High-pressure crystal chemistry of four natural REE(As,P)O4 minerals from Mt. Cervandone, Italy
Authors: Pagliaro Francesco , Lotti Paolo , Guastoni Alessandro , Comboni Davide , Gatta G. Diego , Rotiroti Nicola , Milani Sula 
Published: March 2021

&lt;p&gt;REE orthoarsenates and orthophosphates are common accessory minerals characterized by the general chemical formula REEXO&lt;sub&gt;4&lt;/sub&gt;, where REE represents one of the lanthanides (La-Lu series), Y, Sc, Ca or Th, whereas X stands for As, P or Si. In the framework of a long-term project on the high-&lt;em&gt;T&lt;/em&gt;/high-&lt;em&gt;P&lt;/em&gt; crystal-chemistry and phase-stability of REE-bearing minerals, the high-pressure behavior of chernovite-(Y) (nominally YAsO&lt;sub&gt;4&lt;/sub&gt;), xenotime-(Y) (nominally YPO&lt;sub&gt;4&lt;/sub&gt;) gasparite-(Ce) (nominally CeAsO&lt;sub&gt;4&lt;/sub&gt;) and monazite-(Ce) (nominally CePO&lt;sub&gt;4&lt;/sub&gt;), has been studied. Chernovite-(Y) and xenotime-(Y) show a HREE- (Gd-Lu series) and Y-enrichment, and the same tetragonal symmetry (space group &lt;em&gt;I&lt;/em&gt;4&lt;sub&gt;1&lt;/sub&gt;/&lt;em&gt;amd&lt;/em&gt;), whereas gasparite-(Ce) and monazite-(Ce) share the same LREE (La-Eu) enrichment and monoclinic cell (space group &lt;em&gt;P&lt;/em&gt;2&lt;sub&gt;1&lt;/sub&gt;/&lt;em&gt;n&lt;/em&gt;). All these minerals occur at Mt. Cervandone (Western Alps, Italy), a renowned Alpine REE-bearing mineral deposit. The crystal chemistry of the four minerals has been studied via EPM-WDS analysis. Excluding gasparite-(Ce), which formation is bound to the replacement of the mineral synchisite-(Ce) (CaCe(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;F), a sensible enrichment in Gd and Ho is observed. Moreover, the majority of the chernovite-(Y) show a variable amount of ThO&lt;sub&gt;2&lt;/sub&gt;, up to 13 wt%, and phosphorous as substitute for arsenic in almost every proportion. In the case of the monoclinic series between monazite-(Ce) and gaparite-(Ce), no solid solution has been observed. Experiments at high-pressure were performed by in situ synchrotron X-ray diffraction using a diamond anvil cell. The high-pressure behavior of single crystals of xenotime-(Y), gasparite-(Ce) and monazite-(Ce) has been studied up to ~20 GPa, whereas that of chernovite-(Y) has been studied by powder diffraction up to 8.20(5) GPa. A II-order Birch-Murnaghan equation of state was fitted to the &lt;em&gt;V&lt;/em&gt;-&lt;em&gt;P&lt;/em&gt; data, within the phase stability field of the minerals, yielding the following bulk moduli: &lt;em&gt;K&lt;/em&gt;&lt;sub&gt;P0,T0&lt;/sub&gt; = 125(3) GPa (&amp;#946;&lt;sub&gt;V&lt;/sub&gt;&lt;sub&gt;0 &lt;/sub&gt;= 0.0080(2) GPa&lt;sup&gt;-1&lt;/sup&gt;) for chernovite-(Y); &lt;em&gt;K&lt;/em&gt;&lt;sub&gt;P0,T0&lt;/sub&gt; = 145(2) GPa (&amp;#946;&lt;sub&gt;V&lt;/sub&gt;&lt;sub&gt;0 &lt;/sub&gt;= 0.0069(1) GPa&lt;sup&gt;-1&lt;/sup&gt;) for xenotime-(Y); &amp;#160;&lt;em&gt;K&lt;/em&gt;&lt;sub&gt;P0,T0&lt;/sub&gt; = 106.7(9) GPa (&amp;#946;&lt;sub&gt;V&lt;/sub&gt;&lt;sub&gt;0 &lt;/sub&gt;= 0.0094(1) GPa&lt;sup&gt;-1&lt;/sup&gt;) for gasparite-(Ce), &lt;em&gt;K&lt;/em&gt;&lt;sub&gt;P0,T0&lt;/sub&gt; = 121(2) GPa (&amp;#946;&lt;sub&gt;V&lt;/sub&gt;&lt;sub&gt;0 &lt;/sub&gt;= 0.0083(1) GPa&lt;sup&gt;-1&lt;/sup&gt;) for monazite-(Ce). &lt;em&gt;K&lt;/em&gt;&amp;#8217; = &amp;#8706;&lt;em&gt;K&lt;/em&gt;&lt;sub&gt;V&lt;/sub&gt;/&amp;#8706;&lt;em&gt;P&lt;/em&gt; = 4 (fixed) for all the minerals. Deformation mechanisms, at the atomic scale, were described on the basis of structure refinements. &amp;#160;&lt;/p&gt;&lt;p&gt;Acknowledgments: This research was partly funded by the PRIN2017 project &amp;#8220;Mineral reactivity, a key to understand large-scale processes&amp;#8221; (2017L83S77).&lt;/p&gt;