Micas stoichiometry

Table 5.53 lists the general classification of micas with their main compositional terms. Stoichiometry obeys the general formula XF2 3Z40io(OH,F)2, where X = interlayer cations, Y = octahedrally coordinated cations of the 2 1 mixed layer, and Z = tetrahedrally coordinated cations of the 2 1 mixed layer. It must be noted that several compositional terms are indeed solid mixtures of more elementary components. In particular, glauconite has a complex chemistry and an Al Si diadochy of 0.33 3.67. (R and R terms in table 5.53 identify generic divalent and trivalent cations, respectively.)... 

AFM images were obtained for films constructed, on freshly cleaved mica, from compressed monolayers of DDAB on a subphase of HMP-stabilized CdS (81). Particles, with dimensions of 8 3 nm, were seen to be evenly distributed. The determined area of 58 nm2/particle coincided well with the area per molecule determined for DDAB from its spreading isotherm, implying 1 1 particle/surfactant stoichiometry. This result is puzzling given that freshly cleaved mica is hydrophilic and therefore any particles would be buried under a layer of the hydrophobic tails of the DDAB and unaccessable to the AFM tip. 

The term micelle was introduced in 1877 by Niigeli (from the Latin mica, a crumb) for a molecular organic aggregate of limited size without exact stoichiometry [270]. The existence of surfactant aggregates in aqueous soap solutions was established in 1896 by Kj-afft [271], and the first description of a surfactant micelle was given in 1913 by Reychler [272]. 

I-inure 12. The proton- and oxalate-promoted dissolution of muscovite. The slow weathering kinetics is a characteristic of micas. Oxalate affects the stoichiometry of A1 and Si release, but has not i significant catalytic effect. Figure 12c displays a schematic representation of the muscovite structure. It reveals the 2 1 structure. For example, an A1 layers (black) exists in an octahedral sheet between two tetrahedral sheets (white) whose cations are composed or25% A1 and 75% Si. Siioxane and edge surfaces are exposed lo solution. 

In micas (as well as in many other phyllosilicates) the Pauling model and also the homo-octahedral approximation are abstractions which are very useful, among others, for didactic purposes to gain first knowledge, but also for the calculation of identification diagrams of MDO polytypes, and for the calculation of PID functions, described in sections about experimental identification of mica polytypes below. A better approximation, but still an abstraction, is the Trigonal model, which is important for the explanation of subfamilies and for some features in the diffraction patterns. Also, when speaking of a specific polytype, a characteristic sequence of abstract mica layers is intended rather than deviations from stoichiometry, distribution of cations within octahedral sheets, distortion of coordination polyhedra, etc. 

Before the advent of SIMS, formula recalculation of mica was performed using different normalization schemes [93]. Depending on the particular procedure utilized, the effect was to overestimate, for instance, vacancy-bearing substitutions (i.e., mechanisms 1 and 4 in the list above) or, on the contrary, underestimate them. This was because either perfect stoichiometry was assumed, neglecting deprotonation mechanisms, or the latter were taken into account but, in absence of a direct knowledge of the H content, they could not be properly constrained. 

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