New Mineral with Modular Structure Derived from Hatrurite from the Pyrometamorphic Rocks of the Hatrurim Complex: Ariegilatite, BaCa₁₂(SiO₄)₄(PO₄)₂F₂O, from Negev Desert, Israel

Abstract

Ariegilatite, BaCa₁₂(SiO₄)₄(PO₄)₂F₂O (R$\overline{3}$m, a = 7.1551(6) Å, c = 41.303(3) Å, V = 1831.2(3) ų, Z = 3), is a new member of the nabimusaite group exhibiting a modular intercalated antiperovskite structure derived from hatrurite. It was found in a few outcrops of pyrometamorphic rocks of the Hatrurim Complex located in the territories of Israel, Palestine and Jordan. The holotype specimen is an altered spurrite marble from the Negev Desert near Arad city, Israel. Ariegilatite is associated with spurrite, calcite, brownmillerite, shulamitite, CO₃-bearing fluorapatite, fluormayenite-fluorkyuygenite and a potentially new mineral, Ba₂Ca₁₈(SiO₄)₆(PO₄)₃(CO₃)F₃O. Ariegilatite is overgrown and partially replaced by stracherite, BaCa₆(SiO₄)₂[(PO₄)(CO₃)]F. The mineral forms flat disc-shaped crystals up to 0.5 mm in size. It is colorless, transparent, with white steaks and vitreous luster. Optically, ariegilatite is uniaxial, negative: ω = 1.650(2), ε = 1.647(2) (λ = 589 nm). The mean composition of the holotype ariegilatite, (Ba_0.98K_0.01Na_0.01)_Σ1(Ca_11.77Na_0.08Fe²⁺_0.06Mn²⁺_0.05Mg_0.04)_Σ12(Si_3.95Al_0.03Ti_0.02)_Σ4(P_1.70C_0.16Si_0.10S⁶⁺_0.03V_0.01)_Σ2F_2.04O_0.96, is close to the end-member formula. The structure of ariegilatite is described as a stacking of the two modules {F₂OCa₁₂(SiO₄)₄}⁴⁺ and {Ba(PO₄)₂}⁴⁻ along (001). Ariegilatite, as well as associated stracherite, are high-temperature alteration products of minerals of an early clinker-like association. These alterations took place under the influence of pyrometamorphism by-products, such as gases and fluids generated by closely-spaced combustion foci.

1. Introduction

Ariegilatite, BaCa₁₂(SiO₄)₄(PO₄)₂F₂O (R$\overline{3}$m, a = 7.1551(6) Å, c = 41.303(3) Å, V = 1831.2(3) ų, Z = 3), is the first P-bearing mineral in the nabimusaite group, which combines nabimusaite, КCa₁₂(SiO₄)₄(SO₄)₂O₂F (R$\overline{3}$m, a = 7.1905(4), c = 41.251(3) Å, V = 1847.1(2) ų, Z = 3) and dargaite, BaCa₁₂(SiO₄)₄(SO₄)₂O₃ (R$\overline{3}$m, a = 7.1874(4); c = 41.292(3) Å; V = 1847.32(19) ų, Z = 3) [1,2]. Ariegilatite was found in altered spurrite marble belonging to the pyrometamorphic Hatrurim Complex in the Negev Desert near Arad, Israel. Rocks of the Hatrurim Complex mainly consist of spurrite marbles, larnite pseudoconglomerates and gehlenite hornfelses. Furthermore, pyrometamorphic rocks and by-products of their low-temperature alterations commonly occur along the Dead Sea Rift in the territories of Israel, Palestine and Jordan (the “Mottled zone”) [3,4,5].

Minerals of the nabimusaite group are silicates with additional anions and are isostructural with arctite, a phosphate with an ideal crystal chemical formula of Ba(Ca₇Na₅)(PO₄)₆F₃ [6]. Minerals of the nabimusaite group and arctite can be described as hexagonal intercalated antiperovskites with the general crystal chemical formula AB₁₂(TO₄)₄(TO₄)₂W₃, A = Ba, K, …; B = Ca, Na, …; T = Si, P, S⁶⁺, V⁵⁺, Al, …; W = O²⁻, F⁻ [2,7]. Nabimusaite group minerals show hatrurite-like triple antiperovskite layers {[W₃Ca₁₂](SiO₄)₄}^e+, which alternate with A(TO₄)₂^e− single layers [1,2]. The same type of layers is found in arctite. However, the composition of the triple antiperovskite module is {[F₃(Ca₇Na₅)](PO₄)₄}⁴⁺, and the second module is Ba(PO₄)₂⁴⁻ [6,8].

Minerals of the zadovite group can also be described as intercalated antiperovskites. Their general formula is AB₆(TO₄)₂[(TO₄)_2−x(CO₃)_x]W (A, B, T, W = as above; х ≈ 0 or х ≈ 1): zadovite, BaCa₆[(SiO₄)(PO₄)](PO₄)₂F; aradite, BaCa₆[(SiO₄)(VO₄)](VO₄)₂F; gazeevite, BaCa₆(SiO₄)₂(SO₄)₂O and stracherite BaCa₆(SiO₄)₂[(PO₄)(CO₃)]F. In these minerals, single antiperovskite layers {[WB₆](TO₄)₂}^e+ intercalate with single Ba(TO₄)₂^e− layers [7,9]. In stracherite, about half of the (PO₄)³⁻ tetrahedra are replaced by (CO₃)²⁻ groups [10].

Ariegilatite is named in honor of Dr. Arie Gilat (b. 1939). Arie Gilat is retired from the Geological Survey of Israel, where he was involved in geological mapping, tectonics and geochemical studies for more than 30 years. He is the author and co-author of numerous geological papers. At present, his main interests are related to the study of earthquake physics and processes at the core and mantle interface. His continuous support, consulting and several new unconventional ideas on the genesis of the Hatrurim Complex are greatly appreciated by the authors.

Ariegilatite was approved as a new mineral species by Commission on New Minerals, Nomenclature and Classification, International Mineralogical Association (CNMNC IMA) in March 2017 (IMA2016-100). The material was deposited in the mineralogical collection of the Fersman Mineralogical Museum, Leninskiy pr., 18/k2, 115162 Moscow, Russia, Catalogue Number 4956/1.

2. Methods of Investigation

The crystal morphology and chemical composition of ariegilatite and associated minerals were examined using optical microscopes, as well as the analytical electron scanning microscopes Philips XL30 and Phenom XL, PhenomWorld, ThermoFisher Scientific, Eindhoven, The Netherlands (Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland). Chemical analyses of ariegilatite were performed with a CAMECA SX100 microprobe analyzer (Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Warszawa, Poland) at 15 kV and 10 nA using the following lines and standards: BaLα, SKα, baryte; PKα, apatite; CaKα, wollastonite; MgKα, SiKα, diopside; FeKα, hematite, AlKα, KKα, orthoclase; TiKα, rutile; NaKα, albite, SrLα, SrTiO₃, FKα, fluorphlogopite.

Raman spectra of ariegilatite were recorded on a WITec alpha 300R Confocal Raman Microscope, WITec, Ulm, Germany (Department of Earth Science, University of Silesia, Sosnowiec, Poland) equipped with an air-cooled solid-state laser (532 nm) and a CCD camera operating at −61 °C. The laser radiation was coupled to a microscope through a single-mode optical fiber with a diameter of 3.5 μm. An air Zeiss LD EC Epiplan-Neofluan DIC-100/0.75NA objective was used. Raman scattered light was focused on a broad band single mode fiber with an effective pinhole size of about 30 μm, and a monochromator with a 600-mm⁻¹ grating was used. The power of the laser at the sample position was ca. 40 mW. Integration times of 5 s with an accumulation of 15–20 scans and a resolution 3 cm⁻¹ were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm⁻¹). Fitting of spectra was performed with the help of the “GRAMS” program using the mixed Lorentz–Gauss function.

Single-crystal X-ray diffraction data were collected from a crystal of ariegilatite (∼38 × 32 × 25 μm) using synchrotron radiation at the super-bending magnet beamline X06DA at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland. The multi-axis goniometer PRIGo [11] and a PILATUS 2M-F detector, which was placed at a distance of 120 mm from the crystal with a vertical offset of 67 mm, was used. The wavelength was set to λ = 0.70848 Å. The data were collected in a single 180° omega scan with steps of 0.1° and 0.1-s exposures, controlled by the DA + software [12]. Data evaluation and processing was performed using the CrysAlisPro software package [13]. As a starting model, the structure of the isostructural analogue nabimusaite was used, adapted to the expected composition. With subsequent analyses of difference-Fourier maps, the structure was refined to R₁ = 1.95%. The refinements include anisotropic displacement-parameters and have been carried out with neutral atom scattering-factors, using the program SHELX97 [14].

3. Results

3.1. Occurrence and Description of Holotype Specimen

Ariegilatite was found in a few outcrops of pyrometamorphic rocks of the Hatrurim Complex located in the territories of Israel, Palestine and Jordan. Investigations of a new mineral were performed in samples of spurrite rocks collected in the Negev Desert (Hatrurim Basin, N31°13′ E35°16′) near Arad, Israel. Ariegilatite forms strongly flattened crystals of disc-shaped form. In thin-sections, its pseudo-aciculate morphology is usually observed (Figure 1A). The size of some highly-fractured crystals of ariegilatite reaches 0.5 mm with a thickness of 0.1 mm (Figure 1A). Ariegilatite is associated with spurrite, calcite, brownmillerite, shulamitite, CO₃-bearing fluorapatite, fluormayenite-fluorkyuygenite, periclase, brucite, barytocalcite, baryte, garnets of elbrusite-kerimasite series, unidentified Ca-Fe- and Rb-bearing K-Fe sulfides and a potentially new mineral, Ba₂Ca₁₈(SiO₄)₆(PO₄)₃(CO₃)F₃O [15]. Ariegilatite is often overgrown and replaced by stracherite (Figure 1B). Ariegilatite and stracherite are usually limited to re-crystallization zones of dark-grey fine-grained spurrite rocks, which differ from the surrounding rocks by discoloration, development of thin calcite veins and also by local appearance of large spurrite metacrysts (up to 1 cm in size), as well as the presence of sulfide mineralization.

Ariegilatite is a colorless, transparent mineral with white streaks and vitreous luster. It does not show any fluorescence. The mineral is uniaxial, negative: ω = 1.650(2), ε = 1.647(2) (λ = 589 nm) and non-pleochroic. The hardness was measured using micro-indentation and determined to VHN₅₀ = 356(16) kg·mm⁻², as an average of 16 measured spots ranging from 331–378 kg·mm⁻², which corresponds to 4–4.5 on the Mohs scale. In contrast to minerals of the nabimusaite-dargaite series [1,2], ariegilatite does not show pronounced cleavage on (001), and the fracture is irregular. The small size of separated crystal fragments does not allow measuring the density. Therefore, the density was calculated on the basis of structural data and the mean composition of ariegilatite: 3.329 g·cm⁻³. Compatibility index 1 − (K_p/K_c) = −0.017 (superior) was calculated for the empirical formula of ariegilatite of the holotype specimen (

Table 1. Chemical composition of ariegilatite, wt %.

	L15			YV595			SS20b
Mean	22	s.d.	Range	23	s.d.	Range	14	s.d.	Range
SO3	0.17	0.07	0.05–0.31	0.60	0.22	0.32–1.02	n.d.
V2O5	0.10	0.06	0–0.17	0.32	0.08	0.13–0.51	0.41	0.07	0.26–0.52
P2O5	9.83	0.45	8.96–10.55	10.39	0.55	9.63–12.53	10.61	0.4	10.05–11.40
TiO2	0.12	0.06	0.05–0.25	0.31	0.04	0.24–0.40	0.26	0.16	0.10–0.56
SiO2	19.87	0.26	19.52–20.42	19.21	0.36	17.70–19.62	18.64	0.27	18.21–19.15
Fe2O3				0.34	0.09		0.17	0.05	0.11–0.24
Al2O3	0.12	0.03	0.07–0.18	0.17	0.11	0.10–0.68	0.24	0.06	0.17–0.38
BaO	12.26	0.06	12.14–12.41	12.01	0.35	10.81–12.49	12.29	0.34	11.74–12.94
FeO	0.32	0.06	0.24–0.46	0.49
MnO	0.29	0.08	0.09–0.39	0.26	0.07	0.14–0.41	n.d.
CaO	53.84	0.24	53.19–54.40	53.77	0.30	53.13–54.27	54.24	0.31	54.30–55.62
MgO	0.14	0.03	0.11–0.22	0.38	0.05	0.27–0.53	0.03	0.02	0–0.07
K2O	0.04	0.03	0–0.10	n.d.			n.d.
Na2O	0.22	0.05	0.16–0.36	0.05	0.02	0.02–0.09	0.31	0.05	0.20–0.38
F	3.17	0.10	2.96–3.34	3.24	0.45	2.27–3.27	3.05	0.11	2.84–3.17
CO2 *	0.57			0.00			0.62
–O=F	1.33			1.36			1.28
Total	99.72			100.18			99.58
Ba	0.98			0.96			0.99
K	0.01
Na	0.01			0.02			0.01
Ca				0.02
A	1			1			1
Ca	11.77			11.75			11.88
Mn2+	0.05			0.05
Fe2+	0.06			0.08
Mg	0.04			0.12			0.01
Na	0.08						0.11
B	12			12			12
Si	3.95			3.86			3.81
Ti4+	0.02			0.05			0.04
Fe3+				0.05			0.03
Al	0.03			0.04			0.06
P							0.07
T1	4			4			4
P	1.70			1.80			1.77
Si	0.10			0.07
V5+	0.01			0.04			0.06
S6+	0.03			0.09
C	0.16						0.17
T2	2			2			2
O	0.96			0.91			1.03
F	2.04			2.09			1.97
W	2			2			2
	SS27A			MA5b			IS129			teor
Mean	29	s.d.	Range	11	s.d.	Range	12	s.d.	Range
SO3	0.05	0.08	0–0.34	1.35	0.22	1.10–1.92	n.d.
V2O5	0.42	0.07	0.24–0.62	0.18	0.11	0.04–0.41	n.d.
P2O5	10.52	0.52	9.48–11.65	12.91	0.43	11.89–13.60	9.02	0.49	8.31–10.16	11.53
TiO2	0.27	0.16	0.05–0.59	0.18	0.06	0.09–0.28	n.d.
SiO2	18.85	0.42	18.25–20.30	16.46	0.45	15.81–17.16	19.83	0.27	19.44–20.23	19.53
Fe2O3	n.d.						n.d.
Al2O3	0.25	0.08	0.14–0.48	0.53	0.10	0.40–0.75	0.15	0.1	0.09–0.47
BaO	12.26	0.41	11.39–13.15	12.17	0.49	11.42–13.31	12.36	0.17	12.12–12.66	12.46
FeO				0.17	0.11	0.03–0.38
MnO	n.d.			n.d.			n.d.
CaO	54.26	0.36	54.00–55.58	54.49	0.13	54.25–54.68	54.78	0.15	54.56–55.07	54.68
MgO	0.08	0.16	0–0.93	0.09	0.02	0.06–0.12	n.d.
K2O	n.d.			0.06	0.02	0.02–0.10	0.04	0.02	0.02–0.08
Na2O	0.24	0.08	0.06–0.37	0.23	0.04	0.15–0.32	0.66	0.11	0.49–0.82
F	3.02	0.15	2.68–3.37	2.30	0.07	2.15–2.48	2.92	0.14	2.56–3.22	3.08
CO2 *	0.55			0.18			1.68			0.00
–O=F	1.27			0.97			1.23			1.29
Total	99.50			100.32			100.21			100.00
Ba	0.98			0.97			0.97			1.000
K				0.02			0.01
Na	0.02			0.01			0.02
Ca
A	1			1			1
Ca	11.90			11.87			11.76			12.000
Mn2+
Fe2+				0.03
Mg	0.02			0.03
Na	0.08			0.07			0.24
B	12			12			12
Si	3.86			3.35			3.97			4.000
Ti4+	0.04			0.02
Fe3+
Al	0.06			0.13			0.03
P	0.04			0.50
T1	4			4			4
P	1.78			1.72			1.53			2.000
Si							0.01
V5+	0.06			0.02
S6+	0.01			0.21
C	0.15			0.05			0.46
T2	2			2			2
O	1.04			1.52			1.15			1.000
F	1.96			1.48			1.85			2.000
W	2			2			2			2

* Calculated on charge balance, Fe²⁺: calculated as 13 − (A + B) cations, n.d., not detected.

).

The Raman spectrum of ariegilatite exhibits the following strong bands (Figure 2; cm⁻¹): 129, 179, 229 and 309 (lattice mode, Ba-O, Ca-O vibrations); 403 [υ₂(SiO₄)⁴⁻]; 427 [υ₂(PO₄)²⁻]; 520 [υ₄(SiO₄)⁴⁻]; 569 and 591 [υ₄(PO₄)³⁻]; 834 and 874 [υ₁(SiO₄)⁴⁻]; 947 [υ₁(PO₄)³⁻]; 993 [υ₁SO₄)²⁻]; 1030 [υ₃(PO₄)²⁻]; 1066 [υ₁(CO₃)²⁻]. The main strong Raman bands of ariegilatite are related to vibrations in (SiO₄)⁴⁻ and (PO₄)³⁻ (Figure 2). The band at about 1066 cm⁻¹ can be attributed to υ₁(CO₃)²⁻. Its position is very close to the υ₁(CO₃)²⁻ mode in CO₃-bearing hydroxylapatite and hydroxylellestadite [16,17]. Raman spectroscopy data indicate that H₂O is absent in ariegilatite. Direct determination of the CO₂ content was not feasible due to abundant microscopic inclusions of other minerals.

The mean composition of the holotype ariegilatite, (Ba_0.98K_0.01Na_0.01)_Σ1(Ca_11.77Na_0.08Fe²⁺_0.06Mn²⁺_0.05Mg_0.04)_Σ12(Si_3.95Al_0.03Ti_0.02)_Σ4(P_1.70C_0.16Si_0.10S⁶⁺_0.03V_0.01)_Σ2F_2.04O_0.96, is close to the end-member formula BaCa₁₂(SiO₄)₄(PO₄)₂F₂O. The amount of СО₃ in ariegilatite is calculated assuming charge balance and 13 non-tetrahedral cations (Table 1, L15).

Experimental details and refinement data of ariegilatite are summarized in

Table 2. Parameters for X-ray data collection.

Crystal Data
Crystal system	trigonal
Unit cell dimensions (Å)	a = 7.1551(6), c = 41.303(3) Å, b = 7.1551(6) c = 41.303(3) α, β = 90° γ = 120°
Space group	R$\overline{3}$m (no.166)
Volume (Å3)	1831.2(3)
Z	3
Density (calculated)	3.329 g·cm−3
Chemical formula	~BaCa12(SiO4)4(PO4)1.8F2O
Crystal size (mm)	38 × 32 × 25 μm
Data Collection
Diffractometer	beamline X06DA, SLS multi-axis goniometer PRIGo PILATUS 2M-F detector λ = 0.70848 Å
Max. θ°-range for data collection	32.139
Index ranges	−10 ≤ h ≤ 6
	−8 ≤ k ≤ 10
	−59 ≤ l ≤ 44
No. of measured reflections	3445
No. of unique reflections	825
No. of observed reflections (I > 2σ (I))	822
Refinement of the Structure
No. of parameters used in refinement	58
Rint	0.0199
Rσ	0.0105
R1, I > 2σ(I)	0.0195
R1 all Data	0.0195
wR2 on (F2)	0.0555
GooF	1.128
Δρ min (−e. Å−3)	−0.54
Δρ max (e. Å−3)	0.82

,

Table 3. Atom coordinates, U_eq (Ų) values for ariegilatite.

Atom	x/a	y/b	z/c	sof	Ueq
Ba1	0	0	0	1	0.01731(9)
Ca1	0.16368(3)	0.83632(3)	0.39407(2)	1	0.01059(12)
Ca2	0.15534(3)	0.84466(3)	0.53179(2)	1	0.01036(12)
P1	0	0	0.67473(2)	0.892(4)	0.0085(2)
Si1	0	0	0.20615(2)	1	0.00650(17)
Si2	0	0	0.08404(2)	1	0.00723(17)
O1	0.55031(11)	0.44969(11)	0.64495(3)	1	0.0148(4)
O2	0.12532(10)	0.87468(10)	0.19359(3)	1	0.0130(4)
O3	0.12681(11)	0.87319(11)	0.07376(3)	1	0.0140(4)
O4	0	0	0.36244(5)	1	0.0125(5)
O5	0	0	0.75407(5)	1	0.0132(5)
O6	0	0	0.12420(5)	1	0.0113(5)
O7	0	0	0.5	1	0.0057(6)
F1	0	0	0.43179(4)	1	0.0134(4)

,

Table 4. Anisotropic displacement parameters U^ij for ariegilatite.

Atom	U11	U22	U33	U23	U13	U12
Ba1	0.01604(12)	0.01604(12)	0.01985(15)	0.00802(6)	0	0
Ca1	0.00850(14)	0.00850(14)	0.01499(17)	0.00441(12)	0.00013(5)	−0.00013(5)
Ca2	0.00855(14)	0.00855(14)	0.01337(17)	0.00382(12)	−0.00066(5)	0.00066(5)
P1	0.0087(3)	0.0087(3)	0.0082(4)	0.00437(15)	0	0
Si1	0.0053(2)	0.0053(2)	0.0089(3)	0.00264(11)	0	0
Si2	0.0063(2)	0.0063(2)	0.0091(3)	0.00316(11)	0	0
O1	0.0156(4)	0.0156(4)	0.0144(5)	0.0088(5)	0.0010(2)	−0.0010(2)
O2	0.0113(4)	0.0113(4)	0.0194(6)	0.0079(5)	0.0009(2)	−0.0009(2)
O3	0.0107(4)	0.0107(4)	0.0220(6)	0.0065(5)	0.0015(2)	−0.0015(2)
O4	0.0136(6)	0.0136(6)	0.0103(8)	0.0068(3)	0	0
O5	0.0147(6)	0.0147(6)	0.0102(8)	0.0074(3)	0	0
O6	0.0114(6)	0.0114(6)	0.0110(8)	0.0057(3)	0	0
O7	0.0059(7)	0.0059(7)	0.0054(10)	0.0030(4)	0	0
F1	0.0131(5)	0.0131(5)	0.0140(8)	0.0066(3)	0	0

and

Table 5. Selected interatomic distances (Å) for ariegilatite.

Atom	-Atom	Distance		Atom	-Atom	Distance
Ba1	-O1	2.8346(10)	×6	P1	-O4	1.535(2)
	mean	2.8346			-O1	1.5484(11)	×3
Ca1	-O5	2.3735(11)			mean	1.5451
	-O3	2.4006(9)	×2		bvs	4.859(9)
	-O4	2.4128(12)		Si1	-O2	1.6374(10)	×3
	-O1	2.4628(12)	×2		-O5	1.643(2)
	F1	2.5577(12)			mean	1.6388
	mean	2.4387			bvs	3.843(7)
	bvs	1.921(2)		Si2	-O3	1.6279(10)	×3
Ca2	-O6	2.2495(5)			-O6	1.659(2)
	-F1	2.4431(12)			mean	1.6356
	-O2	2.3301(8)	×2		bvs	3.879(7)
	-O7	2.3303(5)		O7	-Ca2	2.3303(5)	×6
	-O2	2.5114(15)			mean	2.3303
	-O3	2.6028(15)			bvs	2.248(1)
	mean	2.3996		F1	-Ca1	2.5577(12)	×3
	bvs	2.196(2)			-Ca2	2.4431(12)	×3
	bvs	1.024(1)			mean	2.5004

. The structural formula of ariegilatite Ba1Ca1₆Ca2₆[(Si1O₄)₂(Si2O₄)₂](P1O₄)_~1.8F1₂O₂ (Table 4) is simplified to BaCa₁₂(SiO₄)₄(PO₄)₂F₂O according to the general formula of the nabimusaite group minerals [2]. The ariegilatite structure is most easily described as a 1:1 stacking of the two modules {[F₂OCa₁₂](SiO₄)₄}⁴⁺ and {Ba(PO₄)₂}⁴⁻ along (001) (Figure 3). The sites F1 and O7 are coordinated by six Ca atoms in an octahedral arrangement (Figure 3B). The module {Ba(PO₄)₂}⁴⁻ is characterized by (P1O₄) tetrahedra connected to six-coordinated Ba1 (Figure 3C).

The antiperovskite module {[F₂OCa₁₂](SiO₄)₄}⁴⁺ consists of columns formed by Ca-triplets Са₃О₁₄, rotated relative to each other by 60° (Figure 4A,B). Ca-triplets form four layers with (SiO₄)⁴⁻ tetrahedra in structural cavities (Figure 4C). The structure of hatrurite (Ca₃SiO₅) shows similar antiperovskite modules [1,18,19].

Raman and electron microprobe data (Figure 2, Table 1, L15) indicate the presence of small amounts of (CO₃)²⁻ in ariegilatite, replacing (PO₄)³⁻ groups. Refinement of the occupancy factor of P1 (Table 3) shows a small reduction from full occupation (89%), which is in agreement with the substitution by CO₃. The structural formula of ariegilatite calculated assuming a balanced charge, BaCa₁₂(SiO₄)₄ {[(PO₄)_0.89□_0.11](CO₃)_0.11}₂F_1.78O_1.22, is close to the empirical formula obtained from microprobe analyses (Table 1, L15). However, the exact location and orientation of the planar CO₃ group cannot be determined from diffraction data. Most likely, replacement of (PO₄)³⁻ by planar (СО₃)²⁻ groups takes place the same way as in stracherite: СО₃ triangles are randomly located along one of the faces of the replaced tetrahedra excluding the face parallel to (001) [10]. Replacement of (PO₄)³⁻ by (СО₃)²⁻ in ariegilatite takes place according to the schemes: Ca²⁺ + (PO₄)³⁻ ↔ Na⁺ + (СО₃)²⁻, 2(PO₄)³⁻ ↔ (SiO₄)⁴⁻ + (CO₃)²⁻ and (PO₄)³⁻ + F⁻ ↔ (CO₃) ²⁻ + O²⁻.

As ariegilatite occurs only in tiny amounts, and the crystals contain numerous inclusions of other phases, useful X-ray powder diffraction data could not be collected. However, a powder pattern was calculated (Table S1) using the model derived from the single-crystal structure refinements.

3.2. Occurrences of Ariegilatite in Other Localities of the Hatrurim Complex Rocks

Ariegilatite occurs both in larnite and spurrite rocks of the Hatrurim Complex, whereas minerals of the nabimusaite-dargaite series are found in larnite rocks only [1,2]. The first samples of ariegilatite were found in 2011 by M. Murashko in larnite pebble, in the northern part of the Siwaqa pyrometamorphic rock area, 80 km south of Amman, Jordan. Daba-Siwaga is the largest area of the Hatrurim Complex within the Dead Sea rift region [20,21,22]. In this samples, ariegilatite is associated with larnite, gehlenite, spinel, fluormayenite, fluorapatite, perovskite and a bredigite-like Ba-bearing mineral (Figure 5A). It forms poikilitic crystals up to 0.25 mm in size and fine reaction rims on fluorapatite (Figure 5A). The mean ariegilatite composition from larnite rock is (Ba_0.96K_0.02Na_0.02)_Σ1(Ca_11.75Mg_0.12Fe²⁺_0.08Mn²⁺_0.05)_Σ12(Si_3.86Ti⁴⁺_0.05Fe³⁺_0.05Al_0.04)_Σ4(P_1.80S⁶⁺_0.09Si_0.07V⁵⁺_0.04)_Σ2F_2.09O_0.91 (Table 1, YV595); here, Mg is slightly increased in comparison to the holotype specimen (Table 1, L15). The Raman spectrum of ariegilatite from larnite rocks in Jordan shows no characteristic bands of (CO₃)²⁻ groups (Figure 2, YV595). Furthermore, the calculation of the stoichiometric formula does not indicate the presence of (CO₃)²⁻ (Table 1, YV595).

Investigation of spurrite rocks (+calcite, fluorapatite, fluormayenite-fluorkyuygenite, brownmillerite and periclase) occurring within a 1–2-km radius from the place, where the larnite pebble was found at the location in Siwaqa, revealed ariegilatite metacrysts 30–50 µm in size (Figure 3B). These rocks are locally enriched in sphalerite and oldhamite and are interspersed with calcite veins of 1–2 mm in thickness. As well as the holotype material, ariegilatite from spurrite rocks of Jordan contains about 0.15–0.17 (СО₃)²⁻ pfu (Table 1, SS20b and SS27A). The presence of (CO₃)²⁻ is also confirmed by Raman spectroscopy (Figure 2, SS20b and SS27A).

Ariegilatite was also found in the territory of Palestinian Autonomy in an outcrop near the Jerusalem-Dead Sea highway close to Ma’ale Adummim. At this location, the mineral was detected in altered flamite rocks (+fluormayenite-fluorkyuygenite, brownmillerite, fluorapatite, gehlenite) with heterogeneous development of jasmundite. Flamite is an analog of the synthetic phase α-Ca₂SiO₄ stabilized by K, Na and P impurities [23,24]. Flamite usually shows exsolution structures with individual lamellas enriched in P [24]. Ariegilatite replaces flamite lamellas with increased phosphorus content (Figure 5C) and exhibits a mean composition of (Ba_0.97K_0.02Na_0.01)_Σ1(Ca_11.87Na_0.07 Mg_0.03Fe²⁺_0.03)_Σ12(Si_3.35P_0.50Al_0.13Ti⁴⁺_0.02)_Σ4(P_1.72S⁶⁺_0.21C_0.05V⁵⁺_0.02)_Σ2F_1.48O_1.52, and it is characterized by increased (SO₄)²⁻ content (Table 1, MA5b); accordingly, its Raman spectrum shows a strong υ₁(SO₄)²⁻ band near 995 cm⁻¹ (Figure 2, MA5b).

Ariegilatite samples showing the highest (CO₃)²⁻ contents were detected in fluorapatite-calcite-spurrite rocks in the Negev Desert near the junction of Hatrurim (Gurim Anticline, Hatrurim Basin), Israel (Figure 5D,E). In these rocks, also cuspidine, fluorite, brownmillerite, chlormayenite, periclase, sphalerite, millerite, U-bearing lakargiite, vorlanite and oldhamite are identified. The empirical formula of ariegilatite from this locality is (Ba_0.97K_0.01Na_0.02)_Σ1(Ca_11.76Na_0.24)_Σ12(Si_3.97Al_0.03)_Σ4(P_1.53C_0.46Si_0.01)_Σ2F_1.85O_1.15 (Table 1, IS129). Its Raman spectrum reveals a characteristic band of (CO₃)²⁻ groups (Figure 2, IS129). The (СО₃)²⁻ groups can be incorporated according to two schemes: (PO₄)³⁻ + Ca²⁺ ↔ (СО₃)²⁻ + Na⁺ and (PO₄)³⁻ + F⁻ ↔ (СО₃)²⁻ + O²⁻. Interestingly, in spurrite rock, an ariegilatite relic net of dendrite channels (with an oval cross-section of 0.2–0.4 mm in diameter) filled with coarse-crystalline calcite was observed. Walls of the channels are incrusted by small oldhamite crystals up to 1–2 μm in size. Thanks to intensive oldhamite cathodoluminescence, these channels are well visible within the background of the weakly-luminescent minerals of spurrite rocks (Figure 5F).

4. Discussion

Ariegilatite exhibits an ordered anion distribution. Fluorine and oxygen were assigned to the anion sites utilizing the results of bond valence sum (BVS, Table 5) calculations. Oxygen resides in the center (Figure 3A, blue) and fluorine on the two outer octahedral sites (Figure 3A, green) of the antiperovskite module. New BVS calculations revealed the same preference in nabimusaite, where one fluorine atom seems to be distributed over the two outer octahedral sites, in contrast to what has been reported before [1,2,25]. Fluorine may enter the central octahedra when the charge is compensated as, for example, in arctite BaCa₇Na₅(PO₄)₆F₃ [6,8], where Ca atoms (of the central octahedra) are substituted by sodium. Similar behavior is observed in the isostructural synthetic compound Ca_5.45Li_3.55[SiO₄]₃O_0.45F_1.55 [26], where the outer octahedral sites are occupied by fluorine, whereas the inner site seems to be shared by oxygen and fluorine. For charge compensation, the Ca-sites coordinating the inner octahedral site are partly substituted by lithium.

The main schemes of isomorphism, unifying minerals of the nabimusaite group and arctite are Ca²⁺ + SiO₄⁴⁻ = Na⁺ + PO₄³⁻ and Ca²⁺ + O²⁻ = Na⁺ + F⁻. Arctite was the first mineral with a hexagonal intercalated antiperovskite structure to be discovered. We propose to distinguish an arctite super group (hexagonal intercalated antiperovskites), which at present combines two silicate groups: nabimusaite (nabimusaite, dargaite, ariegilatite, with triple-layer antiperovskite modules) and zadovite (zadovite, aradite, gazeevite, stracherite, with single-layer antiperovskite modules) and arctite, as an ungrouped phosphate.

According to Krivovichev [19], polyphite, Na₆(Na₄Ca₂)₂Na₂Ti₂Na₂Ti₂(Si₂O₇)₂(PO₄)₆O₄F₄ [27], also belongs to the hexagonal intercalated antiperovskites. However, we propose that polyphite, the structurally-related sobolevite, Na₁₂Ca(NaCaMn)Ti₂(Ti,Mn)[Si₂O₇]₂(PO₄)₄O₃F₃, and quadruphite, Na₁₄Ca₂Ti₄[Si₂O₇]₂(PO₄)₄O₄F₂ [27], should not be considered in the arctite super group. In these structures, columns of anion-centered octahedra form continuous layers parallel to titanium-silicate sheets. The proposed arctite super group exhibits A(TO₄) layers, which are perpendicular to the “antiperovskite columns” (Figure 3A).

So far, ariegilatite shows the highest fluorine content (>3 wt %, Table 1) of all members of the nabimusaite group. It occurs in spurrite, as well as in larnite rocks (Figure 1 and Figure 5). Ariegilatite from spurrite rocks contain significant amounts of (CO₃)²⁻ (Table 1). It forms relatively large poikilitic crystals standing out against a fine-grained matrix and grows after fluorapatite (Figure 1 and Figure 5). Minerals of the dargaite-nabimusaite series (and also gazeevite), as well as minerals of the ternesite-silicocarnotite series and jasmundite are high-temperature alteration products of minerals of an early clinker-like association of larnite rocks. These alterations took place under the influence of pyrometamorphism by-products, such as gases and fluids generated by closely-spaced combustion foci [1,28,29].

In our understanding, ariegilatite, as well as associated stracherite are the products of high-temperature alteration of early pyrometamorphic rocks with increased phosphorus (fluorapatite) and decreased sulfur (absence of fluorellestadite and ye’elimite) contents under the influence of high-fluorine, CO₂-bearing fluids/gases. Transport of these fluids/gases is realized through cracks and micro-channel nets (Figure 5F), which may have existed for a long time in heated pyrometamorphic rocks.