The unit cell of talc

s u M M A R Y. Six specimens of talc studied by the electron microscope lattice-imaging and diffraction techniques were found to have the triclinic one-layer (I Tr) structure. Since no evidence ofa monoclinic two-layered structure was found in any specimen it is concluded that the i Tr structure is the common one for talc, and it may, moreover, be the only one. Extremely fine polysynthetic twinning and single intrinsic stacking faults were found in talc. GRUNER (I934) analysed the crystal structure of talc by the powder method, and gave the unit cell as monoclinic with a = 5"26, b = 9-IO, c = I8.8I ilk, and fi = 80 ~ Hendricks (1938) obtained a similar result by single-crystal X-ray diffraction methods, and it became accepted that talc has a monoclinic two-layer structure (2M). Stemple and Brindley (196o) studied atalc from Manchuria by 114.6 mm diameter X-ray powder camera and high-angle X-ray diffractometer and reported it as having the 2M structure. However, Ross et al. (I 968)examined talc single-crystal specimens from two localities (Gouverneur, New York, and Balsam, N. Carolina) and determined their unit cells as triclinic, and Rayner and Brown (1973) re-examined the talc from Maryland previously studied by Hendricks and found that the true unit cell was triclinic (pseudo-monoclinic) with a = 5"z93, b=9"I79, c = 9"496/ ~ and ct = 90.57, fi--- 98.9 I, 7 = 9o'o3 ~ space group CL These results cast considerable doubt on the reality of monoclinic talc although it is'possible that both triclinic and monoctinic poly-morphs exist. Since the study by Hendricks, it has become well known that talc commonly exhibits very disordered stacking (see, for example, Zvyagin, I967, who found very disordered talc by observing hkl elec-tron diffraction patterns). The writers tudied the polymorphs and the layer stacking in talc by means of lattice images and diffraction patterns obtained using a high resolu-tion and high-voltage electron microscope (JEM moo) and a normal voltage electron microscop