#igneoous

Is that really sand?

Yes, these are actually sand grains, but not the kind of sand you’re used to seeing. The large majority of sand grains on the planet Earth are the mineral quartz, but these aren’t.

Quartz has properties that make it very common in sand. Quartz is very stable against chemical weathering, so when quartz in in a rock, other minerals tend to weather out first, leaving nothing but quartz behind. Quartz is also very strong against physical weathering, so when sediments are transported downstream, angular quartz grains tend to survive while other minerals are reduced to fine powder and dust.

In order to make a pile of sand composed of something other than quartz, the sediment must be dominated by something else. Sometimes sand can be composed of a mixture of feldspar and quartz since feldspar is “almost as resistant as quartz but not quite” – that creates a rock known as an arkose.

At the far end though, in areas where there is no quartz and no feldspar present, sand grains can be made of whatever rocks there are in the area. Those sand grains show up because erosion is really good at reducing rocks to sand grains, we just don’t see rocks dominated by sands of anything other than quartz or feldspar if those minerals are present.

The most famous examples of sands that aren’t quartz are black sand beaches like those in Hawaii. Those sands made of basalt, igneous rock broken apart into sand by the action of water. These grains come from a black sand beach in New Zealand, produced by the same processes – erosion of basaltic igneous rocks and no supply of quartz or feldspar.

The most unusual thing about this view to me is that the sand grains look nothing like quartz. Quartz sand grains are so often angular - quartz does not have any strong cleavage directions, so when it does break it tends to break in extremely sharp, angular fragments. Instead, these sand grains are not only rounded, they’re pitted. The basaltic rock that made these has interacted with water on the surface, leaving this rounded texture with no sharp points and lots of dissolution pits along it. The different colors in this image could either be caused by slightly different colors/mineral assemblages or just by how the light from the camera is caught.

Really amazing to see how different these sand grains are from quartz sand grains.

-JBB

Image credit: Microscopic Solutions