Chemiographic Analysis of Trioctahedral Smectite-to-chlorite Conversion Series from the O H Y U Caldera, Japan

-The chemical compositions ofchlorite-smectite mixed-layered minerals (C/S) from the Ohyu caldera (lnoue, 1985) are analyzed using M+-4Si-3R 2§ diagrams. The assumed original saponite has the following composition: Si3.54Alo.460 ~ 0Alo. ~7~Fe 2§ ~.385Mg1.z95Mn0.o2(OH)2M+o.56. Random C/S minerals ( 100 to 80% expandable layers) are interpreted as an interstratification of the starting 2:1 smectite layer with a AlxR2+3_x interlayer. The 2: l smectite layer charge remains constant but Ca, Na, K cations are replaced by a A1-R 2§ complex ion. The brucitic layer (produced by the polymerization of the complex ions) and the 2:1 smectite layer form a 14 ~ non-expandable phase having a composition different from a true chlorite. The true chlorite layers first appear in the ordered (corrensite) phase composed of a high charge saponite: Si335A10650 LoR2+3(OH)2M+0.65 and an octahedral vacancy-free chlorite Si2.90A1L~oOtoA1L~0R2§ The recl"ystallization of the original trioctahedral smectite into a high-charge saponite decreases the b-dimension difference with the chlorite component. From these data, it is suggested that the trioctahedral smectite-to-chlorite conversion is controlled by three reactions: 1. fixation and l,~olymerization of AI-R 2+ complex ions in the interlayer region of the original smectite producing a 14 A non-expandable phase (the interlayering of this phase with the original smectite gives the randomly interstratified C/S mineral. 2. dissolution of these random mixed-layered minerals and precipitation of corrensite. 3. dissolution of corrensite and growth of Fe-rich chlorite. Key Words--Chlorite/smectite mixed layer, Corrensite, Ohyu caldera, Smectite-to-chlorite conversion. I N T R O D U C T I O N The conversion of trioctahedral smectite to chlorite through the ordered interstratified structure (corrensite) has been documented in sedimentary rocks (April, 1981), in diagenetic series (April, 1981; Chang et aL, 1986; Bodine, 1985), in contact metamorphic environments (April, 1980), and in hydrothermal areas (Kristmannsdottir, 1979, 1983; Inoue et aL, 1984; Inoue, 1985, 1987). In spite of the fact that the structural aspect of chlorite-smectite interlayering has been well characterized by XRD analysis (Reynolds, 1988), the chemical processes by which an original smectite is converted to a chlorite via corrensite have not been clearly established. Indeed, the most detailed study is that of Bodine and Madsen (1987) who proposed a set of reactions based on mass balance considerations and on the chemical compositions of end products. Unfortunately, the equations that apply to the particular evaporitic environment they studied are difficult to extrapolate to other diagenetic or hydrothermal series. Among studies of the smectite-to-chlorite conversion in diagenetic or hydrothermal series, the chlorite/ smectite mineral sequence from the Ohyu caldera (Inoue, 1985) is one of the best documented because both chemical composition and percent expandability meaCopyright 9 1991, The Clay Minerals Society sured by XRD are available for 36 samples. The conversion is characterized by a discontinuous change in smectite layer percent. Three types of chlorite/smectite interstratified minerals were identified: highly expandable (100 to 80% smectite layers), corrensite (50 to 40% smectite layers) and highly chloritic (15 to 0% smectite layers). On the other hand, the microprobe analyses of the C/S minerals indicated a continuous change in chemical composition (Inoue, 1985, 1987). Inoue presented the following scheme for the smectite-to-chlorite transformation: (1) increase of the tetrahedral layer charge in precursor smectite layers takes place parallel to the formation of brucite-like layer in the interlayer position, (2) beyond the 20% chlorite layer limit, brucite precipitates in every second interlayer and corrensite is formed, (3) chlorite crystal growth includes corrensite domains. Several critical points remain unclear in the above scenario, such as whether or not the compositions of component layers in C/S stay constant during the transformation, and how the formation of the brucite-like layer influences the charge distribution of precursor smectite. Detailed chemiographic analysis can provide answers to these questions, if the structure and chemical composition of a continuous sequence involving the transformation of smectite to chlorite are well