Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Na8(Al6Si6O24)SO4
Mineral Group Feldspathoid: Sodalite Group
Crystal System Isometric (Cubic) - Hextetrahedral
Crystal Habit Dodecahedral, or elongate pseudohexagonal prisms
Cleavage (110) Poor
Color/Pleochroism Colorless, gray, or very pale blue in thin section or grain mount
Optic Sign Isotropic
2V N/A
Optic Orientation N/A
Refractive Indices
n = 
Max Birefringence N/A
Elongation None
Extinction None
Dispersion None
Hardness 5.5 - 6.0
Specific Gravity 2.3 - 2.4
Streak White to bluish-white
Luster Vitreous
Distinguishing Features Has moderate negative relief. It may not be possible to distinguish between the members of the Sodalite mineral group based on their optics. However, Sodalite is usually colorless, lacks abundant inclusions, and is the only one found in plutonic rocks; nosean and hauyne commonly contain dark inclusions that may be concentrated into sectors, planes, cores, or rims.  Both nosean and hauyne are commonly blue in thin section. 
Occurrence The Sodalite group is found in silica-deficient alkalic igneous rocks. Nosean is generally restricted to volcanic rocks. The volcanic rocks that the sodalite group is found in include phonolites, alkali basalts, trachyte, and feldspathoidal basalt. The commonly associated minerals are nepheline, leucite, and sanidine.
Editors Julie Herrick ('02), Meaghan Moy ('03), and Vanessa O'Connor ('05), Billie Cullison ('19)

A crystal of nosean is centered in this photo under cross-polarized light.  By estimating the tick marks on the left margin (each tick mark is 1 mm.) the centered crystal is about 1.2 mm wide. The materials seen around the nosean crystals are leucite (smaller sized than the nosean and showing twinning), augite, and nepheline. This sample is from Schloss Olburck, Brohlthel, Germany.
Click on the image to see the crystal in both plain-polarized and cross-polarized light. This hexagonal-shaped nosean crystal is approximately 4 mm wide. The smaller twinned minerals around the centered nosean are crystals of leucite. This sample is from a fine grained, light brown volcanic rock (sample K-77), but the origin is unknown.
This nosean can be distinguished from the leucite around it because despite the similar color. Leucite has twinning and nosean is much larger.
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