Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 82. (Budapest 1990)

Papp, G.: A review of the multi-layer lizardite polytypes

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 82. Budapest, 1990 p. 9-17. A review of the multi-layer lizardite polytypes by G. PAPP, Budapest G. PAPP: A review of the multi-layer lizardite polytypes - Annls hist.-nat. Mus, natn. hung. 1990, 82_: 9-17. Abstract - Multi-layer lizardite polytypes are reviewed on the basis of published data and the author's investigatipns

. According to the new classification proposed , these minerals can be grouped as (i) multi-layer polygonal serpentines, (ii) multi-layer lizardites sensu stricto

. The majority of the natural examples belongs to the first group. They generally occur in massive serpentine veins. Only a few natural samples from special genetical environment but a lot of synthetic material form the second group. They are of strongly aluminous composition (frequently in the field of amesite). With 3 tables. Introduction - Lizardite, the Mg-serpentine species with flat-layer structure was described by MIOGLEY (1951), who assigned a onelayer monoclinic cell to this mineral. A multi-layer serpentine

1 was found first at Unst, Shetland by BRINDLEY & von KNORRING (1954) and named as "orthoantigorite"

, because its superlattice was thought to be similar to that of antigorite. This misinterpretation had caused a false nomenclature which survived the correction of ZUSSMAN & BRINDLEY (1957) who showed that the superperiodicity of the "Unst-type serpentine" must be in c direction (see for example JASMUND & SYLLA 1971; Selected powder diffraction data for minerals 1974). A lot of multi-layer serpentines were found or synthesized since then. These materials have been considered as polytypes of lizardite according to WICKS & WHITTAKER ( 1975) . THE STRUCTURE OF LIZARDITE The structure'of lizardite had not been known in details until the last decade due to the'lack of suitable crystals for single crystal measurements. Nevertheless, even the first study by MIDGLEY (1951) demonstrated the characteristic features of the idealized structure: two sheets composed of silicate tetrahedra and Mg-centered octahedra are linked together to form a flat layer . WHITTAKER & ZUSSMAN (1956) determined a one-layer trigonal cell from powder patterns, but they also pointed out the high probability of the existence of two-layer hexagonal polytypes. Similar conclusions were drawn by ZVYAGIN (1967) after electron diffraction investigations. The X-ray pattern of the lizardite flakes studied by RUCKLIDGE & ZUSSMAN (1965) and KRSTANOVIC (1968) suggested that these specimens were composed of domains of one-layer trigonal and two-layer hexagonal structures ( lizardite- 1J_ and lizardite-2H according to BAILEY 1969). In this paper the term "serpentine" means Mg-serpentine.