TWIN SYSTEMATIC OF THE MONOCLINIC POTASSIUM FELDSPARS AS A RESULT OF THEIR FADEN GROWTH IN THE ALPINE-TYPE VEINS

Monoclinic potassium feldspars are frequently encountered in the alpin e-type veins in the form of adularia and hyalophane crystals. They can form simple Karlsbad, Manebach, Baveno and Manebach-Baveno twins. Add itionally, more complex twins deriving from their simple basic forms c an be found in these veins. The development of their complex morpholog ical structures can be ascribed to the elongation of the particular cr ystal, twin or combination twin during the vein widening process. Elon gation of the Karlsbad twin results in 6 types of single and 9 types o f double Karlsbad twins. Elongation of the Manebach or Baveno twin res ults in one single and one double twin type. The elongations of two ty pes of Manebach-Baveno combination twins (fourlings) denoted as (010)- and (001)-fourling produce two corresponding single fourlings and two types of double fourlings, i.e. (010)-((010) over bar)- and (001)-((0 10) over bar)-double fourling. Secondary Manebach twinning caused by t he breaking of the primary single or twin crystal has a crucial role i n the formation and development of most elongated twin types.