**Spontaneous Serpentine Carbonation Controlled by Underground Dynamic Microclimate at the Montecastelli Copper Mine, Italy**

**Chiara Boschi 1,\*, Federica Bedini 1, Ilaria Baneschi 1, Andrea Rielli 1, Lukas Baumgartner 2, Natale Perchiazzi 3, Alexey Ulyanov 2, Giovanni Zanchetta <sup>3</sup> and Andrea Dini <sup>1</sup>**


Received: 15 October 2019; Accepted: 5 December 2019; Published: 18 December 2019

**Abstract:** Understanding low temperature carbon sequestration through serpentinite–H2O–CO2 interaction is becoming increasingly important as it is considered a potential approach for carbon storage required to offset anthropogenic CO2 emissions. In this study, we present new insights into spontaneous CO2 mineral sequestration through the formation of hydromagnesite + kerolite with minor aragonite incrustations on serpentinite walls of the Montecastelli copper mine located in Southern Tuscany, Italy. On the basis of field, petrological, and geochemical observations coupled with geochemical modeling, we show that precipitation of the wall coating paragenesis is driven by a sequential evaporation and condensation process starting from meteoric waters which emerge from fractures into the mine walls and ceiling. A direct precipitation of the coating paragenesis is not compatible with the chemical composition of the mine water. Instead, geochemical modeling shows that its formation can be explained through evaporation of mine water and its progressive condensation onto the mine walls, where a layer of serpentinite powder was accumulated during the excavation of the mine adits. Condensed water produces a homogeneous film on the mine walls where it can interact with the serpentinite powder and become enriched in Mg, Si, and minor Ca, which are necessary for the precipitation of the observed coating paragenesis. The evaporation and condensation processes are driven by changes in the air flow inside the mine, which in turns are driven by seasonal changes of the outside temperature. The presence of "kerolite", a Mg-silicate, is indicative of the dissolution of Si-rich minerals, such as serpentine, through the water–powder interaction on the mine walls at low temperature (~17.0 to 18.1 ◦C). The spontaneous carbonation of serpentine at low temperature is a peculiar feature of this occurrence, which has only rarely been observed in ultramafic outcrops exposed on the Earth's surface, where instead hydromagnesite predominantly forms through the dissolution of brucite. The high reactivity of serpentine observed, in this study, is most likely due to the presence of fine-grained serpentine fines in the mine walls. Further study of the peculiar conditions of underground environments hosted in Mg-rich lithologies, such as that of the Montecastelli Copper mine, can lead to a better understanding of the physical and chemical conditions necessary to enhance serpentine carbonation at ambient temperature.

**Keywords:** CO2 mineral sequestration; hydromagnesite; kerolite; serpentinite; Cu mine; Montecastelli; underground microclimate

#### **1. Introduction**

The formation of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) is generally associated with low-temperature alteration of ultramafic rocks and has attracted an increasing number of studies because during its formation CO2 is chemically bound within its structure, representing a natural analogue of CO2 mineral sequestration [1–7]. The study of natural CO2 sequestration through the formation of carbonate from ultramafic rocks can help developing more efficient engineered CO2 storage processes involving both *in situ* and *ex situ* approaches, which are necessary to mitigate anthropogenic CO2 emissions [8]. Southern Tuscany offers several examples of spontaneous low- and high-temperature carbonation of ultramafic rocks [1,9]. Boschi et al., (2017) showed the presence of hydromagnesite and layered double hydroxides (LDH) in association with serpentinized dunite that outcrops nearby the Montecastelli Cu mine. The authors highlighted that the efficiency of carbonation was linked to the substratum lithology on which the carbonation processes took place. The most efficient reactions were found to occur on serpentinized dunites that are rich in brucite. Brucite is preferentially dissolved at surficial conditions as compared with serpentine, producing a Mg-rich fluid necessary for the precipitation of hydromagnesite [1]. Due to the lithological selectivity of the process, only brucite-rich dunite bodies showed significant carbonation with the formation of hydromagnesite and LDH at the outcrop scale. In this study, we focus on hydromagnesite formation (+ kerolite and aragonite) in adits of the Montecastelli Cu mine. We show that the formation of hydromagnesite in an underground environment is triggered by a different process as compared with the surface, which consists of a complex interaction between meteoric water percolation, evaporation and condensation, and interaction with serpentinite fines accumulated on the adits' walls.
