Corrensite and mixed-layer chlorite/corrensite in metabasalt from northern Taiwan: TEM/AEM, EMPA, XRD, and optical studies

Abstract

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 A1 w relative to A1TM, 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 AlVl/A1TM and low FeVl/(Fe+ Mg) vl ratios of "chlorites" are correlated with interstratification of corrensite (or smectite-like) layers in chlorite. The AEM analyses obtained from 200-500 A. 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/Si~V and high FeW/(Fe + Mg) vl. This implies that the trioctahedral smectite-like component of corrensite has significantly lower AlIV/Si TM and FeVl/(Fe + Mg) w. The ratios of FeVI/(Fe + Mg) vt and AllV/Si TM 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-Offprint requests to: Yen-Hong Shau 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.

Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

“…However, data from refined chlorite structures do n o t s u p p o r t the occurrence o f such vacancies. Incorporation o f siliceous phases into chlorite could result in a p p a r e n t octahedral vacancies in a chlorite formula, as suggested by Bailey (1988c), whereas interlayering o f o t h e r phyllosilicates such as smectite or talc c o m m o n l y give the same results (e.g., Shau et al, 1990). In our experience, however, trioctahedral chlorite samples that h a v e b e e n well characterized by T E M a n d p r o v e n to Figure 10.…”

“…be pure phases i n v a r i a b l y display charge b a l a n c e a n d full octahedral occupancy, w i t h i n analytical error (e.g., Shau et al, 1990). N o r m a l i z a t i o n o f f o r m u l a e b a s e d on charge balance t h a t results in octahedral vacancies in pure trioctahedral chlorite, therefore, is n o t b a s e d Normalization is based on 20 tetrahedral and octahedral cations total.…”

Chlorite, Corrensite, and Chlorite-Mica in Late Jurassic Fluvio-Lacustrine Sediments of the Cameros Basin of Northeastern Spain

“…Both phases are commonly found as a product of hydrothermal alteration of various types of igneous rocks (Meunier et al, 1988;Shau et al, 1990;Shau and Peacor, 1992;Schiffman and Staudigel, 1995;Bettison-Varga and Mackinon, 1997;Schmidt and Robinson, 1997) or related to diagenesis of volcanoclastic sedimentary materials (Almon et al, 1976;Chang et al, 1986;Peacor, 1994a, 1994b). In addition, they are frequently associated with ancient marine evaporites (Bodine and Madsen, 1987) or lacustrine mudrocks (April, 1981;Hillier, 1993).…”

Diagenesis of Dioctahedral and Trioctahedral Smectites from Alternating Beds in Miocene to Pleistocene Rocks of the Niigata Basin, Japan

“…Chang et al, 1986) interpreted the C-S reaction as a series of interstratified chlorite-smectite phases, analogous to the smectite-toillite reaction. However, in mafic associations with volcaniclastic sediments, corrensite is considered a discrete mineral, rather than an interstratification of smectite and chlorite layers (Shau et aL, 1990;Inoue and Utada, 1991;Beaufort et al, 1997). According to many studies, the S --~ C-S --~ C transition does not involve a continuous increase in the number of chlorite layers.…”

“…The change of trioctahedral smectite to chlorite is dominant in sediments associated with mafic volcanic or evaporitic rocks (Yoshimura, 1983;Chang et al, 1986;Inoue and Utada, 1991;Hillier, 1993). In contrast to I-S with changes in layer proportions, the reaction involving trioctahedral smectite has been considered as a prograde sequence of three phases: smectite, corrensite (1:1 C-S) and chlorite (Shau et al, 1990;Inoue and Utada, 1991;Beaufort et al, 1997). Thus, corrensite, the intermediate part of the sequence, is a discrete phase, rather than an interstratification of smectite and chlorite layers.…”