*5.6. Results of SEM-EDS Analyses* 5.6.1. Lorenzenite

This mineral has been found in rocks of the Lovozero and Khibiny massifs [73–77]. At Lovozero, it occurs mostly among porphyrites, where it forms euhedral crystals together with eudialyte, astrophyllite, epistolite, and murmanite. Analyses in the SEM-EDS (Figure 11A) showed that this mineral contains small amounts of niobium. In the boundary zones of the crystal, numerous small inclusions of phosphates containing La, Ce, Pr, Nd, and loparite, also containing some of U, were found. In the Khibiny rocks, it was found as an admixture, accompanying rocks of the ore zone; these minerals co-occur with titanite and pyroxenes. Lorenzenite in these rocks has an anhedral shape and is generally small and strongly corroded. Details are shown in Table A1 in the Appendix A.

#### 5.6.2. Loparite

This mineral has been found in all the discussed massifs, although it occurs in them in different proportions [78–85]. In the Lovozero massif, it is relatively common, occurs as an accessory mineral, and is a rock-forming mineral in some pegmatites. In Lovozero rocks, it usually has a euhedral shape and is visible against other minerals. The results of the analysis in the Lovozero loparite SEM-EDS showed that it contains admixtures of uranium and strontium. The mineral also has niobium in it. In the Khibiny Massif, loparite occurs much less frequently. It has been found, inter alia, in the ore zone, where it accompanies apatite-nepheline rocks in the vicinity of titanite. It is typically small in size and corroded. Analysis in the SEM-EDS (Figure 11B) showed that loparite from Khibiny is sometimes slightly doped with strontium. In the Kovdor rocks, loparite is relatively common as a minor admixture. It co-occurs with magnetite, which is often intergrown with perovskite, and with femic minerals such as pyroxene and phlogopite. SEM-EDS analysis has shown that some of these minerals are admixed with Ta and U, and also Th and Cd. Small admixtures of barite are also found in their vicinity. In Afrikanda rocks, loparite is a rare mineral, co-occurring with perovskite and Ce -rich perovskite as minor associated phases.

The results of the SEM-EDS analyses for loparite are shown in Table A2 in the Appendix A. The results of the analyses are shown in Table A3 in the Appendix A.

#### 5.6.3. Perovskite

This mineral has been found in all massifs discussed [46–48,81]. In Afrikanda, it occurs mostly in rocks (derivatives in magnetitites and pyroxenites). The crystals found there are numerous, large, euhedral. Studies in the SEM-EDS (Figure 11C) have shown that it is doped with Ce, Th, Nb, and sometimes W. In the Kovdor massif, perovskite is also relatively abundant accompanying magnetites, growing on these crystals and in the form of admixtures in femic minerals. In the Khibiny massif, it rarely occurs, as a small admixture in rocks of the ore zone co-occurring with titanite. The same is true in the Lovozero massif, where small amounts of this crystal have been studied in jovite rocks (04LV12). These minerals in this massif have numerous admixtures of Nd, Th, and sometimes also Ce and Th [86].

#### 5.6.4. Titanite

This mineral occurs in all discussed massifs [73–75,86,87]. The Khibiny massif sometimes forms significant accumulations, especially in the ore zone, where it can occur as a rock-forming mineral together with arfvedsonite and aegirine in intercalations with apatite-nepheline ore. In other rocks, it is often present along with ilmenite and less often with magnetite. The titanite specimens examined in the SEM-EDS (Figure 11D) often show admixtures of Nb, Ta, and, less frequently, W. Moreover, inclusions of lanthanide-containing phases are common in these minerals. The same is true for titanite from the Lovozero massif, where it is less common, although it occurs along with ilmenite in syenites, lujavrites, and jovites. When examined in the SEM-EDS, it has abundant admixtures of Nb, Th, Ce, and occasionally Nd, as evidenced by SEM-EDS analyses. Among the Afrikanda rocks, titanite sometimes co-occurs with perovskite, forming a phase of a secondary character, while in the rocks of the Kovdor massif, it is a rare phase. The results of analyses in the SEM-EDS of this mineral can be found in Table A4 in the Appendix A.

#### 5.6.5. Calcite

Calcite occurs in all of the discussed massifs in varying proportions. Most of these minerals are found in Kovdor, where they root their rocks (phoscorites) [4,10–13,49–52,88–93]. In this massif, it is usually calcite, sometimes admixed with iron and manganese. Along with calcite, there is also dolomite forming dolomite phoscorites. In the Afrikanda massif, carbonates are represented mainly by calcite. It forms carbonatite veins and nests among pyroxenites and other rocks. Studies of the SEM-EDS showed that there are numerous admixtures of lanthanum and cerium, and less frequently strontium (Figure 11G). In the Khibiny massif, calcite is an accessory mineral and is less common in alkaline rocks except in zones in the northeastern part of the massif where carbonatites and carbonatite breccias occur. The studied calcite from Khibiny has a large admixture of strontium (strontianite). In the Lovozero massif, as in Khibiny, carbonates generally occur less frequently as accessory minerals in some alkalic rock types. The studied rock samples are dominated by a large admixture of strontium similar to Khibiny. The results of the analyses in the carbonate micro-area can be found in Table A7 in the Appendix A.

#### 5.6.6. Eudialyte

Eudialyte occurs in rocks of the Khibiny and Lovozero massifs [94–97]. At Khibiny, eudialyte is an accessory mineral to many different rocks, as it is at Lovozero. Detailed analyses in the SEM-EDS (Figure 11F) showed little variation in the occurrence of zircon, manganese, and occasionally doped titanium contents in these minerals (can be found in Table A6 in the Appendix A).

#### 5.6.7. Apatite

Apatite crystals co-occur in rocks in all discussed massifs [52,77,89]. At Khibiny, they form rock-forming accumulations in the ore zone, where nepheline-apatite rocks occur. In addition to these rocks, apatite is also present in all varieties of agpatite rocks found there, usually as an accessory mineral. Analyses in the SEM-EDS (Figure 11E) showed that hydroxyapatite sometimes dominates with a small admixture of fluorine less frequently chlorite. The same is true for the Lovozero massif, where apatite also occurs frequently as an accessory mineral. Studies in the SEM-EDS also indicate the occurrence of hydroxyapatite admixed with fluorine, rarely chlorine. In addition, admixtures of lanthanum, cerium, strontium less frequently silver are found in this mineral. In the Afrikanda massif, apatite forms an accessory mineral representing as above hydroxyapatite and fluorapatite. In the case of the Kovdor rocks, apatite sometimes forms their breccias, filling their spaces. These are usually phases of apatite, growing in a concentric form on crumbs of phoscorite breccia. Apart from these zones, apatite also occurs as accessory crystals in rocks, usually as hydroxyapatite and less frequently as fluorapatite. Micro-area studies have shown that barite inclusions are also sometimes present in this mineral. The results of micro-area analyses of this mineral can be found in Table A5 in the Appendix A.

**Figure 11.** Example EDS spectra of the studied minerals: Lorenzenite (**A**), loparite (**B**), perovskite (**C**), titanite (**D**), apatite (**E**), eudialyte (**F**), and carbonates (**G**). The elemental contributions to each phase are discussed in the text.

#### **6. Discussion**

The discussed minerals are important components of rocks in the Khibiny, Lovozero, Kovdor, and Afrikanda massifs. Their presence indicates multi-stadial crystallization processes occurring in the discussed intrusions [98–106]. Perovskite is the mineral of the earliest crystallization that occurs mainly in the Afrikanda massif, less frequently in Kovdor, and is a rare mineral in the Khibiny and Lovozero massifs. In contrast, REE-rich phases such as knopite and loparite are phases that crystallized somewhat later in the crystallization phase of the contaminated melt. It is evidenced by the petrographic features, where in the microscopic image, knopite occurs mostly in the outer parts of perovskite or the crack zones of this crystal. SEM-EDS studies showed numerous Nb and Sr substitutions in perovskite, which was also confirmed by single crystals analysis and FTIR studies. In turn, loparite occurs in rocks in an accessory form, crystallizing together with alkaline phases such as eudialyte and lorenzenite. In carbonatite rocks, loparite has a corroded character which may mean that under changing conditions, it was disintegrated. This hypothesis may be supported by the fact that loparite and perovskite occur at Khibiny, mainly as an admixture among femic minerals, most often together with titanite. In addition, analysis of titanite from Khibiny and Lovozero indicates that it often has Nb admixtures as in loparite. The crystallizing phases in the Loparite-Perovskite-Tausonite series have many dopants indicating some contribution of these members to the loparite structure. Their microanalysis and single-crystals, as well as FTIR studies, indicate a small admixture of Nb and Ti, and Ce and Ca in the crystal structure modify the arrangement in which this mineral crystallizes. Titanite occurs much more frequently at Khibiny and Lovozero than at Afrikanda and Kovdor. It is a secondary mineral to perovskite and loparite, which passed to titanite during the impact of secondary processes. In zones of titanite occurrence, one can see relicts of perovskite and loparite, especially in Khibiny. Due to the nature of titanite (usually a secondary mineral after perovskite or loparite), REE dopants are also found in this mineral. Apatite crystallized probably at the same time as loparite. This is evidenced especially by apatites in the ore zone rocks, forming euhedral crystal inclusions along with loparite and perovskite surrounded by titanite. Apatite from the discussed massifs is also rich in REE elements which were identified using SEM-EDS, single-crystals, and FTIR analysis. Minerals such as lorenzenite and eudialyte crystallized in the residual or hydrothermal phase [40,88,93]. This is particularly evident in the porphyrites of the Lovozero massif, where these minerals form skeletal crystals with numerous inclusions of surrounding phases. In the other rocks of the Lovozero and Khibiny massifs, eudialyte usually forms minerals of anhedral character filling the voids in the rocks between mafic phases as well as feldspars. Lorenzenite is present only in a few hydrothermal impact areas like the porphyrites mentioned above. Crystal structure analyses indicated a model crystal structure. However, FTIR studies showed some slight differences between the eudialyte from Khibiny and that from Lovozero. Carbonates in the discussed massifs are usually present in the form of admixtures, although, in Afrikanda, they form their nests and Kovdor rocks. Their position has been the subject of dispute for many researchers, who have emphasized their features indicating both crystallization during the mixing phase of alkali melts with supracrustal material and later [86]. Isotopic studies of these minerals indicate that they were generally formed by fluid crystallization from the Earth's mantle, although some crystallize later, as also evidenced by δ13C values in carbonates [11,89,107] and δ34S sulfides [108–111]. This is also confirmed by data obtained by other researchers. According to Arzamastsev, the age of perovskite from Khibiny is 383 ± 7 Ma, while the age of apatite-nepheline phases is 370 ± 3 Ma, respectively [22]. This indicates the order of crystallization of these phases. Sr-Nd [89] isotopic data indicate a mantle origin of the components of the intrusions in question and subsequent contamination with supracrustal material at the migration stage of these magmas towards the Earth surface and their further evolution during the formation of the intrusions in question. The Afrikanda and Kovdor intrusions have the most primordial character, and the Khibiny and Lovozero intrusions

are the most contaminated [112]. This hypothesis is also supported by the geochemical data and results of helium isotope studies [113,114].

#### **7. Conclusions**

The discussed minerals are indicator phases in intrusions of ultramafic and alkaline rocks. Their presence is associated with different phases of the evolution of the melts from which the intrusions were formed in the Kola-Karelian Alkaline Province [7,20,21,115,116]. Their presence and petrographic character indicate the processes of their crystallization, corrosion, and phase formation at the expense of these minerals (e.g., titanite after loparite and perovskite). These minerals in the discussed massifs are carriers of REE elements. They are interesting from a mineralogical as well as deposit point of view. Their optical, spectroscopic features are closely related to the internal structure, optical properties of these minerals, and the form of crystallization. Their presence in alkaline rocks arouses interest and is an important genetic indicator of the studied rocks. The occurrence of these phases in the Khibiny, Lovozero, Afrikanda, and Kovdor intrusions is unique in the world. These massifs are relatively easy to access; moreover, the Khibiny and Lovozero massifs are large, showing the multi-zone nature of the different rocks. This provides the opportunity to observe these minerals macroscopically and study them with modern methods of analysis.

**Author Contributions:** Conceptualization, M.H.; methodology, M.H., D.K. and G.C.; software, M.H.; validation, M.H., D.K., G.C. and E.K.; formal analysis, M.H.; investigation, M.H.; resources, M.H. and E.K.; data curation, M.H.; writing—original draft preparation, M.H., D.K. and G.C.; writing—review and editing, M.H.; visualization, supervision, and project administration, M.H.; funding acquisition, M.H. and E.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This statement if the study did not report any data.

**Acknowledgments:** Authors would like to acknowledge of V. V. Balaganskiy and F. P. Mintrofanov from Kola Science Center of Russian Academy of Sciences, for help us collect minerals and needed informations.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A**

**Table A1.** Results of the microanalysis of the lorenzenite crystals.


**Table A1.** *Cont.*


**Table A1.** *Cont.*


**Table A2.** Results of the microanalysis of the loparite crystals.


**Table A2.** *Cont.*


**Table A3.** Results of the microanalysis of the perovskite crystals.


**Table A3.** *Cont.*


**Table A3.** *Cont.*


**Table A4.** Results of the microanalysis of the titanite crystals.


**Table A4.** *Cont.*


**Table A5.** Results of the microanalysis of the apatite crystals.


**Table A5.** *Cont.*


**Table A5.** *Cont.*


**Table A5.** *Cont.*


**Table A6.** Results of the microanalysis of the eudialyte crystals.



**Table A7.** Results of the microanalysis of the carbonate crystals.

#### **References**

1. Arzamastsev, A.A. *Unique Paleozoic Intrusions of the Kola Peninsula*; Russian Academy of Science Publ.: Apatity, Russia, 1994; p. 79.

2. Huber, M. *Mineralogical-Petrographic Characteristic of the Selected Alkaline Massifs of the Kola Peninsula*; TMKarpinski Publisher: Suchy Las, Poland, 2015; p. 187.

