ALBERT

Description: 〈div data-abstract-type="normal"〉〈p〉K-rich nepheline with a structural formula of 〈span〉A〈/span〉〈span〉2〈/span〉〈span〉B〈/span〉〈span〉6〈/span〉〈span〉T〈/span〉1〈span〉4〈/span〉〈span〉T〈/span〉2〈span〉4〈/span〉〈span〉T〈/span〉3〈span〉4〈/span〉〈span〉T〈/span〉4〈span〉4〈/span〉O〈span〉32〈/span〉 (〈span〉Z〈/span〉 = 1) within melilite–olivine nephelinite from Hamada, Shimane Prefecture, Japan, was investigated to clarify its crystal structure and to determine cation distributions in the 〈span〉A〈/span〉 and 〈span〉B〈/span〉 structural positions of structural channels and tetrahedral 〈span〉T〈/span〉1–〈span〉T〈/span〉4 sites. The chemical formula of a single-crystal sample was (Na〈span〉5.437〈/span〉K〈span〉2.248〈/span〉Mg〈span〉0.034〈/span〉Ca〈span〉0.031〈/span〉)〈span〉Σ7.750〈/span〉(Si〈span〉8.332〈/span〉Al〈span〉7.445〈/span〉Fe〈span〉3+〈/span〉〈span〉0.158〈/span〉Ti〈span〉0.009〈/span〉Cr〈span〉0.005〈/span〉)〈span〉Σ15.949〈/span〉O〈span〉32〈/span〉, which results in 65.2, 27.8, 2.1, 3.2 and 1.6 mol.% NaAlSiO〈span〉4〈/span〉, KAlSiO〈span〉4〈/span〉, NaFe〈span〉3+〈/span〉SiO〈span〉4〈/span〉, □Si〈span〉2〈/span〉O〈span〉4〈/span〉 and □〈span〉0.5〈/span〉(Ca,Mg)〈span〉0.5〈/span〉AlSiO〈span〉4〈/span〉 end-member components, respectively, where □ is a vacancy. X-ray diffraction data of a single crystal with dimensions of 0.28 mm × 0.15 mm × 0.05 mm measured at 296 K indicate the space group 〈span〉P〈/span〉6〈span〉3〈/span〉. In the structural refinement, the 〈span〉R〈/span〉〈span〉1〈/span〉 factor was reduced to 3.69% by taking twinning by merohedry into the refinement. The refinement accounted for 77.7% of the absolute structure and 22.3% of the 〈span〉a〈/span〉 and 〈span〉b〈/span〉 axes reversed absolute structure. The atomic populations determined in the 〈span〉A〈/span〉 and 〈span〉B〈/span〉 positions were 1.834 K + 0.166 □ and 5.705 Na + 0.198 K + 0.031 Ca + 0.034 Mg, respectively, implying the substitution of K for Na in the 〈span〉B〈/span〉 position. The 〈span〉a〈/span〉 and 〈span〉c〈/span〉 dimensions are 〈span〉a〈/span〉 = 10.0270(3) and 〈span〉c〈/span〉 = 8.4027(3) Å. The average A–O〉 and B–O〉 distances are 3.009 and 2.65 Å, respectively. The substitution of K for Na in the 〈span〉B〈/span〉 channel results in increased volume and bond-length distortion of the 〈span〉B〈/span〉O〈span〉8〈/span〉 polyhedra, which then reduces distortion of the 〈span〉A〈/span〉O〈span〉9〈/span〉 polyhedra. The average 〈span〉T〈/span〉–O distances indicate that the 〈span〉T〈/span〉1 and 〈span〉T〈/span〉4 sites are essentially filled with Al, whereas the 〈span〉T〈/span〉2 and 〈span〉T〈/span〉3 are filled with Si. Despite the deviation of the O1 oxygen from the triad axis and the combination of K〈span〉+〈/span〉 ions and vacancies in the hexagonal channels, an incommensurate structure was not observed in the X-ray diffraction data or using the electron diffraction technique.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉The distribution of Cr〈span〉3+〈/span〉 ions in blue and green diopsides crystallised from a glass with the composition [CaMgSi〈span〉2〈/span〉O〈span〉6〈/span〉 (Di)]〈span〉95〈/span〉[CaCrAlSiO〈span〉6〈/span〉 (CrAlTs)]〈span〉5〈/span〉 (mol.%) was determined using Rietveld refinement of X-ray diffraction data in order to evaluate published results by optical spectroscopic analysis, and to clarify the influence of Cr〈span〉3+〈/span〉–Al〈span〉3+〈/span〉 distribution between the octahedral 〈span〉M〈/span〉1 and tetrahedral 〈span〉T〈/span〉 sites on the crystal structure. The starting material was Di〈span〉95〈/span〉CrAlTs〈span〉5〈/span〉-diopside crystallised from glass at 800°C for 2 days. After another 19 days at 800°C and 1000°C for 7 days, the diopsides remained blue. The blue diopside gradually changed to bluish green by heating at 1200°C for 3 days and to green after 7 days. The stoichiometric compositions of the synthesised phases were confirmed by electron microprobe analysis. The Cr occupancies refined by the Rietveld method resulted in the site populations in the 〈span〉M〈/span〉1 and 〈span〉T〈/span〉 sites: 〈span〉〈span〉M〈/span〉1〈/span〉[Mg〈span〉0.95〈/span〉Cr〈span〉0.030(4)〈/span〉Al〈span〉0.020〈/span〉]〈span〉〈span〉T〈/span〉〈/span〉[Si〈span〉1.950〈/span〉Cr〈span〉0.020〈/span〉Al〈span〉0.030〈/span〉] and 〈span〉〈span〉M〈/span〉1〈/span〉[Mg〈span〉0.95〈/span〉Cr〈span〉0.037(4)〈/span〉Al〈span〉0.013〈/span〉]〈span〉〈span〉T〈/span〉〈/span〉[Si〈span〉1.950〈/span〉Cr〈span〉0.013〈/span〉Al〈span〉0.037〈/span〉] (per 6 oxygens) for the blue diopsides at 800 and 1000°C, respectively: 〈span〉〈span〉M〈/span〉1〈/span〉[Mg〈span〉0.95〈/span〉Cr〈span〉0.042(3)〈/span〉Al〈span〉0.008〈/span〉]〈span〉〈span〉T〈/span〉〈/span〉[Si〈span〉1.950〈/span〉Cr〈span〉0.008〈/span〉Al〈span〉0.042〈/span〉] for the bluish green diopside at 1200°C for 3 days; and 〈span〉〈span〉M〈/span〉1〈/span〉[Mg〈span〉0.95〈/span〉Cr〈span〉0.049(3)〈/span〉Al〈span〉0.001〈/span〉]〈span〉〈span〉T〈/span〉〈/span〉[Si〈span〉1.950〈/span〉Cr〈span〉0.001〈/span〉Al〈span〉0.049〈/span〉] for the green diopside at 1200°C for 7 days. Such Cr and Al distributions effect the volumes and site distortions of the octahedral and tetrahedral coordination polyhedra: the 〈span〉T〈/span〉O〈span〉4〈/span〉 tetrahedron volumes of the blue diopsides (2.251–2.258 Å〈span〉3〈/span〉) are larger than that of the green diopside (2.237 Å〈span〉3〈/span〉); the 〈span〉M〈/span〉1O〈span〉6〈/span〉 octahedron volumes of the former (11.74–11.77 Å〈span〉3〈/span〉) are smaller than that of the latter (11.86 Å〈span〉3〈/span〉); the 〈span〉T〈/span〉O〈span〉4〈/span〉 tetrahedra in the blue diopside (tet〉 =1.006; σ〈span〉θ(tet)〈/span〉〈span〉2〈/span〉 = 24.37–24.69) are less distorted than that of the green diopside (tet〉 = 1.007; σ〈span〉θ(tet)〈/span〉〈span〉2〈/span〉 = 27.94); the 〈span〉M〈/span〉1O〈span〉6〈/span〉 octahedra in the former (oct〉 =1.006; σ〈span〉θ(oct)〈/span〉〈span〉2〈/span〉 = 20.39–21.13) are more distorted than that of the latter (oct〉 = 1.005; σ〈span〉θ(oct)〈/span〉〈span〉2〈/span〉 = 17.58).〈/p〉〈/div〉

Description: The distribution of Cr3+ ions in blue and green diopsides crystallised from a glass with the composition [CaMgSi2O6 (Di)]95[CaCrAlSiO6 (CrAlTs)]5 (mol.%) was determined using Rietveld refinement of X-ray diffraction data in order to evaluate published results by optical spectroscopic analysis, and to clarify the influence of Cr3+–Al3+ distribution between the octahedral M1 and tetrahedral T sites on the crystal structure. The starting material was Di95CrAlTs5-diopside crystallised from glass at 800°C for 2 days. After another 19 days at 800°C and 1000°C for 7 days, the diopsides remained blue. The blue diopside gradually changed to bluish green by heating at 1200°C for 3 days and to green after 7 days. The stoichiometric compositions of the synthesised phases were confirmed by electron microprobe analysis. The Cr occupancies refined by the Rietveld method resulted in the site populations in the M1 and T sites: M1[Mg0.95Cr0.030(4)Al0.020]T[Si1.950Cr0.020Al0.030] and M1[Mg0.95Cr0.037(4)Al0.013]T[Si1.950Cr0.013Al0.037] (per 6 oxygens) for the blue diopsides at 800 and 1000°C, respectively: M1[Mg0.95Cr0.042(3)Al0.008]T[Si1.950Cr0.008Al0.042] for the bluish green diopside at 1200°C for 3 days; and M1[Mg0.95Cr0.049(3)Al0.001]T[Si1.950Cr0.001Al0.049] for the green diopside at 1200°C for 7 days. Such Cr and Al distributions effect the volumes and site distortions of the octahedral and tetrahedral coordination polyhedra: the TO4 tetrahedron volumes of the blue diopsides (2.251–2.258 Å3) are larger than that of the green diopside (2.237 Å3); the M1O6 octahedron volumes of the former (11.74–11.77 Å3) are smaller than that of the latter (11.86 Å3); the TO4 tetrahedra in the blue diopside (〈λtet〉 =1.006; σθ(tet)2 = 24.37–24.69) are less distorted than that of the green diopside (〈λtet〉 = 1.007; σθ(tet)2 = 27.94); the M1O6 octahedra in the former (〈λoct〉 =1.006; σθ(oct)2 = 20.39–21.13) are more distorted than that of the latter (〈λoct〉 = 1.005; σθ(oct)2 = 17.58).