Experimental study of phase equilibria between bastn&#228;site, burbankite and La phosphates in the system La(CO3)F &#8211; CaCO3 &#8211; Na2CO3 - Ca3(PO4)2 at 100 MPa
Authors: Veksler Ilya , Nikolenko Anna , Stepanov Konstantin 
Published: March 2022

&lt;p&gt;Bastn&amp;#228;site [REE(CO&lt;sub&gt;3&lt;/sub&gt;)F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastn&amp;#228;site-like compounds have been precipitated from aqueous solutions by many different methods but previous attempts to model magmatic crystallization of bastn&amp;#228;site from hydrous calciocarbonatite melts were unsuccessful. Here we present the first experimental evidence that bastn&amp;#228;site and two other REE carbonates, burbankite and lukechangite, can crystallize from carbonatite melt in the synthetic system La(CO&lt;sub&gt;3&lt;/sub&gt;)F &amp;#8211; CaCO&lt;sub&gt;3&lt;/sub&gt; &amp;#8211; Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt; at temperatures between 580 and 850 &amp;#176;C and pressure 100 MPa. The experiments on starting mixtures of reagent-grade CaCO&lt;sub&gt;3&lt;/sub&gt;, Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;, La&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt; and LaF&lt;sub&gt;3&lt;/sub&gt; were carried out in cold-seal rapid-quench pressure vessels. The studied system is an isobaric pseudoternary join of a quinary system where CO&lt;sub&gt;2&lt;/sub&gt; and fluorides act as independent components. &amp;#160;Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastn&amp;#228;site, burbankite solid solution (Na,Ca)&lt;sub&gt;3&lt;/sub&gt;(Ca,La)&lt;sub&gt;3&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt; and lukechangite Na&lt;sub&gt;3&lt;/sub&gt;La&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;F. Calcite and bastn&amp;#228;site form a eutectic in the boundary join La(CO&lt;sub&gt;3&lt;/sub&gt;)F &amp;#8211; CaCO&lt;sub&gt;3&lt;/sub&gt; at 780 &amp;#177; 20 &amp;#176;C and 58 wt% La(CO&lt;sub&gt;3&lt;/sub&gt;)F. Phase equilibria in the boundary join La(CO&lt;sub&gt;3&lt;/sub&gt;)F &amp;#8211; Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt; are complicated by peritectic reaction between Ca-free endmember of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L):&lt;/p&gt;&lt;p&gt;Na&lt;sub&gt;4&lt;/sub&gt;La&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt; (Pet) + NaF (L) = Na&lt;sub&gt;3&lt;/sub&gt;La&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;F (Luk) + Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt; (Nc)&lt;/p&gt;&lt;p&gt;The righthand-side assemblage becomes stable below 600 &amp;#177; 20 &amp;#176;C. In ternary mixtures, bastn&amp;#228;site (Bst), burbankite (Bur) and calcite (Cc) are involved in another peritectic reaction:&lt;/p&gt;&lt;p&gt;2 La(CO&lt;sub&gt;3&lt;/sub&gt;)F (Bst) + CaCO&lt;sub&gt;3&lt;/sub&gt; (Cc) + 2 Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt; (L) = Na&lt;sub&gt;2&lt;/sub&gt;CaLa&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt; (Bur) + 2 NaF (L)&lt;/p&gt;&lt;p&gt;Burbankite in equilibrium with calcite replaces bastn&amp;#228;site below 730 &amp;#177; 20 &amp;#176;C. Stable solidus assemblages in the pseudoternary system are: basn&amp;#228;site-burbankite-fluorite-calcite, basn&amp;#228;site-burbankite-fluorite-lukechangite, bastn&amp;#228;site-burbankite-lukechagite, burbankite-lukechangite-nyerereite-calcite and burbankite-lukechangite-nyerereite-natrite. Addition of 10 wt% Ca&lt;sub&gt;3&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt; to ternary mixtures resulted in massive crystallization of La-bearing apatite and monazite, and complete disappearance of bastn&amp;#228;site 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 P&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; 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, bastn&amp;#228;site and burbankite probably crystallized from highly concentrated alkaline carbonate-chloride brines that have been found in melt inclusions and are thought to be responsible for widespread fenitization around carbonatite bodies.&lt;/p&gt;&lt;p&gt;This study was supported by RSF grant &amp;#8470; 19-17-00013.&lt;/p&gt;