**6. Conclusions**

In this study, we have reported a new occurrence of spontaneous CO2 sequestration through the formation of hydromagnesite and kerolite on serpentine mine walls in the Montecastelli Cu mine located in Southern Tuscany, Italy. The formation of hydromagnesite and kerolite is triggered by the interaction between condensed water with serpentinite fines accumulated on the walls of the adits during excavation. The large surface area of the fines strongly increases the reactivity of serpentine and allows its dissolution at low temperature. This provides the required elements for the precipitation of the wall coating assemblage. The precipitation takes place due to the selective evaporation of water on upwind rock surfaces exposed to the downward circulation during summer. The peculiar features of this occurrence include the following: (i) the instauration of unusual microclimatic condition in this artificial underground system and (ii) the spontaneous carbonation of serpentine at low temperature. The latter has usually not been observed in ultramafic outcrops exposed on the Earth's surface, where, instead, hydromagnesite predominantly forms through the dissolution of the more reactive brucite.

This study shows that serpentine carbonation at low temperature is possible and it could be reproduced in ex situ applications by using fine-grained serpentine powder and a cyclical process of condensation and evaporation steps. The efficiency of this process can be tested and implemented through laboratory experiments and could have a significant impact on the applicability of CO2 mineral sequestration because it does not require pretreatment of serpentine or high temperature during the reaction, thus, potentially reducing the costs.

Further study of the peculiar conditions of underground environments hosted in Mg-rich lithologies, such as that of the Montecastelli Cu mine, can lead to a better understanding of the physical and chemical conditions necessary to enhance serpentine carbonation at low temperature, and thus implementation of new strategies for engineered CO2.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-163X/10/1/1/s1, Table S1: Microprobe analyses (wt %) of Montecastelli mine hydromagnesites, Table S2: Microprobe analyses (wt %) of "deweylite"—a variable mixture of kerolite and serpentine—identified in the wall coating, Table S3: Dissolution reactions and thermodynamic equilibrium constants for considered minerals at 25 ◦C and 1.013 bar; and Figure S1: XRD of the coating paragenesis showing the characteristic peaks of hydromagnesite (red bars), aragonite (green bars), and clay minerals (\* symbol). As it can be seen in the inserted closeup, the presence of kerolite in the specimen is demonstrated by the small angular shift and by the definitely larger FWHM of the diffraction peak at near 9.4 Å, where diffractions of kerolite and hydromagnesite overlap. In fact, the other diffraction peak at higher angle, due to the diffraction of hydromagnesite only show a definitely smaller FWHM.

**Author Contributions:** For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used "conceptualization, C.B. and A.D.; methodology, C.B., I.B., L.B., N.P., A.U. and A.D.; software, F.B., I.B., L.B. and A.U.; validation, C.B., I.B., N.P., A.U. and A.D.; formal analysis, C.B., F.B., I.B., N.P., A.U.; investigation, C.B., I.B., F.B. and A.D.; resources, C.B., L.B., N.P. and G.Z.; data curation, F.B., I.B., A.R.; writing—original draft preparation, C.B., F.B., A.R. and A.D.; writing—review and editing, C.B., I.B., A.R., N.P., A.U. and A.D.; visualization, C.B., F.B., I.B., A.R., N.P. and A.D.; supervision, C.B. and A.D.; project administration, C.B.; funding acquisition, C.B., F.B. and G.Z. All authors have read and agree to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by European Horizon 2020 GECO project (https://geco-h2020.eu), grant number 818169. The F.B. was supported by the Pegaso PhD project (Tuscan Region, Italy). C.B. was partially supported by a SNSF grant (Swiss National Science Foundation; i.e., International Short Visits, grant number IZK0Z2\_158572.) and by Fondation Herbette (Universitè de Lausanne) grant for her work at the University of Lausanne.

**Acknowledgments:** C.B. and co-authors thank Gianluca Giorgi, the owner of the Montecastelli Mine (https: //sites.google.com/site/minieradelpavone/) that recently passed away. Gianluca allowed us to study the mine geology and helped us to sample fluids, rocks, and coating. C.B. and F.B. thank Othmar Müntener for his availability and scientific and analytical support during their stay at the University of Lausanne. The authors thank two anonymous reviewers for their critical review of an earlier version of the manuscript and the Guest Editor Giovanni Ruggieri for supporting our proposal and assisting us with this special issue.

**Conflicts of Interest:** The authors declare no conflict of interest.
