Crystallization of bastnäsite and burbankite from carbonatite melt in the system La(CO3)F-CaCO3-Na2CO3 at 100 MPa

Abstract

Bastnäsite [REE(CO 3 )F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastnäsite-like compounds have been precipitated from aqueous solutions by many different methods but previous attempts to model magmatic crystallization of bastnäsite from hydrous calciocarbonatite melts were unsuccessful. Here we present the first experimental evidence that bastnäsite and two other REE carbonates, burbankite and lukechangite, can crystallize from carbonatite melt in the synthetic system La(CO 3 )F -CaCO 3 -Na 2 CO 3 at temperatures between 580 and 850 °C and pressure 100 MPa. The experiments on starting mixtures of reagent-grade CaCO 3 , Na 2 CO 3 , La 2 (CO 3 ) 3 and LaF 3 were carried out in cold-seal rapid-quench pressure vessels. The studied system is an isobaric pseudoternary join of a quinary system where CO 2 and fluorides act as independent components. Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastnäsite, burbankite solid solution (Na,Ca) 3 (Ca,La) 3 (CO 3 ) 5 and lukechangite Na 3 La 2 (CO 3 ) 4 F. Calcite and bastnäsite form a eutectic in the boundary join La(CO 3 )F -CaCO 3 at 780 ± 20 °C and 58 wt% La(CO 3 )F. Phase equilibria in the boundary join La(CO 3 )F -Na 2 CO 3 are complicated by peritectic reaction between Ca-free endmember of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L): Na 4 La 2 (CO 3 ) 5 (Pet) + NaF (L) = Na 3 La 2 (CO 3 ) 4 F (Luk) + Na 2 CO 3 (Nc)The righthand-side assemblage becomes stable below 600 ± 20 °C. In ternary mixtures, bastnäsite (Bst), burbankite (Bur) and calcite (Cc) are involved in another peritectic reaction: 2 La(CO 3 )F (Bst) + CaCO 3 (Cc) + 2 Na 2 CO 3 (L) = Na 2 CaLa 2 (CO 3 ) 5 (Bur) + 2 NaF (L) Burbankite in equilibrium with calcite replaces bastnäsite below 730 ± 20 °C. Stable solidus assemblages in the pseudoternary system are: basnäsite-burbankite-fluorite-calcite, basnäsiteburbankite-fluorite-lukechangite, bastnäsite-burbankite-lukechagite, burbankite-lukechangite-This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Experimental evidence reveals the mobilization and mineralization processes of rare earth elements in carbonatites

“…S2). Depositional temperatures of REE minerals in our experiments (720° to 460°C; table S2) are 50° to 100°C lower than those from anhydrous carbonatite melts (48). Nevertheless, our crystallization temperatures are consistent with the presence of REE (fluor)carbonate minerals in carbonatitic pegmatites that were estimated to crystalize from carbonatite melts between 800° and 600°C (2,29,31,46,58).…”

“…The CO 3 2− concentration was estimated to be ~2.0 mol/kg or 18 wt %, which is consistent with the exsolution of dilute and barren hydrothermal fluids from alkaline carbonatite melts ( 28 ). These results, along with REEs partitioning preferentially into carbonatitic melts over hydrothermal fluids ( 27 ), demonstrate clearly that, upon melt-fluid immiscibility under shallow crustal conditions or during brecciation of the surrounding rocks ( 30 , 35 , 44 ), enrichment and deposition of REEs are limited within the alkaline carbonate melts, irrespective of the melt being potentially dry and enriched in CO 2 ( 45 ): We view this as further evidence for the mineralization of REE carbonate minerals during a late magmatic stage ( 46 – 48 ). Once again, fluoride in both samples remained incompatible during REE deposition and precipitated as villiaumite from the hydrothermal fluid at lower temperatures (<400°C; Fig.…”

The genesis of calcite and dolomite carbonatite-forming magma by liquid immiscibility: a critical appraisal

“…The phenomenon has been recognized for calcite (Kjarsgaard & Hamilton, 1989; Otto & Wyllie, 1993; Lee et al 1994; Mitchell, 2005a; Chebotarev et al 2019), fluorite (Mitchell and Kjarsgaard, unpublished here as Fig. 3; Chebotarev et al 2019) and nyerereite (Nikolenko et al 2022).…”