Serpentinite

Serpentinite is peridotite that has reacted with water. The olivine and pyroxene of the mantle are unstable in the presence of water at low temperature, and they convert to serpentine minerals — a change that expands the rock, weakens it, and turns it a mottled green with a distinctly soapy feel.

Because it forms wherever mantle rock meets seawater, serpentinite is abundant along mid-ocean ridges and in the ophiolite slices where oceanic lithosphere has been thrust onto land. It is also structurally troublesome: the rock is weak, and serpentinite bodies tend to occupy fault zones because they deform in preference to anything around them.

Serpentinisation is a hydration reaction, and an energetic one. Water is taken into the mineral structure, the rock's volume increases by up to forty per cent, heat is released, and the iron in the original olivine is oxidised — which liberates hydrogen. That hydrogen supports microbial communities entirely independent of sunlight, and serpentinising systems such as the Lost City hydrothermal field are studied as models for where life might begin.

The expansion is what makes serpentinite so pervasively fractured, and the fractures are where the fibrous form of serpentine grows. Chrysotile asbestos is serpentine that crystallised across an opening crack, which is why it occurs as cross-fibre veins rather than dispersed through the rock.

Further reaction with carbon dioxide-bearing fluid converts serpentinite to talc and carbonate, and that alteration is the origin of much of the talc mined worldwide.

Serpentinite's economic history is dominated by asbestos, and by the consequences of it. The same fibrous habit that made chrysotile a superb insulator makes it a carcinogen, and the industry that mined it has been largely dismantled — though chrysotile is still produced in a small number of countries and the legacy of what was installed before the bans is a far larger problem than current production.

What remains is talc, chromite recovered from relict grains in the rock, and a decorative stone trade that has sold serpentinite as marble for centuries. There is also a live question about carbon storage: the same reactivity that turned peridotite into serpentinite can be directed at carbon dioxide instead, and serpentinite mine tailings weather fast enough to absorb meaningful quantities without any intervention.