Chlorite hydrothermal studies

Hydrothermal studies by Roy and Romo (1957) indicate that macroscopic vermiculite cannot form above 200-300°C. At these temperatures some of the octahedral ions migrate into the interlayer positions and a chlorite-like phase is formed. 

Figure 1.34. Frequency histogram for MgO/FeO ratios (in wt%) of chlorite from the basalt studied (A) and MORE (B). Data sources are Shikazono and Kawahata (1987), Humphris and Thompson (1978) (M Mid-Atlantic Ridge) and Kawahata (1984) (C Costa Rica Rift, Galapagos Spreading Centre). The data on chlorite from MORE are taken from typical metabasalt and not from quartz-chlorite breccia and veins which formed in a hydrothermal upflow zone (Shikazono et al., 1987).

In a study of chlorite in sedimentary rocks, Hayes (1970) concluded that type-I chlorite most likely represents authigenic chlorite (because of its relative instability) the lib stable poly type, in most cases, would indicate that the chlorite is detrital and reflects formation by igneous or metamorphic processes. Hayes points out that a few of his samples of lib chlorite appear to be authigenic, probably formed in a higher-temperature environment caused by deep-burial or hydrothermal activity. 

A point that often receives insufficient appreciation is that a system in the laboratory often behaves differently in sometimes critical ways from what is nominally the same system in the field. If one is studying rocks that are the products of natural hydrothermal alteration, the partial equilibrium assumption is more likely to be valid than it is in a laboratory hydrothermal apparatus in which one attempts to recreate such alteration. Such differences are commonly manifested in the appearance of different mineral assemblages, though changes in fluid chemistry may be very similar. An example is hydrothermal reaction of seawater and basalt, a process which occurs naturally at midocean ridges (sec refs, i, M, and many sources cited therein). The naturally altered basalts become rich in chlorite or chlorite plus epidote. In experimental systems, smectite clays appear instead. Time appears to be the limiting factor. 

Laboratory studies of vermiculites under hydrothermal conditions were made by Roy and Romo [1955,1957]. Under 10,000 Ib/in. water pressure, they observed partial dehydration at 550°C and only nonexpanding structures above 650°C. At 300°C, a migration of Mg from octahedral sites in the silicate layers to interlayer positions occurs, the product being a chloritelike phase. They conclude that no primary vermiculite could have crystallized under even mild hydrothermal conditions and that the mechanism of its formation is by the low-temperature alteration of mica and chlorite. 

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