Dioctahedral vermiculite

Rich, C.L and Cook, M.G., 1963. Formation of dioctahedral vermiculite in Virginia soils. Ckys Cky Miner., Proc., 10 96-106. 

Vennicnlite (0.6 < x < 0.9) Dioctahedral vermiculite MJAl2KSi4 xAUO10(OH)2 Trioctahedral vermiculite M JMg3](Si4 jcA1x)0, o(OH)2... 

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. 

Dioctahedral vermiculites Trioctahedral vermiculites Dioctahedral illites Trioctahedral illites... 

In soils derived from parent material containing both biotite and muscovite, podzolisation brings about changes in the ratio between mica species. Whereas biotites are decomposed in the top soils of podzols (Mitchell [1955], Lisitsa and Tikhonov [1969]), muscovite remains under the same conditions almost unaltered (Lisitsa and Tikhonov [1969]), but usually exhibits some structural changes. Thus, interstratification of muscovite with dioctahedral vermiculite is regarded to be typical for podzol soils by Sokolova and Shostak [1969]. 

Brown and Jackson [1956], studying the Hiawatha Sandy Soils, find that the dominant layer silicate in all fractions of the B and C horizons is an interstratified vermiculite-chlorite (dominantly dioctahedral). This chlorite collapses on heating to 600° and is, therefore, relatively labile. Rich and Obenshain [1955] give evidence for an aluminum interlayer in dioctahedral vermiculite, which prevents the collapse of the mineral. 

Although both tri- and dioctahedral vermiculites are known to exist, the latter has never been isolated in sufficient purity to determine its thermal characteristics. Trioctahedral vermiculite shows a fairly characteristic differential thermal curve with a large hygroscopic... 

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. 

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. 

Trioctahedral illites have been reported by Walker (1950) and Weiss et al.(1956). Walker s analysis, which he considers only a rough approximation, is given in Table XI. The clay biotite occurs in a Scottish soil and is believed to be authigenic however, it weathers so easily to vermiculite that unweathered material is difficult to find. Due to its instability, it is not likely that much clay-sized biotite exists although trioctahedral biotite-like layers may occur interlayered with dioctahedral illite layers. Such interlayering has been reported by Bassett (1959). 

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. 

A number of Al chlorites in which both octahedral sheets are dioctahedral have recently been described. Dioctahedral Al chlorites have been reported in bauxite deposits (Bardossy, 1959 Caillere, 1962). These chlorites appear to have been formed by the precipitation-fixation of Al hydroxide in the interlayer position of stripped illite or montmorillonite. A similar type of chlorite, along with dioctahedral chlorite-vermiculite, occurs in the arkosic sands and shales of the Pennsylvanian Minturn Formation of Colorado (Raup, 1966). Bailey and Tyler (1960) have described the occurrence of dioctahedral chlorite and mixed-layer chlorite-montmorillonite in the Lake Superior iron ores. Hydrothermal occurrences have been described by Sudo and Sato (1966). 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

Brown, G., 1953. The dioctahedral analoque of vermiculite. Clay Miner., 2 64-70. 

Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering horizons of podzols. Clay Miner., 5 183-193. 

Vermiculite is a widespread hydrated clay mineral of lesser abundance than smectite. Understanding of its diagenetic behavior is complicated by the fact that most of the laboratory measurements on vermiculite have been made on the hydrothermal alteration products of coarse-grained biotites, whereas most soil vermiculites that would be fed into a sedimentary pile like that of the Gulf of Mexico coast are weathered dioctahedral Ulites containing a lot of interlayer Al- and Fe-hydroxides. 

Brown, G. (1953). The dioctahedral analogue of vermiculite. Clay Miner. Bull. 2, 64-70. Buffle, J. (1988). Complexation Reactions in Aquatic Systems An Analytical Approach. 

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