Trioctahedral mica

P. Hinsinger, F. Elsass, B. Jaillard, and M. Robert, Root-induced irreversible transformation of a trioctahedral mica in the rhizosphere of rape. J. Soil Sci. 44 535 (1993). 

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... 

In common trioctahedral micas with AliSi diadochy of 1 3 [(KR3 (Si3Al)Oio(OH)2], the mean T-O distance is 1.649 A (Hazen and Burnham, 1973). Based on this value and on equations 5.113 and 5.114, we obtain... 

Hazen R. M. and Wones D. R. (1972). The effect of cation substitutions on the physical properties of trioctahedral micas. Amer. Mineral, 57 103-129. 

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. 

Hectorite is similar to stevensite in having little or no tetrahedral substitution however, the octahedral sheet has a significant U content (Table XXXVIII). The hectorite from Hector, California, contains 0.33 octahedral U. A sample described by Faust et al. (1959) contains only 0.10 Li and a sample described by Bradley and Fahey (1962) contains 0.04 octahedral Li. There could presumably exist a continuous range, although the upper limit is not known. Values as high as 1.45 are reported for the trioctahedral micas (Radoslovich,1962). Layer charge is due both to Li substitution and cation deficiencies in the octahedral sheet. Appreciable F— is present proxying for OH". 

The upper limit for the number of R3+ cations in octahedral positions is essentially 0.50 or approximately 85% occupancy by R2+ cations. This compares with a minimum R2+ occupancy value of about 75% for trioctahedral micas (Radoslovich, 1963b) and 65% for dioctahedral smectites and illites. The composition of the octahedral sheet of the trioctahedral smectites falls within the compositional limits established for the trioctahedral micas (Foster,1960). All samples fall within the... 

The average Al203/Mg0 ratio for 24 illites is 9.6 and for 101 montmorillonites 6.7. Attapulgite values range from 2.5 to 0.48. The ratios of octahedral Al/octahedral Mg are respectively 5.4, 4.3 and 1.8-0.4. Radoslovich (1963b) found that the 2M muscovite structure required a minimum of 1.7 of the three octahedral sites be filled with Al. The Al occurs in the two symmetrically related sites and the larger divalent cation occurs in the distinctive or unoccupied site. The lower limit of 1.7 Al is equivalent to 85% of the two symmetrically related or occupied sites being filled in a stable muscovite structure. A similar restriction is reported for the trioctahedral micas where an upper limit of 1.00 (R3++ R4+)per three sites was found by Foster (1960). 

Tetrahedral Al3 + values range from 0.10 to 0.76 per two tetrahedral positions and average 0.49 for the ten selected samples. The amount of tetrahedral substitution of the larger Al3+ for the smaller Si4+ is appreciably greater than has been reported for the 2 1 clays, although the higher values are in the range (0.9 1.5 per four tetrahedral positions) reported for the trioctahedral micas (Foster, 1960) and chlorites (Brown and Bailey,1962). 

Ruotsala et al. (1964) reported a partial analysis of a chamosite which contained 18.7% MgO and only 9.70% FeO. This is enough Mg to fill approximately half the octahedral positions and Fe2+ to fill one-seventh. If this analysis is valid, the composition of the octahedral sheet would be similar to that of Mg-biotites (Fig.24), considerably extending the range of isomorphous substitution. If chamosite behaves as the trioctahedral micas (Foster, 1960), then as octahedral Mg increases at the expense of Fe2+, octahedral Al and tetrahedral Al both tend to decrease and the composition... 

D yakonov, Yu.S. and L vova, I.A., 1967. Transformation of trioctahedral micas into vermiculite. Dokl. Akad. Nauk S.S.S.R., 175 127- 129. (Translated from O. Prevrashchenii trioktaedri-cheskikh slyud vvermikulit. Dokl. Akad. Nauk S.S.S.R., 175 432-434.)... 

Foster, M.D., 1960. Interpretation of the composition of trioctahedral micas. U.S. Geol, Surv. Prof. Pap., 354-B. 

Dyar (1987) A review of Mossbauer data on trioctahedral micas evidence for tetrahedral Fe3+ and cation ordering. Amer. Mineral., 72,101-12. 

Dyar, M. D. Bums, R. G. (1986) Mossbauer spectral study of ferruginous one-layer trioctahedral micas. Amer. Mineral., 71,955-65. 

Robbins, D. W. Strens, R. G. J. (1972) Charge transfer ferromagnesian silicates The polarized electronic spectra of trioctahedral micas. Mineral Mag., 38 551-63. 

Aqua regia extraction is a strong partial extraction method that dissolves carbonates, most sulphide minerals, some silicates like olivine and trioctahedral micas, clay minerals and primary and secondary salts and hydroxides (Salminen, 1995). It can be considered a quasi-total extraction method, since actual total concentrations can be higher. On the other hand, this leaching method overestimates the bioavailable amount of toxic elements in a soil since metals trapped in the silicate lattice are released very slowly in the environment and are not easily involved in plant nutrition processes. 

Trioctahedral micas, such as phlogopite or biotite, are characterized by four distinct cation lattice sites tetrahedral (Z) sites occupied by silicon and aluminum octahedral (Y) sites, denoted Ml and M2, occupied by Al, Cr, Fe +, Ti, Fe +, Mg, and Mn and a large 12-fold coordinated interlayer X-site occupied by potassium, sodium, calcium, and other large cations. The low partition coefficients for lanthanides, uranium and thorium in phlogopite (<2X 10 ) indicate, however, that large highly charged cations cannot... 

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.

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. 

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