minerals

Article Synchysite-(Ce) from Cinquevalli (Trento, Italy): Stacking Disorder and the Polytypism of (Ca,REE)-Fluorcarbonates

Giancarlo Capitani

Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 4, 20126 Milan, Italy; [email protected]



Received: 10 December 2019; Accepted: 15 January 2020; Published: 18 January 2020


Abstract: Synchysite-(Ce) at Cinquevalli occurs as fine needles intergrown with quartz in quartz-dikes and in association with altered K-feldspar and oxidized chalcopyrite as major constituents. Synchysite-(Ce) [Ca1.00(Ce0.43La0.26Nd0.17Y0.07Pr0.04Sm0.02Gd0.01)Σ=1.00(CO3)2(F0.58(OH)0.42)], shows an overgrowth rim of bastnäsite-(Ce) [(Ce0.34La0.25Nd0.17Pb0.07C a0.06Y0.06Pr0.04S m0.02Gd0.01)Σ=1.00C O3(F0.75(OH)0.25)]. Unit cell refinement of synchysite (C2/c) and bastnäsite (P62c) led to a = 12.272(4), b = 7.100(2), c = 18.640(5) Å, β = 102.71(5)◦, and a = 7.085(1), c = 9.746(2) Å, respectively. Polysomatic faults are sporadic, but polytypic disorder is widespread. High resolution transmission electron microscopy images taken along [100] or 130 show an apparent order and the related diffraction h i patterns are streak-free. Conversely, along [010] or 110 , a high density of stacking faults is observed h i and the related diffraction patterns show hhl rows with h , 3n affected by streaks. No ordered domain larger than a few unit cells was detected. The stacking sequence of (Ca,REE)-fluorcarbonates can be compared with subfamily-B mica polytypes (2M , 2O and 6H), which are characterized by n 60 2 · ◦ (n = odd) rotations. Subfamily-A polytypes (1M, 2M and 3T), characterized by n 60 (n = even) 1 · ◦ rotations, should not be possible. Synchysite, characterized by 60 rotations, can be likened to the ± ◦ 2M2 polytype.

Keywords: synchysite-(Ce); bastnäsite-(Ce); polytypism; HRTEM

1. Introduction

Synchysite [CaREE(CO3)2F] represents the Ca-rich end member of the (Ca,REE)-fluorcarbonates’ polysomatic series. The REE-rich end member is bastnäsite [REE(CO3)F]. Both are important ore minerals for rare earth elements (REEs). The chemical formula of individual, intermediate polysomes can be expressed as [REE(CO3)F]m[CaREE(CO3)2F]n, where m and n refer to the number of bastnäsite (B) and synchisite (S) basic units of the BmSn series. Notable intermediate members of the series are parisite (BS) and röntgenite (BS2). A number of additional polysomes/polytypes have been observed with transmission electron microscopy (TEM) by several authors [1–7]. Cerium is commonly the dominant REE, but fluorcarbonates of the bastnäsite-synchysite series with dominant La or Y or Nd have been recorded [8–12]. Other REEs normally present as minor constituents are Gd, Pr, and Sm. Bastnäsites with a partial or complete substitution of hydroxyl for fluorine have also been described [13,14]. The (Ca,REE)-fluorcarbonates have a layered topology with planar carbonate groups “standing-on-edge” with respect to the overall structural layering [15]. The most striking consequence of this structural layering is the formation of syntaxial intergrowths and, therefore, ordered and disordered domains with polysomatic and polytypic connections [7,16–18]. Diverse layer definitions in terms of composition, thickness, position (within the sequence), and symmetry have been employed so far, making the situation quite confusing (for a summary of all these

Minerals 2020, 10, 77; doi:10.3390/min10010077 www.mdpi.com/journal/minerals Minerals 2020, 10, 77 2 of 19 Minerals 2020, 10, x FOR PEER REVIEW 2 of 19 diversethese diverse notations, notations, their correspondence,their correspondence, pros and pros cons, and seecons, Reference see Reference [7]). Throughout [7]). Throughout this paper, this thepaper,VB thenotation VB notation code ( Vcode= vaterite; (V = vaterite;B = bastnäsite) B = bastnäsite) will bewill employed, be employ sinceed, since it is it congenial is congenial for thefor interpretationthe interpretation of high of high resolution resolution transmission transmissi electronon electron microscopy microscopy (HRTEM) (HRTEM) images. images. Briefly, Briefly,B in BS B notationin BS notation remains remainsB in VB B innotation VB notation and Sandin BSS innotation BS notation becomes becomesVB in VBVB in notation;VB notation;V alone V alone has has no equivalentno equivalent in BS innotation. BS notation. For example, For example, the sequence the sequenceBS (parisite) BS (parisite) becomes becomesVBB (Figure VBB1). (Figure Moreover, 1). wheneverMoreover, required,whenever arequired, comparison a comparison with the formerwith thedefg formercode, defg where: code, dwhere:= CeF-layer; d = CeF-layer;e = CO e 3=-layer CO3- betweenlayer between CeF-layers CeF-layers (not present (not present in synchysite); in synchysite);f = Ca-layer; f = Ca-layer; and g =andCO g3 =-layer CO3-layer between between a CeF- a and CeF- a Ca-layerand a Ca-layer [16] (Figure [16] (Figure1), is provided. 1), is provided.

Figure 1. StructuresStructures of of bastnäsite-(Ce) bastnäsite-(Ce) [19], [19 ],synchysite-(Ce) synchysite-(Ce) [17], [17 parisite-(Ce)], parisite-(Ce) [18] [18 and] and vaterite vaterite [20], [20 all], allas seen as seen along along [010]. [010]. Yellow Yellow = Ce,= Ce, blue blue = =Ca,Ca, green green = =F,F, black black == C.C. Oxygen Oxygen atoms atoms at at the the vertices of

triangular COCO33groups groups are are omitted omitted for clarity.for clarity. Analogous Analogou layerss layers in the diinff erentthe different structures structures are connected are byconnected arrows andby arrows indicated and with indicate theird relativewith their codes. relative codes.

In thisthis study,study, we we undertook undertook a a HRTEM HRTEM study study of of synchysite-(Ce) synchysite-(Ce) from from Cinquevalli Cinquevalli (Trento, (Trento, Italy) Italy) that have been never studied before, with the aim to contribute to the assessment of the B S polysomatic that have been never studied before, with the aim to contribute to the assessmentm n of the BmSn series.polysomatic Structural series. defects Structural are thoroughly defects are characterized, thoroughly characterized, a widespread a polytypic widespread disorder polytypic detected, disorder and adetected, comparison and witha comparison the polytypism with the of polytypism mica is provided. of mica is provided. 2. Materials and Methods 2. Materials and Methods The studied (Ca,REE)-fluorcarbonates are hosted in quartziferous veins extracted from the gangue The studied (Ca,REE)-fluorcarbonates are hosted in quartziferous veins extracted from the material of the Cinquevalli Mine, sited near Roncegno, Trento, Italy (Figure2). The mining area is gangue material of the Cinquevalli Mine, sited near Roncegno, Trento, Italy (Figure 2). The mining located on the southern slope of the Mount Fravort, along the valley carved by the Argenta stream, area is located on the southern slope of the Mount Fravort, along the valley carved by the Argenta at altitude around 1500 m. The ore body is represented by quartziferous dikes and veins, from a few stream, at altitude around 1500 m. The ore body is represented by quartziferous dikes and veins, centimeters to tens of meters in thickness (not reported in the map either because too small or cropping from a few centimeters to tens of meters in thickness (not reported in the map either because too out only in mine tunnels), cross cutting Paleozoic phyllites and micaschists of the crystalline basement small or cropping out only in mine tunnels), cross cutting Paleozoic phyllites and micaschists of the and the Roncegno granodiorite-monzogranite pluton. The beginning of the mining activity dates back crystalline basement and the Roncegno granodiorite-monzogranite pluton. The beginning of the to the 12th century, as documented by the finding of mining inlets, melting furnaces, and abundant mining activity dates back to the 12th century, as documented by the finding of mining inlets, melting melting slags. In the Middle Ages, silver and especially copper were mined at Cinquevalli. In the furnaces, and abundant melting slags. In the Middle Ages, silver and especially copper were mined last century, until the end of the activity that took place in 1940, the area was exploited for sphalerite, at Cinquevalli. In the last century, until the end of the activity that took place in 1940, the area was galena, and copper. In the last years of activity and for a short period, fluorite was extracted [21]. exploited for sphalerite, galena, and copper. In the last years of activity and for a short period, fluorite Fluorcarbonates are scarce and dispersed and, therefore, have never represented an exploitable ore. was extracted [21]. Fluorcarbonates are scarce and dispersed and, therefore, have never represented Representative hand specimens were sliced with a diamond wheel saw and prepared as an exploitable ore. polished thin sections suitable for both optical and scanning electron microscopy (SEM) observations. Representative aliquots of the samples were powdered and used for energy dispersive X-ray fluorescence (EDXRF) and X-ray powder diffraction (XRPD) bulk analyses. For the TEM sample preparation, the green parts recognized on the hand specimen were scratched with a needle and left to settle directly

Minerals 2020, 10, 77 3 of 19 in an agate mortar, where the grain size was reduced with a pestle. The resulting powder was Mineralsultrasonicated 2020, 10, x in FOR isopropanol PEER REVIEW and a few microliters of solution were pipetted in holey-carbon Cu-grids.3 of 19

Figure 2. Localization and geological map of the Cinquevalli Mine area (elaborated from the on-line source [[22]).22]).

RepresentativeXRPD analyses werehand performed specimens with were a PANanalytical sliced with X’Pert-Proa diamond PW3060 wheel di sawffractometer, and prepared operating as polishedin Bragg-Brentano thin sections specular suitable (θ- θfor) geometry both optical and and equipped scanning with electron a X’Celerator microscopy position (SEM) sensitive observations. detector. RepresentativeDiffractometer scansaliquots were of recorded the samples in the 3–70were◦ 2 θporangewdered with and step used size offor 0.02 energy◦ and 30dispersive s counting X-ray time, fluorescenceat 40 mA and (EDXRF) 40 kV (CuK andα radiation).X-ray powder A Ni diffraction filter along (XRPD) the diffracted bulk beamanalyses. path For was the used TEM to filter sample out preparation,the CuKβ radiation. the green The parts sample recognized holder on was the allowed hand specimen to spinhorizontally were scratched during with measurements a needle and left to toimprove settle directly particle statistics.in an agate mortar, where the grain size was reduced with a pestle. The resulting powderThe was identification ultrasonicated of major in isopropanol and minor and phases a few was microliters done with of solution the X’Pert were High pipetted Score in software holey- (PANanalytical)carbon Cu-grids. using the ICSD PDF2-2004 database. Quantitative phase analyses (QPA) were performedXRPD withanalyses the Rietveld were performed method [23 with,24] as a implementedPANanalytical in theX’Pert-Pro GSAS/EXPEGUI PW3060 programdiffractometer, [25]. X operatingSemi-quantitative in Bragg-Brentano bulk chemical specular analyses (θ-θ) geometry were obtained and withequipped a Pananalytical with a X’Celerator Epsilon 3 positionEDXRF sensitiveinstrument detector. using aDiffractometer standardless method.scans were Volatile recorded components in the 3–70° (H 2Oθ range and CO with2) werestep size determined of 0.02° andthrough 30 s thecounting weight time, loss onat 40 ignition mA and (LOI). 40 kV (CuKα radiation). A Ni filter along the diffracted beam path SEMwas used investigations to filter out were the performedCuKβ radiation. with aThe Tescan sample VEGA holder TS was 5136XM allowed instrument to spin operatinghorizontally at during20 keV andmeasurements equipped withto improve an EDAX particle GENESIS statistics. 4000XMS energy dispersive system (EDS), and with a ® ZeissThe Gemini identification 500 field emission of major gun and (FEG) minor instrument, phases wa equippeds done with with the a Bruker X’Pert XFlashHigh ScoreEDS software detector. The(PANanalytical) standardless using method the and ICSD the ZAFPDF2-2004 correction database. method Quantitative were used for phase semi-quantitative analyses (QPA) analyses. were performedTEM observations with the Rietveld were performed method [23,24] at the as Department implemented of Physical in the GSAS/EXPEGUI Sciences, Earth and program Environment [25]. of theSemi-quantitative University of Siena bulk with chemical a Jeol JEM analyses 2010 instrumentwere obtained operating with a atPananalytical 200 keV and Epsilon equipped 3X withEDXRF an Oxford Link EDS and an Olympus Tengra 2.3k 2.3k 14-bit slow-scan CCD camera. To remove instrument using a standardless method. Volatile× components× (H2O and CO2) were determined throughnoise contrast the weight dueto loss amorphous on ignition materials, (LOI). HRTEM images were rotationally filtered [26] with the HRTEMSEM filter investigations [27], as implemented were performed in the Gatan with Digitala Tescan Micrograph VEGA TS version5136XM 3.9. instrument High resolution operating image at 20 keV and equipped with an EDAX GENESIS 4000XMS energy dispersive system (EDS), and with a Zeiss Gemini 500 field emission gun (FEG) instrument, equipped with a Bruker XFlash® EDS detector. The standardless method and the ZAF correction method were used for semi-quantitative analyses. TEM observations were performed at the Department of Physical Sciences, Earth and Environment of the University of Siena with a Jeol JEM 2010 instrument operating at 200 keV and

Minerals 2020, 10, 77 4 of 19 simulations were performed with JEMS© [28]. Finally, semi-quantitative EDS analyses were obtained with the standardless method using the Cliff & Lorimer approximation [29].

3. Results

3.1. Rock Mineral Assemblage and Composition At the hand-specimen scale, the fluorcarbonate-bearing rocks show a granitic texture made of quartz, K-feldspar, and chalcopyrite as major constituents. The fluorcarbonates are recognizable because of their pale green color (Figure3a) and form fine needles or lamellae intergrown with quartz. ChalcopyriteMinerals 2020, 10 appears, x FOR PEER oxidized REVIEW and reddish oxides decorate rock joints. 5 of 19

Figure 3. (a) Representative hand specimen of a (Ca,REE)-fluorcarbonate-bearing rock from Cinquevalli Figure 3. (a) Representative hand specimen of a (Ca,REE)-fluorcarbonate-bearing rock from Mine. (Ca,REE)-fluorcarbonates are recognizable by the pale green color. Oxidized chalcopyrite Cinquevalli Mine. (Ca,REE)-fluorcarbonates are recognizable by the pale green color. Oxidized (gold metal luster), quartz (white) and K-feldspar (pink) can also be recognized. The latter, however, chalcopyrite (gold metal luster), quartz (white) and K-feldspar (pink) can also be recognized. The is overestimated by visual inspection, because most of the reddish color is due to oxides and hydroxides latter, however, is overestimated by visual inspection, because most of the reddish color is due to that alter sulphides. (b) Photomicrograph (plain polarized light; 50X) showing bands of needle-like, oxides and hydroxides that alter sulphides. (b) Photomicrograph (plain polarized light; 50X) showing randomly oriented crystals of (Ca,REE)-fluorcarbonates (dark) embedded in quartz. (c) Related cross bands of needle-like, randomly oriented crystals of (Ca,REE)-fluorcarbonates (dark) embedded in polarized light photomicrograph. (d) Photomicrograph (cross polarized light) of coarse, euhedral quartz. (c) Related cross polarized light photomicrograph. (d) Photomicrograph (cross polarized K-feldspar (center) altered by white mica and bordered by ribbons of fibrous quartz (chalcedony). light) of coarse, euhedral K-feldspar (center) altered by white mica and bordered by ribbons of fibrous Photomicrograph side ~9 mm. quartz (chalcedony). Photomicrograph side ∼9 mm.

Table 1. EDXRF Bulk Composition (wt%) of a Representative Fluorcarbonate-Bearing Quartz Dike Sample.

MgO 0.12 Rb2O 0.01 Al2O3 7.68 Y2O3 0.15 SiO2 74.62 SnO2 0.01 P2O5 0.04 La2O3 0.76 SO3 2.35 CeO2 1.28 K2O 3.39 Nd2O3 0.65 CaO 0.70 PbO 0.25 TiO2 0.08 Bi2O3 0.04 Fe2O3 3.57 LOI 2.64 CuO 1.47 Tot. 99.8

Minerals 2020, 10, 77 5 of 19

In thin section, under polarized light, the fluorcarbonates appear as opaque needles intergrown with granular quartz. The needle morphology may also result from cross sectioning thin lamellae of fluorcarbonates at high angle to the thin section plane (Figure3b,c). Coarse, euhedral K-feldspar is sometimes altered to white mica (Figure3d). In addition to granular quartz, fibrous quartz recalling chalcedony is present, in the latter case not intergrown with fluorcarbonates (Figure3d). Therefore, two generations of quartz seem present. In the thin section, chalcopyrite shows a reddish rim, suggesting later alteration in hematite/goethite. XRPD Rietveld refinement of a representative aliquot of the sample (Rp = 8.18%; Rwp = 11.22%; X2 = 6.703) gave the following composition (wt%): quartz 79.4(1)%, muscovite 9.5(4)%, K-feldspar 6.9(3)%, chalcopyrite 1.4(1)%, bastnäsite 1.5(1)% and synchysite 1.2(1)%. These results are in keeping with the optical observations, although muscovite, possibly as an alteration of K-feldspar, is probably overestimated because of (001) preferential orientation. Unit cell refinement of bastnäsite (P62c) and synchysite (C2/c) gave the following results: a = 7.085(1), c = 9.746(2) Å, and a = 12.272(4), b = 7.100(2), c = 18.640(5) Å, β = 102.71(5)◦, respectively, which are consistent with literature data, although should be taken with caution because of their low concentration. EDXRF data of a representative aliquot of the studied samples are close to a typical granitic aplite composition, except the Al2O3 content reduced by half, absence of NaO and higher abundance of exotic components, such as REE-oxides (Table1). The low Al 2O3 content and the absence of NaO determines the strong prevalence of quartz over feldspars evidenced by the Rietveld refinement. Among the other major components, K2O is present in K-feldspar and in its white mica alteration; S, Cu and Fe are hosted in chalcopyrite (of course not as oxides); Fe2O3 is present in the chalcopyrite alteration halo; REEs (and CaO) are hosted in fluorcarbonates. On the basis of the texture, mineral assemblage, and chemical composition, the fluorcarbonate-hosting rock can be defined as a quartz dike.

Table 1. EDXRF Bulk Composition (wt%) of a Representative Fluorcarbonate-Bearing Quartz Dike Sample.

MgO 0.12 Rb2O 0.01 Al2O3 7.68 Y2O3 0.15 SiO2 74.62 SnO2 0.01 P2O5 0.04 La2O3 0.76 SO3 2.35 CeO2 1.28 K2O 3.39 Nd2O3 0.65 CaO 0.70 PbO 0.25 TiO2 0.08 Bi2O3 0.04 Fe2O3 3.57 LOI 2.64 CuO 1.47 Tot. 99.8

3.2. Microstructure and Mineral Composition In order to complement optical observations as well as XRPD and EDXRF results, a SEM-EDS investigation was undertaken. As argued from the optical examination, the fluorcarbonates form interwoven needle-like crystals embedded in quartz and K-feldspar (Figure4). An additional phase not detected with the previous techniques is metallic bismuth, which accounts for the Bi2O3 component detected with EDXRF. The presence of metallic bismuth and chalcopyrite are consistent with a reducing environment. However, the pinite alteration of K-feldspar and the Fe-Cu-oxide halo wrapping chalcopyrite (Figure5) suggest a late alteration stage under high oxygen fugacity in the presence of water. Minerals 2020, 10, x FOR PEER REVIEW 6 of 19 Minerals 2020, 10, x FOR PEER REVIEW 6 of 19 3.2. Microstructure and Mineral Composition 3.2. Microstructure and Mineral Composition In order to complement optical observations as well as XRPD and EDXRF results, a SEM-EDS investigationIn order to was complement undertaken. optical As argued observations from the as opticalwell as examination,XRPD and EDXRF the fluorcarbonates results, a SEM - formEDS interwoveninvestigation needle was - undertaken.like crystals Asembedded argued in from quartz the and optical K-feldspar examination, (Figure the 4). fluorcarbonates An additional phase form interwoven needle-like crystals embedded in quartz and K-feldspar (Figure 4). An additional phase not detected with the previous techniques is metallic bismuth, which accounts for the Bi2O3 componentnot detected detected with the with previous EDXRF. techniquesThe presence is of metallic metallic bismuth, bismuth and which chalcopyrite accounts are for consistent the Bi2O3 withcomponent a reducing detected environment. with EDXRF. However, The presence the pinite of alterationmetallic bismuth of K-feldspar and chalcopyrite and the Fe- Cuare- oxideconsistent halo wrappingwith a reducing chalcopyrite environment. (Figure However, 5) suggest the a latepinite alteration alteration stage of K under-feldspar high and oxygen the Fe -fugacityCu-oxide in halo the wrapping chalcopyrite (Figure 5) suggest a late alteration stage under high oxygen fugacity in the Mineralspresence2020 of, 10water., 77 6 of 19 presence of water.

Figure 4. (a) Backscattered electron (BSE) SEM image of a sample from Cinquevalli showing randomly Figure 4. (a) Backscattered electron (BSE) SEM image of a sample from Cinquevalli showing randomly oriented,Figure 4. (needlea) Backscattered-like crystals electron of synchysite (BSE) SEM-(Ce) image (Syn) of embedded a sample fromin quartz Cinquevalli (Qtz) and showing K-feldspar randomly (Kfs). oriented, needle-like crystals of synchysite-(Ce) (Syn) embedded in quartz (Qtz) and K-feldspar (Kfs). Theoriented, saw- teethneedle contact-like crystals between of synchysiteQtz and Kfs-(Ce) is marked (Syn) embedded by a dotted in quartzline in (Qtz)the central and K -topfeldspar part of(Kfs). the The saw-teeth contact between Qtz and Kfs is marked by a dotted line in the central top part of the image.The saw Rarer-teeth aggregates contact between of metallic Qtz bismuthand Kfs is(Bi) marked are also by observed. a dotted (lineb) EDS in the spectrum central oftop the part bismuth of the image. Rarer aggregates of metallic bismuth (Bi) are also observed. (b) EDS spectrum of the bismuth grainimage. (Bi) Rarer imaged aggregates in (a) showing of metallic the bismuth lack of the (Bi) S arepeak also. observed. (b) EDS spectrum of the bismuth a grain (Bi) imaged in (a)) showingshowing thethe lacklack ofof thethe SS peak.peak.

Figure 5.5. (a) Large chalcopyrite chalcopyrite crystal crystal (Ccp) (Ccp) with with a a Fe-Cu-oxide Fe-Cu-oxide reactionreaction rim. rim. ((b)) Compositional Compositional profileprofileFigure along5. (a) theLarge tracetrace chalcopyrite evidencedevidenced crystal in (aa).). (Ccp)Note that with how a Fe the -Cu fine fine-oxide dark reaction rim within rim. ( theb) Compositional oxide yields a CuCu/Feprofile/Fe ratioalong higher the trace than evidenced the surrounding, in (a). Note which that conflictsconflicts how the with fine thedark darkerdark rimer within graygray tone,the oxide suggesting yields a (Cu,Fe)-bearing(Cu,Cu/FeFe) ratio-bearing higher material than withwith the surrounding, aa lowerlower averageaverage which atomicatomic conflicts number,number, with possiblypossibly the dark ananer hydroxide.hydroxide. gray tone, suggesting a (Cu,Fe)-bearing material with a lower average atomic number, possibly an hydroxide. The fluorcarbonate needles show a core richer in Ca and a rim richer in REEs (Figure6), suggesting The fluorcarbonate needles show a core richer in Ca and a rim richer in REEs (Figure 6), an enrichment of REEs in the final stage of crystallization. The related SEM-EDS analyses are reported suggestingThe fluorcarbonate an enrichment needles of REEs showin the afinal core stage richer of crystallization. in Ca and a rim The richerrelated in SEM REEs-EDS (Figure analyses 6), in Table2, and are consistent with synchysite-(Ce), ideally CaCe(CO 3)2F, and bastnäsite-(Ce), ideally aresuggesting reported an in enrichment Table 2, and of are REEs consistent in the final with stage synchysite of crystallization.-(Ce), ideally The CaCe(CO related3 )SEM2F, and-EDS bastnäsite analyses- Ce(COare reported3)F, respectively. in Table 2, and In fact,are consistent Ce is the with dominant synchysite REE- in(Ce), both ideally minerals CaCe(CO (~0.433)2 andF, and ~0.69 bastnäsite a.p.f.u.- (Ce), ideally Ce(CO3)F, respectively. In fact, Ce is the dominant REE in both minerals (0.43 and 0.69 in average in synchysite and bastnäsite, respectively) and the Ca/(Ca + REE) ratio is close to 0.50 a.p.f.u.(Ce), ideally in average Ce(CO in3)F, synchysite respectively. and In bastnäsite, fact, Ce is respectively) the dominant and REE the in Ca/(Caboth minerals + REE) ( ratio0.43 isand close 0.69 to for synchysite and very low in bastnäsite (0.03–0.09). Few analyses show Ca/(Ca + REE) lower than 0.50a.p.f.u. for in synchysite average in and synchysite very low and in bastnäsitebastnäsite, (0.0 respectively)3–0.09). Few and analyses the Ca/(Ca show + Ca/(CaREE) ratio + REE) is close lower to 0.50 (e.g., the n. 4 in Table2), suggesting compositional (polysomatic) faults towards the bastnäsite than0.50 for 0.50 synchysite (e.g., the and n. 4very in Tablelow in 2) bastnäsite, suggesting (0.0 3 compositional–0.09). Few analyses (polysomatic) show Ca/(Ca faults + towards REE) lower the composition, as commonly observed in (Ca,REE)-fluorcarbonates. Lanthanum (~0.26 and ~0.50) and than 0.50 (e.g., the n. 4 in Table 2), suggesting compositional (polysomatic) faults towards the Nd (~0.17 and ~0.33) are the other abundant REEs, whereas Y (~0.07 and ~0.11) and Pr (~0.04 and ~0.08) are less abundant and Sm (~0.02 and ~0.04) and Gd (<0.02) are definitely scarce, especially the latter, which is sometimes below the detection limit. Detectable amounts of Pb (~0.13 a.p.f.u.) are present in bastnäsite, whereas Pb is always below the detection limit in synchysite. Of course, CO2 and any possible presence of H2O could not be detected (ideally, synchysite-(Ce) contains 27.6 wt% of CO2 component and bastnäsite-(Ce) 20.08%). Additionally, the data show that F is lower than expected. In part this deficiency may be due to the presence of OH substituting for F in the structure, but mostly is due to diffusion of F under the highly Minerals 2020, 10, 77 7 of 19

Minerals 2020, 10, x FOR PEER REVIEW 7 of 19 focused electron beam, a problem further exaggerated in TEM-EDS analyses, where F is not detected at allbastnäsite (see ahead). composition, In summary, as commonly taking into observed account in that (Ca,REE) fluorine-fluorcarb can beonates. underestimated Lanthanum and ( the0.26 (OH) and content,0.50) and as Nd calculated (0.17 and by di ff0.33)erence, are overestimated,the other abundant the average REEs, whe compositionreas Y (0.07 of synchysite-(Ce) and 0.11) and and Pr bastnäsite-(Ce)(0.04 and 0.08) read: are Caless1.00 abundant(Ce0.43La 0.26andNd Sm0.17 (Y0.070.02Pr and0.04Sm 0.04)0.02Gd and0.01 Gd)Σ=1.00 (<0.02)(CO3 are)2(F 0.58definitely(OH)0.42 scarce,), and (Ceespecially0.34La0.25 theNd latter,0.17Pb which0.07Ca 0.06is sometimesY0.06Pr0.04Sm below0.02Gd the0.01 detection)Σ=1.00CO limit.3(F0.75 (OH)0.25), respectively.

Figure 6. ((aa)) SEM SEM-BSE-BSE image of fluorcarbonatesfluorcarbonates from Cinquevalli showing elongated crystals of synchysite-(Ce),synchysite-(Ce), embedded in quartz (dark gray), with brighter lamellae parallel to the elongation of the crystals near the borders or fillingfilling cavities. ( b) L Lineine profile profile along the trace evidenced in ( a) crossing crossing twotwo bright lamellae. Note the mutual decrease o off Ca and increase of REEs as wel welll as F in the brighter lamellae. Because the spatial resolution of the EDS is larger than the thickness of the lamellae, the EDS spectra of the latter contain always contribution of the surrounding matrix richer in Ca. The thinner the lamella, thethe higherhigher thethe matrixmatrix contribution, contribution, which which may may explain explain the the higher higher Ca Ca/REE/REE ratio ratio observed observed in thein the thinner thinner lamella. lamella.

Table 2. Composition of (Ca,REE)-Fluorcarbonates from Cinquevalli (Trento, Italy). Atoms Calculated Detectable amounts of Pb (0.13 a.p.f.u.) are present in bastnäsite, whereas Pb is always below on the Basis of Two Cations in the Chemical Formula. Relative Error in Brackets (Values in Italics when the detection limit in synchysite. Of course, CO2 and any possible presence of H2O could not be Below 2σ). detected (ideally, synchysite-(Ce) contains 27.6 wt% of CO2 component and bastnäsite-(Ce) 20.08%). Additionally,1 the data 2 show 3 that F Mean is lower than 4 expected. 5 In 6part this 7 deficiency 8 may 9be due Mean to the Syn Syn Syn Syn Mix Bas Bas Bas Bas Bas Bas presence of OH substituting for F in the structure, but mostly is due to diffusion of F under the highly F 0.539 (17) 0.656 (18) 0.546 (16) 0.580 (17) 0.642 (19) 1.283 (23) 1.652 (25) 1.648 (24) 1.507 (24) 1.390 (23) 1.496 (24) focusedCa electron0.987 (9) beam,0.987 (9) a problem1.012 (9) 0.996further(9) exaggerated0.754 (9) 0.168 in(7) TEM0.098-EDS(6) 0.063analyses,(6) 0.144 where(6) F0.080 is not(6) detected0.111 (6) Y 0.061 (5) 0.082 (5) 0.079 (5) 0.074 (5) 0.094 (5) 0.114 (7) 0.136 (7) 0.109 (7) 0.104 (7) 0.109 (7) 0.114 (7) at allLa (see0.273 ahead).(6) 0.263 In summary,(6) 0.255 (6) taking0.264 into(6) 0.322account(7) 0.475that fluorine(9) 0.496 (9)can 0.520be underestimated(9) 0.496 (9) 0.505 and(9) the0.498 (OH)(9) Ce 0.431 (7) 0.422 (7) 0.421 (7) 0.425 (7) 0.468 (8) 0.704 (10) 0.751 (11) 0.666 (10) 0.659 (10) 0.662 (10) 0.688 (10) content,Pr as0.036 calculated(5) 0.046 (5)by difference,0.038 (4) 0.040 overestimated(5) 0.046 (5) 0.075, the (7)average0.063 (7) composition0.083 (7) 0.087 of synchysite(6) 0.075 (7) -(Ce)0.077 and(7) bastnäsiteNd 0.181-(Ce)(6) 0.163 read:(5) 0.158Ca1.00(5) (Ce0.1670.43La(5) 0.260.237Nd0.17(6) Y0.070.303Pr0.04(8) Sm0.3190.02Gd(8) 0.010.348)=1.00(8)(CO0.3283)2(F(8)0.58(OH)0.355 (8)0.42),0.331 and(8) Sm 0.017 (5) 0.021 (5) 0.020 (4) 0.019 (5) 0.024 (5) 0.033 (7) 0.036 (7) 0.038 (7) 0.037 (7) 0.035 (7) 0.036 (7) (CeGd0.34La0.250.009Nd(5)0.17Pb0.0160.07Ca(5) 0.060.018Y0.06(5)Pr0.040.014Sm0.02(5) Gd0.0230.01)(5)=1.000.024CO3(F(7)0.750.006(OH)(7)0.25),0.021 respectively.(7) 0.012 (7) 0.006 (7) 0.014 (7) Pb 0.004 (3) --- 0.033 (3) 0.104 (4) 0.094 (4) 0.152 (5) 0.134 (4) 0.174 (5) 0.132 (4) ΣREE 1.009 1.013 0.988 1.003 1.214 1.728 1.808 1.785 1.722 1.746 1.758 Ca/Ca + Table0.49 2. Composition 0.49 0.51 of (Ca,REE) 0.50-Fluorcarbonates 0.38 0.09 from 0.05 Cinquevalli 0.03 (Trento, 0.08 Italy). 0.04 Atoms 0.06 REE Calculated on the Basis of Two Cations in the Chemical Formula. Relative Error in Brackets (Values in Italics when Below 2σ). 3.3. Observations at the Nanoscale 1 2 3 Mean 4 5 6 7 8 9 Mean 3.3.1. EDS AnalysisSyn Syn Syn Syn Mix Bas Bas Bas Bas Bas Bas F 0.539 (17) 0.656 (18) 0.546 (16) 0.580 (17) 0.642 (19) 1.283 (23) 1.652 (25) 1.648 (24) 1.507 (24) 1.390 (23) 1.496 (24) CaFluorcarbonate 0.987 (9) 0.987 grains (9) dispersed1.012 (9) 0.996 on (9) the grid0.754 (9) were 0.168 first (7) discerned0.098 (6) from0.063 (6) other 0.144 more (6) abundant0.080 (6) 0.111 grains (6) Y 0.061 (5) 0.082 (5) 0.079 (5) 0.074 (5) 0.094 (5) 0.114 (7) 0.136 (7) 0.109 (7) 0.104 (7) 0.109 (7) 0.114 (7) (quartz,La K-feldspar)0.273 (6) 0.263 on (6) the basis0.255 (6) of a0.264 fast (6) EDS 0.322 spectrum, (7) 0.475 then (9) 0.496 checked (9) 0.520 for (9) their 0.496 orientation (9) 0.505 (9) (see 0.498 ahead) (9) andCe in suitable0.431 (7)cases 0.422 studied (7) 0.421 in (7) HR0.425 mode. (7) The0.468 (8) final 0.704 EDS (10)analysis 0.751 (11) was0.666 taken (10) 0.659 at (10)the end,0.662 (10) in order0.688 (10) to Pr 0.036 (5) 0.046 (5) 0.038 (4) 0.040 (5) 0.046 (5) 0.075 (7) 0.063 (7) 0.083 (7) 0.087 (6) 0.075 (7) 0.077 (7) preserveNd the0.181 crystal (6) 0.163 for (5) HR 0.158 imaging. (5) 0.167 (5) 0.237 (6) 0.303 (8) 0.319 (8) 0.348 (8) 0.328 (8) 0.355 (8) 0.331 (8) SmTEM-EDS 0.017 (5) analyses 0.021 (5) are0.020 reported (4) 0.019 in (5) Table 0.0243.Overall, (5) 0.033 (7) they 0.036 are (7) consistent 0.038 (7) with0.037 (7) SEM-EDS 0.035 (7) analyses,0.036 (7) Gd 0.009 (5) 0.016 (5) 0.018 (5) 0.014 (5) 0.023 (5) 0.024 (7) 0.006 (7) 0.021 (7) 0.012 (7) 0.006 (7) 0.014 (7) althoughPb the0.004 low (3) sensitivity- of-TEM-EDS - did0.033 not (3) allow 0.104 for (4) the0.094 detection (4) 0.152 of (5) Pr, Sm,0.134 (4) and 0.174 Gd. (5) Fluorine 0.132 (4) is alsoREE not detected1.009 because1.013 of0.988 di ffusion1.003 during 1.214 the analysis.1.728 In1.808 fact, in1.785 the TEM1.722 the electron1.746 beam1.758 is focusedCa/Ca + REE on 0.49 a thin foil,0.49 the irradiated0.51 volume0.50 is0.38 therefore 0.09 much 0.05 lower than0.03 in the0.08 SEM and0.04 the electron0.06

Minerals 2020, 10, 77 8 of 19 dose per atom much higher. Moreover, the electron energy is an order of magnitude higher than for the SEM. Both factors determined the diffusion of the total amount of F. Consistently with SEM-EDS of bastnäsite, TEM-EDS analyses also show detectable amounts of Pb in the latter. Few analyses show a Ca/(Ca + REE) ratio slightly lower than 0.50 (e.g., 2 and 4), whereas most analyses show values slightly higher (especially 3 and 5). Whereas lower values can be explained with bastnäsite lamellae within synchysite (polysomatic faults), higher values would suggest the presence of vaterite lamellae, which has never been observed so far and it is considered improbable because metastable [18]. Therefore, unless vaterite lamellae could be imaged, the most plausible explanation of the observed higher values is an overestimation of Ca due to the low accuracy of TEM-EDS analysis.

Table 3. TEM-EDS Analysis of (Ca,REE)-Fluorcarbonates from Cinquevalli (Trento, Italy). Each Column Refers to a Single Grain (if not a Single Spot Analysis, the Number of Averaged Analyses is Indicated in Brackets). Atoms are Calculated on the Basis of Two Cations in the Chemical Formula. Relative Error in Brackets (Values in Italics when Below 2σ).

1(3) 2(3) 3(3) 4(3) 5(3) 6(4) 7(2) 8 9 10 11 12 Syn Syn Syn Syn Syn Syn Syn Syn Bas Bas Bas Bas Ca 1.05 (4) 0.98 (6) 1.08 (7) 0.96 (3) 1.10 (3) 1.01 (7) 1.00 (5) 1.04 (7) 0.18 (7) 0.14 (4) 0.19 (5) 0.20 (2) Y 0.05 (1) 0.03 (1) 0.06 (2) 0.05 (1) 0.04 (1) 0.04 (2) 0.02 (2) 0.03 (2) 0.13 (8) 0.11 (5) 0.14 (5) 0.13 (2) La 0.27 (2) 0.27 (3) 0.25 (3) 0.30 (2) 0.26 (1) 0.29 (4) 0.31 (3) 0.28 (4) 0.52 (15) 0.43 (9) 0.48 (10) 0.47 (3) Ce 0.45 (2) 0.47 (4) 0.41 (4) 0.49 (2) 0.42 (1) 0.48 (4) 0.49 (3) 0.48 (4) 0.80 (17) 0.80 (10) 0.94 (10) 0.79 (3) Nd 0.18 (2) 0.24 (3) 0.20 (3) 0.19 (1) 0.19 (1) 0.18 (3) 0.19 (3) 0.17 (3) 0.37 (13) 0.40 (8) 0.24 (8) 0.34 (3) Pb ------0.12 (3) - 0.08 (1) Ca/Ca + 0.53 0.49 0.54 0.48 0.55 0.51 0.50 0.52 0.09 0.07 0.10 0.10 REE

3.3.2. Suitable Crystal Orientation for Order/Disorder Assessment Stacking order/disorder information in (Ca,REE)-fluorcarbonates, and therefore polytypic and polysomatic analyses, can be achieved along low indexed orientations perpendicular to the c*-axis, which is the stacking directions of (001)-layers. In principle, four different nonequivalent orientations are suitable in C2/c synchysite: 130 , [100], 110 and [010]. However, [100] is indistinguishable from h i h i 130 in any case and [010] is undistinguishable from 110 in case of dynamical scattering, which makes h i h i h0l, l , 2n reflections, normally absent by symmetry, to be indeed present, and level out any intensity difference present under kinematic conditions. Polytypic disorder and related streaking on selected area diffraction (SAED) patterns, further complicate any possible distinction. Therefore, the number of distinguishable orientations reduces to two: [130], [100], and [130] (and related Friedel’s opposites) can be grouped as 100 orientation type; [110], [010], and [110] (and related Friedel’s opposites) can be h i grouped as 110 orientation type. These two distinguishable orientation types alternate every ~30 h i ◦ rotating the crystal around c* (Figure7).

3.3.3. [100] HRTEM Observations HRTEM images of the 100 orientation type (hereafter [100] HR images), mostly show a zig-zag h i contrast which corresponds to a perfectly ordered sequence of synchysite half-cells (repeat unit ~9.1 Å). The zig-zag contrast is caused by the alternating positions of fluorine atoms with respect to Ce atoms in CeF-layers (Figure8). Two adjoining half-cells determine the actual ~18.3 Å (001) periodicity of synchysite. These observations suggest that: i) most samples are free of polysomatic disorder, and ii) any polytypic disorder, if present, can only have a component along the observation direction. The related SAED pattern, which is an average view of a larger area (in this case ~0.5 µm in diameter), is streak-free (Figure8b), suggesting that this apparent order is maintained over larger distances than those reproduced in HR images (~70 nm). The HR image above and the related simulation (Figure8c), reveal that under optimal experimental conditions, i.e., close to Scherzer defocus (35–43 nm) and thin enough crystals (few nm), heavy atoms appear as dark fringes, which are thicker (darker) for Minerals 2020, 10, 77 9 of 19

CeF-layers (d) than for Ca-layers (f ), separated by rows of white dots, corresponding to C-layers (g). Average carbon positions appear offset along b* across the Ca-layers, whereas no shift is seen across the CeF-layers. Unfortunately, this detailed structural information could not be always achieved, because fluorcarbonates have been revealed to be very beam sensitive, and in some cases the samples could not Minerals 2020, 10, x FOR PEER REVIEW 9 of 19 be perfectly oriented.

Figure 7. First row: kinematic simulations of SAED patterns along [100] (left), which is indistinguishable Figure 7. First row: kinematic simulations of SAED patterns along [100] (left), which is from 130 , and 110 (right), which is indistinguishable in case of dynamical scattering from [010] indistinguishableh i h fromi 130, and 110 (right), which is indistinguishable in case of dynamical (indexing according to [010]). These patterns alternate every ~30◦ rotating the crystal around c*. Second scattering from [010] (indexing according to [010]). These patterns alternate every 30° rotating the row: corresponding experimental images. Note that: i) 00l reflection with l , 2n may be present in crystal around c*. Second row: corresponding experimental images. Note that: i) 00l reflection with l experimental images because of dynamical scattering, and ii) the diffuse streaking affecting hhl rows,  2n may be present in experimental images because of dynamical scattering, and ii) the diffuse h , 3n in 110 patterns. streakingh affectingi hhl rows, h  3n in 110 patterns. The only exception to the situation described so far is the presence of sporadic polysomatic faults detected3.3.3. [100] in HRTEM few [100] Observations HR images. They are revealed by the presence of a thicker dark fringe every four darkHRTEM fringes, images whereas of the in100 normal orientation synchysite type a(hereafter thicker dark [100] fringe HR images), occurs everymostly five show dark a zig fringes-zag (Figurecontrast9 ).which The faultedcorresponds lamellae to a measure perfectly ~14.1 ordered Å, which sequence is consistent of synchysite with thehalf lack-cells of (repeat a vaterite unit layer 9.1 (~4.2Å ). The Å) zig within-zag synchysitecontrast is (~18.3caused Å), by therefore the alternating transforming positions the ofVBVB fluorinesynchysite atoms sequencewith respect into toVBB Ce, i.e.,atoms a single in CeF parisite-layers half-cell. (Figure 8 It). shouldTwo adjoining be noted, half however,-cells determine that if a bastnäsite the actual layer 18.3 (~4.9 Å (001) Å) was periodicity missing, insteadof synchysite. of a vaterite These observations one, a slightly suggest thinner that: lamella i) most would samples have are resulted free of polysomatic (~13.4 Å), with disorder sequence, and VVBii) any. The polytypic difference disorder involved, if present, is probably can only close have to thea component measuring along error. the Unfortunately, observation direction. whereas The we canrelated reasonably SAED pattern, assume which that darker is an average fringes correspondview of a larger to heavy area atoms(in this layers, case 0.5 we µ cannotm in diameter), confidently is streak-free (Figure 8b), suggesting that this apparent order is maintained over larger distances than those reproduced in HR images (70 nm). The HR image above and the related simulation (Figure 8c), reveal that under optimal experimental conditions, i.e., close to Scherzer defocus (35–43 nm) and thin enough crystals (few nm), heavy atoms appear as dark fringes, which are thicker (darker) for CeF-layers (d) than for Ca-layers (f), separated by rows of white dots, corresponding to C-layers (g). Average carbon positions appear offset along b* across the Ca-layers, whereas no shift is seen across the CeF-layers. Unfortunately, this detailed structural information could not be always achieved,

Minerals 2020, 10, 77 10 of 19 distinguish between Ca-layers and CeF-layers, because of the slight misalignment affecting this HR Minerals 2020, 10, x FOR PEER REVIEW 10 of 19 image. Therefore, the actual layer sequence cannot be demonstrated, although on the basis of the measurements,because fluorcarbonates the VBB havesequence been revealed seems more to be probable very beam and sensitive in line, withand in the some belief cases that the adjoining samples Ca-layerscould not arebe perfectly metastable oriente [18].d.

Figure 8.8. SynchysiteSynchysite-(Ce)-(Ce) from Cinquevalli (Trento, Italy): ( a) [100] HRTEM image showing a zig zig-zag-zag contrast of perfectly ordered sequence of synchysitesynchysite half-cellshalf-cells (~9.1(9.1 Å Å). ). Every two of them originate the (001) (001) lattice lattice repeat repeat of of 18.3 ~18.3 Å . Å. (b) ( bRelated) Related SAED SAED pattern. pattern. Consistently Consistently with withthe HR the image, HR image, it is free it isof freestreaks. of streaks. 00l reflections 00l reflections with l  with2n shouldl , 2 nbeshould absent beby absentsymmetry by symmetry but are indeed but arepresent indeed because present of becausedynamical of dynamical effects. (c) e Enlargedffects. (c) filtered Enlarged portion filtered of portion (a) compared of (a) compared with a calculated with a calculated image image (inset; (inset;simulation simulation parameters parameters are: defocus are: defocus = 43 nm;= thickness43 nm; thickness = 1 nm, =atomic1 nm, vibration atomic vibration = 0.05 nm).= 0.05 Atom nm).ic Atomicpositions positions are indicated are indicated by dots: byred dots: = Ca; red yellow= Ca; = yellowCe; green= Ce; = F; green blue = averageF; blue =positionaverage of position C atoms). of CNote atoms). how Ca Note-layers how (f Ca-layers) and CeF- (layersf ) and (d CeF-layers) form dark (d fringes) form that dark are fringes separated that areby rows separated of white by rowsdots, ofcorresponding white dots, correspondingto C-layers (g). to Corresponding C-layers (g). Corresponding vaterite-like (V vaterite-like) and bastnäsite (V)- andlike ( bastnäsite-likeB) layers are also (B) layersindicated. are also Under indicated. optimal Under experimental optimal experimental conditions, Ce conditions,-layers form Ce-layers thicker form fringes thicker than fringes Ca-layers than; Ca-layers;moreover, the moreover, average the positions average of positionswhite dots of related white to dots carbon related atoms to carbonis offset atoms along isb* o acrossffset along the Cab*- acrosslayers, thewhereas Ca-layers, no shift whereas is seen no across shift isthe seen CeF across-layers the. CeF-layers.

InThe the only studied exception TEM to samples,the situation free described bastnäsite so grainsfar is the could presence not beof sporadic found. Bastnäsitepolysomatic usually faults formsdetecte fined in lamellae few [100] at HR synchysite images. borders They are with revealed coherent by boundariesthe presence (Figure of a thicker 10). Taking dark intofringe account every thefour poor dark tofringe impossibles, whereas distinguishability in normal synchysite between a thicker certain dark diff ractionfringe occurs patterns every reported five dark above, fringe thes following(Figure 9). crystallographicThe faulted lamellae relationships measure  could14.1 Å be, which established: is consistent [100] withSyn//[1 the10] lackBas; of (001) a vateriteSyn//(001) layerBas; (010)(4.2Syn Å )// within(110)Bas synchysite. A higher ( frequency18.3 Å ), theref of compositionalore transforming faults the consistent VBVB synchysite with VBB sequenceparisite into half-cells VBB, withini.e., a singleVBVB parisitesynchysite half were-cell. observed It should close be tonoted, the contact however, with that bastnäsite. if a bastnäsite The contrast layer in( the4.9 HRTEMÅ ) was imagemissing, shown instead in Figure of a vaterite 10 is characterized one, a slightly by rows thinner of bright lamella spots would ~9.1 have Å apart, resulted corresponding (13.4 Å ), to wi theth widthsequence ofa VVB synchysite. The difference half-cell. In involved several places, is probably the distances close to between the measuring these rows error. are Unfortunately, different from thewhereas 9.1 Å we periodicity, can reasonably indicating assume the that presence darker of fringe compositionals correspond faults. to heavy The fault atoms in layers, the center, we cannot closer toconfidently the boundary distinguish with bastnäsite, between is Ca characterized-layers and CeF by a-layers, larger distance because between of the slight neighboring misalignment bright fringes,affecting i.e., this ~14.1 HR Å,image. corresponding Therefore, tothe an actual additional layerB sequence-layer introduced cannot be in demonstrated, the regular VBVB althoughsequence on ofthe synchysite, basis of thei.e., measurements corresponding, the toVBB a parasite sequence half-cell. seems more The brightprobable fringes and in characterizing line with the belief the faults that inadjoining the upper Ca- partlayers of are the metastable image are [18]. only ~4.9 Å apart, which corresponds to the width of a B-layer. Despite the difference in contrast, also these faults correspond to additional B-layer in the regular VBVB sequence of synchysite. Indeed, as inferred from the SAED, the crystal is slightly misaligned, causing the two VB synchysite half-cells to be clearly distinguished. Therefore, a possible explanation of the different appearance of the VBB lamellae is that the extra B-layer enters the VBVB sequence in two distinguished half-cells, i.e., VBBVB vs. VBVBB.

Minerals 2020, 10, 77 11 of 19 Minerals 2020, 10, x FOR PEER REVIEW 11 of 19

Figure 9. [100] HR image of synchysite showing two polysomatic faults (arrows). The measured width Figure 9. [100] HR image of synchysite showing two polysomatic faults (arrows). The measured width is ~14.1 Å, consistent with a single parisite half-cell with sequence VBB, within VBVB synchysite with is 14.1 Å , consistent with a single parisite half-cell with sequence VBB, within VBVB synchysite with (001) repeat distance ~18.3 Å. Inset filtered region. (001) repeat distance 18.3 Å . Inset filtered region. 3.3.4. [110] HRTEM Observations In the studied TEM samples, free bastnäsite grains could not be found. Bastnäsite usually forms fine lamellaeSimilarly at to synchysite what has already borders been with reported coherent for boundaries parisite-(Ce) (Figure [7], synchysite-(Ce) 10). Taking into imaged account down the 110 shows alternating bright and dark fringes that in the thinnest part of the sample and under hpoori to impossible distinguishability between certain diffraction patterns reported above, the conditions close to Scherzer defocus can be assigned to V- and B-layers, respectively (Figure 11). As for following crystallographic relationships could be established: [100]Syn//[110]¯ Bas; (001)Syn//(001)Bas; parisite-(Ce), V- and B-layers are delimited by rows of white dots that correspond to the C-layers. (010)Syn//(110)Bas. A higher frequency of compositional faults consistent with VBB parisite half-cells Whereaswithin VBVB any changesynchysite in the were regular observed sequence close of to bright the contact and dark with fringes bastnäsite. indicates The the contrast presence in the of polysomaticHRTEM image disorder, shown the in positions Figure of10 theis white characterized dots, whenever by rows the of image bright has spots sufficient 9.1 resolution Å apart, andcorresponding is not too severely to the width affected of bya synchysite beam damage, half-cell. reveal In theseveral presence places, of polytypicthe distances disorder, between if present. these Indeed,rows are polytypism different from and polytypicthe 9.1 Å disorderperiodicity, in fluorcarbonates indicating the arisespresence through of compositional the different possibilitiesfaults. The to link the vaterite layers with the adjacent bastnäsite layers (s. below), which are revealed in 110 fault in the center, closer to the boundary with bastnäsite, is characterized by a larger distanceh i HRTEMbetween imagesneighboring by the bright relative fringes, positions i.e.,  of14.1 the Å , bright corresponding dots in adjacent to an additional synchysite B-layer half-ells. introduced Taking thein the bright regular spots VBVB as corners sequence of a half-cell, of synchysite, the di ff i.e.erent, corresponding stacking possibilities to a parasite can lead half to-cell. three The di ff brighterent geometriesfringes characterizing of the half-cells: the afaults rectangular in the cell upper (labelled part 0of in the Figure image 11), are and only two oblique4.9 Å cells, apart, one which with a positive slant (+) and the other with a negative slant ( ). corresponds to the width of a B-layer. Despite the difference− in contrast, also these faults correspond to additional B-layer in the regular VBVB sequence of synchysite. Indeed, as inferred from the SAED, the crystal is slightly misaligned, causing the two VB synchysite half-cells to be clearly distinguished. Therefore, a possible explanation of the different appearance of the VBB lamellae is that the extra B- layer enters the VBVB sequence in two distinguished half-cells, i.e., VBBVB vs. VBVBB.

Minerals 2020, 10, 77 12 of 19 Minerals 2020, 10, x FOR PEER REVIEW 12 of 19

FigureFigure 10 10.. [100][100] HRTEM HRTEM image image of of synchysite synchysite (Syn) (Syn) with with an an overgrowth overgrowth of of bastnäsite bastnäsite (Bas) (Bas) along along with with thethe SAED pattern pattern of of synchysite synchysite (inset). (inset). According According to the to the high high resolution resolution image image (enlarged (enlarged filtered filtered image of the squared region in the lower right side) bastnäsite is oriented along [110]. Synchysite shows some image of the squared region in the lower right side) bastnäsite is oriented along [110].¯ Synchysite polysomatic faults (arrows). The measured ~14.1 Å width is consistent with a VBB lamella within shows some polysomatic faults (arrows). The measured 14.1 Å width is consistent with a VBB lamella VBVB synchysite (~18.3 Å). The narrow faults present in the upper region measure ~4.9 Å, which is within VBVB synchysite (18.3 Å ). The narrow faults present in the upper region measure 4.9 Å , consistent with an extra B-layer within VBVB synchysite, therefore also these faults correspond to VBB which is consistent with an extra B-layer within VBVB synchysite, therefore also these faults sequences within synchysite (for explanation see text). correspond to VBB sequences within synchysite (for explanation see text). HRTEM images of the 110 orientation type (hereafter [110] HR images), reveal that the order 3.3.4. [110] HRTEM Observationsh i observed along 100 is only apparent. Whereas the (001) lattice periodicity of ~18.3 Å is constant, the h i slantSimilar of thely synchysite to what has half-cells already is been highly reported variable for (Figure parisite 11-).(Ce) Basically, [7], synchysite no ordered-(Ce) sequence imaged down larger than110 fewshows unit alternating cells was observedbright and in dark these fringe samples.s that In in addition, the thinnest SAED part patterns of the alongsample this and zone under are conditionsaffected by close streaks to onScherzerhhl layers defocus with hcan, 3ben, whereasassignedhhl to Vlayers- and with B-layers,h = 3 nrespectivelyare streak-free (Figure (Figure 11). 11 Asb). forThese parisite observations,-(Ce), V- and in agreementB-layers are with delimited previous by studiesrows of [white7], indicate dots that diff usecorrespond polytypic to disorderthe C-layers and. Whereasabsence ofany polysomatic change in disorder.the regular sequence of bright and dark fringes indicates the presence of polysomatic disorder, the positions of the white dots, whenever the image has sufficient resolution and is not too severely affected by beam damage, reveal the presence of polytypic disorder, if present. Indeed, polytypism and polytypic disorder in fluorcarbonates arises through the different possibilities to link the vaterite layers with the adjacent bastnäsite layers (s. below), which are revealed in 110 HRTEM images by the relative positions of the bright dots in adjacent synchysite

Minerals 2020, 10, x FOR PEER REVIEW 13 of 19 half-cells. Taking the bright spots as corners of a half-cell, the different stacking possibilities can lead to three different geometries of the half-cells: a rectangular cell (labelled 0 in Figure 11), and two oblique cells, one with a positive slant (+) and the other with a negative slant (−). HRTEM images of the 110 orientation type (hereafter [110] HR images), reveal that the order observed along 100 is only apparent. Whereas the (001) lattice periodicity of 18.3 Å is constant, the slant of the synchysite half-cells is highly variable (Figure 11). Basically, no ordered sequence larger than few unit cells was observed in these samples. In addition, SAED patterns along this zone are affected by streaks on hhl layers with h  3n, whereas hhl layers with h = 3n are streak-free (Figure 1Minerals1b). These2020, 10observations,, 77 in agreement with previous studies [7], indicate diffuse polytypic disorder13 of 19 and absence of polysomatic disorder.

Figure 11.11. ((aa)) F Filterediltered [110] [110] HRTEM image of synchysite-(Ce) synchysite-(Ce) from Cinquevalli. Cinquevalli. ( (bb)) Related Related FFT FFT hhl h n hhl pattern. The latterlatter showsshows distinct distinct reflections reflections along along hhlrows rows with with= h3 =, and3n, and severely severely streaked streakedrows hhl h n rowswith with, 3 ,h indicating  3n, indicating absence absenceof polysomatic of polysomatic disorder and disorder extensive and polytypic extensive disorder. polytypic Consistently, disorder. the HR image shows (002) half-cells with varying slants, but constant interplanar distance (lower left). Consistently, the HR image shows (002) half-cells with varying slants, but constant interplanar Therefore, the VBVB sequence is maintained throughout the crystal, as well as the composition, but distance (lower left). Therefore, the VBVB sequence is maintained throughout the crystal, as well as the relative orientation of the VB-layers within the sequence may be different from cell to cell. The the composition, but the relative orientation of the VB-layers within the sequence may be different white boxes in (a) include simulations for two different thickness and defocus conditions, which are from cell to cell. The white boxes in (a) include simulations for two different thickness and defocus in part reproduced and magnified in (c) (thickness = 15 nm, defocus = 43 nm, atomic vibrations = conditions, which are in part reproduced and magnified in (c) (thickness = 15 nm, defocus = 43 nm, 0.05 nm) and (d) (thickness = 3.5 nm, defocus = 45 nm, atomic vibrations = 0.09 nm), respectively. atomic vibrations = 0.05 nm) and (d) (thickness = 3.5 nm, defocus = 45 nm, atomic vibrations = 0.09 Although disordered and affected by beam damage, the HR images allow recognition of vaterite (V) nm), respectively. Although disordered and affected by beam damage, the HR images allow and bastnäsite (B) layers as alternating bright and dark fringes. Note, however, as dark and bright recognition of vaterite (V) and bastnäsite (B) layers as alternating bright and dark fringes. Note, fringes invert contrast with increasing thickness of the thin foil (top and bottom arrows in (a) are however, as dark and bright fringes invert contrast with increasing thickness of the thin foil (top and aligned vertically). bottom arrows in (a) are aligned vertically). 4. Discussion

4.1. Rock Mineral Association (Ca,REE)-fluorcarbonates from Cinquevalli are hosted in quartz-dikes with granitic texture made of quartz (79.4%), K-feldspar (6.9%), and chalcopyrite (1.4%) as major, primary, constituents. Quartz is present as: i) equant grains intergrown with (Ca,REE)-fluorcarbonates and ii) as in fibrous crystals recalling chalcedony and unrelated to fluorcarbonates. Muscovite (9.5%), possibly overestimated by Rietveld refinement because of (001) preferential orientation, has not been observed by optical or electron microscopy, but in the alteration of K-feldspar as white mica (pinite). Alteration also affects chalcopyrite, which shows an Fe-Cu-oxide rim. Metallic bismuth, detected by SEM-EDS, completes the mineral association. Minerals 2020, 10, 77 14 of 19

These observations suggest that the crystallization of the quartz-dikes was a multistage event, at least judging from the two generations of quartz. Chalcedony possibly formed in a later stage under higher undercooling conditions. The graphic texture of the intergrowth between primary quartz and (Ca,REE)-fluorcarbonates suggest a simultaneous crystallization of quartz and fluorcarbonates. The presence of metallic bismuth and chalcopyrite are consistent with a reducing environment during the first stage of crystallization, but the pinite alteration of K-feldspar and the oxide rim wrapping chalcopyrite suggest a late alteration stage under higher oxygen fugacity and in the presence of water. SEM-EDS analyses reveal that synchysite and bastnäsite are Ce-rich, with significant amounts of La and Nd, lower amounts of Y and Pr, and minor amounts of Sm and Gd. In total, in the REE-oxides amount to ~2.8 wt% of the rock and are confined in fluorcarbonates finely intergrown with quartz. The small concentration and the dispersion within a resilient silicate matrix make this occurrence scarcely interesting for the ore mineral industry.

4.2. Polytypism of Synchysite The crystal structure of synchysite-(Ce) has been refined and described in the C2/c space group [17]. 1 1 The synchysite structure consists of Ca-layers at z = 0 and z = 2 , alternating with CeF-layers at z = 4 3 and z = 4 , all separated by intercalated CO3-layers. The heavy atoms form columns of Ca and Ce alternating along c* in a hexagonal stacking, as formerly predicted [16]. To accomplish chemical 1 bonding, the CO3-layers above z = 2 are shifted by ~2.37 Å along [110] and those above z = 1 by another ~2.37 Å along [110] with respect to the position that they should have in bastnäsite, therefore breaking the hexagonal symmetry and leading to monoclinic symmetry. The overall shift of the CO3-layers along a is ~4.11 Å, leading to a monoclinic angle of ~102.7◦. It can be easily realized that the layered structure of the (Ca,REE)-fluorcarbonates is at the origin of syntaxial intergrowths with a theoretically unlimited number of polysomatic and polytypic stacking variants. In the studied samples, polysomatic faults are sporadic, but polytypic disorder is widespread. Basically, no ordered domain, i.e., free of polytypic faults, larger than few unit cells was found. Polytypism results from the insertion of a Ca-layer between bastnäsite portions and the different types of linkages it can form with the bastnäsite portions above and below it [18]. As discussed above, in ordered synchysite-(Ce), the operating stacking vectors between CO3-layers in alternating bastnäsite portions are [110] and [110], respectively, which are related by 60 rotations (Figure 12). A consistent ± ◦ bonding pattern, however, is also obtained for shifts in the second bastnäsite portion along [010] and [110], which are rotated by –60◦ and 180◦, respectively, from the former shift (i.e., [110]). In contrast, shift directions rotated either 0 or 120 relative to the first stacking vector (n 60 , n = even) would put ◦ ± ◦ · ◦ the CO3 groups aligned with the heavy atoms columns along c*, impeding a consistent bonding pattern. Therefore, all polytypes and polytypic faults in synchysite-(Ce) are based on n 60 (n = odd) shifts. · ◦ The complete set of all possible shifts and how they are recognized in HR images has been derived for parisite [7] and here reproduced for synchysite (Table4). Such shifts are common to all (Ca,REE)-fluorcarbonates, once a consistent monoclinic unit cell is chosen. When the structure is seen down [100], the [110] and [110] shifts would appear as –b/6 and +b/6 translations in HR images (Table4), leading to a zig-zag sequence, as actually observed in Figure8. Any polytypic departure from the ordered sequence will move the structure towards the allowed [010] shift, if occurring after the [110] one, or [010], if occurring after [110]. Both faults involve translation of b/3. Because of the half-cell periodicity due to the C-centring of the structure, a shift of +b/3 will appear as a –b/6 shift on [100] HR images, and a shift of –b/3 as a +b/6 shift, therefore undistinguishable from the structural changes expected in an ordered sequence. A similar analysis can be done for any of the 100 -type indistinguishable directions. h i As can be seen in [110] HRTEM images, synchysite-(Ce) from Cinquevalli is extensively faulted. In [100] HR images, however, these faults cannot be seen, due to the equality between the projected shifts in regularly stacked sequences and in faulted sequences (and not because it has component along the observation direction only as assumed at first glance). When synchysite is observed along [110], Minerals 2020, 10, 77 15 of 19

the [110] shift, which is collinear with the observation direction, has no component perpendicular to c*, whereas the [110] shift when projected onto the image plane results in a shift component equal to b/3 cos(30◦) ~2.05 Å (or a/6) perpendicular to the stacking direction (Figure 12). Minerals· 2020, 10, x FOR PEER REVIEW 15 of 19

FigureFigure 12.12. ((aa)) ProjectionProjection onon (001)(001) ofof aa fgdgfgdg-layer-layer ofof synchysite-(Ce)synchysite-(Ce) showingshowing thethe allowedallowed shiftsshifts relatedrelated

byby 6060°◦ rotationsrotations (1(1 toto 6)6) ofof thethe carbonate-layercarbonate-layer (arrows). (arrows). FilledFilledcircles circles indicate indicate atom atom positions positions (green (green= = F;F; blueblue= = Ca;Ca; yellowyellow == Ce; greygrey == C) and solid lines thethe monoclinicmonoclinic unitunit cell.cell. ((bb)) DrawingDrawing ofof thethe mainmain crystallographiccrystallographic directions directions along along which which fluorcarbonates fluorcarbonates are are commonly commonly observed observed by HRTEM. by HRTEM. Note Note that, dependingthat, depending on the on observation the observation direction, direction, on the on image the image plane plane (perpendicular (perpendicular to it) ato given it) a given shift mayshift havemay ahave component a component equal toequalb/3 (~2.37to b/3 Å);(2.37b/3 Åcos(30 ); b/3cos(30°)) (~2.05 Å,(2.05 or a /Å6); , or ab/6);/3 cos(60 or b/3)cos(60°) (~1.18 Å ( or1.18b/6); Å oror · ◦ · ◦ 0b/6); (summary or 0 (summary in Table 4in). Table 4).

Table 4. Shift Magnitude on the (001) Plane of the Carbonate-Layer in (Ca,REE)-Fluorcarbonates as The complete set of all possible shifts and how they are recognized in HR images has been Function of Both the Operating Stacking Vector (First Row) and of The Observation Direction (Left derived for parisite [7] and here reproduced for synchysite (Table 4). Such shifts are common to all Column). The Minus Sign Indicates that the Shift Has a Component Along the Negative Semi-Axis of (Ca,REE)-fluorcarbonates, once a consistent monoclinic unit cell is chosen. the Monoclinic Unit Cell.

Table 4. Shift Magnitude[110] on (1) the (001) [110] Plane (2) of[010] the (3)Carbonate [110]-Layer (4) in[1 (Ca,REE)10] (5)-Fluorcarbonates [010] (6) as Function of Both the Operating Stacking Vector (First Row) and of The Observation Direction (Left [110] –a/6 0 a/6 a/6 0 –a/6 Column). The Minus Sign Indicates that the Shift Has a Component Along the Negative Semi-Axis of [100] –b/6 b/6 b/3 b/6 –b/6 –b/3 the Monoclinic[110] Unit Cell.0 a/6 a/6 0 –a/6 –a/6 [130] –b/6 –b/3 –b/6 b/6 b/3 b/6 [1¯10]¯ (1) [110]¯ (2) [010] (3) [110] (4) [110]¯ (5) [010]¯ (6) [010] –a/6 –a/6 0 a/6 a/6 0 [130][110]¯ ––ab/6/3 –0b /6 a/6b/ 6 a/6b /3 0 b/6–a/6 –b /6 [100] –b/6 b/6 b/3 b/6 –b/6 –b/3 Therefore, ordered[110] synchysite0 shoulda/6 appeara/6 in [110] HR0 images– asa/6 VB half-cells–a/6 with alternating “0” and “+” slants[130] (or “0”– andb/6 “–” slants).–b/3 This– situationb/6 isb/6 hardly observedb/3 in synchysite-(Ce)b/6 from Cinquevalli, where[010] random –a/6 repetitions –a/6 of “–”, “0”0 and “+” slantsa/6 dominatea/6 and where0 ordered regions are no thicker than[130]¯ few unit–b/3 cells (Figure–b/6 11a).b/6 Subsequent b/3 “0” slantsb/6 in [110]–b/6 HR images can be interpreted as stacking vectors related by 180 rotations, e.g., vectors 1 and 4 in Figure 12, and ± ◦ alternating “+” and “–” slants as stacking vectors¯¯ related by¯ 180◦ rotations, e.g., vectors 2 and 5. When the structure is seen down [100], the [110] and [110]± shifts would appear as –b/6 and +b/6 A similar analysis can be done for any of the 110 -type indistinguishable directions. translations in HR images (Table 4), leadingh to ai zig-zag sequence, as actually observed in Figure 8. Any polytypic departure from the ordered sequence will move the structure towards the allowed 4.3. A Brief Comparison with the Polytypism of Mica [010]¯ shift, if occurring after the [1¯10]¯ one, or [010], if occurring after [110].¯ Both faults involve translationThe stacking of b/3. sequence Because of synchysite-(Ce)the half-cell periodicity is analogous due to thatthe C of-centring 2:1 mica, of such the asstructure, muscovite, a shift if the of CO+b/33-layer will appear is equated as a to – theb/6 (Si,Al)Oshift on4 -layer, [100] HR the images, CeF-layer and to the a shift AlO of5(OH)-layer, –b/3 as a + andb/6 theshift Ca-layer, therefo tore theundistinguishable K-layer (interlayer) from [17 the]. Micastructural polytypes changes result expected from the in translation an ordered of sequence. the (Si,Al)O A 4similar-layers analysis relative tocan each be done other, for whereas any of synchysitethe 100-type polytypes, indistinguishable from the translation directions. of CO3-layers above the Ca-layer. As can be seen in [110] HRTEM images, synchysite-(Ce) from Cinquevalli is extensively faulted. In [100] HR images, however, these faults cannot be seen, due to the equality between the projected shifts in regularly stacked sequences and in faulted sequences (and not because it has component along the observation direction only as assumed at first glance). When synchysite is observed along [110], the [1¯10]¯ shift, which is collinear with the observation direction, has no component perpendicular to c*, whereas the [110]¯ shift when projected onto the image plane results in a shift component equal to b/3cos(30°) 2.05 Å (or a/6) perpendicular to the stacking direction (Figure 12).

MineralsMineralsMinerals 20 20 20202020, ,10 ,10 10, ,x ,x FORx FOR FOR PEER PEER PEER REVIEW REVIEW REVIEW 161616 of of of 19 19 19 Minerals 20202020,,, 1010,,, xxx FORFOR PEERPEER REVIEWREVIEW 1616 ofof 1919 Therefore,Therefore,Therefore, ordered ordered ordered synchysite synchysite synchysite should should should appear appear appear in in in [110] [110] [110] HR HR HR images images images as as as VB VB VB half half half--cells-cellscells with with with alternating alternating alternating Therefore, ordered synchysite should appear in [110] HR images as VB half--cellscells withwith alternatingalternating “0”“0”“0” and and and “+” “+” “+” slants slants slants (or (or (or “0” “0” “0” and and and “ “ –“–”–” ”slants). slants). slants). This This This situation situation situation is is is hardly hardly hardly observed observed observed in in in synchysite synchysite synchysite--(Ce)-(Ce)(Ce) from from from “0” and “+” slants (or “0” and “–” slants). This situation is hardly observed in synchysite--(Ce)(Ce) fromfrom Cinquevalli,Cinquevalli,Cinquevalli, where where where random random random repetitions repetitions repetitions of of of “ “– “–”,–”,”, “0” “0” “0” and and and “+” “+” “+” slants slants slants dominate dominate dominate and and and where where where ordered ordered ordered Cinquevalli, where random repetitions of “–”, “0” and “+” slants dominate and where ordered regionsregionsregions are are are no no no thicker thicker thicker than than than few few few unit unit unit cells cells cells (Figure (Figure (Figure 1 1 1a11a1a).). ).Subsequent Subsequent Subsequent “0” “0” “0” slants slants slants in in in [110] [110] [110] HR HR HR images images images can can can regionsregions areare nono thickerthicker thanthan fewfew unitunit cellscells (Figure(Figure 11a).). SubsequentSubsequent “0”“0” slantsslants inin [110][110] HRHR imagesimages cancan bebebe interpreted interpreted interpreted as as as stacking stacking stacking v v ectorsvectorsectors related related related by by by   180° 180° 180° rotations, rotations, rotations, e.g. e.g. e.g., ,vectors ,vectors vectors 1 1 1and and and 4 4 4in in in Figure Figure Figure 12 12 12, ,and ,and and be interpreted as stacking vectors related by  180° rotations, e.g.,, vectors 1 and 4 in Figure 12,, andand alternatingalternatingalternating “+” “+” “+” and and and “ “ –“–”–” ”slants slants slants as as as stacking stacking stacking vectors vectors vectors related related related by by by   180° 180° 180° rotations, rotations, rotations, e.g. e.g. e.g., ,vectors ,vectors vectors 2 2 2and and and 5. 5. 5. A A A alternating “+” and “–” slantsslants asas stackingstacking vectorsvectors relatedrelated byby  180° rotations, e.g.,, vectors 2 and 5. A similarsimilarsimilar analysis analysis analysis can can can be be be done done done for for for any any any of of of the the the  110 110110--type-typetype indistinguishable indistinguishable indistinguishable directions. directions. directions. similarsimilar analysisanalysis cancan bebe donedone forfor anyany ofof thethe 110--typetype indistinguishableindistinguishable directions. 4.3.4.3.4.3. A A A Brief Brief Brief Comparison Comparison Comparison with with with the the the Polytypism Polytypism Polytypism of of of Mica Mica Mica 4.3. A Brief Comparison with the Polytypism ofof Mica TheTheThe stacking stacking stacking sequence sequence sequence of of of synchysite synchysite synchysite--(Ce)-(Ce)(Ce) is is is analogous analogous analogous to to to that that that of of of 2:1 2:1 2:1 mica, mica, mica, such such such as as as muscovite, muscovite, muscovite, if if if The3 stacking sequence of synchysite4 --(Ce)(Ce) isis analogousanalogous toto thatthat ofof 2:12:15 mica,mica, suchsuch asas muscovite,muscovite, ifif thethethe CO CO CO3-3-layer-layerlayer is is is equated equated equated to to to the the the (Si,Al)O (Si,Al)O (Si,Al)O4-4-layer,layer,-layer, the the the CeF CeF CeF--layerlayer-layer to to to the the the AlO AlO AlO5(OH)5(OH)(OH)--layer,layer,-layer, and and and the the the C C Caa-a-layerlayer-layer the CO33-layer is equated to the (Si,Al)O44-layer, the CeF-layer to the AlO55(OH)-layer, and the Ca-layer thethe COCO --layerlayer isis equated toto thethe (Si,Al)O(Si,Al)O --layer,layer, thethe CeFCeF--layerlayer toto thethe AlOAlO (OH)(OH)--layer,layer, andand thethe CCa4--layerlayer tototo the the the K K K--layer-layerlayer (interlayer) (interlayer) (interlayer) [17]. [17]. [17]. Mica Mica Mica polytypes polytypes polytypes result result result from from from the the the translation translation translation of of of the the the (Si,Al)O (Si,Al)O (Si,Al)O4-4-layerslayers-layers Mineralsto the2020 K,-10layer, 77 (interlayer) [17]. Mica polytypes result from the translation of the (Si,Al)O44-layers16 of 19 toto the the K K--layerlayer (interlayer) (interlayer) [17]. [17]. Mica Mica polytypes polytypes result result from from the the translation translation of of 3 the the (Si,Al)O (Si,Al)O --layerslayers relativerelativerelative to to to each each each other, other, other, whereas whereas whereas synchysite synchysite synchysite polytypes, polytypes, polytypes, from from from the the the translation translation translation of of of CO CO CO3-3-layerslayers-layers above above above the the the Ca Ca Ca--- relativerelative toto eacheach other,other, whereaswhereas synchysitesynchysite polytypes,polytypes, fromfrom thethe translationtranslation ofof COCO33--layerslayers aboveabove thethe CaCa-- layer.layer.layer. layer. layer.layer. 1 2 TheThe six six standard standard micamica polytypes polytypes ( (11MM, ,2 2MM11, ,3 3TT, ,2 2MM22, ,2 2OO and and 6 6HH) )are are classified classified in in subfamily subfamily-A-A TheThe six six standard standard mica mica polytypes polytypes (1(1MM,, 22MM1, 3T, 2 M 2, ,2 2OO andand 6 6HH) )are are classified classified in in subfamily subfamily-A-A The six standard mica polytypes (1M, 2M11, 3T, 2M22, 2O and 6H) are classified in subfamily-A polytypesThe six (1 standardM, 2M1 and mica 3T polytypes), characterized (1M, 2byM stacking, 3T, 2M vectors, 2O and related 6H) are by classifiedn60° rotations in subfamily (n = even-A), polytypespolytypespolytypes (1 M(1 (1M,M 2,M ,2 2MMand1 1and and 3 T3 3T),T),), characterized characterized characterized byby by stackingstacking vectors vectorsvectors related related by by by n nn60°60°60 rotations rotationsrotations ( (nn (= n= even =eveneven),),), polytypes (1M, 21M11 and 3T), characterized by stacking vectors related by n60° ◦rotations (n = even), polytypesand subfamily (1M,- 2BM polytypes and 3T), (2 characterizedM2, 2O and 6H by), characterizedstacking vectors by nrelated60° rotations by n 60°· (n rotations = odd) [30 (n,31 =] even. These), andandand subfamily-B subfamily subfamily- polytypes-BB polytypes polytypes (2 (2 M(2MM,2 2, ,2O 2OOand and and 6 6 H6HH),),), characterizedcharacterized characterized by by n n60°6060° rotations rotations ( (n n = == odd) odd)odd) [ [3030 [30,31,31,31]]. .These]. These These and subfamily--B polytypes (2M2 22,, 2O and 6H),), characterizedcharacterized byby n60°◦ rotations ((n = odd) [[30,,31]].. TheseThese basicbasicbasic polytypes polytypes polytypes along along along with with with their their their stacking stacking stacking vectors vectors vectors and and and notable notable notable ex ex ex·amplesamplesamples are are are reported reported reported in in in Table Table Table 5. 5. 5. basicbasic polytypes polytypes along along with with their their stacking stacking vectors vectors and and notablenotable examplesexamples are are reported reported in in Table Table 5.5.

TableTableTable 5. 5. 5. Comparison Comparison Comparison Between Between Between Mica Mica Mica and and and (Ca,REE) (Ca,REE) (Ca,REE)-Fluorcarbonate-Fluorcarbonate-Fluorcarbonate Polytypes Polytypes Polytypes. . . TableTable 5. 5.Comparison Comparison Between Between Mica Mica and andand (Ca,REE)-Fluorcarbonate (Ca,REE)(Ca,REE)--Fluorcarbonate Polytypes Polytypes... Notable Examples Polytype Vector NotableNotable Examples Examples PolytypePolytypPolytypee VectorVectorVector NotableNotable Examples Examples Polytypee Vector SymbolicSymbolicSymbolic Charts Charts Charts (Ca,REE)(Ca,REE)(Ca,REE)--- SymbolSymbolSymbolSymbol RotationsRotationsRotationsRotations Symbolic Charts MicasMicasMicas * * * (Ca,REE)- FFluorcarbonatesluorcarbonates(Ca,REE)(Ca,REE)(Ca,REE)--- Symbol Rotations MicasMicas ** * Fluorcarbonates FFluorcarbonatesluorcarbonatesluorcarbonates MuscoviteMuscoviteMuscovite [ 32[ 32[32];]; ];Phlogopite Phlogopite Phlogopite [ 33[ 33[33] ] ] MuscoviteMuscoviteAnnite [ [33 [323232];];];]; Celadonite PhlogopitePhlogopite Phlogopite [ [34[ [333333]] ]] AnniteAnnite [ 33[33];]; Celadonite Celadonite [ 34[34] ] PolylAnniteAnniteithionite [ [333333];];]; [CeladoniteCeladonite Celadonite35]; Clintonite [[ [3434]]] [36] Should not be 1M 0° PolylPolylithioniteithionite [ 35[35];]; Clintonite Clintonite [ 36[36] ] ShouldShould not not be be 11MM 0°0° PolylPolylithioniteNorrishiteithioniteithionite [37 [[ [3535];35 ];Preisw];]; ClintoniteClintonite Clintoniteerkite [ [ [36[363638]]] ] Shouldpossible not be 1M11M 00°0° NorrishiteNorrishite [ 37[37];]; Preisw Preiswerkiteerkite [ 38[38] ] Shouldpossiblepossible not be possible ◦ NorrishiteNorrishite [ [373737];];]; PreiswPreisw Preiswerkiteerkiteerkite [[ [3838]]] possible ChromophylliteChromophylliteChromophyllite [ 39[ 39[39];] ;] ; ChromophylliteChromophyllite [ [393939]]];;; SiderophylliteSiderophylliteSiderophyllite [ 40[ 40[40] ] ] SiderophylliteSiderophyllite [ [404040]]]

MargariteMargariteMargarite [ 41[ 41[41];]; ];Paragonite Paragonite Paragonite [ 42[ 42[42] ] ] Margarite [4141];]; ParagoniteParagonite [[4242]] ShouldShouldShould not not not be be be 2M11  120° MargaritePolylithionite [41]; [ Paragonite43]; Bityite [[4244]] 22MM1  120° 120° PoPolylithionitelylithionite [ 43[43];]; Bityite Bityite [ 44[44] ] Shouldpossible not be 2M22M11 120 120°120° PoPolylithionitelylithionitelylithionite [[ [434343];];]; BityiteBityite Bityite [[ [444444]]] Shouldpossiblepossible not be possible 1 ± ◦ KinoshitaliteKinoshitaliteKinoshitalite [ 45[ 45[45] ] ] possible KinoshitaliteKinoshitalite [ [454545]]] Paragonite [46] ParagoniteParagonite [ 46[46] ] Should not be 3T + 120° PolylithioniteParagoniteParagonite [ [464646 []47]] ] ShouldShould not not be be 33TT ++ 120° 120° PolylithionitePolylithionite [ 47[47] ] Shouldpossible not be 3T33T +++120 120°120° PolylithionitePolylithionite [ [474747]]] Shouldpossiblepossible not be possible ◦ MuscoviteMuscoviteMuscovite [ 48[ 48[48] ] ] possible MuscoviteMuscovite [ [484848]]]

LepidoliteLepidoliteLepidolite [ 49[ 49[49];]; ];Illite Illite Illite [ 50[ 50[50] ] ] Lepidolite [49]; Illite [50] SynchysiteSynchysite-(Ce)-(Ce) [17] [17] 2  Lepidolite [4949];]; IlliteIllite [[5050]] Synchysite-(Ce) [17] 22M2MM2 2  60° 60° 60° PolylithionitePolylithionitePolylithionite [ 51[ 51[51];] ;]Muscovite ;Muscovite Muscovite [ 52[ 52[52] ] ] SynchysiteSynchysite-(Ce)-(Ce) [17] [17 ] 2M2 60◦ Polylithionite [51]; Muscovite [52] SynchysiteParisite--(Ce)-(Ce)(Ce) [18] [17][17] 22M22 ± 60°60° Polylithionite [5151]];; Muscovite [5252]] ParisiteParisiteParisite-(Ce)-(Ce)-(Ce) [18] [18] [18 ] NanpingiteNanpingiteNanpingiteNanpingite [ 53[ [53[5353];]]; ;]Tobelite ;Tobelite TobeliteTobelite [ 54[ 54[54] ] ] Parisite--(Ce)(Ce) [18][18] Nanpingite [5353]];; TobeliteTobelite [[5454]] Possible, few unit Possible,Possible,Possible, few few few unit unit unit cells Anandite [55] cellsPossible, observed few unit at the 2O  180° AnanditeAnanditeAnandite [ 55[ 5555] ]] cellscellsobserved observed observed at the at at the TEMthe 2O22OO 180 180° 180°◦ Anandite [5555]] cellscells observedobserved atat thethe 22O ± 180°180° PhlogopitePhlogopitePhlogopitePhlogopite [ 56[ 56[56] ]] ] TEMTEMTEMscale scale scale scale (this (this (this (this work, work, work, work, Phlogopite [5656]] TEM scale (this work, FigureFigureFigure 11 11 11).). ). Possible,Figure should 1111).). have Possible,Possible, should should have have Possible,Possible,a “0 +should should − 0 − + have0” a Possible, but never observed as aa “0 “0 + + − − 0 0 − − + + 0” 0” 6H +60° Possible,Possible,Possible, but but but never never observed observed as as sequenceaa “0 “0“0 + ++ −− on00 −− [110] +++ 0”0”0” HR 6H66HH ++60°60+60°◦ Possible, but never observed as sequencesequence on− on [110] [110]− HR HR 66H +60°+60° orderedorderedorderedordered single single single crystal. crystal. crystal. sequencesequencesequence onon on [110][110] [110] HRHR HR ordered singlesingle crystal.crystal. images,images,images, not not not found found found images,images,images, not notnot found foundfound yet. yet.yet.yet. yet. ** For* *For For a a acomprehensive comprehensive comprehensive review reviewreview review ofof of of mica-polytypesmica mica mica-polytypes-polytypes-polytypes see see see see Reference R R efRefeferenceerenceerence [57 [ ].57[ 57[57].]. ]. ** For a comprehensive review of mica--polytypes see Refeferenceerence [[5757].]. SynchysiteSynchysiteSynchysite--(Ce)(Ce)-(Ce) isis is characterizedcharacterized characterized byby by  60°60°60° rotationsrotations rotations ofof of subsequentsubsequent subsequent bastnäsitebastnäsite bastnäsite portionsportions portions aboabo aboveveve thethe the Synchysite-(Ce)Synchysite-(Ce) is is characterized characterized by by 6060°◦ rotationsrotations ofof subsequent bastnäsite bastnäsite portions portions abo aboveve the the Synchysite--(Ce)(Ce) isis characterizedcharacterized byby 60° rotations2 of subsequent bastnäsite portions above the CaCaCa--layers,layers,-layers, therefore therefore therefore can can can be be be likened likened likened to to to the ±thethe 2 2M2MM2 2 subfamily subfamilysubfamily--BB- B mica mica mica polytype, polytype, polytype, such such such as as as lepidolite, lepidolite, lepidolite, for for for Ca-layers,Ca--layers,layers, therefore therefore therefore can can can be be be likened likened likened to to to the thethe 2 2MM222 subfamily-B subfamilysubfamily--B mica polytype, polytype, such such as as lepidolite, lepidolite, for for instance.instance.instance. Hypothetical Hypothetical Hypothetical subfamily subfamily subfamily--AA-A (Ca,REE) (Ca,REE) (Ca,REE)--fluorcarbonates,fluorcarbonates,-fluorcarbonates, since since since entailing entailing entailing rotations rotations rotations of of of 0 0 0or or or   120°120°120° instance.instance. Hypothetical Hypothetical subfamily-A subfamily-A (Ca,REE)-fluorcarbonates, (Ca,REE)-fluorcarbonates, since entailing entailing rotations rotations of of 0 0or or 120°120 instance.instance. HypotheticalHypothetical subfamilysubfamily--A (Ca,REE)--fluorcarbonates,fluorcarbonates,3 sincesince entailingentailing rotationsrotations ofof 00 oror 120° ◦ thatthatthat would wouldwould align alignalign the thethe vertical verticalvertical edges edgesedges of ofof the thethe CO COCO3 3 triangles trianglestriangles above aboveabove the thethe heavy heavyheavy atoms atomsatoms columns columnscolumns have, have,have,± as asas thatthatthat would wouldwould align alignalign the thethe vertical verticalvertical edges edgesedges of ofof the thethe CO COCO333 triangles trianglestriangles aboveabove the heavy heavy atoms atoms columnscolumns columns have, have, as as expected, never been observed. In contrast, subfamily-A mica polytypes are even more abundant in nature than subfamily-B mica polytypes. This can be ascribed to the different atomic arrangement in the interlayer. In subfamily-B polytypes, basal tetrahedral oxygens on the adjacent layers are exactly superimposed along the c* direction. In subfamily-A polytypes, these basal oxygens are laterally displaced from each other due to ditrigonal distortion of the tetrahedral rings, thereby reducing the repulsive forces between the oxygens facing across the interlayer and making the structural configuration energetically more favorable [58]. Parisite-(Ce), the other (Ca,REE)-fluorcarbonate whose structure has been refined [18], should be considered again as belonging to the 2M2 polytype, since it is characterized by the same stacking vectors as synchysite-(Ce). Other possible short range (Ca,REE)-fluorcarbonate polytypes should be the 2O and the 6H. The 2O polytype has been observed as a few unit cells repeats at the TEM scale (Figure 11), whereas the 6H has not yet been clearly characterized, not even at the TEM scale. Minerals 2020, 10, 77 17 of 19

Funding: This article is an outcome of MIUR Project—Dipartimenti di Eccellenza 2018–2022. The university grant (Fondi di Ateneo) from the University of Milano-Bicocca is also greatly acknowledged. All authors have read and agreed to the published version of the manuscript. Acknowledgments: Paolo Gentile kindly provided the Cinquevalli samples. Lucia Galimberti carried out EDXRF and XRPD analyses. Simone Suardi prepared the sample and carried out a preliminary characterization of the samples during his bachelor thesis. The manuscript benefits from the revision from two anonymous referees. Conflicts of Interest: The author declares no conflict of interest.

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