TRIOCTAHEDRAL 11 CLAY MINERALS

Fig.24 Composition of the octahedral sheets of the trioctahedral clay minerals. The two solid boundary lines are those established by Foster (1960) for natural biotities. Dashed line separates 2 1 and 1 1 clays.

Fig.27 shows the compositions of the octahedral sheets of the low-temperature dioctahedral and trioctahedral clay minerals calculated in the basis of R3+, Mg, Fe2+ +... 

X-ray diffraction patterns of all synthesis products clearly show the formation of trioctahedral clay minerals within 5 hours, as indicated by the (060/-332) reflection at 1.54A, revealing that Mg, Ni, Zn or Co is incorporated into the octahedral sheet of the clay minerals. After a preparation time of 20 hours, only Zn-saponites clearly display the (001) reflection at 12.5-13.OA, indicating the formation of saponite instead of the non-swelling talc. XRD patterns of Mg-, Zn- and MgZn-saponites after 20 hours of preparation are presented in Figure 1,... 

The least compHcated clay minerals are the 1 1 clay minerals composed of one tetrahedral (T) layer and one octahedral (O) layer (see Fig. 1). These 1 1 clay minerals are also referred to as TO minerals. The TO package has a basal spacing (nominal thickness) of 0.7 nm (7 E) and they are commonly referred to as 7 E minerals. Kaolinite, the dioctahedral 1 1 mineral, has filling two of three octahedral sites, and serpentine [12168-92-2J, (Mg)3Si205(0H)4, the trioctahedral 1 1 mineral has filling all three octahedral sites. The kaolin minerals have limited substitution in the octahedral... 

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2 1 stmcture has a basal spacing (nominal thickness) of 1.0 nm (10 E). Pyrophjlhte [12269-78-2] Al2Si40 Q(0H)2, is the dioctahedral mineral, ie, AF" in the octahedral sites, and talc [14807-96-6], Mg3Si402Q(0H)2, is the trioctahedral, ie, in the octahedral sites. Both these minerals are essentially free of substitution in the octahedral site and therefore do not have a net... 

Talc and Pyrophyllite. Talc (qv) and pyrophjlhte are 2 1 layer clay minerals having no substitution in either the tetrahedral or octahedral layer. These are electrostatically neutral particles (x = 0) and may be considered ideal 2 1 layer hydrous phyUosiHcates. The stmctural formula of talc, the trioctahedral form, is Mg3Si402Q(0H)2 and the stmctural formula of pyrophylUte, the dioctahedral form, is Al2Si402Q (OH)2 (106). Ferripyrophyllite has the same stmcture as pyrophylUte, but has ferric iron instead of aluminum in the octahedral layer. Because these are electrostatically neutral they do not contain interlayer materials. These minerals are important in clay mineralogy because they can be thought of as pure 2 1 layer minerals (106). 

A superficial classification of the most common clay mineral groups can be based upon the number of ions present in the octahedral layer (2 diocatahedral or 3 trioctahedral) and the numbers and the kind of ions... 

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

The 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

With increasing temperature ( 200°C), either due to deeper burial or increase in heat-flow rates, upward migrating K, Mg, and Fe, derived from the underlying sediments, become sufficiently abundant that the remaining expanded layers are lost and some discrete 10A illite (2M) and trioctahedral chlorite are formed however, much of the illite at this stage still contains an appreciable proportion of dioctahedral chlorite and the chlorite contains some 10A layers. This is the typical clay-mineral suite... 

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. 

These clays have considerably more octahedial Al than any of the other trioctahedral smectites. Those with high octahedral Al (0.73—0.79) have a low octahedral population (2.70—2.80). These latter values are tl 3 lowest that have been reported for the clay minerals deposited from solution. A mil mum of approximately 60% of the... 

Foster (1962) calculated the structural formulas for 150 selected chlorite analyses. These formulas indicate that the Si content ranges from 2.34 to 3.45 per four tetrahedral positions. Most samples fall in the 2.40-3.20 range (Fig. 17), the distribution being highly skewed towards the higher Si values. Most chlorites tend to have a much higher tetrahedral A1 content than 2 1 clays. (Some of the 1 1 trioctahedral clays are the only clay minerals with tetrahedral A1 contents as high as that of most chlorites.)... 

Many of the trioctahedral 1 1 minerals that have been studied are not considered to be clay minerals however, it is likely that minerals of similar composition and structure exist as clay-sized material but are seldom concentrated enough to be analyzed or even identified. 

MacEwan, D.M.C., 1947. The nomenclature of the halloysite minerals. Mineral Mag., 28 36-44. MacEwan, D.M.C., 1954. Cardenite a trioctahedral montmorillonite derived from biotite. Clay Miner., 2 120. 

Fig. 2 The part of 2 1 layer of clay minerals with one octahedral sheet (concretely trioctahedral) between two tetrahedral sheets with exchangeable Mg2+ cation and interlayer water molecules.

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. 

Figure 6 Representation of chemical compositions of potassic, low-temperature micas in space. The poles represent feldspar, dioctahedral clays, and trioctahedral clays, respectively. M = Na, Ca, and especially K ions, R = Al, Fe R = Fe Mg. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite), smectite, and mixed layer mica/smec-tites are indicated. Initial materials are kaolinite (kaol) and iron oxides. A second step is the production of an iron-aluminous smectite and then the formation of either illite via an iUite/smectite mixed layer mineral or glauconite via a glauconite mica/iron-smectite mixed layer phase.

Figure 11 Representation of the evolution of clay pellets in shallow shelf sediment areas according to the oxido-reduction conditions locally present. Lower arrow shows berthierine formation through reduction of iron, shifting the pellet composition from the ferric (R = Fe ) pole to the ferrous pole (R = Fe ). This reaction passes through a chemical evolution by the formation of a berthierine/smectite mixed layer mineral (chi in the figure). The arrow towards glauconite indicates the change in composition with increase in potassium and some reduction of ferric iron. The diagram represents feldspar, dioctahedral clays, and trioctahedral clays, respectively. R + = Fe +, R = Al, Fe. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite) and end-member celadonite (Ce) are indicated.

Polyak, V.J. and Giiven, N., 2000, Authigenesis of trioctahedral smectite in magnesium carbonate speleothems in Carlsbad Cavern and other caves of the Guadalupe Mountains, New Mexico, Clays Clay Miner, 48 317-321. 

Figure 10.19. A. Relationship between the 8.45 T Cs MAS NMR room temperature chemical shifts of fully hydrated Cs-exchanged clay minerals and their degree of tetrahedral Al substitution. Open squares denote the dioctahedral minerals, open circles denote the trioctahedral minerals. Note that due to motional averaging in these samples, only one caesium resonance is observed. B. The same relationship for samples fully dehydrated at 450°C. The 2 lines correspond to the 2 Cs resonances observed in these samples. Note the similar behaviour of the dioctahedral and trioctahedral minerals when dehydrated. From Weiss et al. (1990a) by permission of the Mineralogical Society...

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