Thermal Transformation of Talc as Studied by Electron-Optical Methods

Helena De Souza Santos1 and Keiji Yada2
1 Laboratório de Microscopia Eletrônica, Instituto de Física Universidade de São Paulo, São Paulo, Brazil
2 Institute for Scientific Measurements, Tohoku University, Sendai, Japan

Abstract: The thermal decomposition of a fibrous talc was studied by transmission electron microscopy (TEM) and selected-area electron diffraction (SAD). Small changes in the lengths of a and b unit-cell parameters were noticeable at 500°C, but the talc laths did not change morphologically until 800°C. At this temperature striations began to appear in the TEM image, and the talc SAD reflections began to develop faint satellite streaks. At 900°C the striations appeared to be crystallites, and reflections of orthorhombic enstatite were noted. Both TEM and SAD evidence showed that the enstatite crystallites were formed in three orientations corresponding to the three pseudohexagonai a axes of the talc. Thus, triple superposition of the electron diffraction pattern at the three equivalent angles explains the high symmetry star-shaped pattern. At 1000° to 1100°C the enstatite crystailites were shorter and thicker, and the streaks in the SAD pattern were nearly absent. Above 1200°C only one orientation of the enstatite crystailites was found. Noncrystalline regions were also detected at 900°C and became progressively scarce at 1000° and 1100°C. They were not detected at 1200°C. At 1300°C cristobalite was detected in some SAD patterns. The crystallographic axes and unit-cell parameters of the talc and enstatite were also topotactically related as follows: at (5.3 Å)//ce (5.2 Å); bt (9.16 Å)//be (8.8 Å); d(001)t (18.84 Å)//ae (18.2 Å). These results are compatible with an inhomogeneous decomposition mechanism as proposed by earlier workers.

Key Words: Enstatite • Morphology • Selected-area electron diffraction • Talc • Thermal transformation • Topotactic relation • Transmission electron microscopy

Clays and Clay Minerals; August 1988 v. 36; no. 4; p. 289-296; DOI: 10.1346/CCMN.1988.0360401
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)