Interstratification and other reaction microstructures in the chlorite-berthierine series |
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Authors: | H Xu David R Veblen |
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Institution: | (1) Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, USA, TP |
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Abstract: | Transmission electron microscopy (TEM) results show there is a series of periodically and nonperiodically interstratified
structures composed of berthierine and chlorite layers in low-temperature “chlorite” that is one of the alteration products
of granulite-facies Archean ironstone from the eastern border of the Beartooth Mountains, Montana. An antiphase domain structure
suggests that the interstratified structures are intermediate transformation products formed by reaction from berthierine
(serpentine structure) to chlorite. Periodically interstratified structures consisting of chlorite (C) and serpentine-like
(S) (or berthierine) layers include (CS), (CCS), (CCCS), (CCCCS), and (CCCCCCS). The layer sequences in interstratified chlorite-berthierine
are indicative of the reaction mechanisms that produce the interstratified structures (e.g., crystallization from solution
or solid-state transformation). The reaction from berthierine to chlorite is crystallographically much like a cell-preserved
phase transformation, even though it is highly reconstructive. Berthierine can be considered a polymorph of the Fe-rich chlorite
mineral chamosite, with berthierine as the los-temperature phase. Interstratification and integrowths in the chlorite-berthierine
series may be common phenomena in low-temperature layer silicates resembling chlorite. Although such relations are difficult
to recognize from chemical analyses or powder X-ray diffraction patterns, they can be observed readily with TEM method.
Received: 25 April 1995/Accepted 5 April 1996 |
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