Kationenaustausch an Plagioklasen

Abstract

A series of K-plagioclases have been produced metastably by ion-exchanging the plagioclases in KCl-melt. Chemical analyses and re-exchange experiments (with NaCl-melt) have proved that the K-plagioclases are truly isomorphous with plagioclases. Because of their isomorphous nature it was possible to mix them in different proportions and homogenise them, thereby producing ternary feldspars of different compositions in the system Or-Ab-An.

A comparative study of the lattice constants of plagioclases, their K-equivalents and the different ternary feldspars calculated using powder data leads to the following conclusions:

• 1. It is possible to dertermine by X-ray methods the anorthite content of a plagioclase to an accuracy of ±1% An.

• 2. The influence of the ionic size of the bigger cations Na, Ca, and K and that of order/disorder are different on different lattice constants and it is possible to distinguish these two effects.

• 3. The b-parameter indicates a definite structural change in plagioclases at about 25–30% An.

• 4. It is possible to predict the average Al/Si-distributions in the four different positions T_1(o), T_1(m), T_2(o) and T_(m) of all plagioclases of all stable structural states, using the c-parameter of their K-equivalents.