Smith College - Geology 222b - Petrology

Petrographic Data File

Nepheline
Property
Value
Comments
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
nw = 
nE = 
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)
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 cleevage and twinning as is typical of nepheline. In cross-polarized light, the low birefringence which is characteristic of nepheline is apparent.
Two grains of nephelins shown in center. Left grain is oreinted almost perfectly perpendicular with c-axis.
WWW Images U.C.L.A. Petrographic Workshop

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