H₂O^-: water lost at low temperature. This water either (1) has not been incorporated into the structure, but was only absorbed, or (2) was actually present as a loosely bound hydrate (e.g., gypsum). Water of hydration will usually be evolved in large amounts, whereas absorbed water will only be present in trace amounts. H₂O⁺: water which is evolved at high temperature. It must therefore have been part of the chemical structure of the mineral.