easily altered under surface weathering conditions. A similar suite of reactions involves pyroxene-group minerals: Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over. When olivine is abundant, silica activity drops low enough that talc begins to react with olivine: This reaction requires higher temperatures than those at which brucite forms. The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface. Breakdown of diopside and formation of rodingites Ultramafic rocks often contain calcium-rich pyroxene (diopside), which breaks down according to the reaction: This raises both the pH, often to very high values, and the calcium content of the fluids involved in serpentinization. These fluids are highly reactive and may transport calcium and other elements into surrounding mafic rocks. Fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and depleted in silica, called rodingites. Formation of magnetite and hydrogen In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the fayalite-magnetite-quartz (FMQ) buffer. The very low chemical activity of silica during serpentinization eliminates this buffer, allowing serpentinization to produce highly reducing conditions. Under these conditions, water is capable of oxidizing ferrous () ions in fayalite. The process is of interest because it generates hydrogen gas: However, studies of serpentinites suggest that iron minerals are first converted to ferroan brucite, that is, brucite containing , which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization: Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between and when fluids are carbonate undersaturated. If the original ultramafic rock (the protolith) is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen. Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite). During serpentinization, the rock is