A new modification of the 2:1 phyllosilicate layer has been discovered in annealed celadonite, a Fe3+, Mg-rich dioctahedral mica. Plan-view diffraction patterns in TEM indicate a base-centered supercell with A = 3a and B = b, where a and b are the cell dimensions of the original mica. Basic h0l reflections with h = 3n form an orthogonal lattice with one-layer periodicity, which is not expected for normal micas. The high-resolution TEM image along the <100> or related directions is similar to that expected from normal micas but the image along the <010> directions is completely different. From these images, it is concluded that the two tetrahedral sheets in a 2:1 layer are facing each other with no lateral a/3 stagger. In the proposed model that explains the high-resolution TEM images, two thirds of the spaces surrounded by two facing tetrahedral six-member rings accommodate three (Fe3+,Mg) cations and one third of the spaces are completely vacant. The (Fe3+,Mg) cations are coordinated by six or five oxygen atoms forming trigonal prisms or square pyramids, respectively.