Vermiculite from biotite

Most of the vermiculites listed by Foster apparently were formed by the leaching of K from biotite. Biotite has a negative layer charge near 1.00 per Oi0(OH)2 units. Foster s vermiculites have charges from 1.08 to 0.38 with only five of 25 values... 

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. 

These experiments do not prove, but suggest the existence of a qualitative boundary separating saponites from vermiculites. This boundary cannot be found in the structural formula. If it exists, it can only be found in the distribution (ordered and disordered) of isomorphous replacements. The exact structure of saponites is still poorly known. Vermiculites are most often derived from biotite and phlogopite and could inherit the structure of the tetrahedral layers of micas. The A1 atoms in the tetrahedral layers of micas form unidimensional, ordered ensembles (linear chains of substitution Gatineau [1964] and Gatineau and Merino [1966]). 

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]). 

Mixed-layer illite-montmorillonite is by far the most abundant (in the vicinity 90%) mixed-layer clay. The two layers occur in all possible proportions from 9 1 to 1 9. Many of those with a 9 1 or even 8 2 ratio are called illites or glauconites (according to Hower, 1961, all glauconites have some interlayered montmorillonite) and those which have ratios of 1 9 and 2 8 are usually called montmorillonite. This practice is not desirable and js definitely misleading. Other random mixed-layer clays are chlorite-montmorillonite, biotite-vermiculite, chlorite-vermiculite, illite-chlorite-montmorillonite, talc-saponite, and serpentine-chlorite. Most commonly one of the layers is the expanded type and the other is non-expanded. 

Much of the derived expanded clay, even that which resembles montmorillonite (holds two layers of ethylene glycol), will contract to 10 A when exposed to a potassium solution. Weaver (1958) has shown that these clays can obtain sufficient potassium from sea water and readily contract to 10 A. Vermiculite and mixed-layer biotite-vermiculites are rare in marine sedimentary rocks. Weaver (1958) was unable to find any expandable clays in marine sediments that would contract to 10 A when treated with potassium. A few continental shales contained expanded clays that would contract to 10A when saturated with potassium. Most vermiculites derived from micas and illites have high enough charge so that when deposited in sea water they extract potassium and eventually revert to micas and illites. Some layers may be weathered to such an extent that they do not have sufficient charge to afford contraction and mixed-layer illite-montmorillonites form. 

Barshad, I. 1954. Cation exchange in micaceous minerals. II. Replaceability of ammonium and potassium from vermiculite, biotite, and montmorillonite. Soil Sci. 78 57-76. 

Figure 4 Age of moraines in the Wind River Mountains, Wyoming, on which soils of a chronosequence have formed (with the names of the glacial advances labeled) versus the Sr/ Sr of the B-horizon soil exchangeable fraction (circles) and C-horizon total soil digests (open squares). Also plotted are analyses of mineral separates from the granitoid bedrock in the area (filled symbols). The elevated Sr/ Sr in the younger soils was attributed to the release of radiogenic Sr as biotite is altered to form hydrobiotite and vermiculite (source Blum and Erel, 1997).

Forster (1961, 1963) reported a similar release and uptake of K by the mycelium of A. niger and a variety of other soil fungi when incubated with orthoclase and oligoclase. Weed et al. (1969) demonstrated that fungi weathered biotite, muscovite and phlogopite to vermiculite by acting as a sink for the K released from these minerals. Wheat plants apparently function in this manner during the alteration of biotite to vermiculite (Mortland etal., 1956). 

Vermiculite may form from the alteration of biotite or phlogopite, or of other Fe- and Mg-rich aluminosilicates, such as chlorites and hornblende in igneous or metamorphic rocks. 

The formation and survival of unstable or metastable micas and clays in sediments and soils at low temperatures reflects kinetic as well as thermodynamic factors. First, the rates of reactions involving solid-aqueous and especially solid-solid transformations in dilute solutions are very slow at low temperatures (most natural waters are dilute )- The slow kinetics of clay transformations reflects small differences in free energy between stable and metastable clays. Also, the occurrence of specific clays is related to the chemistry and crystal structure of source minerals. Thus, illite often results from the weathering of muscovite, and vermiculite results from the weathering of biotite (cf. Drever 1988), consistent with the similar chemistries and structures of these pairs of T 0 T minerals. 

Figure 20. A HRTEM image of hydrobiotite. The interlayer regions indicated by arrows have a continuous white line contrast compared to adjacent biotite interlayer regions, snggesting that potassium is depleted in these interlayer regions. They are considered to be formed by the collapse of vermiculite layers (modified from Kognre and Mnrakami 1996).

Diquat and paraquat are readily adsorbed from aqueous solutions by soil particles 40, 41, 42, 43, 44, 45, 46, 47), montmorillonite (30, 41, 48, 49, 50, 51, 52, 53, 54, 55, 56), kaolinite 41, 50, 52, 53, 54, 55), vermiculite 29, 30, 49,56), biotite 29, 30), muscovite 29, 30), phlogo-pite (29), muck 46, 48, 51), and cation exchange resins 48, 57). Only small or insignificant amounts were adsorbed by charcoal and anion exchange resins 48, 51, 53). The compounds were adsorbed to cation exchange substances through cation exchange reactions for diquat by clay minerals (Equation 1). 

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]. 

As has been shown by Mortland et al [1956], biotite can be altered to vermiculite if interlayer potassium is used as a nutritional source by plants. In a long-term field experiment, the exchange capacity of the soil from plots that received no potassium over an 80-yr period was found to be increased by about 10% (Scheffer et al. [I960]). This effect, which was attributed to partial expansion of illites, was especially marked where heavy dressings of N fertilizer had been used, and the strain on the natural potassium sources was consequently high. 

Evidence for loss of protons and octahedral iron from oxidized biotites and vermiculites. Min, 38 121-137. 

In 1934, the first insights into the structure of vermicuUte were obtained by two independent workers using X-ray powder methods. Kazantzev, on the one hand, reported that the unit cell is analogous to that of biotite, but of slightly larger dimensions, with K partly replaced by H and Fe by Mg. The other, Gruner, showed that the structure consists of silicate layers resembling those of mica or talc, with double sheets of water molecules between them. These so-called interlayer water molecules occupy a space very nearly equal to that occupied by a brucite layer in the chlorite structure, with the result that the X-ray diffraction effects obtained from vermiculites and chlorites have certain similarities. 

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