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W h e n mica twins are identified f rom single-crystal xray photographs , numbers of par t ic ipat ing individuals ( = orientations), and no t of their lamellae, are recognized. A system of mica-twin symbols is proposed on t he basis of combinat ions of possible orientations. From this concept i t follows t h a t t h e inf requent twin elements [100]IEO° and [110]HO° may be even less frequent t h a n i t h ad been though t . Twinning on [100]ιβο· due to an epitactic coalescence of * Present address: Üstav geologickych ved Universi ty Kar lovy ( Ins t i tu te of Geological Sciences, Charles University), Albertov 6, P r a h a 2, Czechoslovakia. Z. Kristallogr. Bd. 132, 3 11 162 MILAN R I E D E R FRAUZTNI'S A and Β layers was not found in lithium-iron micas. There are indications t h a t twinning m a y be preferred by micas grown a t high temperatures. T h e importance of growth conditions for the mode of stacking is indicated by the epi taxy between two-layer s t ructures and 1 Μ and by the frequency of r andomly stacked polytypes in micas f rom occurrences likely to produce condit ions of chaotic growth. F o u r new polytypes, 5M[(222)22], 14Af[(222)422], 9Tc[(0)725], and 12Μ were identified. The angle β of 1 Μ subcell varies systematically with polytypism: i t is largest in I Μ and smallest in 2 M \ . I n all other polytypes β lies between t h e t w o extremes and can be predicted if the stacking sequence of a polytype is known. This relation holds for l i thium-iron micas and par t ly for muscovite; i t does not hold for magnesium-iron micas. Criteria for identification of twins of 1 M , epitactic overgrowths between I M a n d 2Mi, 2Ms, as well as addit ional criteria for distinction between twins, epi tact ic overgrowths and polytypes are given.

Lithium-iron micas from the Krušné hory Mountains (Erzgebirge): Twins, epitactic overgrowths and polytypes