Chlorite formation from vermiculite

The most widespread of the secondary minerals formed during the development of a laterite weathering profile are iron and aluminium ses-quioxides (Table 3.1). These may form either directly from the alteration of primary minerals, or else via a series of pathways involving the formation of intermediary sheet silicate minerals and clays (e.g. chlorite, illite, smectite, vermiculite and halloysite), which are then themselves broken down, stripped of their mobile ions and silica, and eventually converted to alumina and ferric oxyhydroxide residua (Figure 3.9). It is not possible to describe these mineral transformations in detail, but the key issue is that under tropical-type weathering conditions these transformation pathways lead to... 

Despite the unlikelihood of secondary mineral formation by ion substitution into or movement within an existing solid, some secondary 2 1 layer silicates apparently are formed by solid-phase changes of mica fragments inherited from the parent material. Hydrous mica, for example, is a product of chemical weathering as well as mechanical breakdown of mica. Hydrous mica, in turn, can be modified directly to vermiculite, montmorillonite, or chlorite. The process is not completely understood, but seemingly involves the outward diffusion of K+ from between the layer lattices and a subsequent or simultaneous reduction of charge within the layer lattice. 

Tailings Mineralogy. Tailings minerals consist of sand, clays, amorphous oxides, and trace metals. The sand is 97.5-99% Si02, 0.5-0.9% Al203, and 0.1-0.9% Fe (4, 27, 28). The oil sands, and hence the clay minerals found in the fine tails suspension, come from the McMurray Formation. The majority of clays in this formation are kaolinite and illite with traces of smectites, chlorite, vermiculite, and mixed-layer clays (5, 29). The upper McMurray Formation has a larger amount of smectites, whereas the lower McMurray Formation has larger amounts of vermiculite and mixed-layer clays. However, in both areas, kaolinite and illite are still the predominant clay minerals (5). 

There are also the other reactive aggregates, namely gneiss and mica containing shales [41], In the interfacial transition zone, in the vicinity of aggregate surface— kaolinite and hydromicas, while from cement paste side—gel of sodium-calcium silicate hydrate, respectively are formed. However, in the case of serpentine concrete deterioration is due to the formation of brucite [75]. The clay minerals, such as chlorites, vermiculite, as well as micas and feldspars, are also included to reactive aggregate components. 

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. 

Mixed-layer montmorillonite-chlorite has been found in fireclays in Idaho by Ponder and Keller [1959], and mixed-layer chlorite-montmorillonite-vermiculite in fireclays from Olive Hill district in Kentucky by Patterson and Hosterman [1958]. These chlorites are relatively stable to the action of ethylene glycol. Mixed-layer ilUte-chlorite has been found in the Morrison formation of the Colorado Plateau by Keller [1958, 1962] in Jurassic shales. 

Regular mixed-layer chlorites similar probably to corrensite have been found in various types of limestones, dolomitic limestones, calcareous and quartzitic rocks in the lower part of the Oquirrh formation in Utah by Tooker [1960] and in uranium-containing carbonate rocks from the Cumberland Plateau, Tennessee, by Peterson [1961]. (This mineral was interpreted as a 1 1 regular chlorite-vermiculite interstratification.)... 

---