Dioctahedral micas

H20(a) = diaspore, gibbsite, serpentine, dioctahedral micas H20(b) = chlorites, talc, trioctahedral micas, amphiboles... 

The intermediate octahedral sheet is normally made up of cations of charge 2 or 3 (Mg, Al, Fe, Fe, or, more rarely, V, Cr, Mn, Co, Ni, Cu, Zn), but in some cases cations of charge 1 (Li) and 4 (Ti) are also found. In the infinite octahedral sheet, formed by the sharing of six corners of each octahedron, there may be full occupancy of all octahedral sites ( trioctahedral micas ) alternatively, one site out of three may be vacant ( dioctahedral micas ). Nevertheless, the primary classification of micas is based on the net charge of the mixed 2 1 layer. In common micas this charge is close to 1, whereas in brittle micas it... 

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

Muscovite is the main term of the common dioctahedral micas, and it is found in acidic intrusive rocks (biotite and biotite-muscovite granites), although in subordinate quantities with respect to biotite. It is a common mineral in aplitic rocks and is peculiar to fluorine metasomatism in the contact zones between granites and slates ( greisenization ). 

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 trioctahedral micas can be distinguished by x-ray diffraction from the dioctahedral type. The dioctahedral micas characteristically show distortions that are detected as variations in the bond angles of the hexagonal pattern, Fig. 2.12E (Bailey, 1984). Natural mineral samples often exhibit an occupancy of the B site of greater than 2 and less than 3, producing many variations that cannot be detailed here. 

Figure 7. Phases present along the compositional join muscovite (Mu)— MgFe +. celadonite (Ce) at 2Kb pressure. MU = muscovite-phengite Mica = dioctahedral mica of unidentified composition CE = celadonite mica ...

The mineral types familiar in sediments and sedimentary rocks are present micas, mica-like phases, fully expandable phases and mixed layered series. In a sense, celadonite mica is isolated from dioctahedral mica by a multiphase zone where montmorillonite is stable with a feldspar and mica. It is evident that the only way to. produce celadonite mica under high potassium concentrations is by having a proper bulk composition toward that of celadonite. The possibility of producing celadonite in a potassium deficient system, i.e., where montmorillonite coexists with a non-alkali bearing phase, has not yet been studied experimentally. 

Figure 23. Phase relations in the muscovite-MgAl celadonite compositional join, 2Kb pressure. (Velde, unpublished data.) M = dioctahedral mica ...

Once the illite-chlorite zone is entered, i.e., the facies where dioctahedral mica-montmorillonite mineral solid-solutions are no longer stable, how does the assemblage change into muscovite-chlorite The major... 

Dioctahedral micas (alkali interlayer ions having total charge near +1 per formula weight) which have either lMd or 1M polymorphs are either metastable muscovite forms or are micas with a composition differing from muscovite, e.g., glauconite, celadonite, and illite. ... 

The term sericite is frequently used to describe fine-grained dioctahedral micas. This material is usually coarser than illites and often hydrothermal in origin. Sericites... 

The maximum amount of Al3+ tetrahedral substitution that 2 1 clays minerals formed at low temperatures can accommodate appears to be 0.80—0.90 per four tetrahedra. While this appears to place an upper limit on the amount of R3+ octahedral substitution, it is not clear why the limit should be such a low value. The dioctahedral smectites can accommodate more substitution (R2 + for R3+) in the octahedral sheet than can the dioctahedral micas. The reverse situation exists for trioctahedral equivalents. In the latter clays octahedral R3+ increases as tetrahedral Al increases. Thus, as one sheet increases its negative charge, the other tends to increase its positive charge. This is likely to introduce additional constraints on the structure. In the dioctahedral clays substitution in either sheet affords them a negative charge and substitution in one sheet is not predicted by substitution in the other sheet thus, one might expect more flexibility. 

Most of the celadonite samples lie in the area where some mixed-layering is to be expected. Although celadonite is commonly considered to be non-mixed, the literature suggests that little effort has been made to establish this. Of the 15 analyses examined by Wise and Eugster (1964) six reported adsorbed water and in the others it was not determined. In any event, the sheet structure of the celadonite is distinctly different from that of the other 2 1 dioctahedral clays (Radoslovich,1963a). It has a very thick octahedral sheet all three octahedral positions are of equal size (in the other 2 1 dioctahedral clay the two filled positions are smaller than the vacant position) and the interlayer separation is larger than in other contracted 2 1 dioctahedral micas (Radoslovich,1963a). 

Trioctahedral micas release more rapidly than dioctahedral micas. As a... 

Figure 6.2. View from directly above the hexagonal hole of (a) trioctahedral mica, and (b) dioctahedral mica. The proton of the structural OH is labeled with an "H", while the O and octahedral metal ions are symbolized by large shaded and smaller black circles, respectively.

In dioctahedral micas, one of three octahedral sites is vacant, so that the 0-H bond tilts toward this vacant site, as pictured in Figure 6.2b, to reduce repulsion between the proton and the two octahedrally coordinated ions = Fe " or Al ). As a result, the ion is further from the structural proton and is stabilized in the hexagonal hole. The energy barrier to removal is consequently high in muscovite. 

Studies of potassium release from micas in solutions of 1 M NaCl have revealed that the concentration ratio of K+ to Na+ in solution, [K J/fNa" ], ranges from 1 X 10 for resistant dioctahedral micas to 1.2 X 10" for easily weathered biotites (Newman, 1969). 

High fluoride content in trioctahedral micas impedes release. This is actually a special case and an exception to the first rule. Since F", which can proxy for OH" in mica structures, is not a dipole, it attracts electrostatically regardless of whether the mica is trioctahedral or dioctahedral. As a result, trioctahedral micas in which F isomorphously substitutes for much of the structural OH" release with difficulty. That is, this particular type of trioctahedral mica behaves much like a dioctahedral mica with respect to removal. 

Most of the trioctahedral tme-mica stmctures are M polytypes and a few are 2Mi, 2M2, and 3T polytypes. In dioctahedral micas, the 2Mi sequence dominates, although 3T and M structures have been found. Brittle mica crystal-structure refinements indicate that the IM polytype is generally trioctahedral whereas the 2Mi polytype is dioctahedral. The 10 structure has been found for the trioctahedral brittle mica, anandite (Giuseppetti and Tadini 1972 Filut et al. 1985) and recently was reported for a phlogopite from Kola Peninsula (Ferraris et al. 2000). The greatest number of the reported structures were refined from single-crystal X-ray diffraction data, with only a few obtained from electron and neutron diffraction experiments. 

Chromphyllite and chromium-containing dioctahedral micas. A dioctahedral mica... 

A. A Az of <0.24 A is observed for dioctahedral micas for which Al occupancy reaches 2 apfu. A1 is a cation of relatively small size. For micas with significant amounts of octahedral A1 and where A1 ordering occurs, differences in size between octahedral sites are enhanced and the value of Az increases. Such differences also occur for micas with a low charge cation (e.g., Li in trioctahedral polylithionite) or by vacancies (i.e., in dioctahedral micas), where charge balance occurs within the octahedral sheet only. 

With respect to trioctahedral micas, dioctahedral muscovite and celadonitic muscovite have smaller interlayer separations but similar a values. In dioctahedral micas, the proton position results in part from repulsion by the interlayer cation and the cations in the M(2) sites. Thus, the proton is located in that portion of the structure with minimal local positive-charge concentration, near the M(l) site (Radoslovich 1960 Guggenheim et al. 1987). The six-fold coordination of the interlayer cation with the basal inner O atom is distorted and elongated parallel to c. Both effects (i.e., the distorted coordination of the interlayer cation and the smaller H -K repulsion) thus control the interlayer separation. 

Bailey (1984b) suggested that the large dimensions of the vacant M(l) octahedral site in dioctahedral micas cause an overshift , (i.e., the intralayer shift parameter), where the upper tetrahedral sheet is shifted relative to the lower sheet by a value greater than... 

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