On the symmetry and crystal chemistry of britholite: New structural and microanalytical data

Abstract

We provide in this paper complete structural and micro-chemical characterization of two britholite samples with compositions (Mn 0.(from Los Islands, Guinea). The crystal-chemical formulae were calculated by combining electron-microprobe analyses for intermediate-Z elements (Na, Ca, Mn, Si, P), ion-microprobe analyses for low-Z (H, Li, Be, B, F) and high-Z elements (Ba, Y, REE and actinides), and high-quality (R obs 1.2-2.6) single-crystal structure refinements. Structure refinements indicate that the best approximation to the real symmetry is the P6 3 space group. In britholite, the lowering of symmetry with respect to the P6 3 /m space group of apatite means that the O3 and O3a atoms are no longer equivalent and allows the tetrahedron to rotate up to ~4° around the Si-O1 bond. Consequently, the O3a atom moves closer to the REE1a site, whereas the O3 atom moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite from Latium shows a unique and very broad band in the OH-stretching region at 3437 cm -1 , which is consistent with the ordering of trivalent REE cations at the REE2 site. The calculated integral molar absorptivity ε i is 23600 L⋅mol -1 ⋅cm -2 . A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behavior suggests constraints due to the rigidity of the tetrahedral group, that are further enhanced at high symmetry.

Britholite, monazite, REE carbonates, and calcite: Products of hydrothermal alteration of allanite and apatite in A-type granite from Stupné, Western Carpathians, Slovakia

“…5) show their very similar flat patterns with LREE ≈ HREE; the only different feature is an absence of Eu anomaly in fluorapatite versus a strong negative Eu anomaly in BGM (Eu/Eu* = 0.2 to 0.6). Such negative Eu anomaly is comparable with other britholite occurrences (Oberti et al, 2001); it indicates a partial removal of Eu and their possible deposition (in the Eu 2+ form) into REE carbonates with slightly positive Eu anomalies (Fig. 8) during origin of BGM.…”

“…While thorium concentrations range from 2.5 to 5.2 wt.% ThO 2 (0.07 to 0.15 apfu), uranium reaches only 0.4 to 1.0 wt.% UO 2 (0.01 to 0.03 apfu U), with maximum Th and U concentrations denoting fluorbritholite-(Y) composition. Several possible substitution mechanisms explain the entry of tetravalent Th and U cations into the apatite-britholite structure: (1) (Th,U) 4+ + Si 4+ = (Y,REE) 3+ + P 5+ (Pekov et al, 2011), (2) (Th,U) 4+ + 2Si 4+ = Ca 2+ + 2P 5+ (Luo et al, 2011;MacDonald et al, 2013) (Oberti et al, 2001). Positive correlation between (Th,U) and Si but negative relationship between (Th,U) and P indicate that mechanism (2) which, together with domi- 4E).…”

“…BGM are locally enriched in actinide elements, particularly in Th and less in U which are the regular source of their metamictization. Thorium content in britholite-(Ce), fluorbritholite-(Ce), and fluorcalciobritholite compositions with Ce N Y can reach~3 to 23 wt.% ThO 2 or 0.06 to 0.68 apfu (Contini et al, 1993;Della Ventura et al, 1999;Melluso et al, 2010;Oberti et al, 2001;Orlandi et al, 1989;Pekov et al, 2007). In contrast, BGM compositions with Y N Ce contain notably lesser Th (up to~2 wt.% ThO 2 ; Griffin et al, 1979;Payette and Martin, 1986;Pekov et al, 2011;Smith et al, 2002).…”

Apatite-group minerals from nepheline syenite, Pilansberg alkaline complex, South Africa

“…thus suggesting the presence of several different species, e.g. 'fluorbritholite' akin to material described by Oberti et al (2001), and its F-poor analogues.…”

Britholite ores of the Sakharjok Zr–Y–REE deposit, Kola Peninsula: Geochemistry, mineralogy, and formation stages

“…The variable composition of britholite and diversity of its mineral species in the nepheline syenite and related pegmatites (fluorbritho lite (Ce), birtholite (Ce), fluorbritholite (Y) and fluo rcalciobritholite) (Table 3) The ore bearing nepheline syenite of the Sakharjok Massif contains Ce rich britholites. The contents of Y, REE, P, and Ca in these minerals sharply differ from those of cerium britholites from alkaline rocks (Eliseev et al, 1965;Kolyago and Lapin, 1990;Ripp et al, 2005;Della Ventura et al, 1999;Nash, 1972;Noe et al, 1993;Oberti et al, 2001) in the lower phosphorus and cal cium contents and higher yttrium and heavy rare earth element contents up to the appearance of fluorbritho lite (Y). These differences could be related to the geochemical features of crystallization environment: "low" phosphorus and calcium contents (Batieva and Bel'kov, 1984;Zozulya et al, 2012) and accumulation of HREE and Y during autometasomatism and hydro thermal alteration of the rocks (see above).…”