Karwowskiite, Ca₉(Fe²⁺_0.5□_0.5)Mg(PO₄)₇—A New Merrillite Group Mineral from Paralava of the Hatrurim Complex, Daba-Siwaqa, Jordan

Abstract

Crystals of karwowskiite, Ca₉Mg(Fe²⁺_0.5□_0.5)(PO₄)₇, a new mineral of the merrillite group, were found on an amygdule wall in the central part of an anorthite–tridymite–diopside paralava of the Hatrurim Complex, Daba-Siwaqa, Jordan. The amygdule was filled with a sulfide melt, which after crystallization gave a differentiated nodule, consisting of troilite and pentlandite parts and containing tetrataenite and nickelphosphide inclusions. Karwowskiite crystals are colorless, although sometimes a greenish tint is observed. The mineral has a vitreous luster. The microhardness VHN₂₅ is 365 (12), corresponding to 4 on the Mohs hardness scale. Cleavage is not observed, and fracture is conchoidal. The calculated density is 3.085 g/cm³. Karwowskiite is uniaxial (−): ω = 1.638 (3), ε = 1.622 (3) (λ = 589 nm), and pleochroism is not observed. The composition of karwowskiite is described by the empirical formula: Ca_9.00(□_0.54Fe²⁺_0.23Mg_0.12Na_0.04 Sr_0.03 Ni_0.03K_0.01) _Σ1.00Mg_1.00(PO₄)_7.02. Karwowskiite is distinct from the known minerals of the merrillite subgroup with the general formula A₉XM[TO₃(Ø)]₇, where A = Ca, Na, Sr, and Y; X = Na, Ca, and □; M = Mg, Fe²⁺, Fe³⁺, and Mn; T = P; and Ø = O, in that the X site in it is occupied by Fe²⁺_0.5□_0.5. Karwowskiite is trigonal, space group R-3c with a = 10.3375 (2) Å, c = 37.1443 (9) Å, and V = 3437.60 (17) ų. Karwowskiite crystallizes at temperatures lower than 1100 °C in a thin layer of secondary melt forming on the walls of amygdules and gaseous channels in paralava as a result of contact with heated gases which are by-products of the combustion process.

1. Introduction

Karwowskiite, Ca₉(Fe²⁺_0.5□_0.5)Mg(PO₄)₇, a new mineral of the merrillite group, was discovered in the central part of a basalt-like diopside paralava of the Hatrurim Complex, Daba Siwaqa, Jordan. Large barringerite aggregates (up to 1 cm), five new phosphides, and five phosphates were found at the contact of this paralava with altered carbonate country rocks (Table 1). Among the latter was deynekoite, Ca₉□Fe³⁺(PO₄)₇, which also belongs to the merrillite group [1] (Table 2). Keplerite, Ca₉(Ca_0.5□_0.5)Mg(PO₄)₇, another mineral of the merrilllite group, was recently discovered in hematite-bearing diopside paralava from the “olive unit” of the Hatrurim Complex in the Negev Desert, Israel [2].

Karwowskiite belongs to the cerite supergroup and merrillite group and subgroup. The general formula of minerals of the cerite supergroup is as follows: A₉XM[TO₃(Ø)]₇W₃, where A = Ce, La, Ca, Sr, and Na; X = □ [vacancy], Ca, Na, and Fe²⁺; M = Mg, Fe²⁺, Fe³⁺, Al, and Mn; T = Si and P; Ø = O and OH; and W = □, OH, and F [3]. The known minerals of the merrillite group have a simpler formula, AXM(TO₄)₆(TO₃Ø), as the W site is absent in these minerals. In the merrillite subgroup, Ø = O, and in the whitlockite subgroup, Ø = (OH) (Table 2). Previously, it was held that minerals of the merrillite subgroup are exclusively cosmic minerals and found only in meteorites, and minerals of the whitlockite subgroup are present in terrestrial rocks. But during the last few years, minerals of the merrillite subgroup have been discovered in rocks of the Hatrurim Complex ([1,2,4]. These include solid solutions of merrillite, ferromerrillite, and matyhite and the recently discovered minerals keplerite, deynekoite, and karwowskiite, as well as a potentially new mineral with the formula Ca₈Y□Mg(PO₄)₇, a Mg analogue of Changesite-(Y), Ca₈Y□Fe²⁺(PO₄)₇ that was recently identified in lunar rocks [5].

Table 1. Minerals discovered in diopside paralava from Jordan.

	Mineral	Formula	References
		Contact Facies
1	Transjordanite	Ni2P	[6]
2	Zuktamrurite *	FeP2	[7]
3	Murashkoite *	FeP	[8]
4	Orishchinite	Ni2P	[9]
5	Nickolayite	FeMoP	[10]
6	Crocobelonite	CaFe3+(PO4)O	[11]
7	Moabite	NiFe3+(PO4)O	[12]
8	Yakubovichite	CaNi2Fe3+(PO4)3	[13]
9	Nazarchukite	Ca2NiFe3+2(PO4)4	[14]
10	Deynekoite	Ca9□Fe3+(PO4)7	[1]
		Sulfide Nodule
11	Ferrodimolybdenite	FeMo3+2S4	[15]
12	Karwowskiite	Ca9Mg(Fe2+0.5□0.5)(PO4)7	This study

* Minerals which were also found in Wadi Halamish, Hatrurim Basin, Israel.

Table 1. Minerals discovered in diopside paralava from Jordan.

	Mineral	Formula	References
		Contact Facies
1	Transjordanite	Ni2P	[6]
2	Zuktamrurite *	FeP2	[7]
3	Murashkoite *	FeP	[8]
4	Orishchinite	Ni2P	[9]
5	Nickolayite	FeMoP	[10]
6	Crocobelonite	CaFe3+(PO4)O	[11]
7	Moabite	NiFe3+(PO4)O	[12]
8	Yakubovichite	CaNi2Fe3+(PO4)3	[13]
9	Nazarchukite	Ca2NiFe3+2(PO4)4	[14]
10	Deynekoite	Ca9□Fe3+(PO4)7	[1]
		Sulfide Nodule
11	Ferrodimolybdenite	FeMo3+2S4	[15]
12	Karwowskiite	Ca9Mg(Fe2+0.5□0.5)(PO4)7	This study

* Minerals which were also found in Wadi Halamish, Hatrurim Basin, Israel.

Table 2. Site occupation for the merrillite group minerals, AXM(TO₄)₆(TO₃Ø).

	A	X	M	T2+T3TO4	T1TO3Ø	References
Merrillite Subgroup
Merrillite	Ca9	Na	Mg	(PO4)6	(PO4)	[16]
Ferromerrillite	Ca9	Na	Fe2+	(PO4)6	(PO4)	[17]
Keplerite	Ca9	Ca0.5□0.5	Mg	(PO4)6	(PO4)	[2]
Matyhite	Ca9	Ca0.5□0.5	Fe2+	(PO4)6	(PO4)	[18]
Deynekoite	Ca9	□	Fe3+	(PO4)6	(PO4)	[1]
Changesite-(Y)	Ca8Y	□	Fe2+	(PO4)6	(PO4)	[5]
Karwowskiite	Ca9	Fe2+0.5□0.5	Mg	(PO4)6	(PO4)	This study
Whitlockite Subgroup
Whitlockite	Ca9	□	Mg	(PO4)6	(PO3OH)	[19]
Strontiowhitlockite	Sr9	□	Mg	(PO4)6	(PO3OH)	[20]
Wopmayite	Ca9	□	Mn	(PO4)6	(PO3OH)	[21]
Hedegaardite	Ca7Na2	Ca	Mg	(PO4)6	(PO3OH)	[22]

Table 2. Site occupation for the merrillite group minerals, AXM(TO₄)₆(TO₃Ø).

	A	X	M	T2+T3TO4	T1TO3Ø	References
Merrillite Subgroup
Merrillite	Ca9	Na	Mg	(PO4)6	(PO4)	[16]
Ferromerrillite	Ca9	Na	Fe2+	(PO4)6	(PO4)	[17]
Keplerite	Ca9	Ca0.5□0.5	Mg	(PO4)6	(PO4)	[2]
Matyhite	Ca9	Ca0.5□0.5	Fe2+	(PO4)6	(PO4)	[18]
Deynekoite	Ca9	□	Fe3+	(PO4)6	(PO4)	[1]
Changesite-(Y)	Ca8Y	□	Fe2+	(PO4)6	(PO4)	[5]
Karwowskiite	Ca9	Fe2+0.5□0.5	Mg	(PO4)6	(PO4)	This study
Whitlockite Subgroup
Whitlockite	Ca9	□	Mg	(PO4)6	(PO3OH)	[19]
Strontiowhitlockite	Sr9	□	Mg	(PO4)6	(PO3OH)	[20]
Wopmayite	Ca9	□	Mn	(PO4)6	(PO3OH)	[21]
Hedegaardite	Ca7Na2	Ca	Mg	(PO4)6	(PO3OH)	[22]

In the article describing deynekoite [1], we discussed the recently approved classification of the cerite supergroup [3], in which the main problem was the definition of the end-member formulas. This problem has been solved in a new publication by Atencio et al. [23], but in the case of hedegaardite, approved by the CNMNC-IMA in 2015 [22], the formula (Ca₈Na)(Ca_0.5□_0.5)Mg(PO₄)₆ (PO₃OH) was given, which does not correspond to the end-member formula requirements, as it contains two sites with double-site occupation.

In this article we provide a description of a new mineral: karwowskiite. We consider the mechanism and conditions of its formation from reduced paralava, and we discuss the problem of iron valency in karwowskiite. Because of limited material, the iron valency could not be determined by direct methods. The mineral and its name (mineral symbol Krw) have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA), as IMA 2023-080 [24]. The name karwowskiite is given in honor of the renowned Polish mineralogist Prof. Łukasz Karwowski (1945–2022), who was the founder the Polish Meteoritic Society and its president for many years. Łukasz Karwowski discovered two new phosphate minerals in the Morasko meteorite: moraskoite, Na₂Mg(PO₄)F [25] and czochralskiite, Na₄Ca₃Mg(PO₄)₄ [26]. Type material is deposited in the mineralogical collection of the Fersman Mineralogical Museum, Leninskiy pr., 18/k. 2, 115162 Moscow, Russia, registration number: 6005/1.

2. Materials and Methods

Samples of paralava containing a sulfide nodule with karwowskiite and a sulfide-phosphide nodule with barringerite ~7 mm in diameter (see detailed description of nodule mineralogy in [27]) were collected in 2015 during fieldwork in the area of pyrometamorphic rocks of the Hatrurim Complex in the Daba Siwaqa, Jordan.

The investigation of mineral morphology and composition was performed using an Olympus optical microscope, a Quanta 250 scanning electron microscope (Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia, Sosnowiec, Poland), and an electron microprobe analyzer (Cameca SX100, Micro-Area Analysis Laboratory, Polish Geological Institute–National Research Institute, Warsaw, Poland). Chemical analyses were carried out in WDS mode (wavelength-dispersive X-ray spectroscopy, settings: 15 keV, 20 nA, and ~1 μm beam diameter) using the following lines and standards: NaKα–albite, CaKα–wollastonite, MgKα–diopside, AlKα and KKα–orthoclase, FeKα–Fe₂O₃, NiKα–pentlandite, SrLα–celestine, and PKα–fluorapatite. Other chemical elements were below the detection limit.

Single-crystal X-ray studies of karwowskiite crystals were carried out using a SuperNova diffractometer with a mirror monochromator (CuKα, λ = 1.54184 Å) and an Atlas CCD detector (Agilent Technologies) at the Institute of Physics, University of Silesia, Poland.

The structure of karwowskiite was refined using the SHELX-2019/2 program [28]. The crystal structure was refined starting from the atomic coordinates of keplerite, Ca₉Mg(Ca_0.5□_0.5)(PO₄)₇ [2].

The Raman spectrum of karwowskiite was recorded on a WITec alpha 300R confocal Raman microscope (Department of Earth Science, University of Silesia, Poland) equipped with an air-cooled solid laser (532 nm) and a CCD camera operating at −61 °C. An air Zeiss LD EC Epiplan-Neofluan DIC-100/0.75NA objective was used. Raman scattered light was focused onto a multi-mode fiber and monochromator with a 1800 mm⁻¹ grating. The power of the laser at the sample position was ~30 mW. Fifteen scans with an integration time of 3 s and a resolution of ~2 cm⁻¹ were collected and averaged. The spectrometer monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm⁻¹).

3. Occurrence and Results

Karwowskiite was discovered in a small quarry (prospecting for phosphorite deposits) at the Daba-Siwaqa complex within the Transjordan Plateau, Jordan (31°22′01″ N, 36°11′10″ E). This quarry is the type locality for the 12 new minerals (Table 1). All the minerals mentioned above, with the exception of ferrodimolybdenite and karwowskiite, were found in the thin contact zone (<1 cm) of paralava forming a body about 30 m in diameter within altered carbonate rock of the Muwaqqar Chalk Marl Formation, Jordan. The paralava in the central part of the body consisted of weakly altered basalt-like rock with uneven zonal distribution of porous fragments (Figure 1a). The paralava consisted of anorthite, diopside, wollastonite, tridymite, and a small amount of glass. Fluorapatite, titanite, and spinel of the magnesioferrite–magnetite–chromite series were accessory minerals of the paralava. Cristobalite growing on tridymite in the paralava showed a specific fish-scale-type cracking, indicating that the melt temperature was higher than 1400 °C [1]. The immediate contact of the paralava with altered sedimentary rocks was characterized by the presence of a large amount of iron phosphides, among which barringerite and murashkoite were predominant [29,30,31]. It was notable that the new phosphates described here (crocobelonite, moabite, yakubovichite, nazarchukite, and deynekoite), formed after the phosphides, contained only trivalent iron (Table 1).

Karwowskiite was not present in the rock groundmass, where fluorapatite was the only phosphate phase. Primarily, we found a phase close in composition to karwowskiite at the eastern selvage of the paralava body. The paralava selvage had heterogeneous color and contained a huge number of channels and empty space (Figure 1b–d). On and in the walls of channels covered by silicate glass, a mineral of the merrillite group was found (Figure 1c–e), the composition of which can be expressed using the following empirical formula: (Ca_8.98Na_0.02) _Σ9.00(Mg_0.52Fe³⁺_0.46Fe²⁺_0.02) _Σ1.00(□_0.65Fe²⁺_0.14Ca_0.10Na_0.07Sr_0.02K_0.02) _Σ1.00[(PO₄)_6.95(SiO₄)_0.05] (

Table 3. Chemical data (in wt.%) for karwowskiite-like mineral (1) and karwowskiite (2).

Wt.%	1			2
	n = 20	s.d.	Range	n = 10	s.d.	Range
Na2O	0.25	0.02	0.23–0.30	0.13	0.01	0.11–0.15
K2O	0.10	0.01	0.08–0.13	0.04	0.01	0.03–0.06
MgO	1.92	0.25	1.56–2.43	4.25	0.02	4.21–4.27
CaO	47.12	0.27	46.66–47.72	47.45	0.12	47.18–47.62
SrO	0.17	0.05	0.08–0.26	0.31	0.04	0.20–0.36
NiO	n.d.			0.19	0.07	0.11–0.33
FeO *				1.54	0.07	1.45–1.65
Fe2O3 ♣	4.61	0.00	4.00–5.40
SiO2	0.26	0.07	0.13–0.42	n.d.
P2O5	45.67	0.29	45.00–46.20	46.86	0.34	46.39–47.46
Total	100.10			100.77
Fe2O3 ♦	3.40
FeO ♥	1.09
Calculated on 28O
Ca	8.98			9.00
Na	0.02
A	9.00			9.00
Fe2+	0.14			0.23
Mg				0.12
Ca	0.10
Na	0.07			0.04
K	0.02			0.01
Sr	0.02			0.03
Ni				0.03
X	0.35			0.46
Mg	0.52			1.00
Fe3+	0.46
Fe2+	0.02
M	1.00			1.00
P5+	6.95			7.02
Si	0.05
T	7.00			7.02

* all Fe as FeO, ^♣ all Fe as Fe₂O₃, ^♦/♥ Fe³⁺/Fe²⁺ calculated on charge balance with normalized P+Si to 7 pfu.

, an. 1). This formula can be simplified in two ways: (1) to the karwowskiite formula—Ca₉Mg[Fe²⁺_0.5□_0.5](PO₄)₇, considering M²⁺ > M³⁺ at the M site and (2) to the deynekoite formula—Ca₉Fe³⁺□ (PO₄)₇, considering □ > ∑M at the X site. This non-uniqueness connected with the end-member definition was the reason we chose not to continue studying the structure of this mineral and engaged in a study of the relatively large karwowskiite crystals which were found on the periphery of the sulfide nodule from the central part of the paralava body (Figure 2).

The mineralogy of this sulfide nodule has previously been described in detail [27]. The troilite and pentlandite parts of the sulfide nodule were differentiated and contained inclusions of tetrataenite, nickelphosphide, molybdenite, rudashevskyite, and galena, as well as rare grains of a new mineral, ferrodimolybdenite (Figure 2b–e).

Found on the boundary of the sulfide nodule and the paralava, the karwowskiite formed crystals up to 100 μm in size (Figure 2e and Figure 3). It crystallized on the walls of the amygdule, which was filled with sulfide melt. Usually, karwowskiite crystals are represented by flattened pinacoid trigonal crystals, the lateral faces of which feature rhombohedra and hexagonal prisms (Figure 2e). The pinacoid faces are, as a rule, subparallel to the surface of the gaseous channel or blow hole (Figure 2e and Figure 3).

Karwowskiite crystals have a vitreous luster and are practically colorless, although a greenish tint is sometimes observed. The streak is white. The average measured microhardness was VHN₂₅ = 365 (12), and the range of measurements was 322–384 (in kg/mm²), corresponding to 4 on the Mohs hardness scale. Cleavage is not observed and fracture is conchoidal. The density calculated on the basis of the structural data and the empirical formula is 3.085 g/cm³. Karwowskiite is uniaxial (−): ω = 1.638 (3), ε = 1.622 (3) (λ = 589 nm), and pleochroism is not observed.

The studied crystals of karwowskiite have a homogeneous composition (Table 3, an. 2), which is described by the empirical formula: Ca_9.00Mg_1.00(□_0.54Fe²⁺_0.23Mg_0.12 Na_0.04 Sr_0.03Ni_0.03K_0.01) _Σ1.00(PO₄)_7.02. This can be simplified to Ca₉Mg[(Fe²⁺, Mg, Na, Ni, Sr)_0.5□_0.5](PO₄)₇, and then to the end-member formula Ca₉Mg(Fe²⁺_0.5□_0.5)(PO₄)₇.

4. Raman Spectroscopy

Most of the bands in the Raman spectrum of karwowskiite (Figure 4) are related to vibrations in (PO₄)³⁻ groups: bands in the range 1200–1000 cm⁻¹ are related to the vibrations of ν₃(PO₄)³⁻; 1000–900 cm⁻¹, ν₁(PO₄)³⁻; 550–630 cm⁻¹, ν₄(PO₄)³⁻; and 400–480 cm⁻¹, ν₂(PO₄)^3-. Bands lower than 300 cm⁻¹ are ascribed to Ca-O and the lattice vibrations. These bands are also noted in the spectra of merrillite and keplerite [2]. Bands were absent in the range of the fundamental stretching vibration of the OH groups in the interval 2800–3700 cm⁻¹. The absence of bands at about 930–935 cm⁻¹ provides further evidence of the absence of OH groups in the structure. The band at 934 cm⁻¹ is related to P–OH vibrations in the PO₃OH group and is observed in the spectra of whitlockite [32] and deynekoite, which contain OH groups [1].

5. Structure of Karwowskiite

Single-crystal X-ray diffraction data were collected using a small fragment of karwowskiite crystal (20 × 20 × 20 μm³) by means of a SuperNova diffractometer. The experimental details and refinement data are summarized in

Table 4. Crystal data and structure refinement details for karwowskiite.

Crystal Data
Formula from refinement	Ca8.94Fe0.37Mg1.00O28P7
Crystal system	Trigonal
Space group	R-3c (no. 161)
Unit-cell dimensions	a = 10.3375 (2) Å
	c = 37.1443 (9) Å
	V = 3437.60 (17) Å3
Z	6
Crystal size	0.02×0.02×0.02 mm
Data collection
Diffractometer	SuperNova with Atlas CCD
Radiation wavelength	CuKa, λ = 1.54184 Å
Min. and max. theta	5.46°, 73.64°
Reflection ranges	−12 ≤ h ≤ 12; −10 ≤ k ≤ 12; −34≤ l ≤ 46
Refinement of structure
Reflection measured	7594
No. of unique reflections	1390
No. of observed unique refl. [I > 3σ(I)]	1366
Refined parameters	152
Rint	0.028
R1/Rall	0.0212/0.0217
wR	0.054
Goof	1.086
Δρmin [e Å−3]	−0.605
Δρmax [e Å−3]	0.409

,

Table 5. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų) for karwowskiite.

Site	Atom	x	y	z	Uiso*/Ueq	Occ.
P2	P	0.68604 (13)	0.86098 (13)	0.13393 (6)	0.0089 (3)	1
P3	P	0.65503 (13)	0.84760 (12)	0.03031 (6)	0.0080 (2)	1
Mg	Mg	0	0	−0.00051 (9)	0.0063 (8)	1.014 (13)
O3	O	0.7362 (4)	0.9169 (4)	0.17241 (10)	0.0205 (7)	1
O4	O	0.7574 (4)	0.7727 (4)	0.12004 (11)	0.0202 (7)	1
O5	O	0.7230 (3)	−0.0001 (3)	0.11101 (9)	0.0109 (6)	1
O6	O	0.5142 (3)	0.7593 (3)	0.13104 (11)	0.0120 (7)	1
O7	O	0.6032 (3)	0.9560 (3)	0.04278 (10)	0.0135 (6)	1
O8	O	0.5815 (3)	0.6993 (4)	0.05078 (11)	0.0161 (7)	1
O9	O	0.8257 (4)	0.9231 (3)	0.03630 (10)	0.0107 (6)	1
O10	O	0.6176 (4)	0.8153 (3)	0.99015 (11)	0.0151 (7)	1
P1A	P	0	0	0.26504 (7)	0.0096 (7)	0.825 (6)
P1B	P	0	0	0.2465 (4)	0.0096 (7)	0.175 (6)
O1A	O	0	0	0.3066 (2)	0.0170 (17)	0.825 (6)
O1B	O	0	0	0.2047 (4)	0.0170 (17)	0.175 (6)
O2	O	0.0079 (4)	0.8634 (3)	0.25324 (12)	0.0251 (8)	1
Ca1	Ca	0.72526 (9)	0.85692 (9)	0.43171 (6)	0.0085 (3)	0.985 (6)
Ca2	Ca	0.61681 (10)	0.82312 (9)	0.23007 (5)	0.0099 (3)	1.000 (5)
Ca3	Ca	0.12729 (10)	0.27541 (13)	0.32448 (6)	0.0198 (4)	0.997 (6)
Fe1	Fe	0	0	0.1862 (2)	0.011 (3)	0.245 (11)
Fe2	Fe	0	0	0.1611 (17)	0.083 (16)	0.093 (13)

,

Table 6. Atomic displacement parameters (Ų) for karwowskiite.

	U11	U22	U33	U23	U13	U12
P2	0.0087 (5)	0.0099 (5)	0.0090 (6)	0.0010 (5)	0.0003 (4)	0.0054 (4)
P3	0.0072 (5)	0.0083 (5)	0.0080 (6)	−0.0003 (5)	−0.0006 (4)	0.0035 (4)
Mg	0.0058 (9)	0.0058 (9)	0.0074 (13)	0	0	0.0029 (4)
O3	0.0305 (18)	0.0240 (18)	0.0110 (16)	−0.0003 (14)	−0.0024 (14)	0.0167 (15)
O4	0.0253 (16)	0.0278 (18)	0.0203 (17)	0.0064 (14)	0.0067 (13)	0.0228 (15)
O5	0.0111 (13)	0.0099 (13)	0.0098 (13)	0.0016 (10)	0.0012 (11)	0.0037 (11)
O6	0.0103 (15)	0.0078 (13)	0.0178 (16)	0.0023 (11)	0.0044 (13)	0.0044 (12)
O7	0.0134 (14)	0.0196 (15)	0.0136 (15)	−0.0036 (13)	−0.0008 (12)	0.0128 (12)
O8	0.0170 (14)	0.0127 (16)	0.0139 (15)	0.0039 (12)	0.0030 (12)	0.0039 (12)
O9	0.0077 (14)	0.0120 (14)	0.0127 (16)	−0.0002 (11)	0.0011 (12)	0.0050 (12)
O10	0.0230 (18)	0.0186 (15)	0.0063 (17)	−0.0011 (12)	−0.0026 (12)	0.0124 (13)
P1A	0.0068 (6)	0.0068 (6)	0.015 (2)	0	0	0.0034 (3)
P1B	0.0068 (6)	0.0068 (6)	0.015 (2)	0	0	0.0034 (3)
O1A	0.018 (2)	0.018 (2)	0.014 (4)	0	0	0.0091 (11)
O1B	0.018 (2)	0.018 (2)	0.014 (4)	0	0	0.0091 (11)
O2	0.0198 (15)	0.0102 (15)	0.046 (2)	0.0042 (14)	0.0159 (15)	0.0081 (13)
Ca1	0.0088 (5)	0.0088 (4)	0.0087 (5)	−0.0015 (3)	−0.0008 (3)	0.0051 (3)
Ca2	0.0110 (5)	0.0084 (4)	0.0101 (5)	−0.0003 (4)	−0.0025 (3)	0.0046 (3)
Ca3	0.0146 (5)	0.0353 (6)	0.0151 (6)	0.0087 (4)	0.0017 (4)	0.0167 (4)
Fe1	0.010 (2)	0.010 (2)	0.012 (5)	0	0	0.0048 (11)
Fe2	0.083 (19)	0.083 (19)	0.08 (4)	0	0	0.041 (9)

and

Table 7. Selected bond lengths (Å) and BVS * (bond valence sum) calculation for karwowskiite.

Atom	-Atom	Distance		Atom	-Atom	Distance
P2	O4	1.522 (3)		Ca1	O5	2.467 (3)
	O3	1.532 (4)			O6	2.469 (3)
	O5	1.545 (3)			O7	2.472 (3)
	O6	1.551 (3)			O6	2.497 (3)
	Mean/BVS	1.538/4.96			O4	2.826 (4)
P3	O8	1.530 (3)			Mean/BVS	2.482/2.11
	O7	1.536 (3)		P1A	O2	1.519 (3)	×3
	O10	1.535 (4)			O1A	1.545 (8)
	O9	1.547 (3)			Mean/BVS	1.525/5.12
	Mean/BVS	1.537/4.97		P1B	O2	1.476 (4)	×3
Mg	O9	2.078 (4)	×3		O1B	1.5500 (15)
	O6	2.079 (4)	×3		Mean/BVS	1.495/5.59
	Mean/BVS	2.078/2.11		Ca3	O2	2.996 (5)
O2	Ca2	2.360 (3)	×2		O10	2.471 (3)
O5	Ca2	2.375 (3)			O10	2.545 (3)
O9	Ca2	2.427 (3)			O8	2.646 (4)
O9	Ca2	2.450 (3)			O4	2.489 (3)
O4	Ca2	2.464 (4)			O7	2.379 (3)
O7	Ca2	2.623 (3)			O5	2.413 (3)
O8	Ca2	2.666 (3)			O3	2.586 (4)
	Mean/BVS	2.466/2.17			O1A	2.556 (3)
Ca1	O8	2.312 (3)			Mean/BVS	2.565/1.95
	O10	2.375 (4)		Fe1	O1B	0.689 (18)
	O2	2.439 (3)		Fe2	O1B	1.62 (7)

* Bond valence parameters from [33,34].

. The structure of karwowskiite is shown in Figure 5. The Ca1, Ca2, Ca3, Mg, P2, and P3 cations and O2-O10 anion sites are fully occupied and take part in the formation of intercalated layers of two types (Figure 5a,b). In the structure of karwowskiite, in the [001] direction, there are channels on the threefold axis containing the disordered positions P1, O1, and Fe (Figure 5c,d). A disorder in the ^P1PO₄ tetrahedron was observed, where the P1 and O1 sites split into two parts forming Two atom groups: P1a+O1a and P1b+O1b

(Figure 6a). The occupancy of ^P1PO₄ was constrained to 1 and the occupancies of disordered atoms were refined as part 1 (82.5%) vs. part 2 (17.5%). The Fe site is split into Fe1 and Fe2 sites with low occupation (Figure 6b). The karwowskiite formula derived from refinement, ^Ca1Ca_2.96^Ca2Ca3^Ca3Ca_5.99^Mg1Mg_1.014 (^Fe1Fe_0.245^Fe2Fe_0.093) _Σ0.335(^P2PO₄)₃(^P3PO₄)₃(^P1APO₄)_0.825(^P1BPO₄)_0.175 = Ca_8.95Mg_1.01Fe_0.335(PO₄)₇, is close to the empirical chemical formula Ca_9.00Mg_1.00(□_0.54Fe²⁺_0.23Mg_0.12Na_0.04Sr_0.03 Ni_0.03K_0.01) _Σ1.00(PO₄)_7.02. The main difference is the electron density of the Fe site (=Fe1 + Fe2): 8.79e- (structural data) and 10.03e- (microprobe analysis). This difference can be explained by the influence of heavy-element impurities at the Fe site. A small amount of 0.03Sr + 0.03Ni = 1.98e- in the empirical formula significantly increased the calculated electron density of this site.

Occupation of the Fe1 and Fe2 sites cannot be simultaneous and can take place only if the P1B site is vacant (Figure 6b). The Fe1 site is above the O3-O3-O3 plane and corresponds to the Na site in merrillite and the Ca site in keplerite (X site, z = 0.19). At this site, the cation occupies an asymmetric position in a truncated trigonal prism with the distances Fe1-O3 = 2.47 (1) Å × 3 and Fe1-O2 = 2.88 (1) Å × 3 (Figure 6b). The Fe2 site is below the O3-O3-O3 plane (z = 0.16) in a distorted prism: Fe2-O3 = 2.45 (1) Å × 3, Fe2-O4 = 2.87 (1) Å × 3 (Figure 6b). These distances are longer than typical distances Fe²⁺-O ≈ 2.15 Å. In keplerite, with the empirical formula Ca_9.00(Ca_0.33Fe²⁺_0.20□_0.47) _{Σ 1.00}Mg_1.04P_6.97O₂₈, the distances X1-O3 = 2.44 (1) Å × 3 and X1-O2 = 2.79 (1) Å × 3 [2]. Also, by analogy with synthetic-phase Ca₉Cu_1.5(PO₄)₇ [35], we can assume that the Fe1 and Fe2 sites can shift to the side of the site with three oxygens, O3. The cations at the low-occupied sites Fe1 and Fe2 are probably dynamically disordered.

As karwowskiite occurs only in tiny amounts, powder X-ray diffraction data were not collected. The properties of the mineral could be calculated more reliably from the results of single-crystal structure refinements. The calculated data are listed in Supplementary Table S1. Karwowskiite has unit cell parameters and refractive indexes close to the parameters of other minerals of the merrillite subgroup (Supplementary Table S2).

6. Discussion

6.1. Problem of Iron Oxidation State

The problem concerning the iron oxidation state is a key question for karwowskiite, as at its low iron content (~1.5 wt.% FeO), two variants of the crystal chemical formula calculation (with Fe²⁺ or Fe³⁺ on 28O) give similar results: (1) Ca_9.00Mg_1.00(□_0.54Fe²⁺_0.23Mg_0.12Na_0.04 Sr_0.03Ni_0.03K_0.01) _Σ1.00(PO₄)_7.02 and (2) (Ca_8.97Na_0.03) _Σ9.00Mg_1.00(□_0.56Fe³⁺_0.23Mg_0.12Na_0.02Sr_0.03Ni_0.03 K_0.01) _{Σ 1.00}(PO₄)₇. These formulas lead to the end-member formulas: (1) Ca₉Mg(□_0.5Fe²⁺_0.5)(PO₄)₇ and (2) Ca₉Mg(□_0.667 Fe³⁺_0.333)(PO₄)₇. Each of these formulas corresponds to a potentially new mineral of the merrillite subgroup.

Only a few karwowskiite grains were detected in the boundary paralava and sulfide nodule, which also contained ferrodimolybdenite and nickelphosphide (Figure 2d,e and Figure 3). Because of the limited amount of material, we could not perform a direct study of the valence of iron in the mineral. We consider that iron in karwowskiite is mainly Fe²⁺ because:

• Karwowskiite intimately associates with minerals representing highly reduced mineral association: nickelphosphide, tetrataenite, troilite, pentlandite, rudashevskyite, and ferrodimolybdenite (FeMo₂S₄, the first known mineral with Mo³⁺) (Figure 2d) [15]. Minerals containing Fe³⁺ are absent in the association with karwowskiite.
• Karwowskiite contains troilite and pentlandite inclusions and has an equilibrium boundary (sometimes rounded) with the main minerals in the nodule, pentlandite and troilite (Figure 2e and Figure 3).
• Fe³⁺ can occupy the M octahedron in deynekoite [1]. Na is accommodated in the large X polyhedron in the channel of the merrillite structure. In keplerite, the M polyhedron has incomplete occupation and contains a significant amount of Fe²⁺—^X(Ca_0.33Fe²⁺_0.20□_0.47) [2]. We consider that the larger cation Fe²⁺ is more likely to occupy the X (=Fe1 + Fe2) site of karwowskiite than the smaller Fe³⁺.

6.2. Mechanism of the Genesis of Karwowskiite

Karwowskiite was found in paralava, and its crystals were confined to thin layers enriched with glass on the walls of gaseous channels and blow holes (Figure 1c,d and Figure 2d). At the selvage of the paralava body there were many crystals with intermediate composition (≈50/50) belonging to karwowskiite–deynekoite solid solution. Here, karwowskiite associated with fluorapatite (Figure 7a) and very rarely with phosphides (Figure 7b). Karwowskiite crystals were often covered by a thin film of glass, and the form of the crystals was underlined by the distribution of micron-sized fluorapatite crystals (Figure 7c,d). Karwowskiite from the central part of the paralava body, which did not contain trivalent iron (Table 3), also formed crystals on the walls of an amygdule in which tridymite and glass were identified, as well as an increased amount of fluorapatite (Figure 2e and Figure 3). The amygdule was filled with sulfides (Figure 2b,c).

The mechanism of karwowskiite formation may be presented as follows: The growth of karwowskiite crystals took place in the thin film of melt on the walls of gaseous channels and bowl holes. The existence of melted film on the walls of the empty spaces in the partially crystallized paralava was sustained by the flow of heated gases, which transported phosphorus and fluorine. In rare cases, the empty spaces, on the walls of which karwowskiite formed, were filled with sulfide melt, which formed locally in inhomogeneous protolith. A similar mechanism of crystal formation in paralava on the walls of gaseous channels within a thin film of melt has already been suggested for the genesis of flattened uvarovite crystals from gehlenite–wollastonite paralava of the Hatrurim Complex from the Negev Desert, Israel [36].

β-Ca₃(PO₄)₂ is the archetype of the merrillite structure. Experimental data show that β-Ca₃(PO₄)₂ is stable below 1125 °C [37]. The unit cell of this phase contains Ca¹₁₈Ca²₁₈Ca³₁₈Ca⁴₆Ca⁵₆(P¹O₄)₆(P²O₄)₁₈(P³O₄)₁₈ (R-3c), a = 10.4352 (2), c = 37.4029 (5) Å; [38], and the simplified formula based on the fact that the Ca⁴ site has half occupation is: Ca₆₃(PO₄)₄₂ → Ca₃(PO₄)₂ (Z = 21). The formula β-Ca₃(PO₄)₂ can be presented by analogy with merrillite as Ca₉Ca_0.5□_0.5Ca(PO₄)₇ (Z = 6). All the minerals of the merrillite subgroup discovered in rocks of the Hatrurim Complex—keplerite, deynekoite, and karwowskiite [1,2]—crystallized at temperatures below 1100 °C from melt saturated with phosphorus as a result of gaseous transport. A sedimentary protolith, containing phosphorite inclusions and bituminous substance, played the role of fuel in the combustion process and served as a source of a reducing agent (a graphite-like substance). It was also a source of phosphorus for the phosphates and phosphides found in Jordan paralava [27]. As a result of carbothermal reactions concomitant with the combustion process, phosphorus was reduced [30], which was conducive to its transfer by gaseous phases and determined the formation of karwowskiite on the walls of empty spaces in paralava.