Walstromite, BaCa 2 (Si 3 O 9 ), from Rankinite Paralava within Gehlenite Hornfels of the Hatrurim Basin, Negev Desert, Israel

: Walstromite, BaCa 2 Si 3 O 9 , known only from metamorphic rocks of North America, was found in small veins of unusual rankinite paralava within gehlenite hornfelses of the Hatrurim Complex, Israel. It was detected at two localities—Gurim Anticline and Zuk Tamrur, Hatrurim Basin, Negev Desert. The structure of Israeli walstromite [with P 1 space group and cell parameters a = 6.74874(10) Å, b = 9.62922(11) Å, c = 6.69994(12) Å, α = 69.6585(13) ◦ , β = 102.3446(14) ◦ , γ = 96.8782(11) ◦ , Z = 2, V = 398.314(11) Å 3 ] is analogous to the structure of walstromite from type locality—Rush Creek, eastern Fresno County, California, USA. The Raman spectra of all tree minerals exhibit bands related to stretching symmetric vibrations of Si-O-Si at 650–660 cm − 1 and Si-O at 960–990 cm − 1 in three-membered rings (Si 3 O 9 ) 6 − . This new genetic pyrometamorphic type of walstromite forms out of the di ﬀ erentiated melt portions enriched in Ba, V, S, P, U, K, Na, Ti and F, a residuum after crystallization of rock-forming minerals of the paralava (rankinite, gehlenite-åkermanite-alumoåkermanite, schorlomite-andradite series and wollastonite). Walstromite associates with other Ba-minerals, also products of the residual melt crystallization as zadovite, BaCa 6 [(SiO 4 )(PO 4 )](PO 4 ) 2 F and gurimite, Ba 3 (VO 4 ) 2 . The genesis of unusual barium mineralization in rankinite paralava is discussed. Walstromite is isostructural with minerals—margarosanite, BaCa 2 Si 3 O 9 and breyite, CaCa 2 (Si 3 O 9 ), discovered in 2018. Raman spectra of walstromite and associated minerals were recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Earth Science, of Natural of equipped with an air-cooled solid-state laser (488 nm) and a charge-coupled device (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 µ m. An air Zeiss LD EC Epiplan-Neoﬂuan DIC-100 / 0.75NA objective (Carl Zeiss AG, Jena, Germany) was used. Raman scattered light was focused on a broadband single-mode ﬁber with an e ﬀ ective pinhole size of about 30 µ m 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 3 s with an accumulation of 20 scans and a resolution 3 cm − 1 were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate cm − 1 ). Spectra processing, such as baseline correction and smoothing, was performed using the SpectraCalc software package Bands ﬁtting was performed using a Gauss-Lorentz cross-product function, with a minimum number of component bands used for the ﬁtting process.

In this paper, we report the data on a new genetic type of walstromite and mineral assemblages and the composition of associated minerals. The genesis of unusual barium mineralization in rankinite paralava is discussed.

Walstromite from the North American Localities
Walstromite was first discovered in sanbornite-bearing metamorphic rocks from the Big Creek, eastern Fresno County, California, USA [1]. Its prismatic 0.2-1.2 cm long crystals have been found commonly as clots and layers of interlocking masses. Some isolated crystals were retrieved from the quartz-rich zones in sanbornite-quartz rocks. Walstromite was found at the few places along the western margin of the North American continent spread from Baja California Norte, Mexico in the south to the Brooks Range, Alaska in the north. To the south group localities belong-El Rosario and La Madrelena claim, Baja California Norte, Mexico; and in California, the Baumann Prospect, Tulare

Walstromite from the North American Localities
Walstromite was first discovered in sanbornite-bearing metamorphic rocks from the Big Creek, eastern Fresno County, California, USA [1]. Its prismatic 0.2-1.2 cm long crystals have been found commonly as clots and layers of interlocking masses. Some isolated crystals were retrieved from the quartz-rich zones in sanbornite-quartz rocks. Walstromite was found at the few places along the western margin of the North American continent spread from Baja California Norte, Mexico in the south to the Brooks Range, Alaska in the north. To the south group localities belong-El Rosario and La Madrelena claim, Baja California Norte, Mexico; and in California, the Baumann Prospect, Tulare County; Trumbull Peak, Incline, Mariposa County; and four claims in eastern Fresno County, USA [2][3][4]. Here, the host rocks are metasediments, formed at a contact of granite and Mesozoic sedimentary rocks. In these localities, walstromite is confined to quartz-sanbornite veins containing a number of common and rare Ba-minerals [2]. In the northern USA, at Gun claim locality and Yukon Territory, Canada, walstromite occurs in contact metasomatic rocks, formed on the contact of Paleozoic limestone and porphyric monzonite stock [2,5,6].

Hatrurim Complex
This complex is built of high-temperature rocks (sanidinite facies) and products of its low-temperature alteration. Spurrite marbles, larnite rocks and a few varieties of paralava are the most common types of pyrometamorphic rocks of the Hatrurim Complex [7][8][9]21,22]. Up today, the genesis of these rocks is still debated [23][24][25]. There is a generally recognized fact, that carbonate protolith of the Hatrurim Complex was subjected to combustion processes [7][8][9]21]. Hence, two main hypotheses about the genesis of the Hatrurim Complex are currently considered. The first one assumes the burning of organic matters in the bituminous chalk of the Ghareb Formation. As a supporting evidence, can be considered the work of Picard [26] and Minster [27], which indicates that the mean content of organic carbon in the Ghareb Formation rocks is about 15 wt.% in Negev localities [28,29]. The second hypothesis suggests a "mud-volcanic" activity, causing high-temperature pyrometamorphic alteration of primary rocks as a result of methane fire exhaling from tectonic zones of the Dead Sea rift [9,30]. A spontaneous burning of hydrocarbons at the surface is a well-known phenomenon associated with mud volcanism [10,13,30,31].
The occurrence of paralava in the pyrometamorphic rocks of the Hatrurim Complex suggests that combustion processes had to be locally very intense causing partial or bulk melting of the rocks. Pseudowollastonite in some samples of paralava indicates that the temperature could have reached over 1125 • C [32].

Specific Aspects of Rankinite Paralava
The coarse-grained veins with Ba mineralization occurring within gehlenite hornfelses are classified by Sharygin et al. (2008) [33] as paralava. The main rock-forming minerals of this paralava, apart from rankinite, are wollastonite or pseudowollastonite and minerals of the gehlenite-åkermanite-alumoåkermanite, schorlomite-andradite and fluorapatite-fluorellestadite series. This type of rock has been found at the Negev Desert (Hatrurim Basin, Israel) and Judean Mountains (Nabi Musa, Palestinian Autonomy) [10,11]. Contrary to the traditional definition of paralava, this paralava from the Hatrurim Basin is characterized by the absence of glass and it is fully crystallized rock ( Figure 2). The size of some schorlomite-andradite series garnet crystals is up to 1.5 cm in size. These rocks look much more like pegmatite-veins. A growth of large gehlenite, garnet, wollastonite and rankinite crystals elongated sub-perpendicular to the vein walls is common ( Figure 2).
Generally, the mineral composition of host gehlenite hornfelses and rankinite paralava is similar. In both types of rocks, the main minerals are Ti-bearing andradite, gehlenite, fluorapatite and accessory magnesiochromite. The hornfelses contain more larnite (flamite), whereas irregularly distributed wollastonite and rankinite are predominant in the paralava ( Figure 2). In some cases, rankinite paralava contains both wollastonite and pseudowollastonite [32]. A distinctive feature of this type of paralava is a presence of small aggregates (enclaves) up to 1-2 mm in size enriched in Ba, Ti, P, V, U. The enclaves are composed of rare and recently discovered new minerals, for example, barioferrite, zadovite, aradite, gurimite, vorlanite [10,11,34]. Kalsilite and cuspidine are included in this mineral assemblage. In paralava, commonly garnet and rarely gehlenite and kalsilite crystals host dendritic flamite inclusions, which are interpreted as eutectic intergrowths [35].
Raman spectra of walstromite and associated minerals were recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Earth Science, Faculty of Natural Sciences, University of Silesia, Sosnowiec, Poland) equipped with an air-cooled solid-state laser (488 nm) and a charge-coupled device (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 (Carl Zeiss AG, Jena, Germany) was used. Raman scattered light was focused on a broadband 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 3 s with an accumulation of 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 ). Spectra processing, such as baseline correction and smoothing, was performed using the SpectraCalc software package GRAMS (Galactic Industries Corporation, Salem, NH, USA). Bands fitting was performed using a Gauss-Lorentz cross-product function, with a minimum number of component bands used for the fitting process.
Single-crystal X-ray studies of walstromite were carried out with synchrotron radiation, λ = 0.70849 Å. Diffraction experiments at ambient conditions were performed at the X06DA beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). The beamline is equipped
Raman spectra of walstromite and associated minerals were recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Earth Science, Faculty of Natural Sciences, University of Silesia, Sosnowiec, Poland) equipped with an air-cooled solid-state laser (488 nm) and a charge-coupled device (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 (Carl Zeiss AG, Jena, Germany) was used. Raman scattered light was focused on a broadband 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 3 s with an accumulation of 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 ). Spectra processing, such as baseline correction and smoothing, was performed using the SpectraCalc software package GRAMS (Galactic Industries Corporation, Salem, NH, USA). Bands fitting was performed using a Gauss-Lorentz cross-product function, with a minimum number of component bands used for the fitting process.
Single-crystal X-ray studies of walstromite were carried out with synchrotron radiation, λ = 0.70849 Å. Diffraction experiments at ambient conditions were performed at the X06DA beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). The beamline is equipped with a multi-axis goniometer PRIGo [36] and a PILATUS 2M-F detector. The detector was placed 90 mm from the sample, with a vertical offset of 60 mm. For experiments DA+ acquisition software was used [37]. Determination of lattice parameters was done using CrysAlisPro [38]. The crystal structure refinement was performed using the program SHELX-97 [39] implemented in the WinGX software package [40] to an agreement index R1 = 1.86%. As a starting model we used the structure of walstromite reported by [41], ICSD 24426. Further details of data collection and crystal structure refinement are reported in Table 1. Atom coordinates (x, y, z) and equivalent isotropic displacement parameters (Table 2), as well as, anisotropic displacement parameters (Table 3) and selected interatomic distances are shown (Table 4). Table 1. Crystal data and structure refinement for walstromite.

Walstromite
Crystal system triclinic Unit cell dimensions

Raman Spectroscopy
The Raman spectrum of walstromite from Israel is analogous to the spectrum of walstromite from Big Creek deposit, Fresno County, California [45] and exhibits common features with the spectra of isostructural margarosanite and breyite and also pseudowollastonite ( Figure 4).
The main bands in the Raman spectra of walstromite, margarosanite, breyite and pseudowollastonite are related to vibrations of the (Si 3 O 9 ) 6− three-membered rings [46][47][48][49]. The band from Si-O-Si symmetric stretching vibrations in minerals of the margarosanite group are about 650-660 cm −1 , whereas in pseudowollastonite-at 580 cm −1 (Figure 4). That is connected with the distinct value of the Si-O-Si angle in (Si 3 O 9 ) 6− rings, extending from of 121.2-125.6 • in walstromite, 120.2-123.3 • in margarosanite, 123-123.9 • in breyite to 134. .65 • in pseudowollastonite [13,32,50]. Bands from symmetric stretching vibrations of Si-O (apical oxygen) vibrations in all these minerals is roughly at the same position, in the interval 965-988 cm −1 (Figure 4). In the Raman spectrum of margarosanite taken from RRUFF database [51] this band has a relatively small intensity but then band about 1013 cm −1 , which is absent in the spectra of the other compared minerals, is the most intensive ( Figure 4). This band has an unclear nature and needs in further investigation.

Raman Spectroscopy
The Raman spectrum of walstromite from Israel is analogous to the spectrum of walstromite from Big Creek deposit, Fresno County, California [45] and exhibits common features with the spectra of isostructural margarosanite and breyite and also pseudowollastonite ( Figure 4).
The main bands in the Raman spectra of walstromite, margarosanite, breyite and pseudowollastonite are related to vibrations of the (Si3O9) 6− three-membered rings [46][47][48][49]. The band from Si-O-Si symmetric stretching vibrations in minerals of the margarosanite group are about 650-660 cm −1 , whereas in pseudowollastonite-at 580 cm −1 (Figure 4). That is connected with the distinct value of the Si-O-Si angle in (Si3O9) 6− rings, extending from of 121.2-125.6° in walstromite, 120.2-123.3° in margarosanite, 123-123.9° in breyite to 134. .65° in pseudowollastonite [13,32,50]. Bands from symmetric stretching vibrations of Si-O (apical oxygen) vibrations in all these minerals is roughly at the same position, in the interval 965-988 cm −1 (Figure 4). In the Raman spectrum of margarosanite taken from RRUFF database [51] this band has a relatively small intensity but then band about 1013 cm −1 , which is absent in the spectra of the other compared minerals, is the most intensive (
In the structure of walstromite, three-membered rings (Si 3 O 9 ) 6− intercalate with layers made of Ca1O 8 and Ca2O 6 polyhedra parallel to [101] (Figure 5A-D). The apices of the SiO 4 tetrahedra in neighboring rings are pointing in opposite directions (up or down). In addition, corrugated chains of edge-shared BaO 10 polyhedra are running through the structure along the axis.
The three-membered rings (Si 3 O 9 ) 6− , are formed by highly distorted SiO 4 tetrahedra with bond lengths between 1.576 (2)  In the Ca-layers, two Ca1O 8 and two Ca2O 6 polyhedra are sharing edges, building the Ca 4 O 20 blocks, which are further connected by shared edges to form a two-dimensional network ( Figure 5D). The atom Ca1 is 6 + 2 coordinated and forms antiprism with Ca-O distance in the range of 2.33-2.85 Å (Table 4). Ca2 coordinated by 6 oxygens exhibits deformed octahedra with a mean distance Ca-O = 2.38 Å ( Table 4). The chains of edge-sharing Ba-polyhedron, has 6 + 4 coordination with Ba1-O distance range of 2.563(2)-3.354(2) Å, with four bonds longer than 2.94 Å and mean distance of 2.943 Å (Table 4).
In the structure of walstromite, three-membered rings (Si3O9) 6− intercalate with layers made of Ca1O8 and Ca2O6 polyhedra parallel to [101] ( Figure 5A-D). The apices of the SiO4 tetrahedra in neighboring rings are pointing in opposite directions (up or down). In addition, corrugated chains of edge-shared BaO10 polyhedra are running through the structure along the axis.
A site is occupied by big two-valent cations-Ba (walstromite), Pb (margarosanite); B site is occupied by Ca. Breyite is an exception, both sites in which are occupied by Ca. Breyite, Ca 3 (Si 3 O 9 ), was described by Brenker from Ca-silicate inclusions trapped in a diamond coming from Juina, Brazil in 2018 [18]. It is a phase of high pressure, synthetic analogues of which were known before "wollastonite-II" or "Ca-walstromite" [15]. Margarosanite, Pb(Ca,Mn 2+ ) 2 (Si 3 O 9 ), was described by Ford and Bradley in 1916, from Franklin, Sussex County, NJ, USA [19]. Breyite is a wollastonite and pseudowollastonite polymorph. Pseudowollastonite, as well as minerals of the margarosanite group, has layered structure characterized by intercalation of layers formed by Ca-polyhedra with coordination 8 (deformed cubes) and tetrahedral layers formed by three-membered identically oriented (Si 3 O 9 ) 6− rings ( Figure 6E, F) [32]. In breyite as in walstromite and margarosanite tetrahedral layer is formed by (Si 3 O 9 ) 6− ring, which are alternately oriented in opposite sides ( Figure 6D) [49,50].
In the margarosanite group, the main distinctions in structure are observed for the coordination of A site, which is labelled as Ca2 in breyite [15]. In breyite Ca2 has coordination 6, at that site cation has an untypical position located in the plane of the deformed antiprism base ( Figure 7A). Bigger Pb in margarosanite has coordination 6 + 1 (the next nearest oxygen is located at the distance~3.5 Å) ( Figure 7B). The bigger cation Ba in walstromite has coordination 6 + 4 ( Figure 7C). Only in walstromite Ba polyhedra form columns along the c axis ( Figure 5A,B), whereas in breyite and margarosanite Ca2 and Pb polyhedra form dimers ( Figure 6A,C) [49,50].  [18]. It is a phase of high pressure, synthetic analogues of which were known before "wollastonite-II" or "Ca-walstromite" [15]. Margarosanite, Pb(Ca,Mn 2+ )2(Si3O9), was described by Ford and Bradley in 1916, from Franklin, Sussex County, NJ, USA [19]. Breyite is a wollastonite and pseudowollastonite polymorph. Pseudowollastonite, as well as minerals of the margarosanite group, has layered structure characterized by intercalation of layers formed by Ca-polyhedra with coordination 8 (deformed cubes) and tetrahedral layers formed by three-membered identically oriented (Si3O9) 6− rings ( Figure 6E, F) [32]. In breyite as in walstromite and margarosanite tetrahedral layer is formed by (Si3O9) 6− ring, which are alternately oriented in opposite sides ( Figure 6D) [49,50].
In the margarosanite group, the main distinctions in structure are observed for the coordination of A site, which is labelled as Ca2 in breyite [15]. In breyite Ca2 has coordination 6, at that site cation has an untypical position located in the plane of the deformed antiprism base ( Figure 7A). Bigger Pb in margarosanite has coordination 6 + 1 (the next nearest oxygen is located at the distance ~3.5 Å) ( Figure 7B). The bigger cation Ba in walstromite has coordination 6 + 4 ( Figure 7C). Only in walstromite Ba polyhedra form columns along the c axis ( Figure 5A,B), whereas in breyite and margarosanite Ca2 and Pb polyhedra form dimers ( Figure 6A,C) [49,50]. The genesis of unusual barium mineralization in rankinite paralava of the Hatrurim Basin was discussed by us before in the paper on gurimite and hexacelsian [11]. Basically, a genetic model of enclaves with Ba-mineralization formation sequence in rankinite paralavas can be described by the three stages: I stage-melt formation. Crystallization of gehlenite horfelses in the processes of pyrometamorphism is accompanied by the formation of a small amount of silicate melt. This silicate melt is translocated for a short distance and filled cracks in hornfelses.
II stage-crystallization of residual melt with the formation of rock-forming minerals Relatively quick crystallization of rock-forming minerals from melt begin from cracks walls on already existing crystal seeds (grains of early formed minerals of hornfelses) and comply with geometric selection during the growth, that leads to the formation of elongated crystals sub-perpendicular to the crack walls. Rock-forming minerals of paralava and hornfels are similar: andradite, gehlenite, wollastonite, rankinite, flamite-larnite, magnesioferrite and kalsilite. However, the size of the rock-forming minerals in paralava is 10-100 times bigger, than in hornfels.
III stage-formation of Ba-mineralization. Quick crystallization of rock-forming minerals of paralava leads to the formation of enclaves with residual melt portions. This melt became enriched in Ba, V, P, S, Ti, U, K, F and other incompatible with rock-forming minerals chemical elements. From these melt specific aggregates (enclaves) with Ba-bearing minerals form. The size of these aggregates does not usually exceed the first millimeters. There are differences in the mineral specialization of similar enclaves. For instance, The genesis of unusual barium mineralization in rankinite paralava of the Hatrurim Basin was discussed by us before in the paper on gurimite and hexacelsian [11]. Basically, a genetic model of enclaves with Ba-mineralization formation sequence in rankinite paralavas can be described by the three stages: I stage-melt formation. Crystallization of gehlenite horfelses in the processes of pyrometamorphism is accompanied by the formation of a small amount of silicate melt. This silicate melt is translocated for a short distance and filled cracks in hornfelses.
II stage-crystallization of residual melt with the formation of rock-forming minerals.
Relatively quick crystallization of rock-forming minerals from melt begin from cracks walls on already existing crystal seeds (grains of early formed minerals of hornfelses) and comply with geometric selection during the growth, that leads to the formation of elongated crystals sub-perpendicular to the crack walls. Rock-forming minerals of paralava and hornfels are similar: andradite, gehlenite, wollastonite, rankinite, flamite-larnite, magnesioferrite and kalsilite. However, the size of the rock-forming minerals in paralava is 10-100 times bigger, than in hornfels.
III stage-formation of Ba-mineralization. Quick crystallization of rock-forming minerals of paralava leads to the formation of enclaves with residual melt portions. This melt became enriched in Ba, V, P, S, Ti, U, K, F and other incompatible with rock-forming minerals chemical elements. From these melt specific aggregates (enclaves) with Ba-bearing minerals form. The size of these aggregates does not usually exceed the first millimeters. There are differences in the mineral specialization of similar enclaves. For instance, minerals of the zadovite-aradite series, walstromite and gurimite are not associated with barioferrite and perovskite (titanian specialization) [34].
Detection of pseudowollastonite in rankinite paralava can indicate that the temperature peak of rock formation is higher than 1100 • C [32]. We tested a big number of phases with composition CaSiO 3 from walstromite-bearing paralava from Zuk Tamrur and Gurim Anticline and did not identify neither the one pseudowollastonite crystal. Eutectic intergrowings of walstromite with kalsilite ( Figure 3C) and frequent findings of cuspidine in close contact with it point out its crystallization from the melt enriched in potassium (+sodium) and fluorine, which has an effect on the reduced temperature of walstromite crystallization. In metakaolin-waste glass geopolymers enriched in NaOH walstromite appears at a temperature about 600 • C [54].
We consider that the temperature of walstromite formation was significantly lower than 1000 • C.
Author Contributions: All authors wrote the paper, A.K., I.G., Y.V., E.G. collected samples for investigation during the field works, B.K. performed SC XRD analysis and interpreted structural data, A.K., I.G., E.G. studied thin-sections, investigated the chemical composition of minerals, performed Raman spectroscopy measurements. A.K. compilated all crystal-chemical data on walstromite and associated minerals. All authors have read and agreed to the published version of the manuscript.