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Bastnäsite [REE(CO3)F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastnäsite-like compounds were 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(CO3)F-CaCO3-Na2CO3 at temperatures between 580 and 850 °C and a pressure 100 MPa. The experiments on starting mixtures of reagent-grade CaCO3, Na2CO3, La2(CO3)3, and LaF3 were carried out in cold-seal rapid-quench pressure vessels. The studied system is an isobaric pseudoternary join of a quinary system where CO2 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(CO3)5, and lukechangite Na3La2(CO3)4F. Calcite and bastnäsite form a eutectic in the boundary join La(CO3)F-CaCO3 at 780 ± 20 °C and 58 wt% La(CO3)F. Phase equilibria in the boundary join La(CO3)F-Na2CO3 are complicated by peritectic reaction between Ca-free end-member of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L): N a 4 L a 2 C O 3 5 (P e t) + N a F (L) = N a 3 L a 2 C O 3 4 F (L u k) + N a 2 C O 3 (N c). $$ \begin{equation} \mathrm{Na}_4 \mathrm{La}_2\left(\mathrm{CO}_3\right)_5(\mathrm{Pet})+\mathrm{NaF}(\mathrm{L})=\mathrm{Na}_3 \mathrm{La}_2\left(\mathrm{CO}_3\right)_4 \mathrm{~F}(\mathrm{Luk})+\mathrm{Na}_2 \mathrm{CO}_3(\mathrm{Nc}). \end{equation}$$ The right-hand-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 L a C O 3 F (B s t) + C a C O 3 (C c) + 2 N a 2 C O 3 ( L) = N a 2 C a L a 2 C O 3 5 (B u r) + 2 N a F (L). $$\begin{equation} 2 \mathrm{La}\left(\mathrm{CO}_3\right) \mathrm{F}(\mathrm{Bst})+\mathrm{CaCO}_3(\mathrm{Cc})+2 \mathrm{Na}_2 \mathrm{CO}_3(\mathrm{~L})=\mathrm{Na}_2 \mathrm{CaLa}_2\left(\mathrm{CO}_3\right)_5(\mathrm{Bur})+2 \mathrm{NaF}(\mathrm{L}). \end{equation}$$ Burbankite in equilibrium with calcite replaces bastnäsite below 730 ± 20 °C. Stable solidus assemblages in the pseudoternary system are: basnäsite-burbankite-fluorite-calcite, basnäsite-burbankite-fluorite-lukechangite, bastnäsite-burbankite-lukechangite, burbankite-lukechangite-nyerereitecalcite, and burbankite-lukechangite-nyerereite-natrite. Addition of 10 wt% Ca3(PO4)2 to one of the ternary mixtures resulted in massive crystallization of La-bearing apatite and monazite and complete disappearance of bastnäsite and burbankite. Our results confirm that REE-bearing phosphates are much more stable than carbonates and fluorocarbonates. Therefore, primary crystallization of the latter from common carbonatite magmas is unlikely. Possible exceptions are carbonatites at Mountain Pass that are characterized by very low P2O5 concentrations (usually at or below 0.5 wt%) and extremely high REE contents in the order of a few weight percent or more. In other carbonatites, bastnäsite and burbankite probably crystallized from highly concentrated alkaline carbonate-chloride brines that were found in melt inclusions and are thought to be responsible for widespread fenitization around carbonatite bodies. [ABSTRACT FROM AUTHOR]

Interfacial tension between immiscible liquids is an important thermodynamic parameter of silicate melt unmixing and a property that determines the kinetics of phase separation. In this study, we present experimental measurements of interfacial tension between immiscible Fe-rich and silica-rich melts in the system [K.sub.2]O-FeO-[Fe.sub.2][O.sub.3]-[A1.sub.2][O.sub.3]-Si[O.sub.2]. We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550[degrees]C by the maximum detachment force method employing vertical cylinder geometry and using a gravimetric balance system. We have chosen the most oxidized and contrasting liquid compositions containing 73 and 17 wt% Si[O.sub.2] and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465[degrees]C. Interracial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 [+ or -] 3.1 mN/m at 1500[degrees]C to 7.8 [+ or -] 1.1 mN/m at 1550[degrees]C. Interfacial tension between natural, less compositionally contrasting ferrobasaltic and rhyolitic melts should be even lower by a factor of 2 or 3. Very low interracial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions. Keywords: Immiscibility, interface, tensiometry, nucleation DOI: 10.2138/am.2010.3456

Aluminum orthoposphate (AlP[O.sub.4]) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to [alpha]-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 [degrees]C and 0.1-0.2 GPa [H.sub.2]O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% Si[O.sub.2] on the phosphate side of the solvus and to the maximum of 1 mol% AlP[O.sub.4] on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

We present an experimental study of synthetic peraluminous granite doped with [H.sub.2]O, B, P, and F, which confirms that aluminosilicate melt, hydrous fluid, and hydrosaline melt (high-temperature brine) can stably coexist at 450-900 [degrees] C and 0.1-0.2 GPa in bulk compositions similar to those of natural granitic pegmatites. Hydrosaline melt is not quenchable, unstable at room conditions, and requires special techniques for synthesis and preservation. Raman spectroscopy and electron microprobe analyses of hydrosaline melt synthesized in our experiments show that it is composed of [H.sub.3]B[O.sub.3], [Na.sub.3]A1[F.sub.6], A1P[O.sub.4], [H.sub.2]O, and aluminosilicate components. Aluminosilicate melt saturated in both hydrosaline liquid and hydrous fluid at 850 [degrees] C and 0.2 GPa contains 3.6 wt% F, 4.2 wt% [P.sub.2][O.sub.5], and 4 wt% [B.sub.2][O.sub.3]. Natural hydrosaline melts have previously been found as inclusions trapped in rock-forming minerals. They are not restricted to granites and can be effective agents for enhanced crystal growth, metasomatism, and ore formation. In addition, hydrosaline melts may account for many characteristic features of rare-element and miarolitic pegmatites, such as giant size and perfect shapes of crystals in pegmatite cores, diverse mineralogy, and strong enrichment in rare elements.