NEPHELINE

General Formula:
Na3K(Al4Si4O16)
Sample: Nepheline Syenite System:
Hexagonal
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DESCRIPTION
SCALE
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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
nw = 1.529-1.546
ne = 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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