Application of anisotropic electron diffraction to determine the displacement vector of stacking fault in pyrite

Abstract

Pyrite is of cubic structure (space group Pa3) with the iron atoms and the centres of the S-S dumb-bells located at the regular sites of rock salt structure. The diffraction spots forbidden to the ideal pyrite structure have been observed by X-ray diffraction [1 4] and the variation in the {1 00} intensites is known as the anisotropic diffraction effect. This effect has been attributed to the ordering of the non-metallic atoms [1-.3] and the deviation of the sulphur atoms from the ideal sites of the pyrite structure [2, 4]. The electron diffraction spots forbidden to the ideal pyrite structure have been observed [5], but the anisotropic effect has not been studied. The stacking faults (SF) commonly observed in pyrite of various occurrences [6] have fault planes parallel to the { 1 0 0} planes. However, the displacement vector (R) of the SF is controversial in the literature. A value of R = (0.29, 0, 0> was proposed [7] on the basis of TEM observation, assuming the ~ S dumb-bells to remain intact across the SF. In contrast, Gratias et al. [8] calculated theoretically the value of R to be (0.27, 1/2, 0>, assuming that the SF is a coincidence-site lattice interface (CSLI). This value was later supported by Couderc et al. [6] based on the contrast analysis of the outer fringes at the SF. In the present study the components of R are confirmed directly using lattice imaging a n d g . R contrast analysis by taking advantage of the anisotropic electron diffraction spots (g) which enable the imaging of the { 1 0 0} lattice planes. The pyrite sample selected for the present study was recovered from the podiform chromitite deposit at Heng-Chun, southern Taiwan [9]. The dislocations are arranged as tangles, dipoles and subgrain boundaries, indicating that it has been plastically deformed under the ocean floor metamorphism [10]. In addition to the dislocations, extrinsic SF and loops were also found [10]. The electron probe microanalysis shows the pyrite sample is nearly stoichiometric FeS2 in composition. Thin sections of pyrite samples were ion-milled from TEM (Jeol 200 CX) observations at 200 kV. Selected-area diffraction (SAD) patterns of various zone axes were taken to study the anisotropic diffraction effect. The contrast analyses of the SF were conducted at two beam conditions using the invisibility criterion of g • R = 0 or integers, where g is the diffracted beam vector. The lattice fringes formed by the transmitted beam and the { 1 0 0} spots which are forbidden in ideal pyrite structure, were also used to show any disturbance of the SF. Fig. I a is an electron micrograph of the deformed pyrite sample showing loops and two arrays of SF. The SAD patterns (Fig. l b) have (h, 0, 0) diffraction spots which are forbidden to the ideal pyrite structure for h = odd integers. Note that the (h00) spots are clear but the (0 k 0) spots are absent in Fig. lb, where h and k are odd integrers. The (1 00) spot and its odd multiples were uniformly diffracted from the pyrite specimen indicating that the anisotropic electron