Trioctahedral minerals vermiculites

Dioctahedral micas and vermiculites of clay size can be concentrated by differentially dissolving kaolinite and trioctahedral minerals with a fluoride solution proposed by Rich... 

Nakahira and Sugiura [1960] have studied a regular mixed-layer chlorite-vermiculite mineral from an alteration zone of tertiary mudstones in the Noto Peninsula, Japan. It gives a 29.2 A reflection plus a series of rational orders. The (060) reflection is at 1.53 A suggesting a trioctahedral chlorite-vermiculite interstratification. 

Vermicuhte is an expandable 2 1 mineral like smectite, but vermiculite has a negative charge imbalance of 0.6—0.9 per 02q(0H)2 compared to smectite which has ca 0.3—0.6 per 02q(0H)2. The charge imbalance of vermiculite is satisfied by incorporating cations in two water layers as part of its crystal stmcture (144). Vermiculite, which can be either trioctahedral or dioctahedral, often forms from alteration of mica and can be viewed as an intermediate between UHte and smectite. Also, vermiculite is an end member in a compositional sequence involving chlorite (37). Vermiculite may be viewed as a mica that has lost part of its K+, or a chlorite that has lost its interlayer, and must balance its charge with hydrated cations. 

The chemical composition of vermiculite can be quite variable (145). The megascopic varieties are generally trioctahedral, and the clay-si2e varieties contain both dioctahedral and trioctahedral varieties (144). Smectite minerals do not commonly occur as macroscopic single crystals. 

Roy and Romo (1957) and Boettcher (1966) performed high pressure experiments on natural vermiculites. They observed the production of a 14 X chlorite between 300 and 550°C, talc + enstatite and an unidentified phase above 650°C. The experiments on natural minerals indicate that vermiculite will occur when alkali content or activity in solution is low. This trioctahedral expanding phase is relatively stable at high pressures and temperatures as are interlayered minerals which are composed in part by such layers. It is not stable relative to montmorillonite at low emperature. 

If we look back to the experimental studies on natural expandable minerals at high pressures, it can be recalled that the production of a chlorite-phase occurred when interlayering in the natural dioctahedral mineral had reached about 30% interlayering. It is possible that below this transition only expandable phases are present for most magnesium-iron compositions one is dioctahedral, the other would be trioctahedral. Thus, at temperatures below the transition to an ordered allevardite-type phase, dioctahedral mixed layered minerals will coexist with expandable chlorites or vermiculites as well as kaolinite. The distinction between these two phases is very difficult because both respond in about the same manner when glycollated. There can also be interlayering in both di- and... 

If we consider three components, the phases will be arranged as in Figure 48a at conditions of initial burial. The solid solution series are somewhat abbreviated for simplicity. The phase relations are dominated by fully expanding and mixed layered minerals which cover a large portion of the compositional surface. Notably two dioctahedral expandable minerals exist as does a large undefined series of trioctahedral phases designated as expanding chlorite, vermiculite and trioctahedral montmorillonite. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

Vermiculites occur extensively in soils formed by weathering or hydrothermal alteration of micas. The layer structure of vermiculite resembles that of the mica from which the mineral is derived (Fig. 5.8). Both trioctahedral and dioctahedral vermiculites exist. Weathering or alteration of the precursor micas replaces the interlayer K+ mostly with Mg2+ and expands the c spacing to 1,4-1.5 nm. 

The other key difference is the level and type of isomorphous substitution in each of these minerals. Among the trioctahedral clays, talc has no isomorphous substitution, hectorite has moderate substitution of Li for Mg and vermiculite and mica have high levels of substitution. 

The existence of vermiculitelike minerals in soils was first demonstrated in 1947. Yellow-brown crystals in the sand fractions of certain Scottish podzols were found to be derived from biotite by a process of natural weathering and to have some of the characteristics of vermiculites (Walker [1947,1949a]). In the clay fractions of the same soils, expanding lattice minerals with swelling characteristics reminiscent of, but noticeably different from, those of montmorillonite were encountered. The source of these clay minerals is not the weathered mica of the sand fractions, but a trioctahedral illite, which occurs in unaltered form in the C horizons of the soils and gradually alters to a trioctahedral vermiculitelike mineral with decreasing depth of profile (Walker [1947, 1950]). 

Macroscopic vermiculites are invariably trioctahedral, but the minerals that occur in soil clays may be either trioctahedral or dioctahedral. Brown [1953] was the first to report the existence of dioctahedral clay vermiculites, and since then, many, perhaps the majority of, clay vermiculites recorded have been of the dioctahedral type. Clay vermiculites, dioctahedral or trioctahedral, have now been identified in soils and sediments from many parts of the world, including Australia, Canada, Czechoslovakia, England, France, Ireland, Japan, New Zealand, North Africa, Scotland, South Africa, Spain, Sweden, and many states of the U.S.A. The inadequacy of the tests used for distinguishing between clay vermiculites and montmorillonites, discussed in Section CIII, makes it certain that many clay vermiculites have been wrongly identified as montmorillonites, whereas the opposite would occur to a lesser extent. This suggests that clay vermiculites are even more widely distributed in soil clays than is generally believed. M. L. Jackson (private communication) estimates an overall average ratio of 2 1 of montmorillonite to clay vermiculite in soils of all types. 

The development of vermiculite minerals in soils at the expense of micas is now well established as a common phenomenon, more particularly by the work of Jackson and his collaborators e.g., Jackson et al. [1952], Schmehl and Jackson [1956], Jackson [1959,1963], Brown and Jackson [1958]) as well as by others e.g., Fieldes and Swindale [1954], Rich [1958], Cook and Rich [1962], Millot and Camez [1963], Nelson [1963]). In spite of the frequent occurrence of dioctahedral clay vermiculites in soils, dioctahedral clay micas, in general, appear to resist decomposition better than their trioctahedral counterparts and, where direct comparison is possible, the dioctahedral type may remain unaffected, whereas the trioctahedral mica in the same profile is almost completely altered (Mitchell [1955]). Vermiculitelike minerals, however, may also develop in soils by other routes, for example, from montmorillonite (Bundy and Murray [1959], Jackson [1963]) or from chlorite (Droste and Tharin [1958], Brown and Jackson [1958], Droste et al. [1962], Millot and Camez [1963]). Such alterations are reversible, and they depend on a chemical equilibrium between the mineral and the soil solution. Hence clay chlorites, illites, and montmorillonites may develop from clay vermiculites in an appropriate environment, and intermediate types are common. The alteration of clay vermiculites to kaolinite in podzols has also been proposed (Walker [1950], Brown [1953], Jackson et al. [1954], McAleese and Mitchell [1958a]). 

Although the occurrence of clay vermiculites has been reported frequently in recent years. X-ray data on them are still rather limited, only data for the basal and 060 reflections being usually recorded. There is evidence that the nonbasal reflections can sometimes be indexed hkl, and it is probable that this is a general rule. As with the macroscopic minerals, the basal spacing varies with the nature of the interlayer cation and the hydration state of the specimen. If the 060 reflection lies in the range 1.49 to 1.51 A, the mineral is dioctahedral and, if in the range 1.53 to 1.55 A, trioctahedral (Stevens [1946], Walker [1950]). 

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