Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Na3K(Al4Si4O16)
Crystal System Hexagonal
Crystal Habit Anhedral masses, or (less often) prismatic with square or hexagonal cross section .
Cleavage Cleavage (poor) on (100) and (001), subconchoidal fracture .Cleavage not usually seen in thin section
Color/Pleochroism Colorless in thin section (non-pleochroic) .
Optic Sign Uniaxial (-) Isogyres may be slightly separated in some samples, isogyres may be broad or fuzzy. Ca-rich samples may be optically positive.
2V N/A .
Optic Orientation Uniaxial (optic axis is c)  
Refractive Indices

1.529 - 1.546
1.526 - 1.544
Substitution of K for Na produces slight increase in indices.
Max Birefringence 0.005
Elongation Stubby Longitudinal sections through elongate crystals are length fast.
Extinction Parallel  
Dispersion N/A .
Distinguishing Features Resembles feldspars but nepheline is uniaxial, lacks good cleavage and twinning, and has a low birefringence.  Nepheline is softer than quartz and differs in optic sign.
Occurrence Nepheline occurs in alkali-rich, silicon poor igneous rocks (nepeline syenite, foidite and phonolite). May be found in some contact metamorphosed rocks.
Associated with: K-feldspar, Na-rich plagioclase, biotite, sodic and sodic-calcic amphiboles and pyroxenes, cancrinite, sodalite, melilite and leucite. Almost never occurs with quartz.
Editors Meredith Barrett (00), Lisa Cowley (03), Deanna Gerwin (06), Emily Baker (MHC '15)
Sources Nesse, William D. 2000. Introduction to Mineralogy. Oxford University Press, London, 225 p.

Nepheline under plane polarized light.   This sample shows no twinning which is an identifiable feature of nepheline.
Nepheline under crossed polarized light.  Nepheline exhibits low birefringence which can be observed in this sample.

Nepheline seen in plain polarized light. This sample from a Nepheline-Sodalite Syenite, lacks good cleavage and twinning as is typical of nepheline. In cross-polarized light, the low birefringence which is characteristic of nepheline is apparent.
Two grains of nepheline shown in center. Left grain is oriented almost perfectly perpendicular with c-axis.
Photomicrograph of nepheline sample (K-21) under plane polarized light. Nepheline is colorless under plane polarized light in thin section. Click on the image to see the crystal under crossed polars. This hexagonal crystal is oriented with its optic axis vertical.
Nepheline (K-21) is colorless when viewed under plane polarized light. Click on the image to view this thin section under crossed polars. Nepheline does not exhibit twinning and has low birefringence as evidenced by the gray interference color of the central crystal in this image.
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U.C.L.A. Petrographic Workshop

Marshall University Geology

Brock University Earth Sciences

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