A representation of the average (squared) distance traveled by an atom or particle during a defined period of time at a specific temperature. In a molecular dynamics simulation, the mean square displacement is used to calculate the diffusion coefficient for a given atomic or molecular species (e.g., interlayer cation or H₂O). In an atomic structure determination, the mean square displacement describes the time averaged movement of an atom about its mean equilibrium position at a given temperature. This “movement” involves not only (dynamic) thermal vibrations, but also an apparent movement caused by (static) disorder from the random distribution of atoms over different equilibrium positions at (or near) a site from all the unit cells sampled in the experiment (where perfect periodicity is lacking in a crystal).
Cf., density functional theory, Grand Canonical Monte Carlo simulation, molecular dynamics, Monte Carlo molecular simulation, quantum calculation

A general term for a clay used as an active principle or excipient ingredient in medication.
Cf., active principle, aesthetic medicine, antibacterial clay, geotherapy, healing clay, pelotherapy

Discredited name for a copper-rich smectite, now known to be a mixture of chrysocolla and mica.

An older term for a variety of sepiolite often used to make pipes. Some meerschaum samples may contain amorphous material of similar composition to sepiolite.
Cf., sepiolite

See hydrotalcite group.

A poorly defined material, possibly biotite, stilpnomelane, or cronstedtite.

An obsolete term used for an iron chlorite.

An obsolete varietal term for biotite.

In clay science, mesopores (Figure 3) are cavities with diameters of 2 – 50 nm within a clay sample in accord with IUPAC convention (Rouquerol et al., 1994), although in soil science mesopores are defined as between 30 – 70 μm (Soil Science Society of America, 1997). In clays, these pores are present between mineral stacks of several unit structures. The pore volume (or pore size) distribution of clays is commonly determined by gas adsorption methods (typically H₂O or N₂).
Cf., macropore, micropore

A poorly defined material, possibly a weathering product of biotite.