I recently opened up a old decompressed fire extinguisher out of curiosity. After some research I discovered that the dry powder inside, mono ammonium phosphate, can be used to make beautiful crystals. However, I found this powder to be extremely hydrophobic and could never get its dissolve in water at all. I mean this stuff is amazingly hydrophobic. Its fun to see how it reacts in water. But why cant it dissolve and what kind of preparation of mono ammonium phosphate will dissolve and become 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).