4.1. Characterization of Nb-Bearing Phases
HFSE-bearing minerals have been recognized microscopically, using polarizing microscope and electron probe imaging (Figure 3
). Anhedral to euhedral magnetite is either homogeneous or consists of porous cores and compact overgrowths. Other Fe–Ti(±Nb,Ta) oxides occur as relics of early ilmenite replaced by rutile, rutile intergrowths with the early ilmenite, rutile exsolutions within early ilmenite, rutile-ilmenite-silicate symplectite around resorbed ilmenite grains, and compact rutile rims around columbite associated with the early ilmenite. Late ilmenite occurs together with rutile, orthopyroxene, quartz, and sanidine in the interstitial silicate glass. Ilmenite is an early-crystallizing phase, but its temporal relationship with magnetite and zircon remains unclear.
Zircon crystals, up to 500 μm in size, are coeval with late Ti-magnetite. The zircon grains consist of opaque cores overcrowded with uranothorite inclusions. BSE images (Figure 3
d) document sector-zoned distribution of uranothorite inclusions, indicating a breakdown of the ZrSiO4
solid solution during the initial zircon growth. The opaque uranothorite-rich core is overgrown by a clear inclusion-free rim. Weak magmatic oscillatory growth zoning is discernible in BSE images, whereas cathodoluminescence imaging did not reveal any zoning.
(Y,REE,U,Th)–(Nb,Ta,Ti)-oxides are opaque to brown and translucent along margins in transmitted light. They form isolated anhedral to subhedral isometric grains and clusters enclosed in the interstitial glass and rock-forming quartz and feldspars. Oscillatory growth and domain zonings are discernible in BSE images (Figure 3
c,e). (Y,REE,U,Th)–(Nb,Ta,Ti)-oxides post-date the Th-rich zircon cores, but they are coeval with clear zircon rims. Crystallization succession scheme of Fe–Ti(±Nb,Ta) and (Y,REE,U,Th)–(Nb,Ta,Ti)-oxides based on observations of spatial relationships in thin sections is summarized in Figure 4
Analysis of EPMA data (Electronic Supplementary file, Tables S2–S4
) revealed a total of four clusters and one outlier (Figure 5
). Table S1
refers to cluster 1, projecting within the pyrochlore super-group field. Hence, crystallochemical formulas were recalculated from EPMA as AB2X6Y
, such that large [8
]-coordinated cations (Na, Ca, Sr, Ba, Fe2+
, Pb, Y, REE, U, Th, ± vacancy, ± H2
O), were affiliated to the A
]-coordinated, high-field-strength cations (Nb, Ta, Ti, W, Zr, Hf, Fe3+
, Mg, Al, and Si) without vacancies were attributed to the B
-site contained O plus subordinate OH and F, and Y-
site O, OH, H2
O, F, vacancy, and very large cations, such as K, Cs, Rb [37
]. The dominant species in the Y
-site defined a primary prefix of the mineral name. The secondary prefix was derived from the dominant B
-site cation. Structural formulas of pyrochlore listed in Table S2
are charge-balanced, with the fixed number of 2 cations in the vacancy-free B
-site. The maximum OH concentration that could be accommodated in the crystal structure was calculated as an equivalent to the sum of deficits in the A
- and Y
-sites. The calculated OH content in apfu was then recalculated to wt. % H2
O. Fluorine concentrations were always below the detection limit of 0.03 wt. %.
Maximum OH(A + Y
) concentrations have been sometimes greater than 0.5 apfu, however, owing to the deficient A
-site, only one analysis (Ca3d, an1) showed the predominance of OH in the Y
-site, assuming no vacancies. Due to negligible F and extremely large cations (K, Rb, Cs), oxygen must have been the dominant anion at the Y
-site in all but one analyses. Since calcium (0.774–1.013 apfu) predominated in the A
-site, analyzed minerals could be named as oxycalciopyrochlore according to the dominant constituent rule [42
] and the criteria summarized in [37
]. The remaining analysis (Ca3d, an1) corresponds to hydroxycalciopyrochlore.
One outlier within the samarskite field (open circle in Figure 5
) shows enrichment in Fe, Mn, Th, Y, and depletion in Na compared to the cluster 1. The calcium predominance in the A
-site over Y + REE and U + Th is diagnostic of calciosamarskite [43
], however, structural formula calculated with four oxygen atoms (AB
) showed strong excess of cations summing at 2.28. On the other hand, the cation-to-oxygen proportion was similar to that of pyrochlore, though the empirical formula recalculated using 7 oxygen atoms and all iron as FeO was still deficient in the B
-site with the fully occupied A
-site. Hence, ferric iron was provisionally calculated assuming the full occupancy of X
- and Y
-sites by oxygen and full occupancies of the A
- and B
-sites (2 apfu). The final crystallochemical formula corresponds to the almost ideal A2B2
pyrochlore stoichiometry devoid of vacancies and hydroxyl groups.
EPMA data from Table S2
project within the fergusonite subfield in Figure 5
marked as the cluster 2. Weakly oscillatory growth-zoned fergusonite rarely contained homogeneous silicate glass inclusions (Table 1
), corresponding to a peraluminous, subalkalic (TAS scheme), calcic and ferroan rhyolite.
The ideal formula and the site occupancy of fergusonite are identical with that of pyrochlore, however, vacancies are not present. Indeed, EPMA data recalculated with four oxygen atoms (ABO4) resulted in the almost full cation occupancy of the A- and B-sites. The predominance of yttrium over REE’s is diagnostic of fergusonite-Y.
Clusters 3 and 4 in Figure 5
belong to the columbite family. The Fe,Mn-rich variety of cluster 3 is the most abundant Nb-rich mineral besides fergusonite and pyrochlore. Isolated anhedral to subhedral grains visualized in BSE show domain zoning caused by fluctuating Nb/Ta ratios (Figure 2
). EPMA data and structural formulas (Table S3
) recalculated to 3 cations and 6 oxygen atoms (AB2
) are consistent with columbite. Prevalence of Mn over Fe2+
in the A
-site is diagnostic of columbite-Mn. Mn# = Mn/(Mn + Fe2+
) and Ta# = Ta/(Ta + Nb) ratios cluster at 0.56–0.71 and 0.01–0.14, respectively, without any fractionation trend.
Calcium-dominated niobate (Table S3
, cluster 4 in Figure 5
) is intimately intergrown with columbite-Mn or pyrochlore. Apart from an increased Ca content, it is also typical by enrichment in Mn (up to 4.5 wt. % MnO, 0.21 apfu, Mn# 0.76–0.84) compared to columbite. Simplified formula corresponding to (Ca,Mn)Nb2
refers to fersmite or vigezzite. Fersmite and vigezzite are dimorphic, albeit both with orthorhombic symmetry. While the Pmnb
space group vigezzite [46
] belongs to the aeschynite group, the Pcan
space group fersmite is a member of the euxenite group [47
]. However, EPMA data from granite xenoliths project on the boundary between pyrochlore and samarskite groups.
The remarkably high Mn content in the Ca–niobate from granite xenoliths contrasts with the Mn-poor fersmite/viggezite described in carbonatites [48
] and fractionated albite-rich granite pegmatites [46
]. An intimate intergrowth of fersmite/viggezite with columbite might indicate that the Ca–niobate from Čamovce is simply a Ca-analogue of columbite [54
]. Typical for the Ca–niobate from Čamovce is also the depletion in REE, particularly Ce, similar to the low-REE fersmite/vigezzite from a lithium pegmatite [55
] and carbonatite [48
Rutile is the least abundant Nb-concentrating mineral in the granite xenoliths. The highest Nb and Ta concentrations (up to 51 wt. % oxides total, 0.37 apfu) were recorded in the rutile Rt3 crystallized along ilmenite-columbite contacts. Increased Nb concentrations were also identified at contacts of rutile exsolutions (Rt1) with ilmenite host, and locally also in the non-stoichiometric rutile-resembling phases (Rt2) replacing the early ilmenite. Compositions of the non-stoichiometric phases project between ilmenite, pseudobrookite, pseudorutile and ilmenorutile endmembers (Figure 6
). Newly formed rutile Rt4 crystallizing together with Ilm3 within the interstitial glass is almost Nb-free.
A small amount of Nb is also bound in early ilmenite, which contains significant Mn (3.5–11.8 wt. % MnO, up to 25 mol. % pyrophanite endmember MnTiO3), substituting for Fe in the A-site. The B-site electroneutrality with up to 3.9 wt. % Nb2O5+Ta2O5 (0.01–0.04 apfu) is counterbalanced by the trivalent iron (up to 4.5 wt. % Fe2O3). In contrast, late interstitial ilmenite is practically devoid of Nb.
4.2. Crystal Chemistry of Nb-Bearing Phases
Strong correlations at the rutile-columbite join have been recorded for Nb–Ta, Nb–Ti, Nb–Y and Mn–Fetot
couples. The linear trend from rutile to columbite endmembers (Figure 6
) indicates a heterovalent substitution of three Ti atoms for one divalent and two pentavalent cations [56
], combined with two homovalent Mn2+
substitutions. The correlation coefficient for the exchange vector Ti3
equals to r2
= 0.63, assuming all Fe as Fe2+
. A moderate displacement from the ideal substitution line can be explained by the subordinate substitution expressed by the exchange vector Ti2
= 0.53). Correlation has not been found for another possible exchange vector Ti2
Substitutions in the fergusonite-group seem to be more complicated than those along the rutile-columbite join. Two-element correlation coefficients (Table 2
) reveal two distinct groups of rare earth elements, LREE (La–Eu) and HREE (Gd–Lu) series, with an antithetical behavior against Ca, Ti and Th. The similar contrasting behavior of MREE against LREE and HREE has already been observed in aeschynite [57
]. Yttrium behaves in accord with the HREE group due to similar crystal ionic radii of Y and Gd–Lu, 104 pm and 107–100 pm respectively, compared to much larger ionic radii of the La–Eu group (117–109 pm).
The highest correlation coefficient (r2
= 0.97) pertains to the charge-balanced substitution Ca2+A
, occurring at the A
-site only. The coupled substitution Ca2+A
(REE = Pr, Nd, Sm) proposed for the aeschynite-group [57
] may play a role also in our samples with REE = Y + (Gd–Lu) (r2
= 0.7). Other coupled heterovalent substitutions also exhibit high correlation coefficients > 0.95, e.g., Ca2+A
. In contrast to HREE, LREE’s are poorly correlated with other elements. The highest correlation coefficient r2
= 0.41 pertains to the charge-balanced exchange vector Ca2+
Unlike columbite and fergusonite, major elements in calciopyrochlore exhibit poor correlations and also the non-systematic behavior of rare elements, most likely owing to low concentrations affected by increased analytical uncertainty. Together with the homovalent Nb for Ta substitution (r2 = 0.61), increased correlation also exists for the heterovalent (Fe + Mn)2+ for 2Na+ substitution (r2 = 0.66).