Sodalite is a mineral and also a mineral group. The latter contains minerals sodalite, nosean, and haüyne. Lazurite also belongs to this group, but it can be treated as a variety of haüyne. Sometimes lapis lazuli is erroneously also included. It is a beautiful blue gem but it is not a mineral. It is a rock type that contains lots of lazurite.

Sodalite grains from Namibia. Width of view 15 mm.

All sodalite group minerals are feldspathoids. What does it mean? These are minerals that somewhat resemble feldspars — both structurally and chemically and they have a similar role in igneous rocks. But they contain less silicon in relation to other ions in the crystal structure. Feldspathoids form in silicon deficient igneous rocks. Silicon deficiency means that there is not enough silicon for feldspars, let alone for quartz. Therefore, you will not find quartz together with feldspathoids in igneous rocks. Hence, no hope to find sodalite in granite for example. However, there is no restriction for feldspars to occur in the same rocks with feldspathoids. If there is silicon deficiency, some of the magma will crystallize as feldspathoids instead. Hence, they take the place which normally is reserved for feldspars.

Silicon deficient magma is not rare but then it usually contains lots of iron and magnesium. Such magma is called mafic or ultramafic. The latter also contains no quartz and the former may contain only very small amount of it. However, that is not enough for the feldspathoid minerals. Let’s take a look at the chemical composition of sodalite: Na₈Al₆Si₆O₂₄Cl₂. It is clear that in addition to silicon deficiency one more important condition must be fulfilled — sodalite needs lots of sodium (Na). The combination of low silicon and high sodium is pretty rare and so are rocks that contain sodalite or other feldspathoid minerals.

Now it should become clear that we are dealing with minerals that are not easy to find. However, if the right conditions exist for the minerals to form, then they may form a significant portion of the rocks composition. They often are the most important minerals in the rocks that contain them, just like feldspar is usually the most important mineral in granite. Feldspathoid-bearing igneous rocks are usually called foid-bearing. This is officially accepted and agreed upon simplification.

Sodalite is the most common mineral of the sodalite group but not among feldspathoids. That honour goes to nepheline which often occurs together with it. Sodalite is usually blue but other colors (green, yellow, pink, gray, colorless) are possible also. This mineral group is usually identified by its color. They are also optically isotropic. Hence, polarizing microscope is useful to confirm the identification. Sodalite, nosean, and haüyne are not easily distinguished from each other. More advanced methods like X-ray diffraction or chemical analysis is needed for that but it may be helpful to know that plutonic rocks (foid-bearing syenite for example) usually contain sodalite. Nosean and haüyne are normally restricted to volcanic rocks like phonolite and alkali basalt¹.

In addition to igneous rocks, sodalite group minerals also occur in contact metamorphosed carbonates. Lapis lazuli is such a rock. It usually contains calcite and pyroxene in addition to feldspathoids.

This mineral is not a common sand constituent but may locally comprise a significant amount of some sand samples which are the disintegration products of foid-bearing rocks exposed nearby. Blue sand is highly sought after among sand collectors. It is usually sodalite that gives blue color to such sand.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.