X-ray and chemical studies of twenty samples of (Ba,K,Na)-feldspars from Otjosondu, Namibia reveal that they can be divided into the following three groups: (1) Cn(Ba Al2Si2O8)-poor, Ab-rich ones containing unmixed albite phases (2) Cn-rich varieties showing occasional unmixed celsian phases (3) those with Cn contents intermediate between those of the first two groups. The compositions of the unmixed phases in the second group of specimens, determined with the electron microprobe, are Cn54Or34Ab12 and Cn95Or5. Almost all contain considerable amounts of Fe3+ (up to 10 atomic %), which probably replaces Al3+. As a result, their lattice dimensions are slightly larger than those of other (Ba,K)-feldspars reported previously.

In order to estimate the influence of the Na+-ion on the lattice constants of (Ba,K,Na)-feldspars, all of the specimens were ion-exchanged in KCl-melt. The lattice parameters were corrected for the presence of Fe3+ and Na+ and then correlated with volumes or Cn contents in order to determine their variations with respect to chemical composition and structural state.

It was observed that the cell edges of (Ba,K)-feldspars behave similarly to those of the plagioclases; however, the magnitudes of the changes involved are different in the two groups. This, in turn, can be attributed to differences in their respective chemical composition and to certain structural features of feldspars in general.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.