Carbonation and serpentinization of diopsidite in the Altun Mountains, NW China

Our plain-English summary of Carbonation and serpentinization of diopsidite in the Altun Mountains, NW China, by D. Zhou, S. Cao, J. Liu, et al (Scientific Reports, 2022).

The study

• Authors: D. Zhou, S. Cao, J. Liu, X. Li, Y. Dong, F. Neubauer, J. Bai, H. Li
• Journal: Scientific Reports
• Published: 2022;12(1):21361
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Mineral carbonation of mafic-ultramafic rocks has been highlighted as a promising way for permanent carbon capture and storage. Carbonatization involves the release of Ca, Mg and Fe from silicate minerals by dissolution and reaction in the aqueous phase to form stable carbonate minerals. Diopside is one of the most abundant mafic minerals in the lithosphere and contributes a portion of Mg and Ca to surface weathering. Here, we present detailed processes of the carbonation-coupled serpentinization of diopsidite from the Yushishan Nb-Ta deposit in the Altun Mountain, northwest China. Diopsidite is the prograde metamorphic product of siliceous dolomitic marble by full decarbonation process. Retrograde serpentinization and carbonation of diopsidite lead to the addition of CO2, H2O, light rare earth elements and fluid-mobile elements but the loss of SiO2. The diopsides are replaced by calcite and chrysotile by mineral alteration to form pseudomorphic textures. Dissolution-precipitation processes significantly affect diopside serpentinization and carbonation. The carbonation of diopside-rich rocks may be suitable for permanent CO2 storage.

© 2022. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.

In plain English

This was a geology study of rocks, not a medical study. The authors examined natural rock called diopsidite from the Yushishan niobium-tantalum deposit in the eastern Altun Mountains of northwest China. They compared fresh diopsidite with altered (serpentinized) diopsidite using microscopy, Raman spectroscopy, electron imaging and chemical analysis. In the altered rock, most diopside grains had been completely replaced by chrysotile and calcite, and the chrysotile in these replaced grains had a fibrous form. A later stage formed small veins of antigorite and calcite, 0.3 to 5 mm wide. The altered rock had a loss on ignition of 26.16 wt%, compared with 1.18 to 2.10 wt% in fresh diopsidite. The authors linked this to the serpentine and calcite formed. The authors concluded that dissolution and precipitation processes significantly affected the alteration, and that the carbon-storage significance of diopside-rich rocks should not be neglected. They stated that they could not determine whether lizardite was present, could not estimate the temperature and pressure of the change, and could not say where the lost silica went. The article does not address human exposure or health.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Field and laboratory study of natural rock samples (petrography, mineral chemistry and bulk-rock geochemistry)
• Who was studied: Fresh and serpentinized diopsidite rock samples from the northwestern part of the Yushishan Nb-Ta deposit, eastern Altun Mountains, northwest China (number of samples not clearly stated in the text)
• Exposure: No human exposure was studied. The study concerns natural alteration of diopside to the serpentine minerals chrysotile and antigorite, plus calcite.
• Main results:
  • In the serpentinized diopsidite, most diopside grains were completely replaced by chrysotile and calcite
  • The chrysotile in the replaced grains (pseudomorphs) had a fibrous form
  • A second alteration stage formed small veins of antigorite and calcite, 0.3 to 5 mm wide
  • The serpentinized diopsidite had a loss on ignition (LOI) of 26.16 wt%, which the authors attributed to serpentine and calcite formation
  • The fresh diopsidites had a low LOI of 1.18 to 2.10 wt%, indicating little alteration
  • The serpentinized diopsidites had higher MgO to SiO2 ratios than the fresh diopsidites
  • Many etch pits developed on the relict diopside grains, which the authors interpreted as evidence of dissolution
  • No pervasive fractures were found in the serpentinized diopsidite, although a few microfractures were seen
• The authors' conclusion: The authors concluded that dissolution-precipitation processes significantly affect diopside serpentinization and carbonation, and that the carbon capture and storage significance of exposed diopside-rich rocks should not be neglected, with further work needed
• Limitations the authors state:
  • Whether lizardite was present could not be determined
  • The temperature and pressure of the diopside-to-chrysotile-plus-calcite conversion could not be estimated accurately
  • The tectonic and metamorphic settings of the area are unclear, so the sources of the water and carbon dioxide are uncertain
  • It is unclear where the silica lost during alteration went
  • The global amount of diopsidite cannot be determined

Topics

Medical terms in this study, explained in our Lexicon: Serpentinization, Ultramafic Rocks, Silicate, Carbonate Mineral, Diopside, Decarbonation, Chrysotile.

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• The effect of pH on rates of reaction and hydrogen generation during serpentinization — T.M. McCollom, F. Klein, et al. (2020)
• Effect of secondary phase formation on the carbonation of olivine — H.E. King, O. Plümper, et al. (2010)
• Sumas Mountain chrysotile induces greater lung fibrosis in Fischer344 rats than Libby amphibole, El Dorado tremolite, and Ontario ferroactinolite — J.M. Cyphert, A. Nyska, et al. (2012)
• Mapping asbestos-cement roofing with hyperspectral remote sensing over a large mountain region of the Italian Western Alps — F. Frassy, G. Candiani, et al. (2014)

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