caa a22 4500 465791808 CHVBK 20180323112027.0 cr unu---uuuuu 170327e19900301xx s 000 0 eng 10.1007/BF00678980 doi (NATIONALLICENCE)springer-10.1007/BF00678980 Corrensite and mixed-layer chlorite/corrensite in metabasalt from northern Taiwan: TEM/AEM, EMPA, XRD, and optical studies [Elektronische Daten] [Yen-Hong Shau, Donald Peacor, Eric Essene] Many chloritic minerals in low-grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties, expand slightly upon glycolation ("expandable chlorite”) and/or have excess AlVI relative to AlIV, as well as significant Ca, K and Na contents. Chloritic minerals with these properties fill vesicles and interstitial void space in low-grade metabasalt from northern Taiwan and have been studied with a combination of TEM/AEM, EMPA, XRD, and optical microscopy. The chloritic minerals include corrensite, which is an ordered 1:1 mixed-layer chlorite/smectite, and "expandable chlorite”, which is shown to be a mixed-layer chlorite/corrensite. Corrensite and some mixed-layer chlorite/corrensite occur as rims of vesicles and other cavities, while later-formed mixed-layer chlorite/corrensite occupies the vesicle cores. The TEM observations show that the mixed-layer chlorite/corrensite has ca. 20%, and the corrensite has ca. 50% expandable smectite-like layers, consistent with XRD observations and with their abnormal optical properties. The AEM analyses show that high Si and Ca contents, high AlVI/AlIV and low FeVI/(Fe+Mg)VI ratios of "chlorites” are correlated with interstratification of corrensite (or smectite-like) layers in chlorite. The AEM analyses obtained from 200-500 Å thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low AlIV/SiIV and low FeVI/(Fe+Mg)VI, while chlorite has high AlIV/SiIV and high FeVI/(Fe+Mg)VI. This implies that the trioctahedral smectite-like component of corrensite has significantly lower AlIV/SiIV and FeVI/(Fe+Mg)VI. The ratios of FeVI/(Fe+Mg)VI and AlIV/SiIV thus decrease in the order chlorite, corrensite, smectite. The proportions of corrensite (or smectite-like) layers relative to chlorite layers in low-grade rocks are inferred to be controlled principally by Fe/Mg ratio in the fluid or the bulk rock and by temperature. Compositional variations of "chlorites” in low-grade rocks, which appear to correlate with temperature or metamorphic grade, more likely reflect variable proportions of mixed-layered components. The assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases, such as chlorite-corrensite, corrensite-smectite, or chlorite-corrensite-smectite. A model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1:1 ordered mixed-layer chlorite/smectite. Springer-Verlag, 1990 Shau Yen-Hong Department of Geological Sciences, The University of Michigan, 48109, Ann Arbor, MI, USA aut Peacor Donald Department of Geological Sciences, The University of Michigan, 48109, Ann Arbor, MI, USA aut Essene Eric Department of Geological Sciences, The University of Michigan, 48109, Ann Arbor, MI, USA aut Contributions to Mineralogy and Petrology Springer-Verlag 105/2(1990-03-01), 123-142 0010-7999 105:2<123 1990 105 410 https://doi.org/10.1007/BF00678980 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF00678980 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Shau Yen-Hong Department of Geological Sciences, The University of Michigan, 48109, Ann Arbor, MI, USA aut NATIONALLICENCE 700 1- Peacor Donald Department of Geological Sciences, The University of Michigan, 48109, Ann Arbor, MI, USA aut NATIONALLICENCE 700 1- Essene Eric Department of Geological Sciences, The University of Michigan, 48109, Ann Arbor, MI, USA aut NATIONALLICENCE 773 0- Contributions to Mineralogy and Petrology Springer-Verlag 105/2(1990-03-01), 123-142 0010-7999 105:2<123 1990 105 410 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer