It's either mica or an aluminosilicate. None of which will be affected by acids or heat. It looks like asbestos to me. You should probably figure that out before you grind it up or hit it with a torch. Those fibers are mighty hazardous.

Above we were talking about Seneca, WV.  A friend and I drove up through there Sunday, and on up to Spruce Knob, just looking at leaves.  Circled back around, I guess about 175 miles round trip.  Just a pretty Sunday drive.  Was 44* and blowing snow on top, and windy as all get out.   

I agree with Clay on this one. Mica schist. 

One of its commercial uses was as windows in furnaces.

Mica is a silicate and can easily sustain high temperatures and exposure to acids. Mica has been used commercially for thousands of years, often for just those qualities. Your torches and nitric acid probably had little to no effect.