Intracrystalline Microstructure of Synthetic Merwinite
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Crystals of merwinite were prepared at 1550 °C from chemical reagents, and their intracrystalline microstructures were examined by the combined use of x-ray diffraction and optical microscopy. The crystals were composed of pseudomerohedral twins. The adjacent twin domains were related by the pseudosymmetry two-fold axis parallel to 〈011〉 with the composition surface {811}. The overall twin structure was constructed by introducing the pseudo-symmetry three-fold axis normal to (100), which must originally be a symmetry element of the former high-symmetry phase. The transition from the primitive trigonal (point group 3 m) to the primitive monoclinic (space group P21/a) was accompanied by the combination of reducing the order of the point group and the change in the size of the unit cell. The order of the point group was reduced from 12 to 4, resulting in three twin domains with six different interfaces. This accounted for the experimentally observed microstructure consisting of repeated lamella twins in several orientations. Because the unit lattice translation would be lost during the transition, the formation of antiphase domains was expected. The lost translation vectors were 1⁄2[011], 1⁄2[100], and 1⁄2[111] resulting in four antiphase domains. As a result, the total number of domains possible in the transition was 3 × 4 ⳱ 12.
I. INTRODUCTION
Intracrystalline microstructures in synthetic crystals and natural minerals are often induced by phase transitions, which involve changes in the space group symmetry. When the low-symmetry phase is a subgroup of the high-symmetry phase, two fundamentally different types of microstructures can result: twin domains and antiphase domains (APDs).1 The twin domains arise from the loss of a point symmetry element whereas the APDs arise from the loss of a translational symmetry element. If the high- and low-symmetry phases are of the same Bravais lattices, the twinning is by merohedry. On the other hand, the change in the Bravais lattices results in the formation of pseudomerohedral twins. Because the lattices of such twins are not exactly coincident, the adjacent twins produce slightly split diffraction spots. Merwinite is a major constituent of silicate slags from the manufacture of iron and steel. It has been of particular industrial value partly because the crystal has long been believed to be free from polymorphic transitions. Moore and Araki2 have determined the crystal structure of synthetic merwinite (space group P21/a). A prominent feature of the structure is the linking of MgO6 octahedra at every corner by SiO4 tetrahedra to form slabs parallel to {100}. The paper describes the geometrically idealized arrangement which is of the glaserite [K3Na(SO4)2] structure type. Moore3 has suggested the possibility of a high-temperature merwinite polymorph isostructural 1570
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 7, Jul 2000 Downloaded: 24 Mar 2015
with glaserite. Okada and Ossaka4 have determined the crystal structure of glaserite assuming a merohedral twin, w
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