S. L. Hwang

Oxidation–decomposition facilitated reorientation of nanoparticles in reactively sintered (Ni0.33Co0.67)1−δO polycrystals

Abstract (Ni 0.33 Co 0.67 ) 1− δ O polycrystals with rock salt structure and a bimodal size distribution due to reactive sintering at 1000xa0°C were subject to annealing at 720xa0°C for 2–72 h in air and studied by analytical electron microscopy with regard to the effect of oxidation decomposition on the reorientation of nanoparticles in host grains. Upon annealing, the nanoparticles rapidly oxidized as spinel structure progressively Co-richer, whereas the host protoxide grains with rock salt-type structure progressively Ni-richer. The spinel particles less than 100 nm in size readily detached from grain boundaries and fell into parallel epitaxial relationship with respect to the host protoxide grains sharing a coherent interface. Such a Brownian-type reorientation process, in terms of anchorage release at interphase interface and driven by epitaxy energy cusp, at a rather low apparent homologous temperature ( T / T m =0.45) was facilitated by oxidation decomposition process and nanometer-size effect.

Kuratite, Ca4(FTi2)O4[Si8Al4O36], the Fe2+-analogue of rhönite, a new mineral from the D'Orbigny angrite meteorite

Abstract Kuratite, ideally Ca4(Fe2+10Ti2)O4[Si8Al4O36], the Fe2+-analogue of rhönite and a new member of the sapphirine supergroup, was identified from the D’Orbigny angrite meteorite by electron microscopy and micro-Raman spectroscopy. Based on the least-squares refinement of 25 d-spacings measured from selected-area electron diffraction patterns of 11 zone axes, the symmetry of kuratite was shown to be triclinic (space group P1̅ by analogy to rhönite) with a = 10.513(7), b = 10.887(7), c = 9.004(18) Å, α = 105.97(13), β = 96.00(12), γ = 124.82(04)°, V= 767 ± 2 Å3 and Z = 1 for the 40 oxygen formula. The empirical formula based on eight electron microprobe analyses is (Ca38.8Na0.02REE3+0.03Mn0.03Mg0.01Ni0.02Zn0.01Sr0.01)∑4.01(Fe2+9.989.9Ti2.00)∑11.98(Si7.80Al3.52Fe3+0.64P0.05S0.02)∑12.03O39.98F0.01Cl0.01. The simplified formula is Ca4(Fe2+10Ti2)O4[Si8Al4O36]. Micro-Raman spectroscopy showed four main bands resembling those of lunar rhönite but with higher frequencies due to different chemical composition. Analogous to the occurrence of kuratite in terrestrial basaltic rocks, kuratite coexisting with Al, Ti-bearing hedenbergite, ulvöspinel, iron-sulfide, tsangpoite, Ca-rich fayalite and kirschsteinite in D’Orbigny angrite most probably was formed at > 1000°C by rapid cooling of an interstitial melt, which is subsilicic, almost Mg-free but enriched in Al-P-Ca-Ti-Fe.