NEPHELINE
Nepheline is the most common member of a silicate
mineral group called the feldspathoids. Feldspathoids are
chemically similar to the feldspars, but they are depleted in silica (SiO2)
and end up enriched in sodium and/or potassium (typically).
Nepheline is a sodium potassium aluminosilicate
((Na,K)AlSiO4). It's an igneous mineral, and occurs in
alkaline intrusive & extrusive rocks. It generally has the appearance
of plagioclase feldspar. Nepheline has a nonmetallic, vitreous to greasy
luster, typically has a mottled grayish-white color, a white streak, and a
hardness of 5.5 to 6. Nepheline lacks the two well-developed planes of
cleavage at ~90° that plagioclase feldspar has. Instead, nepheline has
poor cleavage.
Nepheline (5.6 cm across) from Dungannon Township, Ontario, Canada.