TEM-investigation of the sanidine/microcline transition across metamorphic zones; the K-feldspar varieties

The diffusive transformation sanidine arrows left and right microcline results in transformation psuedomorphs which display a variety of K-feldspar intergrowths. Several varieties may be distinguished. From a TEM study of approx 70 samples taken across a regional metamorphic zone and a contact metamorphic aureole the K-feldspar components of the observed perthites can be classified. In order to reduce the confusion in K-feldspar definitions, an attempt is made to correlate these 'TEM varieties' with 'optical varieties' as well as 'X-ray varieties'. In addition they are compared to the K-feldspar modifications sanidine and (low) microcline. An interpretation of the different Al,Si ordering paths is given.