Some rocks with appreciable amount of felsic and mafic minerals have been crushed. In order to melt the rock at low temperature and/or to leach water-soluble cations, either boric acid (H3BO3) or mono-ammonium phosphate (NH6PO4) is mixed with the crushed rock and heated.

boric acid is commercial grade and mono-ammonium phosphate is fertiliser grade. Enough provision is made to vent out ammonia. Heating source is household gas cooking oven. Container is made of cast iron. The rocks are mixed, chiefly Granite and Gabbro. i.e. holocrystalline (pegmatite?) rocks with physically discernible grains. Rocks are crushed to about 2-5 mm size, heating period is below 3 hours on open deep bowl. The heated mixture is leached with rainwater to extract the soluble minerals.

My question is, which of these two chemicals would be able to form more water-soluble cation? Or which one would cause more melting temperature drop of the flux+ crushed rock mixture? Extraction of Na, and K cations are of first priority. Please also mention the ratio of rock vs flux as well.