Well, Southdown and its counterparts (Yardright, Old Castle, etc) are what I would call "aragonite based". There are little things like shells and small rocks and stuff in them. Not much. I would say it's about 95%-98% aragonite. I have a bag of it in my trunk if you would like to see it and you are at the next meeting. I'm donating it to the COMAS UCO Tank. It doesn't say on the bag though and is often labeled "not for aquarium use". But thousands of people, including me, use it all the time. A test for sand is to take one cup of distilled vinegar and place one teaspoon of the sand you want to test in. Over a 24 hour period, most likely less, it should be almost completely dissolved. Aragonite sand will fizz alot in vinegar because it is breaking up the Calcium Carbonate. Regular silica sands will only fizz slightly. This is usually from other things be dissolved and not the silica. You can tell silica sand automatically because of the fine pieces of clear quartz crystal (Silicon Dioxide [SiO2]) that is in them.
FYI, back when I first joined in Dec '04, someone (dustin_Combs, maybe?) had brought back a bunch of sand for COMAS members to buy. Maybe "Dustin?" can chime in with the real story, but I thought I remembered it as someone had bought a pallet full in Texas, and "Dustin?" brought enough back to sell to the COMAS members that wanted it. I was planning to do something similar.
***Disclaimer: You can use silica sand in the home aquarium, but it has a tendency to scratch your tank if caught in your scrapers and does not provide the buffering ability when your pH drops. When aragonite dissolves, it releases Calcium and Carbonate (Alkalinity) back into the system to help keep Ca/Alk elevated and maintain the pH. Of course, this isn't a fix all solution of just sticking sand in the bottom, but if an emergency was to happen, at least the sand will slow down the change of the water parameters to buy you time to fix what went wrong.
