Dioctahedral structure

Experimental work in the systems K-Mg-Si-Al-Fe- O concerning celadonites has also produced expandable minerals (Velde, 1972 Velde, unpublished). In both the muscovite-MgAl celadonite and MgFe-MgAl celadonite compositional series, fully expandable phases were produced below 300°C at 2Kb pressure. These expandable phases can coexist with a potassic feldspar (Figure 23). Their (060) reflection near 1.50 X indicates a dioctahedral structure which can apparently be intimately... 

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. 

The EXAFS results reported for the untreated samples (see Section 8.3.4) led to the conclusion that Zn may form highly ordered inner-sphere sorption complexes with gibbsite surfaces or substitute into an octahedral Al-hydroxide layer of some sort. The use of sequential extraction enabled more concrete conclusions to be made. For the nonextracted soil samples (bulk and coarse), second-shell Al coordination numbers did not exceed four, in fine with the dioctahedral structure of gibbsite sheets (only two out of three metal positions are occupied). Elsewhere, a gradual increase was observed in Al coordination up to six with each extraction step, indicating that Zn is part of a fully occupied, trioctahedral Al-Zn2+ layer and not part of gibbsite or another dioctahedral Al compound.67 While dioctahedral Al-hydroxide layers are... 

Trioctahedral and dioctahedral structures If the octahedral cations are bivalent, each apical anion of octahedron is surrounded by three cations. In that case all the octahedra attached to the /-sheet have bivalent cations in their centres. 

In three-layered silicates, an octahedral layer is sandwiched between two tetrahedral layers. They are further divided into those having a dioctahedral structure and those having a trihedral structure. The former have the idealized formula Al2(Si40io)(OH)2 in electrically neutral structure, where only two-thirds of all possible octahedral sites are occupied by Al. The latter have the idealized formula Mg3(SUOio)(OH)2, and the Mg ions occupy all three such sites in a unit cell. The diversity of clays arises from deviations with respect to the ideal formulas. Aluminum... 

Folster and Kalk [1967], investigating deeply weathered igneous and metamorphic rocks in the tropical zone, observed a reduction of the index of refraction associated with expansion and bleaching of biotites, and suggested this to be correlated with a conversion into the dioctahedral structure. 

Table 5.52 Structural parameters for some trioctahedral and dioctahedral micas (from Smyth and Bish, 1988).

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

Fig. 2.12 Structural components and variations in the micas. (A) Plan view of the continuous aluminosilicate sheet (T), [Si,Al205] , a portion of the mica structure. (B) Stereographic representation of an idealized mica. The structure is composed of continuous layers containing two tetrahedral aluminosilicate sheets (T) that enclose octahedrally coordinated cations, or Mg (O). This layer or sandwich," the T-O-T or 2 1 aggregate, is held together by or Na ions. (C) The two possible positions (I and II) of octahedral cations in the micas. Sets of three locations for each are superimposed on the tetrahedral hexagonal aluminosilicate sheet. (D) The three possible directions of intralayer shift when octahedral set I (upper) or II (lower) are occupied. The dashed lines and circles represent ions below the plane of the paper. (E) Distorted hexagonal rings of apical oxygens in the tetrahedral sheet of dioctahedral micas compared with the undistorted positions of the apical oxygens in the tetrahedral sheet of trioctahedral micas.

The main method used to distinguish the relative quantities of neoformed illite is by the polymorph or structure of the material. Using the criteria that 2M and 3T polymorphs of dioctahedral potassic mica are high temperature forms (Velde, 1965a), the determination of the relative quantities of lMd, and 1M vs. 2M, 3T polymorphs permits a semi-quantitative estimation of the proportion of neo-formed or low temperature illite present in a specimen. A method commonly used is a determination of relative intensities of X-ray diffraction peaks of non-oriented mica (Velde and Hower, 1963 Maxwell and Hower, 1967). Usually only 2M and lMd polymorphs are present in illite specimens which simplifies the problem. The 1M polymorph is typical of ferric illites and celadonite-glauconites, the more tetrasilicic types. 

High pressure studies using natural sepiolite and palygorskite (Frank-Kameneckiji and Klockova, 1969) indicate that these minerals can contain variable quantities of silica because they exsolve quartz while retaining their basic structural and mineral identity. These experiments also demonstrate that the natural minerals are compositionally intermediate between talc or montmorillonite and quartz. These latter phases are formed upon the thermal breakdown of sepiolite and palygorskite under conditions of 1 and 2Kb total pressure. Both sepiolite and palygorskite appear to remain stable in sequences of buried rocks, at least up to the depth where fully expandable dioctahedral montmorillonite disappears (Millot, 1964). 

Figure 48c. Phase relations above the stability of expanding dioctahedral phases combined as mixed layered structures.

The dioctahedral subgroup is by far the most abundant. The layer charge on the expanded clays ranges from 0.3 to 0.8 per Oi0 (OH)2 unit of structure. The low-charged (0.3-0.6), expanded minerals are called montmorillonite, montmorillonids, and smectites, among others. Subdivision of the expanded clay group is still a problem. 

Some of the lMd material (either illite or mixed-layer illite-montmorillonite) presumably formed authigenically on the sea bottom or on land from the weathering of K-feldspars however, much of it was formed after burial. Studies of Tertiary, Cretaceous, and Pennsylvanian thick shale sections (Weaver, 1961b) indicate that little lMd illite was formed at the time of deposition. These shales and many others contain an abundance of expanded 2 1 dioctahedral clays with a lMd structure, some of which is detrital and some of which formed by the alteration of volcanic material on the sea floor. With burial the percentage of contracted 10A layers systematically increases. 

The converse is true of the Mg ion. It is more abundant in the octahedral sheets of the low-temperature 2 1 dioctahedral minerals, attaining an average value of 3.55% in the montmorillonites and even higher values in glauconite and celadonite. Mg in the octahedral position increases the size of the octahedral sheet and decreases structural strain. 

---