Brittle mica

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 this section, we consider and discuss the structural and chemical features of more than 200 micas. Most are true micas (146 trioctahedral and 55 dioctahedral). Brittle-mica crystal-structure refinements number about twenty, of which only three are dioctahedral (Tables 1-4, at the end of the chapter). Of the six simple polytypes first derived by Smith and Yoder (1956) and reported by Bailey (1984a, p. 7), only five (i.e., IM, 1M, 3T, IM2, and 20) have been found and studied by three-dimensional crystal-structure refinements. 

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. 

In true micas, the tetrahedral mean bond distance varies from 1.57(1) A in boromuscovite-2Mi (Liang et al. 1995 Table 4) to 1.750(2) A in an ordered (A1 vs. Si) ephesite-2Mi (Slade et al. 1987 Table Id) in brittle micas, the (T-0> mean bond distance varies from 1.620(2) to 1.799(2) A, both values are from anandite-20 (Filut et al. 1985 Table 3a). 

Clintonite. Clintonite is the trioctahedral brittle mica with ideal composition of Ca(Mg2Al)(SiAl3)Oio(OH)2. This structure violates the Al-avoidance principle of Loew-enstein (1954). It crystallizes in H20-saturated Ca-, Al-rich, Si-poor systems under wide P-T conditions. Clintonite, usually found in metasomatic aureoles of carbonate rocks, is rare in nature because crystallization is limited to environments characterized by both alumina-rich and silica-poor bulk-rock chemistry and very low CO2 and K activities (Bucher-Nurminen 1976 Olesch and Seifert 1976 Kato et al. 1997 Grew et al. 1999). The IM polytype and IMj sequences are the most common forms. The 2Mi form is rare (Akhundov et al. 1961) and no 3T forms have been reported. Many IM crystals are twinned by 120° rotation about the normal to the 001 cleavage. Such twinning causes extra spots on precession photographs that simulate an apparent three-layer periodicity (MacKinney et al. 1988). 

Subsequent to an extensive review of brittle micas (Guggenheim 1984), additional crystal-chemical details of clintonite-IM (space group C2/m) were reported by MacKinney et al. (1988) and Alietti et al. (1997). These studies confirmed that natural clintonite crystals do not vary extensively in composition (i) the octahedral sites contain predominant Mg and Al with Fe to <7% of the octahedral-site occupancy (ii) the extent of the substitution Mg.2 " Si (A1,D), which involves the solid solution of... 

In some naturally occurring true micas. Si nearly fills all the tetrahedral sites (e.g., polylithionite, tainiolite, norrishite, and celadonite), whereas in the most common mica species (i.e., muscovite and phlogopite) Al substitutes for Si in a ratio near 1 3. In some true micas and brittle micas, the Al for Si substitution corresponds to a ratio of Al Si = 1 1 (e.g., ephesite, preiswerkite, siderophyllite, margarite, and kinoshitalite), whereas the... 

A more general relationship derived here including both trioctahedral and dioctahedral true and brittle micas (Tables 1-4, Appendix II) between tetrahedral mean bond distances (T-O) and tetrahedral chemistry (in apfu) is ... 

The displacement obtained was used to isolate the x value from the influence of the divalent interlayer cation. The x values of tetrahedrally disordered brittle micas which was thus isolated (i.e., x ) follow the same trend defined for true micas, confirming the influence of interlayer cations on x (Fig. 4). 

Figure 4. Relationship between x and Si tetrahedral content. x refers to the X valne isolated from the inflnence of the interlayer cation for the brittle micas clintonite and kinoshitalite. Regression eqnation x (°) = 2.920 X f" Si + 101.98, r = 0.950. Symbols and samples as in Fignre 3.

The plane of basal oxygen atoms approaches the tetrahedral cation in flattened tetrahedra (the distance between the tetrahedral cation and the basal oxygen-atom plane decreases with respect to the T-Oapkai distance), whereas the tetrahedral cation shifts toward the tetrahedral apex (the distance between the tetrahedral cation and basal-oxygen atom plane increases with respect to the T-Oapkai distance) in elongated tetrahedra. In preiswerkite and in boromuscovite the tetrahedral cation shifts from its ideal position toward the plane of basal oxygen atoms (x < 109.47°). In the brittle mica clintonite, the tetrahedral cation more closely approaches the center of the tetrahedron (x <= 109.47°), whereas in other micas the cation shifts toward the tetrahedral apex (x > 109.47°). The maximum shift was observed in norrishite (Tyrna and Guggenheim 1991) and in polylithionite (Takeda and Burnham 1969). 

In conclusion (i) x increases as the distance between the tetrahedral cation and the basal oxygen-atom plane increases from its ideal value (ii) x increases as <0-0>basai decreases, thus reflecting a dimensional a ustment between the tetrahedral sheet and octahedral sheet and (iii) x increases with Si content. Differences between x values of brittle micas from the true micas are related in part to electrostatic features. It is useful to understand why the tetrahedral cation moves from its ideal position. Drits (1969) stated that the position of the tetrahedral cation depends not only on the degree of substitution of Si by A1 in the tetrahedra (Brown and Bailey 1963), but also in the position and distribution in compensating positive charges. This assumption is related to electrostatic forces in the following way (see Appendix I for derivation) ... 

Figure 11 shows the effect of A1 octahedral content ( Al) on Az. Where Al occupancy is less than 1 apfu, Az is approximately zero (trioctahedral true and trioctahedral brittle micas). In trioctahedral Li-rich micas (polylithionite, trilithionite and siderophillite) and in preiswerkite, Al occupancy is nearly 1 apfu and Az is as large as... 

In K-rich trioctahedral micas, both a and interlayer separation increase from norrishite to tetra-ferriphlogopite (and aluminian phlogopite) toward values for Fe-rich polylithionite, Fe-rich phlogopite, Mg-rich annite, and phlogopite. Annite deviates from the trend of trioctahedral true micas owing to a larger interlayer separation. In the Ba-rich brittle mica, ferrokinoshitalite (M sites mainly occupied by Fe ), a- and interlayer-separation values are smaller with respect to those of kinoshitalite (M sites mostly occupied by Mg). With respect to trioctahedral micas, the interlayer separation in both muscovite and celadonitic muscovite is smaller, but a values are similar. To explain this behavior, the octahedral, tetrahedral, and 0(4) site chemistry must be considered. 

Ordering of tetrahedral cations is quite unusual in the common mica species such as muscovite-2Mi, phlogopite-lM and annite-lM (Bailey 1975, 1984c), whereas it is common in brittle micas. Margarite, bityite and anandite are examples of minerals with Si,Al (or Fe ) tetrahedral ordering (Guggenheim 1984). 

In trioctahedral brittle micas, octahedral cation ordering was found for bityite and for anandite. No evidence of ordering, except for the usual mica ordering with M(l)... 

TABLE 3a. STRUCTURAL DETAILS OF TRIOCTAHEDRAL BRITTLE MICAS... 

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