Micas
Polytypes
Polytypes can now be visualized in the transmission electron microscope
by viewing the layers from the side.
The first image 
shows a normal layer 10 Å progression for biotite. The white
dots corresond to the K+ layer. (Xu, H. and Veblen, D.R.
1995. Periodic and nonperiodic stacking in biotite from the Bingham Canyon
porphyry copper deposit, Utah. Clays and Clay Minerals 43, 159-173.)
and the second image 
shows a mixed layer polytype with 20, 30, 40 and 50 Å stackings [1
nm = 10 Å] within a smal volume of a single crystal. (Ma Chi, 1997,
in preparation, obtained on the TEM at JPL)
shows a normal layer 10 Å progression for biotite. The white
dots corresond to the K+ layer. (Xu, H. and Veblen, D.R.
1995. Periodic and nonperiodic stacking in biotite from the Bingham Canyon
porphyry copper deposit, Utah. Clays and Clay Minerals 43, 159-173.)
