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

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

the same shape in every case (YADA & WE I 1987, PAPP 1988). On the boundary of the sectors an inversion of the layers is assumed by DÚD0NY (1988, pers. comm.). A systematic TEM study of ion-thinned serpentine samples (PAPP 1988) revealed that serpentine "rods" thicker than 0.1 >um in diameter are polygonal serpentines without exception. (Polygonal serpentine of smaller diameter does exist, but normal cylindrical chrysotile does not exceed this diameter limit.) The greatest part of the Unst material is composed of such "thick rods" according to the TEM study of the powdered sample available (PAPP 1988; cf. also Fig. 7c of ZUSSMAN et al. 1957). According to HALL et al. (1976) "several flat laths in different orientations parallel to X, rather than curling" may explain the electron diffraction pattern of the Unst serpentine. Based on these observations "lizardite-6J_i " from Unst can be classified with no doubt as a mineral composed predominantly of polygonal serpentine, even in the lack of direct imaging of the polygonality for the time being. Evidently the other multi-layer lizardites showing "lath-like" fragments by TEM must be polygonal serpentines, too. This statement is also supported by the observations of KRSTANOVIC & PAVLOVIC (1967) who called the attention to the similarity between the morphology of a six-layer serpentine and of the "Povlen-type chrysotile", both studied by them. ( "Povlen-type chrysotile" is an early name for the polygonal serpentine. For lack of observations from the axial direction the authors could not recognize the polygonality, but they pointed out that the structure of Povlen-type chrysotile was not based on a cylindrical lattice.) The similarity between several multi-layer serpentine and the "Povlen-type chrysotile" is also emphasized by BARLAKOV & GURYEV (1985). Another indirect proof of the polygonality of "lathy" multi-layer serpentines is supplied by JIANG & LIU (1984) and DÚD0NY (1988, pers. comm.) who observed 2-,6- and 2-,4-,6-layer repeat in high resolution TEM images of ionthinned polygonal serpentine samples, respectively. These observations are in accordance with X-ray data showing that multi-layer polygonal serpentines are of type 6(2), that is their structure approximates 2-layer cells. For example the layer sequence of Unst serpentine can be described as a stacking of two unit cells of 2H

Z polytype plus one cell of 2]H

1 polytype (HALL et al. 1976). It is easy to understand the experience that "most of the multi-layer lizardites described in the literature have some degree of disorder" (WICKS & WHITTAKER 1975) if we take into account their real nature. In all probability, these materials are mixtures composed of polygonal serpentine and a considerable amount of chrysotile (and in some cases lizardite as well). Polytypism is confined to the polygonal serpentine component, which is itself composed of many individual "crystallites": assuming a uniform diameter of 0.5 >um

, hexagonal array, and parallel axes for polygonal serpentine about 3,500,000 individual "rods" can be found within a "bundle" measuring 1 mm in diameter! Disordered character of X-ray patterns may be caused either by actual disorder within the single "rods" or by ordered but different layer sequence in different "rods". Multi-layer lizardite sensu stricto The number of the known natural examples of multi-layer platy serpentine (lizardite s

. s t r

. ) seems to be. lower than that of the multi-layer serpentine. (On the other hand, synthetic materials exclusively belong.to this group.) The "pseudo-multi-layer" nature of some 6(3)- and 3-layer serpentine may further decrease this number. BAILEY & TYLER (1960) described a 6(3)-layer material intermixed with type 6(2). (A 6(3) structure approximates a 3-layer cell.) Later BAILEY (1969) pointed out that this mineral did not produce the reflections distinctive for a 6-layer structures in fact, and could be correlated either with 1M or 3J_ polytype (which are indistinguishable by powder methods). Recent recognition of a "pseudo-three-layer" serpentine (PAPP 1988) permits another interpretation of this mineral (and perhaps other 6(3)- or 3-layer serpentines ). An Al-lizardite sample from Zalahaláp, Hungary gave an X-ray diffractogram comparable with that of the above mentioned mineral (Table 3, see hkl indices, d^t values are different because of the larger unit cell size of the Zalahaláp mineral). TEM investigations (DÚD0NY in PAPP 1988) showed that