Lanthanite-(Nd), Nd2(CO3)3·8H2O

Lanthanite-(Nd), ideally Nd2(CO3)3·8H2O [dineodymium(III) tricarbonate octahydrate], is a member of the lanthanite mineral group characterized by the general formula REE2(CO3)3·8H2O, where REE is a 10-coordinated rare earth element. Based on single-crystal X-ray diffraction of a natural sample from Mitsukoshi, Hizen-cho, Karatsu City, Saga Prefecture, Japan, this study presents the first structure determination of lanthanite-(Nd). Its structure is very similar to that of other members of the lanthanite group. It is composed of infinite sheets made up of corner- and edge-sharing of two NdO10-polyhedra (both with site symmetry ..2) and two carbonate triangles (site symmetries ..2 and 1) parallel to the ab plane, and stacked perpendicular to c. These layers are linked to one another only through hydrogen bonding involving the water molecules

Structural and vibrational properties of single crystals of Scandia, Sc2O3\mathrm{Sc_{2}O_{3}} under high pressure

We report the results of single-crystal X-ray diffraction and Raman spectroscopy studies of scandiumoxide, Sc$_{2}$O$_{3}$, at ambient temperature under high pressure up to 55 and 28 GPa, respectively.Both X-ray diffraction and Raman studies indicated a phase transition from the cubic bixbyitephase (so-called C-Res phase) to a monoclinic C2/m phase (so-called B-Res phase) at pressuresaround 25–28 GPa. The transition was accompanied by a significant volumetric drop by ~ 6.7%. Inaddition, the Raman spectroscopy detected a minor crossover around 10–12 GPa, which manifestedin the appearance of new and disappearance of some Raman modes, as well as in softening of oneRaman mode. We found the bulk modulus values of the both C-Res and B-Res phases as B$_{0}$ - 198.2(3) and 171.2(1) GPa (for fixed B' - 4), respectively. Thus, the denser high-pressure latticeof Sc$_{2}$O$_{3}$ is much softer than the original lattice. We discuss possible mechanisms that mightbe responsible for the pronounced elastic softening in the monoclinic high-pressure phase in this“simple” oxide with an ultra-wide band gap

Extreme redox variations in a superdeep diamond from a subducted slab

: The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation1. Here we investigate the redox conditions of the deep mantle recorded in inclusions in a diamond from Kankan, Guinea. Enstatite (former bridgmanite), ferropericlase and a uniquely Mg-rich olivine (Mg# 99.9) inclusion indicate formation in highly variable redox conditions near the 660 km seismic discontinuity. We propose a model involving dehydration, rehydration and dehydration in the underside of a warming slab at the transition zone-lower mantle boundary. Fluid liberated by dehydration in a crumpled slab, driven by heating from the lower mantle, ascends into the cooler interior of the slab, where the H2O is sequestered in new hydrous minerals. Consequent fractionation of the remaining fluid produces extremely reducing conditions, forming Mg-end-member ringwoodite. This fractionating fluid also precipitates the host diamond. With continued heating, ringwoodite in the slab surrounding the diamond forms bridgmanite and ferropericlase, which is trapped as the diamond grows in hydrous fluids produced by dehydration of the warming slab

Dual origin of ferropericlase inclusions within super-deep diamonds

Ferropericlase [(Mg,Fe)O] is one of the major constituents of Earth’s lower mantle and the most abundant mineral inclusion in sub-lithospheric diamonds. Although a lower mantle origin for ferropericlase inclusions has often been suggested, some studies have proposed that many of these inclusions may instead form at much shallower depths, in the deep upper mantle or transition zone. No straightforward method exists to discriminate ferropericlase of lower-mantle origin without characteristic mineral associations, such as co-existing former bridgmanite. To explore ferropericlase-diamond growth relationships, we have investigated the crystallographic orientation relationships (CORs), determined by single-crystal X-ray diffraction, between 57 ferropericlase inclusions and 37 diamonds from Juina (Brazil) and Kankan (Guinea). We show that ferropericlase inclusions can develop specific (16 inclusions in 12 diamonds), rotational statistical (9 inclusions in 7 diamonds) and random (32 inclusions in 25 diamond) CORs with respect to their diamond hosts. All measured inclusions showing a specific COR were found to be Fe-rich (XFeO>0.30). Coexistence of non-randomly and randomly oriented ferropericlase inclusions within the same diamond indicates that their CORs may be variably affected by local growth conditions. However, the occurrence of specific CORs onlyfor Fe-rich inclusions indicates that Fe-rich ferropericlases have a distinct genesis and are syngenetic with their host diamonds. This result provides strong support for a dual origin for ferropericlase in Earth’s mantle, with Fe-rich compositions likely indicating redox growth in the upper mantle, while more Mg-rich compositions with random COR mostly representing ambient lower mantle trapped as protogenetic inclusions