#conchoidal

Stephanie Holt, Conchoidal Ring,

From the ‘U.F.O.’ (Unique Finger Ornaments) Collection, 

Close-up of sand

Ok, I’ll confess. I have a review of a paper I should be working on, but instead I’m sitting here watching baseball. Go ahead, yell at me, it’s a Sunday. At least I’m still thinking about Earth Science.

Couldn’t help but stop and think about the dirt this game is being played on. Yes, you’ve got a writer appearing on your wall that watches a baseball game and decides to write several hundred words about the dirt. It happens.

The most common constituent of most sand on Earth is the mineral quartz. Quartz isn’t the most abundant mineral on Earth, but it’s still very common and it is very resistant to erosion. It doesn’t dissolve in water easily, and it doesn’t fracture very easily, so by the time things become sand, it’s often all that is left. It’s very hard, it scratches most things; in fact, when I’m trying to figure out if a rock is made of quartz out in the field, I try to scratch either my hammer or the band on my watch. If the rock scratches the hammer, then it’s made of quartz; the mineral actually has a greater hardness than most metals.

In this image you see a number of properties of quartz. The numbers are from the paper it was published in, if you’d like to see the exact details they’re listed below. These images are of individual quartz sand grains taken using a scanning electron microscope. The scale bars in (a) and (b) are 100 μm, (c) has zoomed in so that the scale bar is only 10 μm.

Ever wind up sliding on a sandy surface and having your skin scratched up pretty badly? Take a look at these jagged grains, I think it’s pretty self explanatory how that happened. Quartz doesn’t like to fracture, and when it does, it fractures into angular, jagged fragments. It’s called a “conchoidal fracture”, a fracture that has no natural planes of weakness on which it can expand, so it just fractures however it can, with the angle of the fracture changing all the time.

When quartz fractures into sand grains, it just rips apart. All sorts of sharp points are formed, all sorts of harsh looking shapes, not exactly what you’d like rubbing against your skin.

You can probably look at the shapes in this image and make a good guess as to why sandpaper works. Its abrasive, grains that don’t want to abrade, covered with sharp edges. Great way to wear down anything rubbing against it.

When quartz sand grains pile up and turn into a rock, they make sandstone. Sandstone is a rock with high porosity, meaning there is a lot of space between the grains. Sandstones make great reservoirs for oil or groundwater because liquids can get in and fill the gaps. In a simple SEM image like this, you can see quite readily why sandstones are so porous; how tightly could you pack these grains together? The odd shapes and the fact that they’re not easy to break make it so that sandstones have all this extra useful space. In many major cities around the globe, when you walk to the sink and take a drink, the water you’re drinking has been pumped out of a sandstone.

Anyway, that’s a few minutes talking about sand. It’s everywhere, it often gets into your clothes and annoys you, but it’s a very important substance, and occasionally you can have some fun playing in it.

-JBB

Image credit and paper: http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1116&context=natrespapers