NEPHELINE


General Formula: Na₃K(Al₄Si₄O₁₆)	Sample: Nepheline Syenite	System: Hexagonal

TITLE DESCRIPTION SCALE	TITLE DESCRIPTION SCALE	Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties


Colour Pleochroism	colourless non pleochroic	Form	anhedral to sudhedral in intrusive rocks, subhedral to euhedral in extrusive rocks
Relief RI	low negative to low positive n_w = 1.529-1.546 n_e = 1.526-1.544	Cleavage	rarely seen in thin section, irregular fractures
Birefringence Interference Colours	0.003 - 0.005 first order grey	Twinning	not observed
Interference Figure Optic Sign 2V	uniaxial negative	Optic Orientation	longitudinal sections through euhedral crystals as length fast, with parallel extinction
Composition	most nepheline has a 3:1 ratio of Na:K. At high temperatures complete solid solution exists between pure nepheline (100% Na) and kalsilite (100% K)	Alteration	alters to clay minerals, analcime, sodalite, calcite, and cancrinite
Occurrence	common in syenite, nepheline syenite and related alkalic rocks. Nepehiline is never associated with primary quartz.	Distinguishing Features	low relief, looks like quartz but may be altered and is uniaxial negative.

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