▪ Abstract Mixed-layer clay minerals are intermediate products of reactions involving pure end-member clays They come from natural environments ranging from surface to low-grade metamorphic and hydrothermal conditions Most often mixed layering is essentially two component, but more complicated interstratifications have also been documented Variable tendency to form regular 1:1 interstratifications has been observed and explanations of this phenomenon have been proposed Mixed-layer clays are either di- or trioctahedral; di/trioctahedral interstratifications are rare Most mixed-layer clays contain smectite or vermiculte as a swelling component Exceptions are all trioctahedral: serpentine/chlorite in low-temperature environments, and mica/chlorite and talc/chlorite at high temperatures Solid state transformation and dissolution/crystallization are the two mechanisms responsible for the formation of different mixed-layer clays In general, the weathering reactions that produce mixed layering are revers

Nature of mixed-layer clays and mechanisms of their formation and alteration

The chemical composition of about 500 diagenetic chlorites, determined by electron microprobe, has been studied in six different sedimentary sequences spanning conditions from early diagenesis to low-grade metamorphism, in the temperature range 40–330°C. The range of Fe/(Fe + Mg) is almost complete and is positively correlated with Al. Five sequences show the same compositional variation. In each, the most siliceous chlorites have the lowest R2+, substantially more octahedral than tetrahedral Al, and the lowest octahedral totals. Conversely, the least siliceous have the highest R2+, nearly equal octahedral and tetrahedral Al, and octahedral totals close to that for an ideal trioctahedral mineral. A dioctahedral substitution Si[]R2−2 (where [] represents a vacant octahedral site) which decreases with temperature, describes this variation. Low octahedral totals are, however, induced by the method of calculation and need not indicate vacancies; for published wet chemical analyses of metamorphic chlorites they may simply indicate oxidation of Fe. Intergrown dioctahedral phyllosilicates may partly account for apparent vacancies in diagenetic chlorites. Nevertheless, the correlation of composition with temperature and similarities to the temperature-related evolution of synthetic chlorites, suggest that diagenetic chlorites are compositionally distinct from, but metastable with respect to, fully trioctahedral metamorphic chlorites. Temperature-related trends are modified by bulk composition, complicating their potential use for low-temperature geothermometry.

Octahedral Occupancy and the Chemical Composition of Diagenetic (Low-Temperature) Chlorites

Formation of clay minerals under hydrothermal influence is the result of rock alteration by circulating hot water in the Earth’s crust. A pre-existing rock-forming mineral assemblage is altered to a new set of minerals which are more stable under the hydrothermal conditions of temperature, pressure, and fluid composition. The interaction of hot water and rocks forms a spatially and temporally regular zonal pattern of new clay minerals, as the fluid with cooling temperature moves through the surrounding rock mass. This chapter discusses the formation of clay minerals in such dynamic processes of hydrothermal alteration. The approach is one of clay-mineral facies formed under conditions of massive alteration in the rocks. The chemical and mineralogical changes which occur on the scale of a rock or rock mass are considered to have been dealt with in the preceding chapter. The exact process of change via local, vein-influenced exchange processes is ignored for simplicity (see Chap. 6).

Formation of Clay Minerals in Hydrothermal Environments

In well NJ-15 of the Nesjavellir geothermal field, Iceland, the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300° C. At temperatures below 180° C, the clay fractions contain mixtures of di- and trioctahedral smectites, whose layer charge increases with depth. Between 200 and 240° C, discrete smectites have transformed into smectite-rich, randomly interstratified chlorite and smecite (R0 C/S). Because the abundance of chlorite interlayers in this C/S is generally <20%, its presence can be detected only by electron microprobe techniques and not by X-ray diffraction. Between 245 and 265° C, both regularly (R1) and randomly interstratified C/S are the predominant layer silicates. Discrete chlorite first appears at approximately 270° C and coexists with minor amounts of R0 C/S at higher temperatures.

R0 and R1 C/S form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end-members. The interlayer cation and Si content of smectites and C/S decrease with increasing temperature. The Mg/(Mg + Fe) content of smectite, C/S, and chlorite is unrelated to temperature. The percentage of chlorite in C/S, as determined by electron microprobe analyses, increases continuously with increasing temperature, except for occurrences of smectite-rich C/S in fresh basaltic dykes which have not thermally equilibrated with the higher grade country rocks.

The smectite–chlorite transition in drillhole NJ‐15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations

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 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/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.

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

Conversion of Trioctahedral Smectite to Interstratified Chlorite/Smectite in Pliocene Acidic Pyroclastic Sediments of the Ohyu District, Akita Prefecture, Japan

Hiroshi Nagata

Papers

Conversion of Trioctahedral Smectite to Interstratified Chlorite/Smectite in Pliocene Acidic Pyroclastic Sediments of the Ohyu District, Akita Prefecture, Japan