Nepheline

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

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First order gray interference color.
Never occurs with quartz.
Occurrence is an important clue: found in alkalic igneous rocks rich in K-feldspar. The illustration above shows two nepheline crystals surrounded by microcline and perthite.
Alteration is common. The usual alteration product is cancrinite , which gives first-order interference colors (A). Cancrinite grains are typically larger and coarser than the sericite that occurs in potassium feldspar, and indivudual grains are sometimes recognizable.
If all else fails, can be distinguished from quartz by its optic sign. Quartz is uniaxial positive, nepheline is uniaxial negative.

Two views of nepheline in crossed polarizers. Note the cancrinite alteration and thefeldspar-rich surroundings.

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Created 16 December 1999, Last Update 20 December 1999