INTERSTRATIFICATION AND OTHER REACTION MICROSTRUCTURES IN THE CHLORITE-BERTHIERINE SERIES

Transmission electron microscopy (TEM) results show there is a series of periodic ally and nonperiodically interstratified structures compos ed of berthierine and chlorite layers in low-temperature ''chlorite'' that is one of the alteration products of granulite-facies Archean iro nstone from the eastern border of the Beartooth Mountains, Montana. An antiphase domain structure suggests that the interstratified structur es are intermediate transformation products formed by reaction from be rthierine (serpentine structure) to chlorite. Periodically interstrati fied structures consisting of chlorite (C) and serpentine-like (S) (or berthierine) layers include (CS), (CCS), (CCCS), (CCCCS), and (CCCCCC S). The layer sequences in interstratified chlorite-berthierine are in dicative of the reaction mechanisms that produce the interstratified s tructures (e.g., crystallization from solution or solid-state transfor mation). The reaction from berthierine to chlorite is crystallographic ally much like a cell-preserved phase transformation, even though it i s highly reconstructive. Berthierine can be considered a polymorph of the Fe-rich chlorite mineral chamosite, with berthierine as the los-te mperature phase. Interstratification and integrowths in the chlorite-b erthierine series may be common phenomena in low-temperature layer sil icates resembling chlorite. Although such relations are difficult to r ecognize from chemical analyses or powder X-ray diffraction patterns, they can be observed readily with TEM method.