In phyllosilicates, the “layer charge”, “net layer charge”, or “permanent layer charge” is the total negative charge deviation from an ideal, unsubstituted dioctahedral or trioctahedral composition. In addition, phyllosilicates may have other charge effects on their surface, commonly referred to as the “variable layer charge”. For example, for an R³⁺-rich dioctahedral 2:1 layer, the layer composition is ideally: R₂Si₄O₁₀(OH)₂. In muscovite mica where R = Al and there is an Al substituted Si site, the layer composition is: Al₂(Si₃Al)O₁₀(OH)₂ and because an Al³⁺ substitutes for an Si⁴⁺, there is an unsatisfied residual charge on the layer that results, a layer charge of -1. In muscovite, this residual charge is compensated by an interlayer cation, K⁺, so that the structure is charge neutral. Because of the anion framework of O₁₀(OH)₂, layer charges are always negative, but may be reported in the literature as either a positive or a negative value. A negative layer charge results from either a solid solution where a cation of lesser positive charge substitutes for a cation of greater charge or by a vacancy (no charge) substitution for a cation. Anion substitutions [e.g., O for (OH)] are also possible but uncommon. The location and size of the substitution has a profound effect on the physical properties of clays. The layer charge is used in the classification scheme for phyllosilicates. The variable layer charge depends on the pH of the suspension. Assuming a simple pK model, low pH values lead to protonation of the surface species OH⁰ group located at the edges or the surface and hence, to a positive variable layer charge of OH₂⁺. Increasing pH values may lead to deprotonation and hence, to a negative variable charge of O^–. The pH point where the net charge of the entire particle is zero (e.g., for a clay mineral, the positive variable change is equal to the negative permanent charge) is called “point of zero charge” (pzc).
See point of zero charge.

The sum of the intralayer displacement plus the interlayer displacement, which defines the total relative displacement between adjacent layers, as shown in Figure 1. For 2:1 layers, the layer displacement is measured from the geometric center of the ditrigonal ring. The “intralayer displacement” is the shift that originates from the octahedral slant within one layer and is measured from the geometric center of the ditrigonal ring from the lower to the upper tetrahedral sheet of that layer (Figure 1). Layer displacement should be used instead of “interlayer shift”.
Cf., interlayer, layer

Composites produced on nearly any substrate, including textiles, where the composite is fabricated by successive dipping/rinsing/drying of the substrate in two different solutions, one solution containing a clay, usually montmorillonite, and the second solution containing a complimentary polymer (e.g., any polycationic polymer). These composites are typically transparent, and generally 40 to 50 bilayers thick. Layer-by-layer composites lower flammability substantially and improve gas barrier properties.
See flame retardancy

A poorly defined material, possibly interstratified biotite and vermiculite.

An obsolete varietal term for Fe²⁺-rich saponite

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A description of the interactive forces occurring between a pair of neutral atoms or molecules. The potential is comprised of force-field terms: at long-separation distances, van der Waals attraction predominates, whereas at short-separation distances, strong repulsion predominates as a result of the Pauli exclusion principle. The Lennard-Jones potential is accurate for noble gas interactions and a relatively good model for most neutral atoms and molecules. The Lennard-Jones potential is computationally simple and thus commonly used in modeling programs.

Mg analogue of stilpnomelane.
See stilpnomelane

An obsolete name for altered material, probably vermiculite.

See boehmite.

A series name for trioctahedral micas on or close to the trilithionite-polylithionite join. Also used to describe light-colored micas with a significant amount of lithium. Lepidolite is useful as a field term for micas that have not been completely analyzed compositionally, that are commonly found in pegmatite, that have a pink or whitish color. In general, lepidolite, as distinguished from muscovite, commonly crystallizes as the 1M polytype, whereas muscovite is commonly the 2M₁ polytype. Lithium is not a chromophore and does not impart the pink color to lepidolite; the presence of Mn probably imparts the pink color to lepidolite.