Serpentinite - Formation and Petrology - Serpentinite Reactions

Serpentinite Reactions

Serpentinite is formed from olivine via several reactions, some of which are complementary. Olivine is a solid solution between the magnesium-endmember forsterite and the iron-endmember fayalite. Serpentinite reactions 1a and 1b, below, exchange silica between forsterite and fayalite to form serpentine group minerals and magnetite. These are highly exothermic reactions.

Reaction 1a:
Fayalite + water → magnetite + aqueous silica + hydrogen

3Fe₂SiO₄ + 2H₂O → 2Fe₃O₄ + 3SiO₂ + 2H₂

Reaction 1b:
Forsterite + aqueous silica → serpentine

3Mg₂SiO₄ + SiO₂ + 4H₂O → 2Mg₃Si₂O₅(OH)₄

Reaction 1c:
Forsterite + water → serpentine + brucite

2Mg₂SiO₄ + 3H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂

Reaction 1c describes the hydration of olivine with water only to yield serpentine and Mg(OH)₂ (brucite). Serpentine is stable at high pH in the presence of brucite like calcium silicate hydrate, (C-S-H) phases formed along with portlandite (Ca(OH)₂) in hardened Portland cement paste after the hydration of belite (Ca₂SiO₄), the artificial calcium equivalent of forsterite.

Analogy of reaction 1c with belite hydration in ordinary Portland cement:
Belite + water → C-S-H phase + portlandite

2 Ca₂SiO₄ + 4 H₂O → 3 CaO · 2 SiO₂ · 3 H₂O + Ca(OH)₂

After reaction, the poorly soluble reaction products (aqueous silica or dissolved magnesium ions) can be transported in solution out of the serpentinized zone by diffusion or advection.

A similar suite of reactions involves pyroxene-group minerals, though less readily and with complication of the additional end-products due to the wider compositions of pyroxene and pyroxene-olivine mixes. Talc and magnesian chlorite are possible products, together with the serpentine minerals antigorite, lizardite, and chrysotile. The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed 600°C 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. Fluids involved in serpentinite formation commonly are highly reactive and may transport calcium and other elements into surrounding rocks; fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and called rodingites.

In the presence of carbon dioxide, however, serpentinitization may form either magnesite (MgCO₃) or generate methane (CH₄). It is thought that some hydrocarbon gases may be produced by serpentinite reactions within the oceanic crust.

Reaction 2a:

Olivine + water + carbonic acid → serpentine + magnetite + methane

→

or, in balanced form:

→

Reaction 2b:

Olivine + water + carbonic acid → serpentine + magnetite + magnesite + silica

→

Reaction 2a is favored if the serpentinite is Mg-poor or if there isn't enough carbon dioxide to promote talc formation. Reaction 2b is favored in highly magnesian compositions and low partial pressure of carbon dioxide.

The degree to which a mass of ultramafic rock undergoes serpentinisation depends on the starting rock composition and on whether or not fluids transport calcium, magnesium and other elements away during the process. If an olivine composition contains sufficient fayalite, then olivine plus water can completely metamorphose to serpentine and magnetite in a closed system. In most ultramafic rocks formed in the Earth's mantle, however, the olivine is about 90% forsterite endmember, and for that olivine to react completely to serpentine, magnesium must be transported out of the reacting volume.

Serpentinitization of a mass of peridotite usually destroys all previous textural evidence because the serpentine minerals are weak and behave in a very ductile fashion. However, some masses of serpentinite are less severely deformed, as evidenced by the apparent preservation of textures inherited from the peridotite, and the serpentinites may have behaved in a rigid fashion.