Abstract: Ultramafic rock material containing, or which is serpentinized so as to contain, substantially or completely serpentinized material, or alternatively, clay material containing structural water, is subjected to a temperature and pressure for a period of time and optionally in the presence of added elemental or ferrous iron to cause deserpentinization or dehydroxylation and consequent formation of hydrogen and hydrocarbons and formation of a solid residue having a mineral content containing one or more of magnesium, nickel, iron, magnetite, calcium or other metal values. When the material is deserpentinized or dehydroxylated in the vicinity of 700.degree. C. or subsequently heated at between about 700.degree. and 815.degree. C., the mineral content of the residue has a transition lattice structure whereby the magnesium, nickel, calcium and other mineral values can be extracted by trituration with mineral acid, e.g., hydrochloric acid, to form an aqueous solution of the mineral acid salts of such metal values.