Hematite twinning

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Epitaxial twins (Fig. 4.11) consist of a hematite centre with outgrowths of acicular goethite. As the structures of both goethite and hematite are based on an hep anion array, some of the interplanar spacings in the two compounds are similar and this fa-... 

Watari, F. Delavignette, P. Van Landuyt, J. Amelinckx, S. (1979) Electron microscopic study of dehydration transformations. I. Twin formation and mosaic structure in hematite derived from goefhite. J. Solid State Chem. 

Figure 12.9. Spiral step pattern observed on both sides of a twin boundary on a 0001 face of a hematite crystal [7]. T.B. is the twin boundary, J. and T indicate orientations. Phase contrast photomicrograph.

I. Sunagawa, Mechanism of crystal growth, etching and twin formation of hematite. Mineral. J 3,1960, 50-89... 

Langmnir D (1971) Particle size effect on the reaction goethite = hematite + water. Am J Sci 271 147-156 Lee WH, Shen PY (1999) On the coalescence and twinning of cube-octahedral Ce02 condensates. J Crystal Growth 205 169-176... 

Nord, G.L., Lawsort, C.A. (1989) Order-disorder transition-induced twin domains and magnetic properties in ihnenite-hematite. Am Mineral 74 160-176... 

Figure 13. Model of self-reversal in the ilmenite-hematite solid solution (after Hoffman 1992). Boxes represent (001) cation layers (viewed down the c-axis), shaded according to their Fe-occupancy (Fe = dark, Ti = light). Two ordered ferrimagnetic domains are shown (left and right) separated by a twin wall (central) with a canted antiferromagnetic stmeture. (a) The twin wall orders first with its parasitic moment parallel to the external field. The ferrimagnetic domains order perpendicular to external field and antiparallel to each other, (b) The domain moments tilt away from the wall moment at lower temperatnres, creating a large reverse component of magnetization.

Figure 71. Sniall particles of twinned hematite formed in the electron microscope as a decomposition product of goethite 1215]...

The interfaces between finescale exsolution lamellae of ilmenite in hematite have been investigated and most were shown to be coherent and dislocation free [251]. Due to differences in lattice parameters, dislocation arrays decorate the interface [Fig. 8(b)]. Contrast analysis revealed a hexagonal grid of rhombohedral dislocations [250] rather than dislocations with basal Burgers vectors [252]. The ordering phase transition in ferrian ilmenite from the high-temperature R3c disordered structure to a R3 lower-temperature ordered structure results in the creation of twin domains [253,254]. No dislocations were involved. 

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