X-RAY AND ELECTRON-DIFFRACTION STUDY OF PENFIELDITE - AVERAGE STRUCTURE AND MULTIPLE CELLS

Penfieldite is a lead hydroxychloride mineral with composition Pb2Cl3(OH). It belongs to the hexagonal system, space group P6BAR, a = 11.393(3), c = 4.024(1) angstrom. The 4 angstrom c parameter corresponds to the basic sub-cell, whereas modulated structures are known with a true c axis 12 times longer. The average crystal structure of penfieldite has been solved with direct methods and refined to Rw = 0.041 for 871 reflections collected with Mo-Kalpha radiation. The chemical and structural relationships between penfieldite and laurelite, Pb2F3(F,Cl,OH), are briefly discussed. An electron diffraction study of penfieldite revealed the occurrence, besides the common modulated structure with C = 12c, of domains with a 15c periodicity. Moreover, a 9c periodicity has been observed in crystals heated at 180-degrees-C. Penfieldite is quickly destroyed above 200-degrees-C.