ORIENTED PRECIPITATE COMPLEXES IN IRON-RICH OLIVINES PRODUCED EXPERIMENTALLY IN AQUEOUS OXIDIZING ENVIRONMENT

Iron-rich olivines (Fa(100) similar to Fa(70)Fo(30)) experimentally al tered in alkaline and acidic aqueous oxidizing environment were examin ed in transmission electron microscope to determine the precipitate ph ases and their crystallographic relationships to the host. The planar zones consist of precipitates of laihunite and hematite in alkaline aq ueous oxidizing environment while hematite and amorphous silica in aci dic aqueous oxidizing environment. The directions of planar zones prod uced in alkaline aqueous oxidizing environment are parallel to (100) s imilar to (001) of olivine dependent on the composition of the host ol ivines, while those produced in acidic aqueous oxidizing environment a re always parallel to (001) of olivine independent from the compositio n of the host olivine. These differences are interpreted by the dissol ution kinetics of olivine. The present experimental results explain wh y the planar zones found in oxidized iron-rich olivines in carbonaceou s chondrite are parallel to (100) of olivine while those in terrestria l olivines are parallel to (001) of olivine. That is, the difference i s attributed to the pH of the alteration environment.