Abstract: Pyrophyllite gives a diffraction pattern consisting of sharp and diffuse reflections. The unit cell for all the reflections is monoclinic with a = 5.17 Å, b = 8.92 Å, c = 18.66 Å, β = 99.8°. From the absences, the space group is either C2/c or Cc but there are additional absences of hkl reflections with k ≠ 3n and l even. The additional systematic absences show that the structure is partially disordered, the disordered state being based on a small subcell defined by the sharp reflections. This subcell is monoclinic with a′ = a = 5.17 Å, b′ = b/3 = 2.97 Å, c′ = c/2 = 9.33 Å, β′ = β = 99.8° and belongs either to the space group Cm or C2/m. Co-ordinates have been found in the space group C2/m by Fourier and least-squares methods, which give R = 0.180 for h0l reflections and R = 0.148 for hkl reflections with h = 0, 1, 2. The Si—O tetrahedra are twisted 10°–10.5° from the “ideal” arrangement, leading to a ditrigonal array of oxygens on the surfaces of the layers. The average Si—O bond length is 1.610 Å and the average octahedral site—O distance is 2.025 Å. The surfaces of the layers can come together in three ways; the O—O contacts for any pair of layers are either approximately parallel to (010) with contacts 2.890 Å and 3.028 Å alternately or approximately parallel to planes making angles of ±120° with (010) with contacts 3.009 Å, 3.066 Å, 3.009 Å, 3.343 Å sequentially.