Cylindrical Mg-amesite from Yakiyama, Iizuka, Fukuoka Prefecture, Japan

Abstract

Mg-amesite and Al-serpentine with cylindrical microstructures were found in white patches near the edge of rodingite, which were mixtures of white and opaque parts comprising fine grains and light-brown transparent parts comprising coarse grains, in a serpentinite body in the Yakiyama area of Iizuka, Fukuoka Prefecture, Japan. In the transmission electron microscopy images, the white parts comprised a cylindrical structure with a large hollow in the center (cylindrical Mg-amesite), space-filling irregular grains, and fibrous grains. Cylindrical Mg-amesite may have one or more lath grains with widths close to the diameter of the hollow or none at all. The selected area electron diffraction pattern of cylindrical Mg-amesite shows diffraction spots on b* and c* aligned in a straight line. Therefore, we conclude that the cylindrical Mg-amesite has a cylindrical structure elongated along the a-axis, similar to chrysotile or polygonal serpentine. Although the lath grain inside the cylindrical Mg-amesite shows mainly 7 Å reflections, slight reflections are present, indicating the presence of a long-period structure. The chemical composition shows trends corresponding to the microstructure, which can be roughly divided into three phases: most parts of the cylinder and some space-filling irregular grains are of Mg-amesite composition, whereas the inner wall of the cylinder and irregular grains are of Al-serpentine composition; lath grains inside the cylinder and fibrous grains show compositions that can be explained by serpentine-kaolin substitution or a mixed layer of serpentine-amesite and talc.