Nepheline
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Nepheline, also called nephelite, is a rock-forming mineral in the feldspathoid group – a silica-undersaturated aluminosilicate, Na₃KAl₄Si₄O₁₆, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites. Wikipedia
Birefringence: δ = 0.003–0.004
Cleavage: Poor
Color: White, gray, brown, brownish gray, or reddish white
Crystal class: Pyramidal (6); H–M symbol: (6)
Crystal habit: Massive granular to prismatic
Crystal system: Hexagonal
Formula mass: 146.08 g/mol