The image posted is probably of particles way too big to help us that much, but it's interesting to see the color difference and also the note about how acids will dissolve non-silica particles. Maybe there's a way to use their knowledge against them...
I’m talking about impurities in the sand, like iron oxides, clays, feldspars, and carbonates.
These are particles other than the silica sand grains.
These can be seen under the scope, and they tend to be much darker than the pure silica sand. Some are round, which help the sphericity and roundness test.
Here is a microscopic picture of nice silica sand particles next to some impurities and non-silica particles.
Others are nasty jagged creeps.
An additional way to determine how many of these impurities (“creepy clowns”) exist is to perform an acid solubility test.
It’s a very exacting process, using a tight procedure and nasty acid to dissolve the impurities.
So, we are dissolving some of the “creepy clowns” in acid.
It’s kind of satisfying.
As long as the weight dissolved is less than 2% or 3% of the total sample weight (depends on the size of the sand being tested), the sand will meet the American Petroleum Institute standards for frac sand.