Rare earth elements (REE) bear special characteristics that make them potential candidates for various applications in modern high technology. Many rare earth deposits occur in magmatic carbonatites (e.g., Nabyl et al., 2020; Beland and William-Jones, 2021) and examples include those in the Panxi Cenozoic orogen (Hou et al., 2006, 2009), Qinling Early Mesozoic orogen, and Trans-North China Proterozoic orogen in China (Xu et al., 2010), the Naantali Proterozoic orogens, Finland (Woodard and Hetheringto, 2014), and the Trans-Hudson orogen, Canada (Chakhmouradian et al., 2008), as well as the Cenozoic orogen in Pakistan (Tilton et al., 1998). The origin of the well-known Bayan Obo carbonatite REE-Nb-Fe deposit, which is the world’s largest supplier of REE metals (Song et al., 2018) occurring along the northern margin of the North China Craton is linked to partial melting of oceanic carbonate and REE-rich sediments subducted to the mantle (Hou et al., 2006; Ling et al., 2013; Xu et al., 2010, 2015). However, there are no high pressure experimental studies that substantiate this hypothesis.
Some natural hydroxyl rare earth carbonate minerals occur as hydroxylbastnasite-(Ce) Ce(CO3)OH (Kirillov, 1964; Michiba et al., 2011, 2013), hydroxylbastnasite-(Nd) Nd(CO3)OH (Maksimovic and Panto, 1985; Farkas et al., 1985), hydroxylbastnasite-(La) La(CO3)OH (Maksimovic and Jovic, 2006; Voigt et al., 2016), kozoite-(Nd) Nd(CO3)(OH), (Miyawaki et al., 2000), kozoite-(La) La(CO3)(OH), (Miyawaki et al., 2004; Miyawaki, 2014) and ancylite - (Ce) CeSr(CO3)2(OH)•H2O (Dal Negro et al., 1975), calcioancylite - (Ce) CeCa(CO3)2(OH)•H2O (Orlandi et al., 1990), and gysinite-(Nd) NdPb(CO3)2(OH)•H2O (Chabot and Sarp, 1985). Among these, hydroxylbastnasite-(La, Ce, Nd) and kozoite-(La, Nd) were reported from Hungary, Yugoslavia and Japan (theAlps-Carpathian orogen and Circum-Pacific island ar), implying that it is possible to form hydroxyl rare earth carbonate minerals in subduction settings through a recombination of oceanic carbonate and REE-bearing sediments. Therefore, understanding the stability of hydroxyl rare earth carbonate minerals at high pressure is important in understanding the potential pathway of REE, carbon and water transfer to the deep Earth in subduction zones.
Some carbonate minerals, such as calcite, aragonite, magnesite and their solid solution phases (Oganov et al., 2006; Gao et al., 2015), as well as zinc, lead, barium, cadmium carbonates (Ono et al., 2008; Minch et al. 2010; Merlini et al., 2012; Ono et al., 2007a; Ono et al., 2007b; Lin and Liu, 1997), have been investigated using high pressure and high temperature experiments. Hydrous carbonate minerals such as malachite and azurite, were studied at high pressure (Merlini et al., 2012; Xu et al., 2015a). Several hydroxyl rare earth carbonates were experimentally synthesized such as Nd(CO3)OH (Christensen, 1973), RE(CO3)OH (RE = Y, La, Gd, Er) (Chai et al., 1978), Y(CO3)OH (Tareen et al., 1980), Dy(CO3)OH (Kutlu and Meyer, 1999)and RE(CO3)OH (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y) (Tahara et al., 2007). Hydroxylbastnasite-(Ce), Ce(CO3)OH was determined to be hexagonal (Kirillov,1964), whereas kozoite-(Nd), Nd(CO3)OH and Kozoite-(La), La(CO3)OH are orthorhombic (Dexpert et al. 1974; Miyawaki et al. 2004). With regard to RE(CO3)OH minerals, there are no high pressure experimental studies to the best of our knowledge.
In this study, high-quality single crystals of hydroxylbastnasite-(Sm), Sm(CO3)OH, was synthetized at 3 GPa and 1073 K using cubic-anvil-type apparatus. The structure of hydroxylbastnasite-(Sm) was determined with single crystal X-ray diffraction (XRD). Its high-pressure behavior was studied using in-situ synchrotron radiation XRD and Raman spectroscopy. The results and implications of the study are discussed in this paper.