Microscopic Visualization of Crystal Twinning in Accessory Zircon

Abstract In general, crystal twinning in accessory zircon represents a rare, but in almost all granites and gneisses, encounterable phenomenon. Since only the crystal face {101} occurs as a twin plane, formation of crystal twins turns out to be relatively simple. Besides geniculate twins, which develop by rotation of the crystal structure about the line [101], parallel twins that are arranged parallel to the crystallographic main axis of zircon can also be observed by light and electron microscopy. While geniculate twins usually show a frequency within the region of a permille, the number of parallel twins can rise to several percent depending on the investigated rock. In the present contribution, a brief insight into the microscopic world of zircon twinning is provided.