The blurb on the Nature's Guide Texas Greensand bag says
Texas Green Sand is glauconite - a naturally occuring mineral rich in iron. Formed from ancient seabed deposits, Texas Green Sand helps hold moisture in the ground so that it is released more slowly. When incorporated into clay soils its friable nature works to decrease compaction. Its texture and chemical composition also make it an ideal amendment for sandy soils where it adds iron and potassium in a slow release form.
I've used some of the Espoma organic fertilizer types, but I haven't seen their greensand. I find it online, though, and they say it is glauconite. http://www.espoma.com/p_consumer/org_trad_overview.html
It looks like you'll be fine with the product you found.