X-ray and Electron Diffraction Study of Penfieldite: Average Structure and Multiple Cells

S. Merlino, M. Pasero, N. Perchiazzi and A. Gianfagna
Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 1-56126 Pisa, Italy
Dipartimento di Scienze della Terra, Università di Roma ‘La Sapienza’, P. le A. Moro 5, I-00185, Roma, Italy

Abstract: Penfieldite is a lead hydroxychloride mineral with composition Pb2Cl3(OH). It belongs to the hexagonal system, space group P¯6, a = 11.393(3), c = 4.024(1) Å. The 4 Å 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 refections collected with Mo-Kα radiation. The chemical and structural relationships between penfeldite 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°C Penfieldite is quickly destroyed above 200°C.

Keywords: penfieldite • lead halides • crystal structure • laurelite • modulated structures • TEM

Mineralogical Magazine; June 1995 v. 59; no. 395; p. 341-347; DOI: 10.1180/minmag.1995.059.395.17
© 1995, The Mineralogical Society
Mineralogical Society (www.minersoc.org)