A phase or mixture of two phases (the status remains unknown) generally showing a non-regular interstratification of illite-like layers with smectite-like layers. The ratio of smectite-like layers (i.e., one 2:1 layer and one interlayer capable of swelling) to illite-like layers (i.e., one 2:1 layer) may vary. Regularity in stacking may be found at specific ratios of layers (I = illite-like, S=smectite-like), for example, ISISIS… or IIISIIISIIIS…, but these patterns of regularity are not of a sufficient long-range nature to designate a separate species. Commonly abbreviated as “I/S” or “I-S”.