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.

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