Ab intitio mineral physics simulations discovered several novel phase transitions in major Earth and planetary materials under (multi) megabar pressure conditions. Here we report some recent remarks on the ultrahigh-pressure structure search of some important oxides and silicates.
MgSiO3 dominant perovskite is believed to be the most abundant constituent mineral in the Earth’s lower mantle. Generally minerals form solid solutions and their nature should affect on physical properties of minerals. In this paper, we will introduce our recent studies about incorporation mechanism of FeAlO3 component into MgSiO3 perovskite and its crystal chemistry.
Considerable efforts have been devoted to the structural studies of spinel group minerals or type compounds because of their importance as constituents of the Earth’s crust and mantle. Despite their simple structures, many spinel type compounds exhibit complex disordering phenomena involving the mixing of cation on two sites, which have important consequences for both thermodynamic and physical properties. The cation distributions and the structural variation in MgAl2-xGaxO4 solid-solution have been clarified using 27Al MAS NMR measurements and single crystal X-ray diffraction. The determined local distance in the solid solution corresponds with the bond distance expected from the effective ionic radii except Al-O distance in the tetrahedral site. We have revealed that the Al-O distance in the tetrahedral site in spinel solid solution is about 0.15 Å longer than the expected value. Boron is the same group element as Al and Ga and its ionic radius is considerably small. Single crystals of MgAl2-xBxO4 spinel were synthesized under high pressure and high temperature. The maximum content of boron was about x = 0.13 at 1273 K and 11 GPa. The smallest B ion occupies the octahedral site in top priority in the spinel solid solution of the Mg-Al-B systems. The B3+ ions can replace considerably bigger Al3+ ion under pressure. These spinel solid-solutions are largely disordered crystals. Only the positional shifts of oxygen ion have been relaxing the disorder in the solid solution.
This paper reviewed the hydrogen incorporations in crystal structures of some slab and mantle minerals based on the recent author’s and the group’s studies. The topics included 1) hydrogen positions in the crystal structures of Mg2SiO4 polymorphs, humite minerals and DHMS phases, and 2) the effects of hydrogen on the physical properties of these minerals.
The crystal structure of η-Al(OH)3, which is a high pressure form of gibbsite (γ-Al(OH)3), was solved by single crystal X-ray diffraction and molecular dynamics simulations. The phase transition mechanism is considered to be the layer-shift type, which may frequently occur for layered hydrous minerals with non-close packing structure of oxygens. The layer-shift transition involves breaking and recombination of hydrogen bonds so that the activation energy of phase transition could directly relate to the strength of hydrogen bonds.
A structural approach was attempted to estimate rigid ion contributions to pyroelectric coefficients, employing atomic position vectors determined in the least-squares refinement of a polar mineral, cancrinite. The two sets of positions, obtained from the centers of gravity (centroids) and modes of asymmetric probability density functions (pdf) of atoms, were applied to calculate electric polarizations, whose temperature derivatives may correspond to the rigid ion contributions to the pyroelectric coefficient, γ(σ), normally measured under the condition of constant stresses and to the secondary pyroelectric coefficients, γ(2), respectively. The primary pyroelectric coefficient γ(ε) was given as the difference γ(σ)-γ(2).
An isosymmetric phase transition from orthopyroxene to a new high-temperature orthorhombic phase was observed by molecular dynamics simulations for the Mg end-member composition of pyroxene (enstatite : Mg2Si2O6) and by high-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca0.06Mg1.94)Si2O6. This new phase has the same space group as orthoenstatite, Pbca. The discontinuous changes of the cell volume and cell parameters during the transition indicate a first-order transition. The transition is characterized by the switching of bonds between Mg atoms at the M2 sites and the coordinated O3 atoms. This high-temperature phase is thermodynamically distinct from the low-temperature phase, i.e., orthoenstatite in the Mg-rich portion of Mg2Si2O6 - CaMgSi2O6(diopside) phase diagram.
Vesuvianites are classified into three types, P4/nnc, P4/n, and P4nc. These varieties are interpreted as “rod polytypism”. A rare type of high-temperature skarn is developed in Kushiro (Tojyo, Shobara Hiroshima) southwestern Japan. Vesuvianites formed by the retrograde metamorphism of gehlenite, and several structural types were found corresponding to various stages of the metamorphism. The crystal structure of a unique type of vesuvianite with the space group P4/n [a=15.576 (2), c=11.835 (2) Å] is determined by single-crystal X-ray diffraction. Significant vacancies (29%) occur only on one of the two pseudo-equivalent Z1 sites (Z1b).
The 3D packing structures of the microcrystals in framboidal pyrite have been studied through morphological examinations and crystallographic orientation analysis to understand the self-organization process. The structures are divided into (i) face-centered cubic, (ii) icosahedral and (iii) random packings. The detailed crystallography of pyrite framboids is characterized by high-angle (∼90°) misorientations shown by about a half of the microcrystals in a framboid. This means that the crystallographic orientation of microcrystals is not uniform even in highly ordered framboids, suggesting that their self-organization process is not crystallographically controlled, but occurs by the physical rotation of individual microcrystals.
Stacking disorder is a common phenomenon in phyllosilicates but difficult to be understood using conventional diffraction techniques. In contrast, recent investigations using high-resolution transmission electron microscopy (HRTEM) have elucidated the nature of stacking disorder in various phyllosilicates, by directly observing individual layers and stacking sequences. Furthermore, simulations of X-ray or electron diffraction patterns using the information from the HRTEM results can complement the limited analysis area in TEM and quantify the degree of the stacking disorder. In this review, several examples of such analyses to reveal the stacking disorder in natural phyllosilicates are described.
A mineral substance is defined as a naturally occurring solid that has been formed by geological processes, either on earth or in extraterrestrial bodies by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMMC-IMA). A mineral species is defined with the chemical composition and crystallographic properties. So far approximate 4,300 mineral species have been endorsed and approved by the CNMMC-IMA. The crystal structure is the fundamental key of the crystallography in the description of mineral. Owing to the recent developments in the hardware and software for crystal structure analysis, many of new mineral species have been described with data of crystal structures. Minor differences in atomic arrangement such as chemical orderings provide the information on the genesis of mineral.
Although amphibole is one of the most common rock-forming minerals on the earth, it rarely occurs in meteorites. This is due to anhydrous and low-pressure environment of most meteorite parent bodies. In this paper we report two different occurrences of amphibole from extraterrestrial materials. One is hornblende from an R chondrite, and the other is kaersutite from Martian meteorites. Both amphiboles bear unique crystal chemistry different from terrestrial ones and provide important information about their thermal history especially related to the water in extraterrestrial environment.
We have improved the application of multi-disciplinary methods for crystal evaluation of fine-grained planetary materials : micrometeorites, a small particular portion of a meteorite, fine-grained samples retrieved from Wild2 comet and Asteroid Itokawa, and so on. In this paper, we give an example of the application of multi-disciplinary methods when we investigated secondary minerals in nakhlite Martian meteorites retrieved from Yamato Mountains, Antarctica. We also touch upon an issue of sample preparation and analysis of fine-grained dust retrieved from Asteroid Itokawa.
A variety of ultrahigh-pressure phases of silicates have been discovered in the vicinity of shocked-induced melt veins in stony meteorites by analytical transmission electron microscopy. The natural ultrahigh-pressure minerals provide us not only the clues to understand the impact events of meteorite parent bodies but also insights into the structure and dynamics of the deep Earth.
Microtextures of belite induced by polymorphic phase transitions and a remelting reaction have been reviewed, together with the melt differentiation reaction of interstitial melt. In the α-to-α'H phase transition, the α'H-phase nucleates as lamellae within the parent α-phase so as to realize a good lattice matching across the interface. The remelting reaction, in which the α-phase belite decomposes into a liquid and the α'H-phase during cooling, is necessarily preceded by the α-to-α'H phase transition. The lamella boundaries provide heterogeneous nucleation sites for the exsolving liquid. A variety of microtextures results depending on the surface tension between belite lamellae and exsolved liquid as well as on the cooling rate. As the simultaneous crystallization of zoned ferrite and belite proceeds during the cooling process of clinkers, the coexisting melt progressively increases the Al2O3/Fe2O3 ratio. After the termination of the ferrite crystallization, the aluminete and belite crystallize out of the differentiated melt.
Apatite-type RE9.33(SiO4)6O2 rare-earth silicate compounds are of great interest for their oxide ion conductivity at moderate temperatures. Crystallographic space group and structural parameters of La/Nd9.33(SiO4)6O2 were investigated based on single-crystal X-ray diffraction data at room temperature for La9.33(SiO4)6O2 and at various temperatures from 150 to 900 K for Nd9.33(SiO4)6O2. Based on the most accurate diffraction data to date, these compounds have an apatite prototype structure (space group P63/m) with no symmetry lowering, site splitting nor interstitial oxide ion in the structure. Most striking feature of the structure is large anisotropy in thermal motions of the oxide ion at the anion channel site. Even at room temperature, the mean-square displacement of the channel oxide ion in La9.33(SiO4)6O2 is 16 times larger in  than those normal to the direction. Not striking but important point is that there is no sign of interstitial oxide ion which had been proposed as a charge carrier in La9.33(SiO4)6O2 and related compounds. These findings imply straight migration of oxide ion inside the channel as a primary mechanism of oxide-ion conductivity in these compounds. Size and shape of the anion channel might be a key to achieve high oxide-ion conductivity in these compounds.