SODALITE & LAZURITE
Sodalite & lazurite are two of several silicate
minerals called feldspathoids. Feldspathoids are chemically
similar to the feldspars, but they have far less silica (SiO2) and
end up enriched in sodium and/or potassium (typically). These two
sodalite-group feldspathoids are famous for their brilliant blue coloration.
Sodalite
(field of view ~4.7 cm across) from a seam in Proterozoic-aged nepheline
syenite at the Princess Sodalite Quarry in Bancroft, Ontario, Canada.
Sodalite is a sodium chloro-aluminosilicate (Na4(Al3Si3)O12Cl).
It is moderately hard (H=5.5 to 6). Sodalite is uncommon, but can be found
in some intrusive igneous rocks and some extrusive igneous rocks (lava
flows).
Sodalite
(7.5 cm across at its widest) from a pegmatitic nepheline syenite in the Poços
de Caldas Alkaline Complex (mid-Campanian Stage, late Late Cretaceous, 76 to 78
million years); Poços de Caldas Plateau, Minas Gerais State, southeastern
Brazil.
Lazurite
(blue - the actual has a hint of purplish coloration not apparent in the photo)
in pyritic marble (field of view ~2.5 cm across) from contact metamorphic
marbles of the Sakhi Formation (Neoarchean) in the Hindu Kush Mountains of
northeastern Afghanistan. In this area, lazurite can be a rock-forming
mineral. Rocks dominated by lazurite are called lapis lazuli.
Lazurite
is similar to sodalite, but is much rarer.
It has the moderately complex chemical formula ((Na,Ca)8(AlSiO4)6(SO4,S,Cl)2). Lazurite has a nonmetallic luster, is
intensely blue in color, and has a hardness of about 5 to 5.5. Lazurite
tends to occur in contact metamorphic rocks.