This post is a follow-up to the article about sodalite. This time I am writing about the sand sample from which I picked these beautiful blue sodalite grains. It was an easy task to pick them because the grains are large and there are lots of them. Most of the sand is made of sodalite. Other major component is dolomite (gray crystals).

Sand sample from Namibia containing blue sodalite and gray dolomite. The width of the view is 14 mm.

As much as I know this sand sample is one of the most desirable gems sand collectors wish to have in their collections. The other highly sought after sample is probably the one from Japan that contains star-like tests of forams. I wrote a post titled star sand and sun sand where you can see how these forams look like.

The sodalite sand from Namibia is probably not a natural sand. I can’t say it for sure because I have never visited the collecting place but the composition (only dolomite and sodalite which are both rare in sand) and angularity of the grains do not leave much room for alternative explanations. The sample is from a sodalite mine in NW part of Namibia.

The sand itself may not be natural. It is probably what is left of crushed stones but for me this is no problem. I like them all, no matter whether they come from crushed rocks or natural sand.

It is of course interesting to know what type of rocks were the source material of the sand. Sodalite is usually magmatic mineral¹. Sometimes sodalite forms in contact metamorphosed carbonate rocks but magma is involved there as well. The most likely interpretation is that silicon-deficient and sodium-rich magma intruded into the dolomite formation and solidified there as an extremely feldspathoid-rich plutonic rock known as foidolite. This rock type contains more than 60 percent feldspathoids in a ternary diagram of alkali feldspar, plagioclase, and feldspathoids. Since I can’t find feldspars, I assume that it is foidolite. I did some googling as well and one of the sources confirmed that there is indeed a mine in Namibia where sodalite-bearing foidolite is mined.

It may come as a surprise but it seems to me that we have to thank the European Union and its regulations for that. In EU it is mandatory for dimension stone dealers to use scientific terminology. No more can they classify all of their rocks into marble and granite. It seems unbelievable but sometimes EU and its huge bureaucracy machine does something that seems to be genuinely useful, at least for me and other geologists. Rock dealers, I am afraid, would probably not agree with me.

References

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

You may also like the gallery of colors in sand.