Fine-grained biotite crystals within primary actinolite from a quartz monzonite body of the Bingham Canyon porphyry copper deposit, Utah, consist of 1M, 5M1, and 1Md polytype structures. HRTEM images directly show the stacking sequences of ordered biotite polytypes, stacking faults in ordered polytypes, and stacking sequences in disordered polytypes, if the stacking vectors for the 2:1 layers involve only 0° and ± 120° rotations. The most common type of stacking fault in the 1M biotite is a layer with −120° rotation, followed by a layer with +120° rotation, which corresponds to one unit cell of the 2M1 polytype inserted in the 1M structure. Disordered (or semi-random) biotite is composed primarily of thin domains of the 1M and 2M1 polytypes, with stacking faults. The structure of a new 5-layer (5M1) polytype has been determined from SAED and HRTEM results. The stacking sequence of the polytype is [02022].

A model of structural oscillation among 1M, 2M1, and 3T structural states is proposed to interpret nonperiodic stacking sequences in biotite crystals formed during non-equilibrium crystallization. The model also provides qualitative insights into the structure of complex long-period polytypes and may help to explain intergrowths of ordered and disordered polytypes that form during crystallization far from the equilibrium state.