nb								0.01
Cr3+							0.01	0.00
Fe3+	0.15	0.37					1.85	0.30
Fe2+	0.01	0.01	0.00	0.01	0.03	0.05	0.04		0.00	0.00
Ni+Zn+Cu							0.04
Mn	0.00	0.00					0.01	0.00
Mg	0.02	0.02	0.00	0.00			0.85	0.00
Ca+Sr	2.00	2.00	1.94	1.83	9.97	9.95	0.05	1.00	0.05	0.03
Ba									1.00	0.99
Na	0.00	0.00	0.04	0.05
K	0.00	0.00	0.02	0.09
P5+			0.05	0.17	5.31	4.12
V5+					0.06	0.06
S6+					0.10	0.78			0.58	0.29
Cr6+									0.41	0.70
F					1.26	1.66

Cl is below detection limit (<0.005 wt.%). n.a.—not analyzed; n.d.—not detected. FeO and Fe₂O₃ are calculated on charge balance.

.

Note that among main minerals, only gehlenite shows essential core-to-rim variations with increasing of the ferrigehlenite end-member. Individual grains of fluorapatite are variable in the P₂O₅, SiO₂ and SO₃ contents. Magnesioferrite is rich in CuO (0.4), ZnO (0.7), NiO (0.8), and CaO (up to 1.5 wt.%). Larnite and flamite form regular lamellar intergrowths like in Ca-Al-rich paralavas at Hatrurim Basin [52], but rarely occur as individual grains. Both minerals differ in the contents of CaO, SiO₂, P₂O₅, and total alkalis (Table 1). Perovskite, barite, and hashemite occur very rarely. In general, the petrography of the studied sample indicates that this rock seems to be formed due to high-temperature solid-phase reactions without melting.

In general, individual pebbles and even different parts of concrete pebbles in the Nahal Morag Canyon “pseudoconglomerate” section may have contrasting colors and mineral compositions (Figure 2). For example, larnite-gehlenite pebbles, which are nataliakulikite-bearing rock neighbours, do lack nataliakulikite and may include either rankinite, Ti-andradite-schorlomite, khesinite, trevorite, hematite, perovskite and kalsilite (yellowish W11-2-1) or magnesioferrite, sharyginite, perovskite and khesinite (brownish W11-2-2).

5. Morphology, Optical and Physical Properties of Nataliakulikite

Nataliakulikite commonly occurs in the holotype sample as a subhedral or prismatic grains (up to 20 µm) and their intergrowths (up to 50 μm). Usually, the size of nataliakulikite grains is less than 10 μm (Figure 4, Figure 5 and Figure 6); it is smaller than that of neighboring magnesioferrite and silicates. The color of this mineral is brown and it is easily distinguishable optically from the dark red magnesioferrite in transmitted and reflected light and on BSE images. The color of the powdered mineral is light brown. Nataliakulikite has submetallic to opaque luster. Its hardness is ≈5.5–6 (Mohs), microhardness VHN₂₀ = 531 kg/mm² (n = 1). Cleavage and parting are none observed, fracture is uneven. Density was not measured directly because of small grain size and common inclusions. Density (4.006 g/cm⁻³) was calculated from unit-cell dimensions and results of electron-microprobe analyses. Under reflected light nataliakulikite is gray to light gray and shows yellowish brown internal reflections. Bireflectance and anisotropy are weak, pleochroism is distinct from gray to light gray.

The reflectance data for the mineral are given in

Table 2. Reflectance values for nataliakulikite.

λ (nm)	Rmax/Rmin	λ (nm)	Rmax/Rmin
400	16.70/16.34	560	13.36/13.34
420	14.79/14.41	580	13.25/13.22
440	14.23/14.06	589 (COM)	13.20/13.15
460	14.17/14.09	600	13.13/13.09
470 (COM)	14.15/14.08	620	13.05/12.97
480	14.03/14.02	640	12.99/12.90
500	13.86/13.85	650 (COM)	12.98/12.83
520	13.69/13.68	660	12.90/12.78
540	13.48/13.48	680	12.78/12.64
546 (COM)	13.45/13.43	700	12.70/12.52

. Reflectance percentages for the four R_max and R_min COM (Commission on Ore Mineralogy) wavelengths are: 14.15, 14.08 (470 nm); 13.45, 13.43 (546 nm); 13.20, 13.15 (589 nm), and 12.98, 12.83 (650 nm).

Larnite, gehlenite, and magnesioferrite occur as inclusions in nataliakulikite (Figure 4, Figure 5 and Figure 6). This phase is commonly homogeneous, but rarely shows zoning or overgrows the Fe-rich perovskite cores (Figure 5). There is no obvious difference between two perovskite phases in both transmitted and reflected light. Moreover, they are also hardly distinguished on BSE images.

6. Chemical Composition of Nataliakulikite

The averaged empirical formula of nataliakulikite (based on 8 cations and 11 oxygens, EMPA-WDS, n = 47) is (Ca_3.992Sr_0.014U_0.004)_4.010(Ti_1.933Zr_0.030Nb_0.002)_1.965(Fe³⁺_0.610Fe²⁺_0.405Cr_0.005Mn_0.005)_1.025 (Si_0.447Fe³⁺_0.337Al_0.216)_1.000O₁₁ and shows the predominance Si in tetrahedral site (

Table 3. Chemical composition (WDS, wt.%) of nataliakulikite from larnite-gehlenite rock (W11-3), Nahal Morag Canyon, Hatrurim Basin, Israel.

Component	Nataliakulikite				Ntk-TEM	Ntk-1	Ntk-2	Ideal-1	Ideal-2
	n = 47	sd	min	max	n = 2	n = 5	n = 6
SiO2	5.05	0.63	3.71	6.87	5.11	5.11	5.17		4.53
TiO2	29.04	1.71	25.48	32.64	29.51	29.69	29.44	29.38	30.14
ZrO2	0.68	0.07	0.53	0.84	0.71	0.75	0.69
Nb2O5	0.04	0.02	0.00	0.10	0.03	0.04	0.04
Cr2O3	0.08	0.02	0.04	0.17	0.09	0.08	0.08
Al2O3	2.07	0.41	1.24	3.02	2.14	2.37	2.47		1.92
Fe2O3	14.23	2.52	9.24	19.62	13.17	12.56	12.76	29.37	15.07
FeO	5.47	1.15	2.75	8.07	6.18	6.29	6.01	0.00	5.42
MnO	0.07	0.06	0.00	0.25	0.04	0.05	0.14
CaO	42.10	0.16	41.82	42.50	42.01	42.11	42.09	41.25	42.37
SrO	0.27	0.04	0.20	0.34	0.24	0.31	0.32
UO2	0.20	0.07	0.07	0.33	0.13	0.28	0.28
Sum	99.30				99.36	99.63	99.47	100.00	99.45
Formula based on 8 cations and 11 oxygens
Si	0.447				0.451	0.450	0.456		0.400
Al	0.216				0.223	0.247	0.256		0.200
Fe3+	0.338				0.325	0.303	0.288	1.000	0.400
Sum T	1.000				1.000	1.000	1.000	1.000	1.000
Ti	1.933				1.961	1.967	1.953	2.000	2.000
Zr+Nb	0.031				0.032	0.034	0.031
Cr	0.006				0.006	0.006	0.006
Fe3+	0.610				0.551	0.530	0.559	1.000	0.600
Fe2+	0.405				0.456	0.463	0.443	0.000	0.400
Mn	0.005				0.003	0.004	0.010
Sum B	2.990				3.009	3.003	3.001	3.000	3.000
Ca	3.992				3.977	3.976	3.977	4.000	4.000
Sr	0.014				0.012	0.016	0.016
U	0.004				0.003	0.005	0.005
Sum A	4.010				3.991	3.997	3.999	4.000	4.000
End-members (mol.%)
Ca4Ti2Fe3+Fe3+O11	33.75				32.53	30.30	28.81	100.00	40.00
Ca4Ti2Fe3+AlO11	21.59				22.32	24.65	25.64	0.00	20.00
Ca4Ti2Fe2+SiO11	44.66				45.15	45.04	45.55	0.00	40.00

MgO, CuO, NiO, and ZnO are below detection limits (<0.005 wt.%). Ntk-TEM—nataliakulite grain used for high-resolution transmission electron microscopy (HRTEM) study (see Figure 9); Ntk-1-2—nataliakulite grains used for EBSD and Raman studies (see Figure 10 and Figure 11); Ideal-1—Ca₄Ti₂Fe³⁺Fe³⁺O₁₁ and Ideal-2—Ca₄Ti₂(Fe³⁺_0.6Fe²⁺_0.4)(Fe³⁺_0.4Si_0.4Al_0.2)O₁₁. FeO and Fe₂O₃ are calculated on charge balance.

). In general, individual grains are commonly homogeneous in composition and rarely show chemical zoning (Figure 6). Although, different grains strongly vary in the content of Ti, Si, and Fe (Figure 7). The Ti content shows pronounced positive correlation with that of Fe²⁺ + Mn²⁺ and Zr + Nb and negative correlation with Si and Fe³⁺. All these indicate two possible isomorphic schemes: 2^VIFe³⁺ ↔ ^VITi⁴⁺ + ^VIFe²⁺ and ^IVFe³⁺ + 2^VITi⁴⁺ + ^VIFe²⁺ ↔ ^IVSi⁴⁺ + 3^VIFe³⁺.

In general, the composition of nataliakulikite changes from Ca_4.00Ti_1.72(Fe³⁺_0.96Fe²⁺_0.32)_1.28 (Si_0.61Fe³⁺_0.20Al_0.19)_1.00O₁₁ to Ca_4.00Ti_2.25(Fe³⁺_0.22Fe²⁺_0.53)_0.75(Si_0.33Fe³⁺_0.46Al_0.21)_1.00O₁₁ (Figure 7). The mineral chemistry database is given in Table S1 (Supplementary materials). The simplified formula of nataliakulikite may be presented as Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁. However, according to the IMA-CNMNCC dominant-valency rule [77] the ideal formula should be given as Ca₄Ti₂Fe³⁺Fe³⁺O₁₁ or Ca₄Ti₂Fe³⁺₂O₁₁ because the Si content is low in some grains and total (Fe,Al)³⁺ is commonly higher than Si⁴⁺ in tetrahedral sites. Moreover, its ideal formula is equal to synthetic Ca₄Ti₂Fe³⁺₂O₁₁ [8,9] and nataliakulikite may be considered as a Si-Fe²⁺-rich natural equivalent of the synthetic phase (2CaTiO₃∙Ca₂Fe³⁺₂O₅ in simplified form). General variations may be expressed in hypothetical end-members as (in mol.%): 20.3–45.8 Ca₄Ti₂Fe³⁺Fe³⁺O₁₁; 12.9–28.6 Ca₄Ti₂Fe³⁺AlO₁₁; 33.1–60.6 Ca₄Ti₂Fe²⁺SiO₁₁.

The impurities of Si and Fe²⁺ in nataliakulikite seem to stabilize its structure and prevent its decay into two perovskite phases (intergrowths/domains of Ca₄Ti₂Fe₂O₁₁ + Fe³⁺-perovskite or Ca₃TiFe₂O₈ + Ca₄Ti₂Fe₂O₁₁). Some nataliakulikites in the holotype larnite-gehlenite sample (W11-3) grow over rare grains of Fe³⁺-rich perovskite, which is compositionally close to CaTi_0.6Fe³⁺_0.4O_2.8, x = 0.2, or ideally Ca₅Ti₃Fe³⁺₂O₁₄ (Figure 5; Table 1 and Table S2). The nataliakulikite-Fe³⁺-rich perovskite assemblage has never been reported before from the Hatrurim Basin. Previously one mineral of the perovskite-brownmillerite series (Fe-rich perovskite, shulamitite, sharyginite, brownmillerite) or two minerals: brownmillerite + shulamitite (or sharyginite), Fe-rich perovskite + shulamitite (or sharyginite) were indicated in the different larnite rocks [1,5,26,28,30,73].

7. Crystal Structure Data for Nataliakulikite

It was difficult to obtain single-crystal and X-ray powder diffraction data for nataliakulikite because of its very small crystal size (<10 μm), and its mineral structure was resolved by alternative methods (EBSD, HRTEM, and Raman spectroscopy). Before considering the structure of nataliakulikite, it is pertinent to dwell upon the structure of the Ca₄Ti₂Fe₂O₁₁ synthetic phase.

7.1. The Crystal Structure of Synthetic Ca₄Ti₂Fe₂O₁₁

The crystal structure of a perovskite-related synthetic oxide with a composition Ca₄Ti₂Fe₂O₁₁ has been studied in 1980× using X-ray diffraction and electron microscopy [8,9]. Initially Gonzáles-Calbet and Valet-Regí [8] suggested the following parameters of the phase: orthorhombic symmetry; Pnma or Pn2₁a space group, a = 5.437(1) Å, b = 30.22(1) Å, c = 5.489(1) Å, V = 901.9(3) ų. The structural model was based on a stacking sequence of …OOOTOOOT’… along the b axis. A more detailed refinement of the crystal structure of Ca₄Ti₂Fe₂O₁₁ has been done by high-resolution electron microscopy and crystallographic image processing (CIP) [9]: the space group is Pnma; the structure is composed of MeO₆ octahedra and MeO₄ tetrahedra, arranged as …OOOTOOOT… sequence along the long b-axis. The crystal structure of synthetic Ca₄Ti₂Fe₂O₁₁ is shown in Figure 8.

7.2. HRTEM Data for Nataliakulikite

The unit cell parameters of nataliakulikite were calculated from HRTEM data obtained from FIB-milled foil (Figure 9). They are: a = 5.254 Å; b = 30.302 Å; c = 5.488 Å; V = 873.7 ų; Z = 4; orthorhombic symmetry. Suggested space group is Pnma (no. 62) according to [9]. Additionally, the dark and bright field images and SAED reveal that nataliakulikite is a homogeneous phase, and no striking, twining, oriented intergrowths, or different stacking were indicated in its crystal structure.

The theoretical powder diffraction pattern was calculated using the unit cell dimensions obtained by HRTEM, structural model for Ca₄Ti₂Fe₂O₁₁ [9], and the empirical formula of nataliakulikite. Data for nataliakulikite (CuKα₁ = 1.540598 Å, Bragg-Brentano geometry, fixed slit, no anomalous dispersion, I > 1) are compared in

Table 4. Calculated powder diffraction data for nataliakulikite and synthetic Ca₄Ti₂Fe₂O₁₁ (CuKα₁ = 1.540598 Å, Bregg-Brentano geometry, I > 1; data were calculated using PowderCell 2.4 [78]).

			Nataliakulikite		Synthetic [8,9]
h	k	l	dcalc, Å	Irel	dcalc, Å	Irel
0	2	0	15.151	19	15.110	18
0	4	0	7.576	5	7.555	5
0	6	0	5.050	1	5.037	1
0	3	1	4.822	6	4.820	6
0	5	1	4.068	7	4.063	7
1	0	1	3.795	8	3.863	9
0	8	0	3.788	4	3.778	4
1	2	1	3.681	1	3.742	1
1	3	1	3.553	3	3.607	3
0	7	1	3.399	2	3.393	2
1	4	1	3.393	1
1	5	1	3.217	1	3.255	1
0	9	1	2.870	4	2.864	4
0	0	2	2.744	23	2.745	22
1	8	1	2.681	100	2.701	100
2	0	0	2.627	26	2.719	28
2	1	0	2.617	1	2.708	1
2	3	0	2.542	2	2.625	2
1	0	2	2.432	1	2.450	1
1	3	2	2.365	5	2.381	5
0	8	2	2.222	6	2.220	6
2	8	0	2.159	4	2.207	4
0	13	1	2.145	1	2.141	1
1	7	2	2.121	1	2.131	1
1	8	2	2.047	1	2.056	1
2	8	1	2.009	2	2.047	2
2	0	2	1.898	30	1.931	31
0	16	0	1.894	22	1.889	21
2	5	2	1.811	1	1.840	1
1	13	2	1.683	1	1.686	1
3	0	1	1.668	1	1.721	2
3	3	1	1.646	1	1.696	1
3	5	1	1.609	1	1.655	1
1	8	3	1.572	14	1.576	13
3	8	1			1.566	10
0	16	2	1.559	5	1.556	5
2	16	0			1.551	6

The strongest diffraction lines are given in bold.

with those for synthetic Ca₄Ti₂Fe₂O₁₁ [8,9].

7.3. EBSD Data for Nataliakulikite

The additional structural data were obtained using the EBSD technique (Figure 10) and fitted to the structural model of synthetic Ca₄Ti₂Fe₂O₁₁ [8,9]. Fitting of the EBSD patterns for a Ca₄Ti₂Fe₂O₁₁ model with the cell parameters [8,9] resulted in the parameter MAD = 0.18°–0.36° (best-good fit). The EBSD studies reveal structural identity of nataliakulikite and synthetic Ca₄Ti₂Fe₂O₁₁ compound.

7.4. Raman Spectroscopy for Nataliakulikite

Main Raman bands, which are characteristic for nataliakulikite, are (in cm⁻¹, Figure 11): 128, 223, 274, 468 (shoulder), 562, 790, 915 (shoulder), and 1450 (overtone). No bands have been detected in the 3000-4000 cm⁻¹ region (main for OH vibrations). The Raman spectra for nataliakulikite are interpreted following the results for shulamitite and sharyginite [30,31]. Bands below 400 cm⁻¹ are ascribed to the polyhedral CaO₈ and octahedral (Fe³⁺,Ti)O₆ vibrations. Bands at 468 and 562 cm⁻¹ are related to tetrahedral (Fe³⁺,Si,Al)O₄ bending vibrations (ν₄ + ν₂). The strongest band in the Raman spectra of nataliakulikite at 790 cm⁻¹ represents symmetric stretching vibrations ν₁ of (Fe³⁺,Si,Al)O₄ tetrahedra. The broadening of the bands at 562 and 790 cm⁻¹ may indicate disordering of Fe³⁺, Si and Al in tetrahedral sites.

8. Discussion

Composition and structural data obtained by HRTEM, EBSD, and Raman techniques remarkably show that nataliakulikite is a Si-Fe²⁺-rich analogue of the synthetic phase Ca₄Ti₂Fe₂O₁₁ (Pnma, no. 62, [8,9]). It belongs to the perovskite-brownmillerite pseudobinary series with the generalized formula CaTi_1–2xFe_2xO_3–x (0≤ x ≤ 0.5) [6,8,10,20,21,22] and may be represented in the end members of this series as 66.66% CaTiO₃ and 33.33% Ca₂Fe₂O₅. All members of the series in accordance with the material science terminology [6,7,8,9,10,11,12] may be classified as “layered perovskites”. According to the recent nomenclature of the perovskite supergroup [23] nataliakulikite belongs to the anion deficient perovskites (brownmillerite subgroup, non-stoichiometric perovskites group). The only difference between nataliakulikite and synthetic Ca₄Ti₂Fe₂O₁₁ phase is that the tetrahedral Fe³⁺ is partially substituted by Si and Al in the structure of natural mineral. No strict constraints on the order-disorder in the tetrahedral layer are available because the nataliakulikite grains were too small for structure refinement. Nevertheless, the Raman data indicate a possible disordering of Fe³⁺, Si and Al in tetrahedra.

8.1. Phase Relations in the System Perovskite-Brownmillerite

The synthetic compounds of the CaTiO₃-CaFe₂O₅ series have been exhaustively covered by previous workers (since 1960–70s) due to specific properties of oxygen-deficient Fe-rich perovskites, such as superconductivity, oxygen ionic conductivity, as well as electronic conductivity. Numerous compounds, corresponding to the general formula CaTi_1–2xFe_2xO_3–x (0≤ x ≤ 0.5), were synthesized within this system [6]. However, only four orthorhombic compounds are fully ordered in oxygen vacancies and seem to be most stable: CaTiO₃ (x = 0), Ca₄Ti₂Fe₂O₁₁ (x = 0.25), Ca₃TiFe₂O₈ (x = 0.33) and Ca₂Fe₂O₅ (x = 0.5) [6,7,8,10,11,12,20,21,22,79]. According to the phase diagram CaTiO₃–CaFeO_2.5 (Figure 12), the coexistence of perovskite and brownmillerite is impossible in any associations, because phases Ca₃TiFe₂O₈ and Ca₄Ti₂Fe₂O₁₁ are more stable in energy than the perovskite + brownmillerite paragenesis [79]. The Ca₄Ti₂Fe₂O₁₁ phase seems not to be in association with Ca₂Fe₂O₅; it transforms into Ca₃TiFe₂O₈ or Ca₃TiFe₂O₈ + Ca₂Fe₂O₅. The intermediate phases Ca₄Ti₂Fe₂O₁₁ and Ca₃TiFe₂O₈ should be stoichiometric in composition without vacancies of cations and oxygen. Any deviations in Ti content should lead to the appearance of nanoscale intergrowths/domains of Ca₄Ti₂Fe₂O₁₁ + Fe-perovskite, Ca₃TiFe₂O₈ + Ca₄Ti₂Fe₂O₁₁ or Ca₃TiFe₂O₈ + Ca₂Fe₂O₅. In addition to these four compounds, the ordered Ca₅TiFe₄O₁₃ occurs in the Ca₃TiFe₂O₈ - Ca₂Fe₂O₅ region [13].

8.2. Natural Equivalents for Phases of the Perovskite-Brownmillerite Series

At present, all key ordered compounds on the phase diagram CaTiO₃-CaFe₂O₅ [13,20,21,22] have been found as minerals in natural combustion metamorphism environments (mainly in Ca-rich metacarbonate rocks). Nataliakulikite was the latest mineral in the system discovered in natural occurrence. The natural phases close to synthetic Ca₅TiFe³⁺₄O₁₃-Ca₅TiFe³⁺₂Al₂O₁₃ [13] were previously chemically described in some rocks [26,80,81], but they are not yet approved by the CNMNC IMA as a new mineral species. The perovskite - brownmillerite series now includes perovskite and Fe³⁺-perovskite CaTiO₃-CaTi_1–2xFe_2xO_3–x (0≤ x ≤ 0.4), brownmillerite Ca₂Fe³⁺AlO₅, srebrodolskite Ca₂Fe³⁺Fe³⁺O₅, shulamitite Ca₃TiFe³⁺AlO₈, sharyginite Ca₃TiFe³⁺₂O₈ and nataliakulikite Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁ [18,19,24,25,26,27,28,29,30,31]. In addition, according natural observations and experimental data, the following solid solutions towards Al-rich compositions are well identified: srebrodolskite-brownmillerite Ca₂Fe³⁺Fe³⁺O₅-Ca₂Fe³⁺AlO₅; sharyginite-shulamitite Ca₃TiFe³⁺Fe³⁺O₈-Ca₃TiFe³⁺AlO₈, individual members of which are subdivided due to structural phase transition and the Fe³⁺ ↔ Al substitution in tetrahedral sites [26,28,29,30,31,82,83,84]. The same solid solutions may exist for both nataliakulikite line Ca₄Ti₂Fe³⁺Fe³⁺O₁₁-Ca₄Ti₂Fe³⁺AlO₁₁ and line Ca₅TiFe³⁺₂Fe³⁺₂O₁₃-Ca₅TiFe³⁺₂Al₂O₁₃ [5]. Surprisingly, the Hatrurim Basin is now sole CM outcrop, where all members of the perovskite-brownmillerite series have been found. The general structural data for all these minerals are summarized in

Table 5. Structural data for ordered phases (natural and synthetic) of the perovskite-brownmillerite pseudobinary series.

Mineral	Perovskite	Natalia- kulikite	Sharyginite	Shulamitite	Synthetic phase	Srebrodol- skite	Brown- millerite
Formula	CaTiO3	Ca4Ti2Fe3+Fe3+O11	Ca3TiFe3+Fe3+O8	Ca3TiFe3+AlO8	Ca5TiFe3+2Al2O13	Ca2Fe3+Fe3+O5	Ca2Fe3+AlO5
Orthorhombic space group	Pnma	Pnma	P21ma	Pmma	Body-centered	Pnma	I2mb
Unit cell:
a, Å	5.544	5.254	5.423(2)	5.4200(6)		5.420(3)	5.57
b, Å	7.6412	30.302	11.150(8)	11.064(1)	18.6	14.752(3)	14.52
c, Å	5.381	5.488	5.528(2)	5.5383(7)		5.594(3)	5.34
V, Å3	223.8	873.7	334.3(3)	332.12		447.27	431.88
Z	4	4	2	2		4	4
Sequence	OOOOOO	OOTOOO	OTOOTO	OT’OOT’O	OTOTOO	OTOT’OT	OTOTOT
Density (g/cm3)	4.04	4.006	3.943	3.84		3.94	3.74
Reference	[21]	This study; [8,9]	[31]	[30]	[13]	[25,83]	[24,83]

O, T—octahedral and tetrahedral layers. T, T’—different stacking sequence of tetrahedral chains.

.

The natural relations among minerals of the perovskite-brownmillerite series also do not contradict the phase diagram CaTiO₃-CaFe₂O₅ [20,21,22]. Different types of Ca-rich assemblages in the Hatrurim Basin and other CM outcrops worldwide may contain a single mineral of the series (usually brownmillerite, rarely shulamitite or sharyginite), or two such minerals (brownmillerite/ srebrodolskite + shulamitite/sharyginite, Fe-rich perovskite + shulamitite/sharyginite, Fe-rich perovskite + nataliakulikite) [18,19,24,25,26,27,28,29,30,31]. The “prohibited” associations of Fe-rich perovskite + brownmillerite and nataliakulikite + brownmillerite were not observed in any CM rocks. Although, individual grains of perovskite and brownmillerite may occur in some rock samples. The triple Fe-rich perovskite + shulamitite/sharyginite + brownmillerite/srebrodolskite assemblage was found only in metacarbonate xenoliths of the Bellerberg volcano, Germany [28,29,30], but in this case there is no contradiction with the phase diagram CaTiO₃-CaFe₂O₅ because perovskite and brownmillerite have no immediate contacts.

Assuming the CaTiO₃-Ca₂Fe₂O₅ phase diagram (Figure 12), the phase Ca₄Ti₂Fe₂O₁₁ (synthetic Fe³⁺-dominant analog of nataliakulikite [8,9]) should be stoichiometric without vacancies of cations and oxygen. Any deviations from Ti = 2 (apfu) should lead to intergrowths/domains of either Ca₄Ti₂Fe₂O₁₁ + Ca₃TiFe₂O₈ or Ca₄Ti₂Fe₂O₁₁ + Fe³⁺-perovskite. The holotype nataliakulikite is very close to stoichiometric composition in respect to the Ti content (Table 3, Figure 7). HRTEM study reveals that holotype nataliakulikite is structurally homogeneous and does not contain any domains and intergrowths (Figure 9). It is different from the synthetic compound Ca₄Ti₂Fe₂O₁₁ in both high silica and alumina contents, and by believable disordering of Fe³⁺, Al³⁺, and Si⁴⁺ in tetrahedral sites, and the appearance of Fe²⁺ in octahedral sites. The impurities of Si and Fe²⁺ in nataliakulikite seem to stabilize its structure and prevent its decay into two perovskite phases (intergrowths/domains of Ca₄Ti₂Fe₂O₁₁ + Fe³⁺-perovskite or Ca₃TiFe₂O₈ + Ca₄Ti₂Fe₂O₁₁).

The nature of Si-enrichment of nataliakulikite, and related Fe-perovskite in the studied larnite-gehlenite rock, remains unclear. Possibly it deals with nano-sized inclusions of Ca-silicates in these phases. On other hand, it may be related to high Si activity at high-temperature formation of the rock. Note that SiO₂ does not exceed 3 wt.% in other minerals of the perovskite-brownmillerite series in the Hatrurim Basin larnite and larnite-gehlenite rocks [5,26,30,73]. However, some synthetic phases and minerals in high-temperature Ca-rich paralavas may be extremely rich in SiO₂ (up to 11 wt.%) [80,81,85].

8.3. Temperature Estimation for Nataliakulikite at the Hatrurim Basin

The formation temperature of the Fe-perovskite + shulamitite/sharyginite assemblage from the Hatrurim Basin larnite-mayenite rock was reconstructed previously using the CaTiO₃-Ca₂Fe₂O₅ phase diagram, and the assemblage was found out to be stable at >1160 °C [26]. The minimum temperature of its formation was estimated to be 1170–1200 °C proceeding from the Fe-perovskite composition and the modal proportion of phases in the rock [26]. Note that the presence of Al, Cr, and some other impurities in Fe-perovskite can lead to higher temperature estimates. The same assemblages in other CM rocks worldwide gave similar temperature values [28,29,30,31]. Moreover, the assemblage of Fe-perovskite + nataliakulikite is also applicable for estimation of formation temperatures of rocks [19].

The detailed petrography (larnite-flamite intergrowths, Fe-perovskite + nataliakulikite) has shown that studied larnite-gehlenite rock seems to be formed by high-temperature solid-phase reactions without any signs of melting. The presence of nataliakulikite in the rock indicates the upper temperature limit as 1160 °C, whereas chemical composition of Fe³⁺-rich perovskite in association with nataliakulikite gave minimal temperature estimation >1160 °C (Figure 12). However, the true formation temperature may be higher because both perovskite and nataliakulikite are enriched in Si.

Additionally, we provided independent evaluation for the assemblage khesinite + Fe-perovskite + sharyginite from larnite-gehlenite pebble (Figure 13), which adjoins the nataliakulite-bearing ones in “pseudoconglomerate” layer at Nahal Morag Canyon. The presence of natural Ca-silicoferrite (khesinite) strongly suggests the upper temperature limit as 1220–1255 °C [86,87]. Whereas, perovskite composition in association with sharyginite gave a broader range (1200–1160 °C, Figure 12). In general, this estimation is very similar to that of previous data for the Hatrurim larnite rocks [26]. We can suggest that the temperature gradient for neighboring larnite-gehlenite pebbles, within “pseudoconglomerate” layer, at Nahal Morag Canyon, was very minimal and the difference in mineralogy of Ti-Fe-bearing assemblages seems to be related to composition of sedimentary protolith.

9. Concluding Remarks

Thus, the detailed mineralogical and petrographic studies for larnite-gehlenite rocks at the Hatrurim Basin gave a possibility to describe physical properties, chemical composition and some structural affinities for a new mineral, nataliakulikite, Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁. On the basis of the dominant-valency rule [77] (R³⁺-cations > Si⁴⁺ and Fe³⁺ > Al), the ideal formula should be Ca₄Ti₂Fe³⁺Fe³⁺O₁₁ or Ca₄Ti₂Fe³⁺₂O₁₁. This mineral belongs to the perovskite supergroup (brownmillerite subgroup, non-stoichiometric perovskites group, anion deficient perovskites).

Assuming the CaTiO₃–Ca₂Fe₂O₃ phase diagram and in addition to the Fe-perovskite + sharyginite/shulamitite paragenesis [26], nataliakulikite and its association with Fe³⁺-rich perovskite may be used as new thermometers for metacarbonate rocks of the high-temperature region of the spurrite-merwinite facies.