Surface microtopography of metamorphic white micas
Tóm tắt
Gold decoration and platinum-carbon-shadowing techniques of electron microscopy have been applied to study the surface microtopographs of noncleavage (001) faces of porphyroblastic white micas and sericite crystals collected from the chlorite, biotite, and garnet zones in the Shiragayama area in the Sanbagawa metamorphic terrain, Shikoku, Japan. Two different types of surface microtopograph have been observed; parallel step system and saw-tooth step system. The former is interpreted as representing the surface microtopographs formed either by growth or under near equilibrium conditions, the latter either by dissolution or under rigorous kinetic conditions. The former has been observed on porphyroblastic white micas occurring in the middle portion of each metamorphic zone and on all sericite crystals, the latter on most sericites and on porphyroblastic white micas occurring along the boundaries of the neighboring metamorphic zones. It is suggested that the observed variations in the surface microtopographic characteristics are due to Ostwald ripening during a kind of sintering process in which interstitial water plays an essential role, and that the drastic changes along the metamorphic zone boundaries are due to dehydration reactions. Retrogressive metamorphism is considered to have an almost negligible effect, if any, upon the surface microtopographs.
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