Where is it quarried? What minerals does it contain? Can it be used to lighten garden soil/clay?

No, it can not be use to lighten garden soil/clay

Don't know about the mineral content, but as to mixing it with clay...I've done some internet research on that (including several academic papers), and it seems to be a bad idea. From what I've read, it could be done effectively only if you mix it at a rate of about 50%. Now do some quick math on the back of an envelope and you'll see that would be a lot of material, expense, and hard work. First, you'd need to remove 50% of your garden soil. 50% of a 10'x10' area 12" deep would be 50 cu ft (or almost 2 cu yards). That's 2 large truck loads. And I imagine your garden is much bigger than 10'x10'. Mine is about 50x50, so I would be looking at about 50 truck loads. That's just not feasible. And where would you put all that dirt? A better solution seems to be adding organic matter to your soil. Examples of this might be grass clippings, straw, leaves, cow manure, etc. But it needs to be decomposed pretty good. The organic material will provide many nutrients for your plants while greatly improving the soil structure. Better soil structure means that the clay is more broken up and "airy" (less compact) so plants can more easily grow a healthy root system. A good time to add your organic matter is in the fall. Just dumping it on top will help prevent weeds over the winter and it can continue decomposing. Then in the spring you can till it in.