Mineralogy of Silicate-Natrophosphate Immiscible Inclusion in Elga IIE Iron Meteorite †

Rare type of silicate inclusions found in the Elga iron meteorite (group IIE) has a very specific mineral composition and shows silicate (≈90%)–natrophosphate (≈10%) liquid immiscibility due to meniscus-like isolation of Na-Ca-Mg-Fe phosphates. The 3 mm wide immiscible inclusion has been first studied in detail using optical microscopy, scanning electron microscopy, electron microprobe analysis and Raman spectroscopy. The silicate part of the inclusion contains fine-grained quartz-feldspar aggregate and mafic minerals. The relationships of feldspars indicate solid decay of initially homogenous K-Na-feldspar into albite and K-feldspar with decreasing of temperature. Some mafic minerals in the silicate part are exotic in composition: the dominant phase is an obertiite-subgroup oxyamphibole (amphibole supergroup), varying from ferri-obertiite NaNa2Mg3FeTi[Si8O22]O2 to hypothetical NaNa2Mg3Fe0.5Ti1.5[Si8O22]O2; minor phases are the aenigmatite-subgroup mineral (sapphirine supergroup) with composition close to median value of the Na2Fe5TiSi6O18O2-Na2Mg5TiSi6O18O2 join, orthopyroxene (enstatite), clinopyroxene of the diopside Ca(Mg,Fe)Si2O6–kosmochlor NaCrSi2O6-Na(Mg,Fe)0.5Ti0.5Si2O6 series and chromite. The alteration phases are represented by Fe-dominant chlorite, goethite and hydrated Na2O-rich (2.3–3.3 wt.%) Fe-phosphate close to vivianite. Natrophosphate part consists of aggregate of three orthophosphates (brianite, czochralskiite, marićite) and minor Na-Cr-Ti-clinopyroxene, pentlandite, rarely taenite. Czochralskiite Na4Ca3Mg(PO4)4 is rich in FeO (2.3–5.1 wt.%) and MnO (0.4–1.5 wt.%). Brianite Na2CaMg(PO4)2 contains FeO (3.0–4.3 wt.%) and MnO (0.3–0.7 wt.%) and marićite NaFe(PO4) bears MnO (5.5–6.2 wt.%), MgO (5.3–6.2 wt.%) and CaO (0.5–1.5 wt.%). The contact between immiscible parts is decorated by enstatite zone in the silicate part and diopside–kosmochlor clinopyroxene zone in the natrophosphate ones. The mineralogy of the studied immiscible inclusion outlines three potentially new mineral species, which were first identified in meteorites: obertiite–related oxyamphibole NaNa2Mg3Fe0.5Ti1.5[Si8O22]O2, Mg-analog of aenigmatite Na2Mg5TiSi6O18O2 and Na-Ti-rich clinopyroxene Na(Mg,Fe)0.5Ti0.5Si2O6.

In addition to previous data, the numerous petrographic and mineralogical evidences about local impact melting in the meteorite (both in metal and in silicate inclusions) were outlined in recent publications [9,10,13,[18][19][20]. However, the pressure interval during the impact melting event in the Elga meteorite is still discussible. It is suggested that pressure may be up to [12][13][14][15] GPa (by the presence of tuite in silicate inclusions) and even up to 20 GPa (by the presence of the Fe 3 P-Fe 3 S solid solution in quenched Fe-Ni-P-S melt pockets) [13,20]. Unfortunately, the appearance of the above mineral phases in the Elga meteorite is questionable: identification of tuite was supported only by Raman spectroscopy [13], whereas the real existence and homogeneity of the proposed Fe 3 (P,S) phase were not proven [20].
The mineralogy and geochemistry of the meteorite and its features of impact metamorphism were described in detail . The mineralogical data for the Elga meteorite are summarized in Table 1. This publication provides a detailed description of the mineralogy for an Elga silicate inclusion with silicate-natrophosphate immiscibility. This is a new and rare type of silicate inclusions, which was recently found in the Elga meteorite and which drastically differs from other types in unique mineralogy [12,13,17].   The identification of all 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 IGM, Novosibirsk, Russia. The EDS analyses of minerals were operated at an accelerating voltage of 20 kV and a probe current of 1 nA in high-vacuum mode and at an accumulation time of 20-40 s. The synthetic compounds, pure metals and minerals were used as reference standards for the elements: SiO 2 (Si and O), Al 2 O 3 (Al), diopside (Mg and Ca), albite (Na), orthoclase (K), Ca 2 P 2 O 7 (P), BaF 2 (Ba and F), Cr 2 O 3 (Cr), CsRe 2 Cl 6 (Cl), LaPO 4 (La), CePO 4 (Ce), SrF 2 (Sr), metallic Ti, Fe, Mn, Zn, Ni, V and Cu. Matrix effects were corrected 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). The overlapping of TiKβ and VKα was specially checked using the TiO 2 and metallic V standards. The precision of the EDS analysis for major elements was better than 2% relative.
Electron microprobe analyses (EMPA) in wavelength-dispersive (WDS) mode were performed for all minerals of the Elga iron meteorite, using a JXA-8100 microprobe (Jeol Ltd., Tokyo, Japan) at the IGM. Grains (sizes > 5 µm) 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 10 (for silicates and phosphates) and 50 nA (metals, sulfides and phosphides), and counting time of 10 (5 + 5) s.
The Raman spectra were recorded on a LabRAM HR 800 mm (HORIBA Scientific Ltd., Kyoto, Japan) spectrometer equipped with a 1024 pixel LN/CCD detector and coupled to an Olympus BX40 confocal microscope (objective ×100) at the IGM. A semiconductor laser emitting at 514.5 nm with a nominal power output of 50 mW was used for excitation. In each case, 10-20 spectra were recorded for 10-20 s each at a hole diameter of 100-200 µm and integrated. Most spectra were recorded between 100 and 1200 cm −1 , and some spectra were made for the 100-4000 cm −1 and 3000-4000 cm −1 region. The monochromator was calibrated using the 520.7 cm −1 Raman line of elemental Si.
The main results are presented in Figures 1-15 and Tables 1-9. Some data are given in Supplementary section (Figures S1-S14).

Description of Studied Cut-off from the Elga Meteorite
The polished cut-off (35 × 18 × 2 mm, Figure 2) was used for detailed studies by optical and scanning electron microscopy, EMPA and Raman spectroscopy. The sample contains 12 silicate inclusions of different composition (Elga-1, Elga-3-Elga-12a, Elga-12c) as well as areas with impact melting (Elga-12b, Elga-13) and zone of surface oxidation of the meteorite (Elga-2). The phase composition of silicate inclusions varies from glassy to partially and completely crystallized (Figure 2), which corresponds to previous description of the inclusions in the Elga meteorite [5,8,9,[11][12][13]15]. Some inclusions indicate the abundance in troilite (Elga-8, Figure 2 and Figure S6), even up to the appearance of individual sulfide nodules. Milky white inclusions are dominant in glass, but, in anyway, micron-sized microlites of silicates are fixed in them under high magnification ( Figures S4 and S5). Areas of local impact melting are confined to cracks in metal (Figure 2, Figure 3 and Figure S1) and visible in the inclusions. Sometimes the cracks joint the neighboring silicate inclusions (Figure 2, line Elga-11-Elga-12). In general, the texture and phase composition of such impact assemblages are similar to those described previously in the Elga meteorite [8,10,14,16,19,20]. In the studied sample, the impact associations in Fe-Ni-metal are mainly represented by quenched structures of kamacite + schreibersite + troilite, sometimes with silicate or silicate-phosphate blebs ( Figure 3 and Figure S1).

Inclusion with Silicate-Natrophosphate Immisciblity (Elga-4)
Among all silicate inclusions found in the studied Elga sample, only one inclusion (Elga-4) drastically differs in mineralogy and petrography (Figures 4 and 5). In general, it is a new type of silicate inclusions for the Elga meteorite, which was discovered in recently studied samples, and at present day, only few inclusions have been found [12,13,17]. Litasov and Podgornykh [13] described them as type 3-"silicate/phosphate inclusions with liquid immiscibility". The most remarkable features of such inclusions are the occurrence of natrophosphate globule (isolation) and exotic mineralogy of both silicates and phosphates, enriched in Na.

General Description of the Immiscible Inclusion
The studied silicate inclusion (Elga-4) is oval, 3 mm in size and brown in color. It has a specific texture due to pronounced silicate (≈90 vol.%)-natrophosphate (≈10 vol.%) liquid immiscibility ( Figure 4). Schreibersite and troilite form rim zones around the silicate inclusion: their distribution

Inclusion with Silicate-Natrophosphate Immisciblity (Elga-4)
Among all silicate inclusions found in the studied Elga sample, only one inclusion (Elga-4) drastically differs in mineralogy and petrography (Figures 4 and 5). In general, it is a new type of silicate inclusions for the Elga meteorite, which was discovered in recently studied samples, and at present day, only few inclusions have been found [12,13,17]. Litasov and Podgornykh [13] described them as type 3-"silicate/phosphate inclusions with liquid immiscibility". The most remarkable features of such inclusions are the occurrence of natrophosphate globule (isolation) and exotic mineralogy of both silicates and phosphates, enriched in Na.

General Description of the Immiscible Inclusion
The studied silicate inclusion (Elga-4) is oval, 3 mm in size and brown in color. It has a specific texture due to pronounced silicate (≈90 vol.%)-natrophosphate (≈10 vol.%) liquid immiscibility ( Figure 4). Schreibersite and troilite form rim zones around the silicate inclusion: their distribution is Minerals 2020, 10, 437 7 of 29 well fixed into the characteristic radiations of P, S, Fe and Ni ( Figure 5). It should be noted that host kamacite around the inclusion is virtually free in taenite and "plessite" (≈100 µm zone). Compositions of schreibersite, troilite and host kamacite are given in Table 2. The natrophosphate part mainly forms a meniscus-like isolation of Na-Ca-Mg-Fe phosphates with minor Na-Ti-rich pyroxene, although individual grains of phosphates sporadically occur in the periphery of the silicate part of the inclusion in direct contact with schreibersite or troilite (Figures 6 and 8). They decorate an individual inner zone like troilite and schreibersite ones; this is clearly visible in the characteristic radiations of P and Ca ( Figure 5). Silicate part of the inclusion contains mainly alkali feldspars, quartz, glass and Na-Ti-Cr-rich silicates (amphibole, clinopyroxene, aenigmatite). In contrast with other inclusion types in the Elga meteorite, this inclusion drastically differs in high abundance of alkalis (namely, Na) and Ti. is well fixed into the characteristic radiations of P, S, Fe and Ni ( Figure 5). It should be noted that host kamacite around the inclusion is virtually free in taenite and "plessite" (≈100 µ m zone). Compositions of schreibersite, troilite and host kamacite are given in Table 2. The natrophosphate part mainly forms a meniscus-like isolation of Na-Ca-Mg-Fe phosphates with minor Na-Ti-rich pyroxene, although individual grains of phosphates sporadically occur in the periphery of the silicate part of the inclusion in direct contact with schreibersite or troilite (Figures 6 and 8). They decorate an individual inner zone like troilite and schreibersite ones; this is clearly visible in the characteristic radiations of P and Ca ( Figure 5). Silicate part of the inclusion contains mainly alkali feldspars, quartz, glass and Na-Ti-Cr-rich silicates (amphibole, clinopyroxene, aenigmatite). In contrast with other inclusion types in the Elga meteorite, this inclusion drastically differs in high abundance of alkalis (namely, Na) and Ti.

Mineralogy of Silicate Part
The silicate part of the studied immiscible inclusion mainly contains fine-grained (sometimes symplectitic) quartz-feldspars aggregate (former glass) and mafic silicates; other minerals (chromite, pentlandite and altered phases) are minor or accessory ( Figure 6 and Figures S7-S9). Ontogeny (hollow and skeletal crystals) and relationships of minerals indicate about quenching and rapid crystallization within the inclusion. The relations of feldspars ("layered" grains) assume solid decay of initially homogenous K-Na-feldspar into albite and K-feldspar with temperature decreasing (Figure 6 and Figure  S9). Silicate glass is present in small amounts and locally preserved in the outer zone of the inclusion ( Figure 8). Mafic silicates are very specific in composition: dominant phase is an obertiite-subgroup oxyamphibole; minor phases are low-Ca-pyroxene (orthopyroxene), aenigmatite-subgroup mineral (sapphirine supergroup) and the diopside-kosmochlor-Na(Mg,Fe) 0.5 Ti 0.5 Si 2 O 6 clinopyroxene series.

Mineralogy of Silicate Part
The silicate part of the studied immiscible inclusion mainly contains fine-grained (sometimes symplectitic) quartz-feldspars aggregate (former glass) and mafic silicates; other minerals (chromite, pentlandite and altered phases) are minor or accessory (Figures 6 and S7-S9). Ontogeny (hollow and skeletal crystals) and relationships of minerals indicate about quenching and rapid crystallization within the inclusion. The relations of feldspars ("layered" grains) assume solid decay of initially homogenous K-Na-feldspar into albite and K-feldspar with temperature decreasing (Figures 6 and  S9). Silicate glass is present in small amounts and locally preserved in the outer zone of the inclusion ( Figure 8). Mafic silicates are very specific in composition: dominant phase is an obertiite-subgroup oxyamphibole; minor phases are low-Ca-pyroxene (orthopyroxene), aenigmatite-subgroup mineral (sapphirine supergroup) and the diopside-kosmochlor-Na(Mg,Fe)0.5Ti0.5Si2O6 clinopyroxene series. Ti-rich oxyamphibole is omnipresent in this part, whereas pyroxenes are commonly confined to the boundary between silicate and natrophosphate parts. In general, orthopyroxene decorates the Ti-rich oxyamphibole is omnipresent in this part, whereas pyroxenes are commonly confined to the boundary between silicate and natrophosphate parts. In general, orthopyroxene decorates the silicate-natrophosphate contact ( Figures 5 and 7) and the growth of these crystals was directed from the contact into the silicate part. This is clearly visible in the characteristic radiation of Mg ( Figure 5, Figure 7 and Figure S12).
Minerals 2020, 10, x FOR PEER REVIEW 10 of 29 silicate-natrophosphate contact ( Figures 5 and 7) and the growth of these crystals was directed from the contact into the silicate part. This is clearly visible in the characteristic radiation of Mg (Figures 5, 7 and S12).  The occurrence of chromite is also related to this contact ( Figure 6, Figure 9 and Figure S10). Previously this mineral was chemically labelled as eskolaite due to very small sizes [12,17]. The relations of mafic silicates indicate that orthopyroxene is an earlier mineral, whereas oxyamphibole is a later phase ( Figure 6). The appearance of Fe-rich chlorite, goethite and hydrated phosphate (close to vivianite Fe 2+ 3 (PO 4 ) 2 ·8H 2 O and with Na 2 O-2.3-3.3 wt.%) is attributed to alteration process within the inclusion ( Figure 6, Figure 8 and Figures S8 and S9). The hydrated phosphate seems to be alteration product of former anhydrous Na-Fe-phosphate.

Mineralogy of Natrophosphate Part
Natrophosphate part forming meniscus-like isolation (globule) consists of aggregate of three orthophosphates (brianite, czochralskiite and marićite) and minor complex Na-Cr-Ti-clinopyroxene, pentlandite, rarely taenite ( Figure 4, Figure 5, Figure 6, Figure 7, Figure 9 and Figures S10-S12). The individual grains of anhydrous phosphates occurring beyond of the globule ( Figure 8) are represented by brianite and czochralskiite. Like orthopyroxene in the silicate part complex clinopyroxene also forms a discrete zone, which decorates the contact into the natrophosphate part and well-fixed in the characteristic radiation of Cr ( Figure 7). The relationships of minerals within the natrophosphate globule indicate that clinopyroxene is earlier phase than phosphates, and pentlandite and taenite form blebs in phosphates. In addition, the silicate vein cross-cuts the natrophosphate globule ( Figure 7, Figure 9 and Figure S11), evidencing about earlier crystallization of phosphatic melt rather than silicate ones.

Feldspars, Quartz and Siliceous Glass
As mentioned above, the fine-grained quartz-feldspars aggregate is the main component of the studied inclusion and is a result of complete crystallization of residual silicate melt or quenched product after siliceous glass. In general, compositions of feldspars very strongly vary due to the alternation of the albite and K-feldspar "layers" in most grains. Homogeneous grains of albite or K-feldspar occur rarely ( Figure S9). Maximal compositions correspond to Ab 92.4 Or 7.5 An 0.1 for albite and Ab 27.8 Or 71.3 An 0.9 for K-feldspar, whereas intermediate compositions presented in Table 3 seem to be result of trapping of both minerals in "layered" grains during the WDS-EDS analyses. Large grains of quartz (supported by Raman spectroscopy, see below) contain mainly FeO (up to 0.5 wt.%) as admixture. Siliceous glass is virtually free in CaO and strongly varies in the major components depending on the degree of total crystallization (in wt.%): SiO 2 68.1-79.1; Al 2 O 3 10.6-16.9; FeO 1.1-3.4; MgO 0.2-0.6; Na 2 O 5.8-8.7; K 2 O 2.3-3.9. In general, such variations are common for the Elga glass-containing inclusions [5,8,9,13,15], which are virtually free in water (0.06-0.12 wt.%, SIMS data) [9]. The occurrence of chromite is also related to this contact ( Figures 6, 9 and S10). Previously this mineral was chemically labelled as eskolaite due to very small sizes [12,17]. The relations of mafic silicates indicate that orthopyroxene is an earlier mineral, whereas oxyamphibole is a later phase ( Figure 6). The appearance of Fe-rich chlorite, goethite and hydrated phosphate (close to vivianite Fe 2+ 3(PO4)2•8H2O and with Na2O-2.3-3.3 wt.%) is attributed to alteration process within the inclusion (Figures 6, 8 and S8-S9). The hydrated phosphate seems to be alteration product of former anhydrous Na-Fe-phosphate.

Mineralogy of Natrophosphate Part
Natrophosphate part forming meniscus-like isolation (globule) consists of aggregate of three orthophosphates (brianite, czochralskiite and marićite) and minor complex Na-Cr-Ti-clinopyroxene, pentlandite, rarely taenite (Figures 4-7, 9 and S10-S12). The individual grains of anhydrous phosphates occurring beyond of the globule ( Figure 8) are represented by brianite and czochralskiite. Like orthopyroxene in the silicate part complex clinopyroxene also forms a discrete zone, which decorates the contact into the natrophosphate part and well-fixed in the characteristic radiations of Cr ( Figure 7). The relationships of minerals within the natrophosphate globule indicate that clinopyroxene is earlier phase than phosphates, and pentlandite and taenite form blebs in phosphates. In addition, the silicate vein cross-cuts the natrophosphate globule (Figures 7, 9 and S11), evidencing about earlier crystallization of phosphatic melt rather than silicate ones.

Feldspars, Quartz and Siliceous Glass
As mentioned above, the fine-grained quartz-feldspars aggregate is the main component of the studied inclusion and is a result of complete crystallization of residual silicate melt or quenched product after siliceous glass. In general, compositions of feldspars very strongly vary due to the alternation of the albite and K-feldspar "layers" in most grains. Homogeneous grains of albite or K-feldspar occur rarely ( Figure S9). Maximal compositions correspond to Ab92.4Or7.5An0.1 for albite and Ab27.8Or71.3An0.9 for K-feldspar, whereas intermediate compositions presented in Table 3 seem to be result of trapping of both minerals in "layered" grains during the WDS-EDS analyses. Large grains of quartz (supported by Raman spectroscopy, see below) contain mainly FeO (up to 0.5 wt.%) as admixture. Siliceous glass is virtually free in CaO and strongly varies in the major components depending on the degree of total crystallization (in wt.%): SiO2 68.1-79.1; Al2O3 10.6-16.9; FeO 1.1-3.4; MgO 0.2-0.6; Na2O 5.8-8.7; K2O 2.3-3.9. In general, such variations are common for the Elga glass-containing inclusions [5,8,9,13,15], which are virtually free in water (0.06-0.12 wt.%, SIMS data) [9].  Gt-goethite; Pn-pentlandite. Some details see also Figures S10 and S11-S12 in Supplementary data.    Na-Ti-rich oxyamphibole is dominant silicate in the studied inclusion. It should be mentioned that minerals of the amphibole supergroup rather rarely occur in meteorites (see review in [24]). Moreover, Ti-rich oxyamphiboles and Na-rich amphiboles are exceptional rarities in meteorites: the kaersutite subgroup minerals are known in the Martian samples [25][26][27][28][29][30] and magnesio-arfvedsonite was described in the Kaidun meteorite [31]. The chemical compositions for amphibole in the Elga meteorite are given in Table 4. Its averaged formula (N = 54) is (Na 0.85 K 0.13 )(Na 1. 67    This amphibole belongs to the obertiite root name (subgroup) of the w(O)-dominant amphibole group according to the recent nomenclature for the amphibole supergroup [32,33]. In general, it is approximately close to ferri-obertiite NaNa 2 Mg 3 Fe 3+ Ti(Si 8 O 22 )O 2 . However, the Elga mineral drastically differs from terrestrial oxyamphiboles of the obertiite subgroup [34][35][36][37][38] in its very high Ti content (1.03-1.57 apfu, Table 4 and Figure 10). The core-to-rim chemical deviations within individual grains of the Elga obertiite mineral are negligible. Nevertheless, the compositional variations for all grains are very essential (Table 4 and Figure 10). In general, the mineral shows a pronounced positive correlation between Ti and Fe 2+ and negative ones for Ti with Mg, Fe 3+ and Si. Other Ti correlations (with Al, (Na+K) and Ca) are weaker ( Figure 10). Thus, the main isomorphic schemes of the Elga oxyamphibole are 2Fe 3+ ↔ Ti 4+ + Fe 2+ and 2Mg 2+ + 2Fe 3+ ↔ Ti 4+ + 3Fe 2+ , which cover variations from ferri-obertiite NaNa 2 Mg 3 Figure 11). Other isomorphic schemes like Ca 2+ + 2Fe 3+ + Si 4+ ↔ Na + + 2Ti 4+ + Al 3+ are negligible. In addition, the Elga mineral also contains moderate concentrations of K 2 O (up to 1.2 wt.%, 0.23 apfu) and F (up to 0.5 wt.%, 0.23 apfu). The Raman spectra did not show any evident bands in the 3000-4000 cm −1 region (vibrations for OH-group, see below).
As shown in Figure 11, the part of the Elga amphibole compositions is dominant in the hypothetical NaNa 2 Mg 3 (Fe 2+ 0.5 ,Ti 0.5 )Ti(Si 8 O 22 )O 2 end-member (>50 mol.%). It seems to give support for declaration of a potentially new mineral species using the 50% rule of IMA. However, it needs further detailed studies of the crystal structure and chemical composition. It is known that in addition to the octahedral environments, Ti may occupy the tetrahedral site in the crystal structure of some amphiboles [34,39]. Moreover, this mineral should be carefully checked up on the real concentrations of TiO 2 and Ti 2 O 3 , FeO and Fe 2 O 3 , Li 2 O, F and H 2 O. All these components may essentially influence on the formula calculation.
As shown in Figure 11, the part of the Elga amphibole compositions is dominant in the hypothetical NaNa2Mg3(Fe 2+ 0.5,Ti0.5)Ti(Si8O22)O2 end-member (>50 mol.%). It seems to give support for declaration of a potentially new mineral species using the 50% rule of IMA. However, it needs further detailed studies of the crystal structure and chemical composition. It is known that in addition to the octahedral environments, Ti may occupy the tetrahedral site in the crystal structure of some amphiboles [34,39]. Moreover, this mineral should be carefully checked up on the real concentrations of TiO2 and Ti2O3, FeO and Fe2O3, Li2O, F and H2O. All these components may essentially influence on the formula calculation.  Minerals of this subgroup (aenigmatite, krinovite and wilkinsonite) rarely occur in meteorite environment [24]. The mineral close to aenigmatite was found in the Elga-4 studied immiscible inclusion, it is scarce and mainly confined to the periphery in the silicate part. Its averaged composition (N = 11) is Na2.00(Mg2.67Fe 2+ 2.00Fe 3+ 0.16Na0.08Mn0.07Cr0.03)Ti0.99(Si5.91Al0.01Fe 3+ 0.08O18)O2 indicating the predominance of Mg over Fe 2+ (Table 5, Figure 10). According to the recent nomenclature for the sapphirine supergroup [40,41], this phase is related to the aenigmatite subgroup and its compositions correspond to the median values of the series aenigmatite Na2Fe 2+ 5TiSi6O18O2-hypothetical phase Na2Mg5TiSi6O18O2 ( Figure 12). The compositional variations are shown in Figure 12 and Table 5. The core-to-rim deviations within individual grains of the Elga mineral are negligible. Some surplus of Na (>2.0 apfu) and Fe 3+ may be explained by moderate isomorphism in octahedral environment: 2(Mg,Fe 2+ ) ↔ Na + + Fe 3+ (Figure 12).
Thus, the Elga meteorite seems to be a first occurrence for a potentially new mineral species Na2Mg5TiSi6O18O2 (Mg-analog of aenigmatite) assuming the 50% rule of IMA. Aenigmatitic mineral was previously mentioned in the Elga immiscible silicate-phosphate inclusions, but without clear identification by chemical composition [12,13].
In general, extraterrestrial aenigmatite was previously found in the clasts of the meteorites Allende, Kaidun and Adzhi-Bogdo (stone) [31,[42][43][44], but all these compositions are dominant in the Na2Fe 2+ 5TiSi6O18O2 end-member. It should be noted that phases with 0.40-0.48 mol.% of the Na2Mg5TiSi6O18O2 end-member were described in the terrestrial alkaline rocks [45,46], but it was not enough to qualify them as new mineral species.

Minerals of the Aenigmatite-Subgroup (Sapphirine Supergroup)
Minerals of this subgroup (aenigmatite, krinovite and wilkinsonite) rarely occur in meteorite environment [24]. The mineral close to aenigmatite was found in the Elga-4 studied immiscible inclusion, it is scarce and mainly confined to the periphery in the silicate part. Its averaged composition (N = 11) is Na 2.00 (Mg 2. 67 Figure 10). According to the recent nomenclature for the sapphirine supergroup [40,41], this phase is related to the aenigmatite subgroup and its compositions correspond to the median values of the series aenigmatite Na 2 (Figure 12). The compositional variations are shown in Figure 12 and Table 5. The core-to-rim deviations within individual grains of the Elga mineral are negligible. Some surplus of Na (>2.0 apfu) and Fe 3+ may be explained by moderate isomorphism in octahedral environment: 2(Mg,Fe 2+ ) ↔ Na + + Fe 3+ (Figure 12).
Thus, the Elga meteorite seems to be a first occurrence for a potentially new mineral species Na 2 Mg 5 TiSi 6 O 18 O 2 (Mg-analog of aenigmatite) assuming the 50% rule of IMA. Aenigmatitic mineral was previously mentioned in the Elga immiscible silicate-phosphate inclusions, but without clear identification by chemical composition [12,13].

Orthopyroxene
As mentioned above, abundant low-Ca pyroxene is confined to the silicate-natrophosphate contact (Figures 5 and 7) and also dispersed in the silicate part. Unfortunately, the position of the studied Elga silicate inclusion in Fe-Ni metal cannot give a possibility to determine optical and structural features for such pyroxene (orthorhombic or monoclinic). According to previous data for the Elga meteorite [5,8,15], it seems to be orthopyroxene. In general, it is enstatite (Table 6) and its compositional variations for all grains are essential: from En 81.3 Fs 17.4 Wo 1.3 to En 65.3 Fs 33.0 Wo 1.7 . The core-to-rim deviations within individual enstatite grains are sometimes drastic (especially for zoned grains in the silicate-phosphate contact) and directed to increasing of the ferrosilite end-member (Table 6, Figure 6, Figure 9 and Figure S12). All enstatites also contain minor TiO 2 (up to 0.  Table 6).

Na-Ca-Cr-Ti-Mg-Clinopyroxenes
These exotic clinopyroxenes are confined to the silicate-natrophosphate contact in the Elga-4 inclusion and occur both in the silicate and in phosphate sides ( Figure 6, Figure 7, Figure 9, Figures S8 and S10-S12). Unfortunately, we cannot obtain more analyses for the mineral from the contact and natrophosphate globule because of minute size of most grains (<5 µm, Figure 9, Figures S10 and S12). All compositions from the Elga-4 inclusion are poor in alumina and correspond to the series of diopside Ca(Mg,Fe)Si 2 O 6 -kosmochlor NaCrSi 2 O 6 -hypothetical Na(Mg,Fe) 0.5 Ti 0.5 Si 2 O 6 with minor content of the aegirine NaFe 3+ Si 2 O 6 and hedenbergite CaFe 2+ Si 2 O 6 end-members (Table 7). All this does not allow providing correct classification of these clinopyroxenes within the pyroxene supergroup [47]. Although the compositional inhomogenity of some grains is indicated on BSE images and elemental maps ( Figure 6, Figures S8, S11 and S12), it is impossible to establish the sequence and tendency in the changing of the end-member contents during crystal growth. In general, clinopyroxene from the silicate part is richer in the kosmochlor end-member, the mineral from the contact is abundant in the diopside and aegirine end-members and the phase from the phosphate globule has high content of the hypothetical Na(Mg,Fe) 0.5 Ti 0.5 Si 2 O 6 end-member ( Figure 13).
Clinopyroxene from the natrophosphate globule of the Elga-4 inclusion contain 47-64 mol.% of the Na(Mg,Fe)0.5Ti0.5Si2O6 end-member (Table 7 and Figure 13) and this gave reason to consider this composition as a potentially new mineral species Na(Mg,Fe)0.5Ti0.5Si2O6. However, like with the obertiite-subgroup mineral, it needs further detailed studies of the crystal structure and chemical composition with precise determination of TiO2 and Ti2O3, FeO and Fe2O3, and Li2O. All these components may essentially influence during the formula calculation.
Clinopyroxene from the natrophosphate globule of the Elga-4 inclusion contain 47-64 mol.% of the Na(Mg,Fe) 0.5 Ti 0.5 Si 2 O 6 end-member (Table 7 and Figure 13) and this gave reason to consider this composition as a potentially new mineral species Na(Mg,Fe) 0.5 Ti 0.5 Si 2 O 6 . However, like with the obertiite-subgroup mineral, it needs further detailed studies of the crystal structure and chemical composition with precise determination of TiO 2 and Ti 2 O 3 , FeO and Fe 2 O 3 , and Li 2 O. All these components may essentially influence during the formula calculation.

Other Minerals
Chromite is an accessory mineral in the Elga-4 immiscible inclusion and confined to the contact between silicate and natrophosphate parts ( Figure 6, Figure 9 and Figure S10). Unfortunately, we cannot obtain the precise analyses for this mineral due to the very small size (<3 µm) and entrapment of neighboring phases. Anyway, it is a Fe-Cr-dominant phase (FeO > 25, Cr 2 O 3 > 55.2 wt.%) and contains TiO 2 (0.5), MnO (1.7), ZnO (2.7) MgO (2.2) and Al 2 O 3 (0.9 wt.%). That is similar to chromite found in another immiscible inclusion in the Elga meteorite [13].
Fe-rich chlorite and Na-rich hydrated Fe-phosphate are mainly restricted to the silicate part of the Elga-4 inclusion ( Figure 6, Figure 8 and Figure S9). The chemical composition of chlorite is (in wt.%, N =  Table 8. The same phase is also mentioned in other silicate inclusions of the Elga meteorite [15]. This mineral seems to be an alteration product of former anhydrous Na-Fe-phosphate.

Raman Spectroscopy for Minerals of the Elga-4 Inclusion
The main minerals in the Elga-4 inclusion were also studied by Raman spectroscopy. The unoriented Raman spectra of minerals are shown in Figures 14-16 and Figures S13-S14.
In general, the intensity of the Raman bands depends on the orientation of mineral grains with respect to the laser incident beam. Commonly, their orientation is not under control; therefore, the intensity of the resultant Raman bands is not indicative of the amounts of the different chemical components in the studied minerals. Nevertheless, the shifting of the Raman bands characteristic for particular minerals may outline the changing in chemical compositions. Minerals 2020, 10, x FOR PEER REVIEW 21 of 29 Most silicates show complex chemical formulas and different electrovalence substitutions in equivalent crystallographic sites. In general, the characteristic frequencies of the Raman spectra for silicates (for example, amphiboles) may be interpreted as: 100-300 cm −1 -lattice vibrations modes; 300-625 cm −1 -vibrational modes of the non-tetrahedral cations, bending modes of SiO4 tetrahedra, OH libration and translation modes, and translational M-OH modes; 625-750 cm −1 -symmetric stretching modes of bridging oxygen atoms that link the adjacent SiO4 tetrahedra (Si-Ob-Si); 750-950 cm −1 -modes of symmetric SiO4 stretching; 950-1000 cm −1 -modes of antisymmetric SiO4 stretching; 1000-1215 cm −1 -modes of antisymmetric Si-Ob-Si bond stretching; 3600-3800 cm −1 -OH bond stretching modes [64][65][66]. Most silicates show complex chemical formulas and different electrovalence substitutions in equivalent crystallographic sites. In general, the characteristic frequencies of the Raman spectra for silicates (for example, amphiboles) may be interpreted as: 100-300 cm −1 -lattice vibrations modes; 300-625 cm −1 -vibrational modes of the non-tetrahedral cations, bending modes of SiO 4 tetrahedra, OH libration and translation modes, and translational M-OH modes; 625-750 cm −1 -symmetric stretching modes of bridging oxygen atoms that link the adjacent SiO 4 tetrahedra (Si-O b -Si); 750-950 cm −1 -modes of symmetric SiO 4 stretching; 950-1000 cm −1 -modes of antisymmetric SiO 4 stretching; 1000-1215 cm −1 -modes of antisymmetric Si-O b -Si bond stretching; 3600-3800 cm −1 -OH bond stretching modes [64][65][66].
The Raman spectra of the Elga aenigmatite-subgroup minerals show characteristic scattering peaks near 446, 530-533, 660-663 (strongest), 693 (shoulder), 890 (strong) and 1050-1051 cm −1 ( Figure  15) resembling those of terrestrial aenigmatite [67]. The chemical similarity of the Elga aenigmatite and its Mg-analog does not indicate any essential difference in their Raman spectra.     Figure 16). Variable intensity and shifting of some Raman signals seem to record different chemical composition. The Raman spectra of meteoritic and terrestrial kosmochlor and diopside-kosmochlor have been reported in a few publications [54,56]. In general, the Elga clinopyroxene data presented here are quite similar to the published spectra for the Morasko iron meteorite [56].
The Raman spectra of the Na-Ca-Mg-Fe-orthophosphates, quartz, feldspars and siliceous glass are shown in Figures S13 and S14. Brianite, czochralskiite and marićite were analyzed within natrophosphate globule ( Figure S13). In general, the data presented here for the Elga-4 orthophosphates are very similar to the Raman spectra for the holotype czochralskiite from the Morasko iron meteorite [58] and Na-rich phosphates from other silicate inclusions with immiscibility, found in the Elga meteorite [13]. The latter publication contains detailed interpretation of the Raman spectra for studied phosphates. The Raman spectra for the SiO2 phase in the Elga-4 inclusion are clearly shown that it is to be quartz ( Figure S14) and not to be tridymite or cristobalite as it was assumed previously for the Elga silicate inclusions [8]. Unfortunately, Raman spectroscopy does not give any direct information about structural state of K-feldspar (orthoclase, microcline or sanidine) ( Figure S14).   Figure 16). Variable intensity and shifting of some Raman signals seem to record different chemical composition. The Raman spectra of meteoritic and terrestrial kosmochlor and diopside-kosmochlor have been reported in a few publications [54,56]. In general, the Elga clinopyroxene data presented here are quite similar to the published spectra for the Morasko iron meteorite [56].
The Raman spectra of the Na-Ca-Mg-Fe-orthophosphates, quartz, feldspars and siliceous glass are shown in Figures S13 and S14. Brianite, czochralskiite and marićite were analyzed within natrophosphate globule ( Figure S13). In general, the data presented here for the Elga-4 orthophosphates are very similar to the Raman spectra for the holotype czochralskiite from the Morasko iron meteorite [58] and Na-rich phosphates from other silicate inclusions with immiscibility, found in the Elga meteorite [13]. The latter publication contains detailed interpretation of the Raman spectra for studied phosphates. The Raman spectra for the SiO 2 phase in the Elga-4 inclusion are clearly shown that it is to be quartz ( Figure S14) and not to be tridymite or cristobalite as it was assumed previously for the Elga silicate inclusions [8]. Unfortunately, Raman spectroscopy does not give any direct information about structural state of K-feldspar (orthoclase, microcline or sanidine) ( Figure S14).

Discussion and Concluding Remarks
It is assumed that IIE irons have genetic relations to H-chondrite-like bodies according to isotopic and geochemical characteristics [68][69][70][71]. The origin of fractionated IIE irons, including the Elga meteorite, is explained by a complex model evolving disruption and reassembly of partially molten asteroids and their complex impact history [15,[69][70][71]. Silicate-bearing iron meteorites may be also explained by formation at the core-mantle boundary of small bodies due to separation of silicate and metal parts or by condensation of silicate and metal liquids in solar nebula (see review in [13,15]).
The previous and recent studies for the Elga IIE iron meteorite showed mineralogical diversity for silicate inclusions [5,[8][9][10][11][12][13]15,19]. It is suggested that the modal and bulk compositions of glassy-containing silicate inclusions in the Elga meteorite and chemical composition of their phases are consistent with the model of 25% partial melting of H-chondrite and subsequent equilibrium crystallization of this melt (with merrillite liquidus phase) and then quenching at about 1090 • C [15]. However, this model does not explain the appearance of the silicate-phosphate liquid immiscibility observed in some Elga inclusions. It should be noted that in addition to the Elga meteorite meniscus-shaped Ca-Na-phosphate segregations are known within the silicate inclusions in the Colomera and Sombrerete iron meteorites [68][69][70][71].
The study of the Elga immiscible inclusions implies that silicate-natrophosphate immiscibility is a high-temperature process (probably >1000-1100 • C, quartz-feldspar eutectic). In the studied inclusion, the silicate vein cross-cuts the natrophosphate globule (Figure 7, Figure 9 and Figure S11) and this is evidence about earlier crystallization (or quenching) of natrophosphatic melt rather than silicate liquid. However, maximal PTX-conditions for separation of initial melt into silicate and natrophosphate components is not easy to evaluate. It is not excluded that this silicate inclusion was previously subjected to complete melting during shock event.
In general, the inclusions with silicate-natrophosphate immiscibility are rare type of silicate inclusions found in the Elga meteorite [12,13,17]: about 8% of total inclusions based on Figure 2. Such inclusions drastically differ from other types both in chemical composition (high abundance of Na and Ti) and exotic mineralogy. It is quite possible the appearance of Ti-rich silicates (oxyamphibole, aenigmatite, complex clinopyroxenes) and Na-Ca-Mg-Fe-orthophosphates (brianite, czochralskiite, marićite) is strongly related to silicate-natrophosphate immiscibility within silicate inclusions in the Elga meteorite and maybe in other IIE meteorites.
The mineralogy of the studied immiscible inclusion revealed three potentially new mineral species: obertiite-related oxyamphibole NaNa 2 Mg 3 Fe 2+ 0.5 Ti 1.5 [Si 8 O 22 ]O 2 , Mg-analog of aenigmatite Na 2 Mg 5 TiSi 6 O 18 O 2 and Na-Ti-rich clinopyroxene Na(Mg,Fe) 0.5 Ti 0.5 Si 2 O 6 . Formula calculation for these phases was based on the traditional method: charge balance is due to division of total FeO into FeO and Fe 2 O 3 . In general, it seems to be correct because the Mössbauer spectroscopy for the Elga diopsidic clinopyroxene showed the high amount of Fe 3+ (≈50% of total Fe) [8]. In addition, the previous and recent data on mineral chemistry for the Elga meteorite also indicated the abundance of Fe 3+ in silicates [5,8,9,13,15,17,19]. All this suggests a high oxygen fugacity during crystallization of silicate melt [8]. In the case of the Elga-4 immiscible inclusion, silicates are variable in the content of Fe 3+ (in dependence of mineral position-silicate part, contact or phosphate globule), which suggests local variations in oxygen activity during crystallization. There are no strong evidences about highly reduced conditions during crystallization of the Elga-4 inclusion. It is assumed that all Ti in minerals is in tetravalent state, like in other silicate inclusions of the Elga meteorite containing rutile, ilmenite or panethite [8,9,12,13,15]. In general, the abundance of Fe 3+ should be in hypothetical contradiction with the high content of Ti 3+ (different oxygen fugacity conditions). Nevertheless, the presence of trivalent titanium in the Elga minerals, especially for potentially new mineral species found in this meteorite, may be suggested. The different variants of ideal compositions (with Ti 4+ and Ti 3+ ) for the above Elga oxyamphiboles and clinopyroxenes are presented in Table 9. It should be noted that the occurrence of minerals containing Ti 4+ or Ti 3+ or both components is common of carbonaceous meteorites [24,[72][73][74][75][76][77][78][79][80]. It is especially remarkable for the Allende meteorite, in which Ti 4+ -Ti 3+ -bearing minerals have been recently found: Ti 3+ -containing rhönite [73] [80]. Thus, the mineralogy of the Elga inclusion with silicate-natrophosphate immiscibility first outlines three potentially new Ti-rich minerals according to the 50% rule of IMA: obertiite-related oxyamphibole, Mg-analog of aenigmatite and Na-Ti-rich clinopyroxene. However, they need further detailed studies of both the crystal structure and chemical composition. These minerals should be carefully checked for the real concentrations and valence state of titanium and iron.
Supplementary Materials: The following figures are available online at http://www.mdpi.com/2075-163X/10/ 5/437/s1: Figure S1: Elemental maps for the area of local impact melting and further quenching in the Elga metal; Figure S2: BSE images and elemental maps for the area of surface alteration, Elga meteorite; Figure  S3: BSE images and elemental maps for silicate inclusion Elga-1; Figure S4: BSE images for areas in silicate inclusions Elga-6 and Elga-11; Figure S5: BSE images and elemental maps for silicate inclusion Elga-5; Figure S6: BSE images and elemental maps for silicate inclusion Elga-8 enriched in troilite; Figure S7: Relationships of the obertiite-group mineral and aenigmatite in silicate part of immiscible inclusion Elga-4 (BSE image and elemental maps); Figure S8: Relationships of minerals in silicate part of immiscible inclusion Elga-4 (BSE image and elemental maps); Figure S9: BSE image and elemental maps for secondary phases from immiscible inclusion Elga-4; Figure  S10: BSE images and elemental maps for chromite and related minerals on the contact between silicate and natrophosphate parts of immiscible inclusion Elga-4; Figure S11: Relationships of minerals in natrophosphate globule of immiscible inclusion Elga-4 (BSE image and elemental maps); Figure S12: BSE image and elemental maps for the silicate-natrophosphate contact in the immiscible inclusion Elga-4; Figure S13: Raman spectra of Na-rich orthophospates from natrophosphate globule in the Elga-4 immiscible inclusion; Figure S14: Raman spectra of quartz, glasses and feldspars from the Elga-4 immiscible inclusion.
Author Contributions: V.V.S. wrote the paper, performed the mineralogical description, measurements of chemical composition (EMPA, SEM) and Raman studies for minerals of the immiscible inclusion in the Elga meteorite. The author has read and agreed to the published version of the manuscript.