Kübler (1964, 1967) attempted to define a “crystallinity” index for illite (“IC”) by examining the powder X-ray diffraction of intergrown illite and muscovite, originally to identify the anchizone (diagenesis) and the anchizone-epizone (metamorphic) boundaries. Measured values are expressed as small changes in the d value based on the width for the 10-Å peak at half height above the background for Cu radiation. If the procedure is used, it should not be characterized as a “crystallinity” index, as it is unclear if “crystallinity” is actually being measured because such patterns also reflect the presence of smectite and other K-rich micas, different mean crystallite sizes, lattice strain, layer stacking order, instrument parameters and other features.
Cf., crystalline, crystallinity index, Hinckley index, Arkai index

A regular interstratification of talc-like layers and trioctahedral (tri,trioctahedral) chlorite in a ratio of 1:1 (Abraham et al., 1980). The ideal formula is Mg₈Al(Si₇Al)O₂₀(OH)₁₀, although substitutions of NaAl = Si to about Si_0.4 are known.

See astrophyllite group.