Fluorcalciobritholite, (Ca,REE)5[(Si,P)O4]3F, a new mineral: description and crystal chemistry

Abstract

The new mineral fluorcalciobritholite, ideally Ca 3 Ce 2 (SiO 4) 2 (PO 4)F, has been found at Mount Kukisvumchorr, Khibiny alkaline complex, Kola Peninsula, Russia, in veinlets which contains aggregates of orthoclase, nepheline, sodalite and biotite in association with grains of fayalite, gadolinite-(Ce), zircon, monazite-(Ce), zirconolite ("polymignite"), fluorapatite, fluorite, molybdenite, löllingite and graphite. Fluorcalciobritholite forms long-prismatic hexagonal crystals up to 0.5 x 10 mm; the main crystal form is the hexagonal prism {10-10}. The mineral is transparent, with a pale pinkish to brown colour and a white streak. The hardness (Mohs) is 5.5, and the observed density is 4.2(1) g/cm 3. Optically, it is uniaxial (-) with K 1.735(5), 5 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu: [Ca 2.80 (Ce 0.93 La 0.54 Nd 0.26 Y 0.18 Pr 0.08 Sm 0.03 Gd 0.03 Dy 0.02 Yb 0.02 Er 0.01) 7 2.12 Th 0.04 Mn 0.03 Sr 0.02 ] 7 4.99 [(Si 1.94 P 1.06) 7 3 O 12 ] [F 0.76 O 0.22 Cl 0.01 ] 7 0.99 (Z = 2). The IR spectrum of metamict fluorcalciobritholite from Siberia showed a marked similarity with those of hydroxylbritholite-(Ce) and hydroxylbritholite-(Y). The strongest lines of the X-ray powder pattern [d in Å (I) (hkl)] are: 3.51 (45) 002, 3.15 (70) 102, 2.85 (100) 211, 121, 2.78 (60) 300. The mineral is hexagonal, space group P6 3 /m, with a = 9.580(7), c = 6.985(4) Å, V = 555.2(7) Å 3. The crystal structure was refined from single-crystal X-ray diffraction data to R F = 0.029. Fluorcalciobritholite, whose simplified formula is (Ca,REE) 5 [(Si,P)O 4 ] 3 F, differs from fluorbritholite in having Ca 8 7 REE, and differs from fluorapatite in having Si 8 P. Its compositional field falls within the limits Ca 2.5 REE 2.5 (SiO 4) 2.5 (PO 4) 0.5 F (boundary with fluorbritholite) and Ca 3.5 REE 1.5 (SiO 4) 1.5 (PO 4) 1.5 F (boundary with fluorapatite). Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.

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

“…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).…”

“…Occurrences of primary magmatic BGM are limited to peralkaline granites and syenite pegmatites (e.g., Griffin et al, 1979;Larsen, 1996;Pekov et al, 2007Pekov et al, , 2011Vilalva et al, 2013) or peralkaline rhyolitic rocks (Min et al, 2006;Payette and Martin, 1986). Formation of BGM is usually connected with late-to post-magmatic or hydrothermal alteration processes in diverse alkaline rocks, for example, in alkali syenitic ejecta (Della Ventura et al, 1999), REE-enriched pegmatite segregations (Arden and Halden, 1999), carbonate-amphibole-clinopyroxene rocks (Zaitsev and Chakhmouradian, 2002), REE-rich mineralized zones associated with potassic alkaline to subalkaline volcanic to plutonic rocks (Harlov et al, 2003), or carbonatites and related skarn deposits (Ahijado et al, 2005).…”

“…On the other hand, britholite group minerals isostructural to the apatite group are relatively rare in nature, and their occurrences are linked especially to late-magmatic crystallization or subsolidus alteration of primary minerals in alkali-rich granites, monzonites, (nepheline) syenites, and their pegmatites, carbonatites, and dikes and metamorphic rocks related to them (e.g., Ahijado et al, 2005;Arden and Halden, 1999;Della Ventura et al, 1999;MacDonald et al, 2013;Pekov et al, 2007;Smith et al, 2002;Vilalva et al, 2013).…”

Age, petrochemistry, and origin of a REE-rich mineralization in the Longs Peak-St. Vrain batholith, near Jamestown, Colorado (U.S.A.)

“…However, the occurrence of allanite-(Ce) and fluorbritholite-(Ce) with significant aegerine-augite, titanite, and apatite, but without monazite-(Ce), in their study area differs from the mineral assemblages at Jamestown. Whereas the Jamestown mineralization is associated with an anorogenic peraluminous granite intrusion (two-mica monzo-to syenogranite), the few other known localities with fluorbritholite are associated with alkaline (e.g., Della Ventura et al 1999;Liferovich and Mitchell 2006;Pekov et al 2007), peralkaline (agpaitic; e.g., Jiexiang et al 1994;Sørensen 1997), and carbonatite intrusions (e.g., Feldman et al 1987). Pekov et al (2007) describe the mineral and its holotype occurrence on Mount Kukisvumchorr in the Kola Peninsula in Russia, where it appears as veinlets in fenitized gneiss xenoliths.…”

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

“…The minerals were identified in the alkali and felsic magmatic rocks, as well as in the metasomatites and pegmatites associated with subalkali and alkali granites, alkali, and nepheline syenites, and carbon atites (Kolyago and Lapin, 1990;Mel'nikov and Gre chanovskaya, 2010;Ripp et al, 2005;Arden and Halden, 1999;Della Ventura et al, 1999;Griffin et al, 1979;Gu et al, 1994;Orlandi et al, 1989;Pekov et al, 2007Pekov et al, , 2011Uher and Ondrejka, 2008). It is suggested that britholite in most of the occurrences was crystal lized at the late to postmagmatic (pegmatite and hydro thermal) stages.…”