I just tried finding out what it would take to get water from the OH groups in ringwoodite and had the frustrating experience of reading through a short paper in which I recognized everything and back in my university days would have followed it but I’ve forgotten too much to make sense of more than the most basic chemistry any longer. Mere heat, in a test tube, might separate an OH from a mineral, but the situation in the mantle isn’t much like that in a test tube.
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Water of crystallization (like in Epsom salts) is actual water molecules within the crystal structure, and can be released simply by heating the crystal. Adding water to the dessicated crystals will reconstitute the original crystal form. It’s a bit like the way silica gel can absorb or release water under rather ordinary conditions.
That is quite different from having some -OH groups in ringwoodite. Heat that up a lot, and it eventually melts, but getting water from it requires chemical reactions. Stick a lump of ringwoodite in water, and it only gets wet; it does not absorb the water the way anhydrite (aka plaster of Paris) would.
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