Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Carbonated peridotite

Near-surface reaction of CO₂-bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO₂ storage, and perhaps for direct capture of CO₂ from near-surface fuids.

Peridotite-hosted carbonates come in essentially three types:

(a) partially hydrated peridotite with carbonate veins, sometimes termed ophicalcite, especially in Alpine localities,
(b) fully carbonated peridotite (magnesite or dolomite + quartz) known as listwanite (also known as listvenite, listvanite, or listwaenite),
(c) travertine deposited from springs in peridotite catchments.

The fastest mineral carbonation rates known are for the mineral olivine and among abundant rock-forming minerals, olivine, has the highest molar proportion of divalent cations needed to form natural carbonate minerals such as magnesite and dolomite.

Carbonation of the mineral olivine occurs together with hydration via the simplified reactions:

1) 4Mg₂SiO₄ (Mg-olivine) + 4H₂O + 2CO₂ = 2Mg₃Si₂O₅(OH)₄ (serpentine) + 2MgCO₃ (magnesite)
2) 4Mg₂SiO₄ (Mg-olivine) + H₂O + 5CO₂ = Mg₃Si₄O₁₀(OH)₂ (talc) + 5MgCO₃
3) 4Mg₂SiO₄ (Mg-olivine) + 8CO₂ = 4SiO₂ (quartz) + 8MgCO₃ (magnesite)

After hydration, carbonation occurs via the simplified reactions:

4) 4Mg₂SiO₄ (Mg-olivine) + 6H₂O = 2Mg₃Si₂O₅(OH)₄ (serpentine) + 2Mg(OH)₂ (brucite)
5) Mg₃Si₂O₅(OH)₄ (serpentine) + 2Mg(OH)₂ (brucite) + 2CO₂ = 2Mg₃Si₂O₅(OH)₄ (serpentine) + 2MgCO₃ (magnesite) + 2H₂O
6) 2Mg₃Si₂O₅(OH)₄ (serpentine) + 2MgCO₃ (magnesite) + 2H₂O + 3CO₂ = Mg₃Si₄O₁₀(OH)₂ (talc) + 5MgCO₃ (magnesite) + 5H2O
7) Mg₃Si₄O₁₀(OH)₂ (talc) + 5MgCO₃ (magnesite) + 5H₂O + 3CO₂ = 4SiO₂ (quarzo)+8MgCO₃ (magnesite) + 6H₂O

These reactions are written with no components other than H₂O and CO₂ in fluids, but of course they can take on infinite variety when other key components such as Ca²⁺, aqueous SiO₂, H₂, sulfur species, and O₂ are considered to be transported in fuids.

The second most abundant minerals in peridotite are Ca-poor pyroxene and Ca-rich pyroxene. To understand natural peridotite carbonation, it is important to add the simplified reaction:

4Mg₂SiO₄ (Mg-olivine) + CaMgSi₂O₆ (pyroxene) + 6H₂O + CO₂ = 3Mg₃Si₂O₅(OH)₄ (serpentine) + CaCO₃ (calcite).
This reaction often takes place in stages; in the subsurface we have:

4Mg₂SiO₄ (Mg-olivine) + CaMgSi₂O₆ (pyroxene) + 7H₂O = 3Mg₃Si₂O₅(OH)₄ (serpentine) + Ca²⁺(aq) + 2OH^-(aq)

And then, near the surface we have:

Ca²⁺(aq) + 2OH^-(aq) + CO₂ (aq or gas) = CaCO₃ (calcite) + H₂O

ophicalcite2020(1).jpg

White carbonate veins in partially serpentinized peridotites. Semail Ophiolite of the Hajar Mountains of Oman. From Lynn Recker.

White carbonate veins in partially serpentinized peridotites. Ophiolite from Oman. From Sarah Wild.

White carbonate veins in partially serpentinized peridotites. Ophiolite from Liguria, Italy. From Deep carbon.

Carbonated peridotite. Dark peridotite (to the right) and fully carbonated area (to the left). Botro Sanguigna (Livorno province), Italy. Field of view = 1cm.

Bibliography

• Kelemen, P. B., Matter, J., Streit, E. E., Rudge, J. F., Curry, W. B., & Blusztajn, J. (2011). Rates and mechanisms of mineral carbonation in peridotite: natural processes and recipes for enhanced, in situ CO2 capture and storage. Annual Review of Earth and Planetary Sciences, 39, 545-576.

Dark peridotite (to the left) and fully carbonated (dolomite) area (to the right). Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dark peridotite (to the left) and fully carbonated (dolomite) area (to the right). Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dark peridotite (to the left) and fully carbonated (dolomite) area (to the right). Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dolomite veins in a carbonated peridotite. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)
Dolomite veins in a carbonated peridotite. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dark peridotite (to the right) and fully carbonated (dolomite) area (to the left). Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dark peridotite (to the right) and fully carbonated (dolomite) area (to the left). Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)
Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dark peridotite (to the right) and fully carbonated (dolomite) area (to the left). Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dark peridotite (to the right) and fully carbonated (dolomite) area (to the left). Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)
Dolomite veins in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)
Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. PPL image, 1x (Field of view = 9mm)	Dolomite vein in a carbonated peridotite. Note the alternating arrangement of dolomite crystals. Botro Sanguigna (Livorno province), Italy. XPL image, 1x (Field of view = 9mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)	Dolomite crystals in a vein. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 2x (Field of view = 7mm)
Dolomite crystals with regular inclusions. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. PPL image, 10x (Field of view = 2mm)	Dolomite crystals with regular inclusions. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 10x (Field of view = 2mm)	Dolomite crystals with regular inclusions. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 10x (Field of view = 2mm)
Dolomite crystals with regular inclusions. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. PPL image, 10x (Field of view = 2mm)	Dolomite crystals with regular inclusions. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 10x (Field of view = 2mm)	Dolomite crystals with regular inclusions. Carbonated peridotite from Botro Sanguigna (Livorno province), Italy. XPL image, 10x (Field of view = 2mm)