Mineral - Nepheline
Nepheline
Chemical Properties
Mineral Class
Silicates
Toxicity
none
Formula
Na₃K(Al₄Si₄O₁₆)
Unit Cell
a=9.99 c=8.37 Z=8
Physical Properties
Hardness
5.5 - 6
Streak
white
Density
Normal (2-3,5)
Cleavage
Indistinct after {10 1 0} and {0001}
Fracture
conchoidal
Tenacity
Brittle
Optical Properties
Color
whitebrowngrayredcolorless
Birefringence
0.004
Luster
Vitreous,
Greasy
Transparency
translucent,
opaque
Pleochroism
Geomineralogical Properties
Crystal System
hexagonal
Point Group
hexagonal-pyramidal - 6
Space Group
P6 3 (173)
Habit
blocky, coarse, prismatic, tabular
Formation
(1) Primarily magmatic, formed by crystallization from silica-undersaturated, alkali-rich melts in plutonic rocks (syenite, feldspatite) or volcanic rocks (phonolite, nephelinite).
(2) Contact metamorphic, formed by the interaction of alkaline magmas with carbonate host rocks, resulting in the formation of skarns.
Paragenesis
Twinning
Rarity
Rather rare
Synonyms
Nepheline
Composition
Elements
Mass
SiO₂
43.55
Al₂O₃
34.66
MgO
0.05
CaO
4.44
Na₂O
12.09
K₂O
4.87
H₂O
0.50
Italien (Dana 1895)
Groups and Members
Feldspathoid-Group