Dendritic cordierite in argillaceous hornfels from the Toki area, Gifu Prefecture, central Japan

Abstract

Dendritic cordierite occurs in argillaceous hornfels from the Toki area, Gifu Prefecture, Japan. The cordierite crystal consists of ‘c-arms’ elongated parallel to the c-axes and ‘a-arms’ perpendicular to the c-axis. The latter arms could be divided into six kinds of untwinned ‘a-arms’ with different growth directions elongated parallel to the respective a-axis and twinned ‘a-arms’ elongated parallel to the (110) twin plane. ‘A-arms’ branch out from ‘c-arms’ or other ‘a-arms’ with different growth directions and ‘c-arms’ sometimes branch out from a-arms, leading to a tree-like structure. Each of the ‘c-arms’ contains three kinds of domains related by a three-fold axis about the c-axis. These domains are irregularly distributed without any relation to the shape of the ‘c-arm’ and the domain boundaries are of zigzag shape. This domain arrangement suggests that ‘c-arms’ grew as hexagonal cordierite and were later transformed into orthorhombic cordierite. The fact that each untwinned ‘a-arm’ has a fixed growth direction to its orientation suggests that the ‘a-arms’ grew as an orthorhombic cordierite. From the growth directions of ‘c-’ and ‘a-arms’, orthorhombic and hexagonal phases are considered to grow preferentially along the c- and a-axes, respectively. The branching of a new arm is explained by nucleation on an old arm with a different preferred growth direction. The preferential growth is discussed in terms of a significant chemical potential gradient of the cordierite component. This has been preserved in the mineral zoning observed in the matrix around the cordierite porphyroblast.