Can I use the mono ammonium phosphate found in fire extinguishers to make crystals?
If you google for MAP, you'll find out that it is quite soluble in water, ranging from 370 to 401 g/litre of H2O at temperatures around 25 C. Thus, what you report contradicts literature data on MAP's solubility. So, maybe the powder you found is not MAP, it's something else (CaCO3 comes to my mind, because it is insoluble in water and yields CO2 when heated, but I may be wrong).