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

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

the sample was composed of thin lamellae (thickness varied from a few nm to nxlOO nm) with different structure. Selected area electron diffraction patterns revealed that in one set of lamellae there was no shift between the serpentine layers, while in the other set successive layers were shifted by a/3 (or -a/3). Both type of lamellae proved to be disordered according to b. Therefore Zalahaláp material is neither 1M nor 3J_ polytype but an intimate "mixture" of (aluminous) lizardite-lMj and -lTd giving a "pseudo-three-layer" X-ray powder pattern (d denotes disorder on the analogy of kaolinite-lMy ) . Chemistry of multi-layer serpentinesFlat-layer character suggests a considerable Al- (and/or Fe

3+-) substitution in both types, although pure Mg-serpentines with flat-layer structure were also synthesized (CHERNOSKY 1975). Microprobe studies of WICKS & PLANT (1979) also showed that "the multi-layer lizardites generally have more substitution than the single-layer polytype". From the analytical data of the bulk sample-of multi-layer polygonal serpentines it is impossible to draw a firm conclusion to the connection of polytypism and chemical composition. As it was previously discussed, these materials also contain chrysotile (and sometimes lizardite as well), and a considerable amount of the polygonal serpentine component may be of singlelayer character. Synthetic multi-layer lizardites are characterized by significant Alsubstitution. Regarding the idealized formula ( Mgg-x Al^.) ( Sii,_x Alx)0io (ü"H)g multi-layer lizardite formed in the case of 0.75^x^2 or l<x^2, according to GILLERY (1959) and CHERNOSKY (1975), respectively (compositions with x>l are in the field of amesite in fact). WICKS & WHITTAKER (1975) pointed out that the misfit-reducting Al-substitution increases the possibility of the formation of polytypes "because the better-formed structures have more stacking possibilities than the deformed structures". The role of Al-substitution in the stabilization of the flat-layer serpentine structure is emphasized from another point of view by MELLINI (1982). He also called attention to the meaningful fact that multi-layer serpentine polytypes suitable for single crystal study were found only among amesites. Nevertheless, 6-layer lizardite without Al-substitution was synthesized by JASMUND & SYLLA (1971). Occurrence and genetical conditionsOverviewing the descriptions of multi-layer serpentines found up to now, a definite tendency can be outlined, namely multi-layer polygonal serpentines occur most frequently in massive serpentine veins cutting serpentinites

. This is in good correlation with the frequency of polygonal serpentine although the latter is widespread in brittle, splintery veins, too (probably it will be also proved for the multi-layer polytypes in the future). In all probability pseudomorphous replacement of pre-existing minerals does not provide favourable conditions for the development of polygonal serpentines (even for well-ordered crystals) and serpentine minerals in veins formed under different physico-chemical conditions than those in the rock. Synthesis of polygonal serpentine has not been reported yet. Natural multi-layer lizardite rarely found in greater amounts. The occurrence described by MÜLLER (1963) is similar to the above mentioned veins, but the minerals from Tracy Mine and from beside Lake Superior (BAILEY & TYLER 1960, JAHANBAGLOO & ZOLTAI 1968) represent an entirely different type (cf. also the "pseudo-three-layer" Al-lizardite from Zalahaláp; PAPP 1988). Taking into consideration the also highly aluminous composition of the synthetic samples, multi-layer lizardite can be formed in a considerable quantity under genetical conditions similar to those of amesite. PERSPECTIVES OF FURTHER STUDIES High-resolution transmission electron microscopy coupled with analytical devices may significantly advance the study of multi-layer serpentines. From this point of view, the following problems may be of interest: (i) verification of the supposed polygonality of the "Unst-type" multilayer serpentines;