Detailed single-crystal studies of two biotites of the 1M polytype, containing respectively 6.7% and 11.5% by weight iron, are presented. Both the electric field gradient (EFG) and mean squared displacement ((MSD)) tensors are determined for Fe²⁺ in the cis- and trans-octahedral sites with considerable precision. Evidence of rather novel ordering of the iron in the sites is presented and the data fitted to two species (or two families of species) related by rotation of their EFG and (MSC) principal directions about the monoclinic axis.