Abstract
Microprobe analyses of optically zoned nephelines show that the zoning can be due either to replacement of aluminum by iron, or to variation in the relative amounts of silica and alkalis. Iron-rich nepheline from 01 Doinyo Lengai, Tanzania contains as much as 4.6 percent iron. In nepheline with patchy extinction, there is a large variation in the amount of excess silica. It is thought that both the optical and analytical inhomogeneity are the result of incipient unmixing of alkali feldspar from these nephelines.
This content is PDF only. Please click on the PDF icon to access.
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
