Microtextural Characteristics of Ultramafic Rock-Forming Minerals and Their Effects on Carbon Sequestration

Abstract

Ultramafic rocks are promising candidates for carbon sequestration by enhanced carbon dioxide (CO₂) mineralization strategies due to their highly CO₂-reactive mineral composition and their abundant availability. This study reports the mineralogy and microtextures of a representative ultramafic rock from the Ma-Hin Creek in northern Thailand and provides evidence of CO₂ mineralization occurring through the interaction between CO₂ and the rock in the presence of water under ambient conditions. After sample collection, rock description was determined by optical petrographic analysis. The rock petrography revealed a cumulated wehrlite comprising over 50% olivine and minor amounts of clinopyroxene, plagioclase, and chromian spinel. Approximately 25% of the wehrlite had altered to serpentine and chlorite. A series of CO₂ batch experiments were conducted on six different rock sizes at a temperature of 40 °C and pressure of 1 atm over five consecutive days. The post-experimental products were dried, weighed, and geochemically analyzed to detect changes in mineral species. Experimental results showed that product weight and the presence of calcite increased with reducing grain size. Additionally, the modal mineralogy of the wehrlite theoretically suggests potential CO₂ uptake of up to 53%, which is higher than the average uptake values of mafic rocks. These findings support the rock investigation approach used and the preliminary assessment of carbon mineralization potential, contributing to enhanced rock weathering techniques for CO₂ removal that could be adopted by mining and rock supplier industries.

1. Introduction

The long-term cumulative emission of carbon dioxide (CO₂) into the atmosphere has increased the global atmospheric CO₂ concentration, resulting in a drastic rise in the global surface temperature and consequent climate change. In 2018, the Intergovernmental Panel on Climate Change (IPCC) suggested several climate actions that aim to limit the global temperature increase in response to the Paris Agreement [1,2]. One of the promising strategies for contributions to CO₂ emission reduction and atmospheric CO₂ removal is that of carbon capture and storage (CCS) technology [3,4,5]. CCS is an integrated technology comprising (ⅰ) CO₂ capture, whether from an emitting source or the atmosphere, combined with a CO₂ separating procedure, (ⅱ) source-to-sink transportation, and (ⅲ) CO₂ storage, permanently and effectively. The most suitable options for CO₂ storage include sequestration in subsurface geological formations and carbon mineralization [4,6,7,8].

Carbon mineralization, herein called CO₂ mineralization, requires the occurrence of mineral formations rich in divalent cations, including Mg²⁺, Ca²⁺, and Fe²⁺, to combine with molecules of CO₂, resulting in the permanent locking away of CO₂ in the form of stable solid carbonate minerals. CO₂ mineralization is a safe CCS technique increasingly used in many countries, especially where underground CO₂ injecting and storing activities are not applicable [9,10]. Mineralized carbonate products and non-crystalline silica byproducts are used in many industries [11,12,13]. The most common CO₂-reactive minerals are formed in magnesium-, calcium-, or iron-bearing silicate groups, such as olivine, serpentine, pyroxene, amphibole, and plagioclase. Each mineral has a different capacity for CO₂ uptake potential, ranging from 63 to 16%, depending on the thermodynamic properties and elemental availability [11,14,15,16,17,18,19]. Uncertainty in the mineral assemblage of real-world rock formations results in diverse values of CO₂ uptake estimation.

CO₂ mineralization of relevant minerals is spontaneous and exothermal and occurs naturally as rock chemical weathering and subsequent atmospheric CO₂ removal [20,21,22,23], but involves slow reaction rates. Over recent decades, numerous experimental methods and conditions for accelerating the CO₂ mineralization of mineral or rock targets have been proposed (e.g., [24,25,26,27,28,29,30,31,32]). The methods include element extraction, mineral processing, and specific treatments using heat, pressure, and chemicals, which, unsurprisingly, require considerable energy consumption, infrastructure, and costs. An alternative option for carbon removal is also worth exploring. Enhanced rock weathering (ERW) is an attractive engineering practice that can speed up natural CO₂ removal from the air [2,7,19,33,34,35,36]. The technique involves increasing the surface area of CO₂-reactive rocks by finely grinding them and then applying the rock powder over the land, coasts, or in the oceans [8,35,37,38,39]. Selecting suitable rock types for ERW is very important in maximizing the reaction rates and ultimately offering the greatest CO₂ uptake capacity [10,36,40,41]. In addition to the rock types available, the abundance of rock resources essentially offers an unlimited capacity for CO₂ sequestration.

Ultramafic igneous rocks and their metamorphosed versions play an important role in the global supply of favorable CO₂-reactive minerals. They consist of over 90% olivine, pyroxene, and amphibole, with minor amounts of plagioclase and trace quantities of other minerals. Ultramafic rocks form through the crystallization of deep magmas rich in Mg, Ca, and Fe silicates, occurring from the middle-to-lower crust to the upper mantle of the Earth. The rock formations are normally uplifted to near-surface conditions or are exposed to the surface driven by tectonic events, resulting in gigantic massifs of ophiolites or various scaled layered intrusions, which later undergo natural hydration of the anhydrous silicate minerals and subsequent carbon mineralization at varied rates [42,43,44,45,46]. Mineralization of CO₂ in ultramafic rocks occurs when the formations interface with CO₂ in the air or CO₂-dissolved groundwaters flowing through rock fractures. These result in the formation of carbonate minerals, especially calcite or magnesite, coexisting with quartz or non-crystalline silica formed as open-space filling veins. Based on the relatively high CO₂ reactivity and the global availability of ultramafic rock resources, these rocks are potentially considered the most promising materials for the development of ERW methods supporting atmospheric carbon dioxide removal technology.

Based on mineralogical variations, ultramafic rocks are divided into several groups, such as dunites, peridotites, pyroxenites, amphibolites, and serpentinites, which can also be subdivided into numerous types. Each type provides a specific amount of promising minerals that can affect the CO₂ uptake potential [31,40,47,48,49]. Besides the mineralogy, their microtextural characteristics, including the mineral grain size and the alteration level, additionally influence the CO₂ mineralization potential when pursuing ERW strategies. To closely assess the CO₂ mineralization potential of any rock, it is necessary to understand in-depth the rock mineralogy in terms of mineral types, quantities, textures, and alteration degrees.

This study focuses on a primary mineralogical assessment of real-world ultramafic rock exposure in Thailand, SE Asia, for CO₂ uptake potential based on modal composition and batch experiments. In this research, the potential occurrence of CO₂ mineralization was assessed through experimental study of the interaction between selected ultramafic samples and CO₂-bearing water. The experiments were undertaken using different particle sizes, and all were conducted under ambient conditions resembling the natural rock weathering process. The primary evidence for CO₂ mineralization was observed and described in terms of changes in the mineralogical composition and weight of the obtained products compared to the initial rock. This study additionally sought to provide a fundamental strategy for the determination and evaluation of suitable rocks to support ERW techniques that could be applied in mining and rock supplier industries elsewhere.

2. Geological Background

The study area is located in the Phrao District of Chiang Mai Province, northern Thailand. The geology in this area comprises seven units associated with local structures, as shown in Figure 1. From the oldest to youngest formations, there are Carboniferous Mae Tha sandstone, mafic-to-intermediate volcanic rocks (mélange), sandstone–chert–igneous mélange, Permian Pha Huat limestone, and Triassic sandstone and shale, which are mostly formed in subaqueous environments [50]. These units are slightly metamorphosed and are overlaid by Quaternary terrace and alluvial sediments. The outcrops and floated rocks of the ultramafic rocks, such as dunite and peridotite, were discovered along the Ma-Hin Creek and its distributary channels in volcanic terrain [51]. These units have been interpreted as part of the Chiang Rai-Chiang Mai Volcanic Belt formed in back-arc and oceanic basin environments and potentially represent relics of the Palaeo-Tethys Ocean, which separated the Shan-Thai and Indochina cratons [52,53]. The emergence of ultramafic rocks in the study area is a result of a closure of the Palaeo-Tethys Ocean in the Late Triassic induced by an oceanic crust subduction and a subsequent collision of the cratons forming a suture zone. These particular tectonic events mainly influenced the formations of mélange associated with thrust faults.

3. Materials and Methods

3.1. Rock Sampling and Petrographic Analysis

The rock floats were collected in the Ma-Hin Creek, located in the Phrao district, Chiang Mai, northern Thailand. The rock specimens were massive and dense exhibiting dark green to black fresh colors. The tarnished colors were found to be yellowish-to-reddish brown and dark brown. The standard petrographic thin sections were prepared with equipment from the thin-section commercial lab. The finished product was a polished slice of 30-micron-thick rock specimen mounted onto a 27 × 46 mm glass slide glued with a covered glass. Two thin sections were observed under the transmitted light of a conventional petrographic microscope at the Department of Geological Sciences, Chiang Mai University, to obtain data including the modal mineral composition, microtextural characteristics, and alteration features. The modal mineralogy was determined by 400-point counting per thin section and was normalized to 100%. The rock description here is based on the standards of the International Union of Geological Sciences, known as IUGS [55].

3.2. Experimental Procedure

Batch experiments between the rock sample and CO₂-bearing water were performed in an ambient condition resembling as closely as possible the rock-weathering process. The procedures were conducted in the lab at the Department of Mining and Petroleum Engineering, Chiang Mai University.

Prior to the batch experiments, the sample underwent a mechanical pre-treatment procedure to increase its possible reactive surface area for the interaction as follows: First, the rock specimens were cut and ground into small pieces using a jaw-crusher and a grinding machine. Then, the ground sample was separated into six groups based on their particle sizes using a dry-sieving analysis. This included the U.S. mesh sizes of 18, 35, 60, 120, 230, and the pan. The top three coarsest particle sets were individually weighed to 10 g, whereas the remaining sets were weighed to 5 g.

Each sample was put into a 600 mL beaker filled with 250 mL of deionized water, producing a slurry. The CO₂ in the gaseous phase was then introduced to the slurry through a 3 mm rubber hose under ambient pressure conditions with a constant rate of CO₂ bubbling. Lastly, a glass-coated magnetic stirring bar was added. The beaker was partially sealed with aluminum foil and placed on a hotplate magnetic stirrer. The mixture was stirred at a speed of approximately 1000 rpm and a temperature of 40 °C for 60 h. The flow diagram is presented in Figure 2. Thereafter, the stirring bar and the CO₂ hose were carefully removed from the retrieved mixture. The beaker containing the mixture product was subsequently placed in an oven set to 60 °C until the mixture completely dried. The weights of the final products were measured to determine the relationship between weight change and particle size.

3.3. Mineralogical Analysis

The post-experiment products were analyzed mineralogically and geochemically. X-ray diffraction analysis (XRD) was conducted at the Department of Geological Sciences, Chiang Mai University, to characterize the mineralogy and crystal structures of the initial rock sample and its products. The obtained products were powdered and analyzed using a BrukerD8Advance diffractometer equipped with a copper anode. An X-ray wavelength of 1.540598 Å and an accelerating voltage of 40 kV were used, with a filament current of 30 mA. Step scan XRD data were collected over a range of 2°–70°2θ, with a step width of 0.04°2θ and a counting time of 1 s per step. Mineral identification and quantification of the products were performed using the EVA software and referencing the patterns from the International Centre for Diffraction Data (ICDD) database. The availability of mineral components and the XRD patterns of these materials were then compared.

The selected products were imaged using a TESCAN CLARA Schottky field emission scanning electron microscope (FE-SEM) at the Institute of Product Quality and Standardization, Maejo University. The materials were individually mounted on aluminum stubs and gold-coated. The images were taken in secondary electron mode with a working distance of 8.19–10 mm and an acceleration voltage of 15.0 kV. The elemental compositions of these materials were obtained and analyzed using an Oxford Ultim Max energy-dispersive X-ray spectrometer (EDS), which was coupled with the FE-SEM and the Aztec Version 6.0 software.

4. Results

4.1. Mineral Composition and Microtextural Characteristics

Under a microscopic view, the sample exhibits a cumulate texture comprising coarse-grained cumulus crystals of olivine and spinel and fine-grained intercumulus crystals of pyroxene and fully altered plagioclase (Figure 3a,b). The displayed texture indicates intrusive igneous origins. Based on the modal mineralogical investigation, the sample has 74.5% primary minerals consisting of approximately 56.75 modal% olivine, 9.25% clinopyroxene, 6.5% plagioclase, and 2% chromian spinel. The secondary minerals, on the other hand, have resulted from the alteration of the primary minerals. They comprise 25.5 modal% serpentine and chlorite existing in the relict shapes of the primary minerals. Trace amounts of clay minerals are barely observed. The detailed mineralogy and microtextures of each mineral are provided below.

Olivine is the most abundant mineral forming euhedral elongate-to-equant crystals with irregular fractures. Their average size ranges from 0.1 to 2.5 mm. Approximately 30%–70% of the olivine is altered to serpentine and chlorite. The remnant olivine shapes are still preserved, as shown in Figure 3c,d.

The second most abundant mineral is pyroxene, which forms both anhedral intercrystals and euhedral-to-subhedral elongated-to-prismatic crystals, 0.1–0.7 mm in size (Figure 3c,d). The pyroxene distinctly exhibits an inclined extinction involving clinopyroxene members, such as diopside, hedenbergite, pigeonite, and augite. The alteration of clinopyroxene to become chlorite and brownish amphibole occurs at 10%–30%.

The plagioclase exhibits anhedral crystals. They have a size ranging from 0.1 to 0.5 mm. The minerals are highly altered to clay minerals and fine-grained micas known as sericite (Figure 3c,d). The chromian spinel comprises euhedral opaque crystals with a diameter ranging from 0.1–0.7 mm with less than 30% degree of alteration.

Following the standard IUGS ultramafic rock classification, this study utilizes the ternary diagram for olivine, orthopyroxene, and clinopyroxene, known as the Ol–Opx–Cpx ternary plot. The recalculated modal proportion of the primary mineralogy of this rock falls into the zone of wehrlite as the rock contains 85.98% olivine and 14.02% clinopyroxene with the absence of orthopyroxene, as shown in Figure 4. For detailed information, see the Supplementary Materials, PetrographicAnalysis Word document and Figures S1–S7.

4.2. Weight Changes

The batch experiments on the wehrlite samples in all particle sizes resulted in increase in their weights, as recorded in

Table 1. Weight comparisons of the wehrlite samples in various sizes measured between pre-test and post-test conditions.

Sample ID	Mesh No.	Particle Size (mm)	Weight (g)		Difference (g)	% Change
			Pre-Test	Post-Test
PR-18	18	≥1.0	10.000	10.004	+0.004	0.04
PR-35	35	<1.0–0.5	10.000	10.006	+0.006	0.06
PR-60	60	<0.5–0.25	10.000	10.010	+0.010	0.10
PR-120	120	<0.25–0.125	5.000	5.016	+0.016	0.32
PR-230	230	<0.125–0.0625	5.000	5.015	+0.015	0.30
PR-silt	Pan	<0.0625	5.000	5.018	+0.018	0.36

. The finest sample experienced the highest percentage of weight gain, whereas the coarsest sample showed the opposite. Possible factors influencing the sample weight gain are discussed in the following sections.

4.3. Mineralogical Changes

The powder X-ray diffraction data of the original wehrlite sample and its post-experiment products show discrete peak patterns, as presented in Figure 5. The XRD pattern of each sample is influenced by the mineral phases formed in that sample. The minerals forming well-defined crystalline structures result in narrow and sharp peaks, whereas the non-crystalline or amorphous phases induce a peak broadening. The diffraction pattern is, thus, a combination of the diffraction pattern of each phase. However, the heterogeneity in the mixture composition, especially the whole-rock sample provided by this study, certainly affects the complicated diffraction patterns. The raw XRD data of all-sized products have been provided in the Supplementary Materials, see the PowDLLXRD Excel Worksheet and individual EMF files.

The compositional variations, including mafic minerals (olivine, augite, albite, antigorite), clay minerals (chlorite, illite), and carbonates (calcite), are presented in

Table 2. X-ray diffraction analysis of the initial wehrlite and its products obtained from the batch experiments.

Sample Material	Major Phase [>30%]	Moderate Phase [10%–30%]	Minor Phase [2%–10%]	Trace Phase [<2%]
Wehrlite	Antigorite	Augite Olivine	Calcite Albite Chlorite	Illite
18-mesh product	n/d	Antigorite Augite Olivine Albite	Chlorite Mg-bearing calcite	Illite
35-mesh product	Olivine	Albite Calcite Augite	Antigorite	Chlorite Illite
60-mesh product	Antigorite	Ca-bearing albite Calcite	Chlorite Illite Olivine	Augite
120-mesh product	Albite	Antigorite Illite	Calcite Olivine Augite Cr-bearing chlorite	n/d
230-mesh product	n/d	Antigorite Olivine Augite Illite Calcite	Chlorite Albite	n/d
Silt-sized product	Antigorite	Augite Calcite Albite Olivine	Chlorite	Illite

. Antigorite, an indicative of serpentine-group mineral, is a major mineral (35.62%) of the initial rock; however, this mineral becomes a minor phase (8.83%) in the 35-mesh-sized product. Olivine and augite are found as moderate phase minerals (22.14% and 25.33%, respectively) in the initial wehrlite and become minor phases (3.99%–7.28%) in the products exhibiting mesh sizes of 60 and 120. On the other hand, calcite is originally observed as a minor phase (9.95%) and turns to a moderate phase (10.84%–19.18%) in the 35-mesh, 60-mesh, 230-mesh, and below 230-mesh-sized products. Magnesium-bearing calcite is detected as a minor phase (3.56%) only in the 18 mesh-sized product, whereas calcium-bearing albite is only found in the 60-mesh-sized product as a moderate phase (21.08%).

The products in the 120-mesh-sized and silt-sized particles were selected for FE-SEM imaging and EDS analysis due to their significant amounts of calcite, their surface area, and weight gain. The FE-SEM images indicate two different surface morphologies, including irregular surfaces and smooth surfaces. The EDS spectra of the two features show discrete elemental compositions. In Figure 6, the irregular surfaces, which are locally observed in the 120-mesh-sized product, are in the form of encrusting materials. The surfaces display the main contributions of oxygen, carbon, and magnesium. The EDS spectrum of the smooth surface, on the other hand, shows magnesium, silicon, and oxygen signals. In Figure 7, an aggregate of very-fine-grained polycrystalline solid materials is observed on the silt-sized product forming irregular surfaces. This aggregate exhibits a strong calcium and oxygen signal on the EDS spectrum. Its smooth surface, unsurprisingly, displays a strong magnesium, silicon, and oxygen signal.

5. Discussion

5.1. Evidence for the Occurrence of Carbon Mineralization

Most previous analytical experiments account for the presence of carbonate species and non-crystalline silica (SiO₂) as a final result of carbon mineralization. Magnesite (MgCO₃) is formed as a consequence of olivine (Mg₂SiO₄) and/or serpentine (Mg₃Si₂O₅(OH)₄) carbonation [12,13,14,26]. The two minerals are both abundant magnesium-rich silicates and their reactions with CO₂ are presented as follows:

Mg₂SiO₄ _(s) + 2CO₂ _(g) + H₂O _(l) = 2MgCO₃ _(s) + SiO₂ _(s) + H₂O _(l)

(1)

Mg₃Si₂O₅(OH)₄ _(s) + 3CO₂ _(g) + H₂O _(l) = 3MgCO₃ _(s) + 2SiO₂ _(s) + 3H₂O _(l)

(2)

On the other hand, calcite (CaCO₃) is a product of carbon mineralization of calcium-rich pyroxene (CaSiO₃) and plagioclase (CaAl₂Si₂O₈) [56,57]. The mineral reactions with CO₂ are as follows:

CaSiO₃ _(s) + CO₂ _(g) + H₂O _(l) = CaCO₃ _(s) + SiO₂ _(s) + H₂O _(l)

(3)

CaAl₂Si₂O₈ _(s) + CO₂ _(g) + 2H₂O _(l) = CaCO₃ _(s) + Al₂Si₂O₅(OH)₄ _(s)

(4)

The general reaction for carbon mineralization of these mafic-to-ultramafic rock-forming minerals has been proposed as follows [30,58]:

(Mg,Ca,Fe)_xSi_yO_x+2y-t(OH)_2t + xCO₂ = x(Mg,Ca,Fe)CO₃ + ySiO₂ + tH₂O

(5)

This reaction type is spontaneous, exothermic, and slow at near ambient temperatures [20,21,22,23]. It starts with the dissolution of reactive silicates supplying divalent cations into the CO₂-bearing aqueous solution and proceeds with carbonate precipitation [43,59,60,61]. According to the reaction process shown, minerals and/or rocks containing complex elemental compounds potentially influence variation in the formation of carbonate species. The batch experiments of the wehrlite conducted in this study resemble the mineral reactions with carbon dioxide and water in surficial ambient conditions, which naturally occur through the rock weathering processes of ultramafic geologic formations. However, the experimental setup, including temperature, ambient pressure, no acid treatment, and relatively short timing, may affect the occurrence of the mineral–CO₂–H₂O interactions and the emergence of each carbonate formation.

The existence of the interactions between CO₂ and rock at any degree essentially affects the transformation of the properties of that particular rock, such as its mineral assemblages, mass or weight, and microtextural characteristics. These physicochemical changes can be detected on any scale as described below.

The increasing weight of the all-sized products after the batch experiments is associated with transformation of the mineral abundance. Changes in the rock-forming mineral composition subsequently cause the initial mass of the rock to change. A previous study by Cutts et al. (2021) considered the relationships and changes in physical and chemical properties during the serpentinization and carbonation of ophiolitic ultramafic rocks. They suggested that this coupling process results in systematic fluctuation of the dry and wet masses induced by the current mineral assemblages formed in the rocks [47]. This potentially supports one of the findings of this study, as the product weight changes reflect the changes in their mineralogy. Thus, the weight gains of products obtained from this study provide primary evidence of mineralogical changes affected by enhanced weathering interactions. Small degrees of weight change of less than 1% might be effects of slow reaction rates and reaction efficiency. However, it should be noted that determining the sample weight change alone is not likely to be sufficient.

The discrete XRD peak patterns and peak heights among the before- and after-experiment samples indicate their mineralogical and crystallographic variations. Compared to the initial wehrlite, the mineral phases of all-sized products were changed in various degrees and directions, which are consequences of the reaction with CO₂-bearing waters. In this study, there is an absence of pure magnesite. Calcite, on the other hand, is mostly found as a dominant carbonate mineral precipitated in all products. Magnesium-bearing calcite and calcium-bearing plagioclase are locally detected in coarse-to-medium-grained products. The lack of magnesite and silica formation in this study can be explained by the reduced availability of magnesium cations in the aqueous solution, which is possibly limited by the experiment conditions (temperature, pressure), chemistry of the medium (pH, salinity, ion activity), purity of rock (amount of ion supply), and the time (for dissolution, for precipitation). The increasing amounts of calcite phases detected in the products are adequate for carbonate formation as a result of CO₂ mineralization. This study confirms that differences in the diffraction patterns between the initial rock and its CO₂ batch products indicate changes in crystallographic structures and mineral assemblages resulting from the existing CO₂-rock interaction. These data are necessary to provide further mineral evidence for the occurrence of CO₂ mineralization through the rock weathering process.

In addition, imaging with FESEM-coupled EDS analysis of the selected products reveals morphological evidence of new phase precipitation during the CO₂-rock interaction. The irregular plates encrusted on the original smooth surfaces of the samples are formed by reactions between dissolved CO₂ and target CO₂-reactive rock-forming minerals. The reactions result in aggregates of tiny grains of new mineral phases occurring along the interfaces between the rock and aqueous solution. The presence of carbon signals on the EDS spectra of the irregular crusts in combination with oxygen and calcium signals could potentially be indicative of the formation of carbonates.

5.2. Grain-Size Effects

The physiochemical changes of the products vary with the particle sizes. The microtextural characteristics of the studied wehrlite confirm that the olivine crystals range from 0.1 to 2.5 mm in diameter, whereas the other minerals have average crystal sizes of 0.1–0.5 mm. As larger reactive surface areas tend to be more favorable for the interaction process, decreasing the sample size by crushing it into pieces and grinding it into a fine powder effectively increases the magnitude of the sample surface area. This method potentially breaks down and enables the extraction of individual target minerals, such as olivine and pyroxene, from the rocks, allowing increase in CO₂-reactive interfaces. For example, reducing the size of olivine particles from 2.5 mm to 0.25 mm subsequently produces 10 times more surface area in the remaining volume.

The increasing weight of the finest product less than 0.0625 mm in diameter probably reflects the relatively high interaction rate associated with the largest surface area. Identical results are observed for the products having sizes of 0.0625–0.25 mm. Based on the weight observations, these findings are consistent with the relationships between the grain sizes and the interacting surface areas. According to the X-ray diffraction patterns, significant increases in calcite counting rates are found in the products having sizes of 0.25–1.0 mm and <0.0625–0.125 mm. These are equal to the average sizes of the reactive olivine, pyroxene, and other minerals formed in the rock. Particles lacking calcites are detected in the coarser grains due to their smaller surface areas. The results of this study confirm that the occurrence of CO₂–rock interaction of ultramafic rocks is driven by the particle sizes as well as their mineralogy.

In general, the average grain size of a single mineral differs in different rock types, especially in igneous rocks [62]. The rocks formed by rapid crystallization of magmas at shallow depths produce finer textures compared to those formed by steady crystallization in deep-seated magmatic chambers, which produce relatively coarse-grained textures. According to the thin-section observation, different minerals in the same rock formation differ widely in average grain size, shape, and area. This feature has been interpreted to be a result of the sequence of magmatic crystallization and reaction series in which the rock formed. The microtextural characteristics of the wehrlite studied here visibly indicate the specific magmatic sequence as it is formed; first, by crystallization of coarse-grained chromian spinel, followed by olivine crystallization, with formation of relatively fine-grained plagioclase, clinopyroxene, and Fe-Ti oxides in the final stage. The microtextural investigation provides the basis for the optimal grain-size selection of any potential rocks for ERW. Understanding the detailed mineralogy and rock characteristics can assist greatly in both the development of engineering designs for future carbon mineralizing plants and improvement in the mining process and production in the rock supplier industries.

5.3. Rock Mineralogy Versus CO₂ Uptake Prediction

CO₂ uptake generally refers to an amount of CO₂ equivalent that can potentially be absorbed by a mineral or material through carbon mineralization. It can be determined by several approaches including from direct measurement of the CO₂ content of the minerals obtained from laboratory chemical reactions or theoretical calculation based on an ideal mineral reactivity with CO₂ [63,64,65]. Prediction of potential CO₂ uptake of the wehrlite in this study is based on the sum of mineral reactivity with CO₂ in the presence of water as reported in the previous literature [11,14,15,18,56]. The minerals used in the calculation of this study include olivine, pyroxene, serpentine, and plagioclase. The subsequent numerical product refers to a potential amount of CO₂ uptake by that rock when the reactions for carbon mineralization have been fully completed. CO₂ uptake is generally expressed in a unit of a percent weight by weight.

As each mineral varies in chemical composition, they show different capacities for potential CO₂ uptake. Mg-rich olivine has the highest uptake potential, which is approximately 62.5% [11,14,15], while pyroxene, in the form of augite, has an uptake potential of 37% [56]. This means a smaller amount of olivine is required to store a ton of CO₂ compared to pyroxene. In other words, to sequester a ton of CO₂, 1.6 tons of olivine [32] or 2.7 tons of augite are required. Serpentine, in the form of antigorite, has a 47.6% uptake potential, which requires 2.1 tons for a ton of CO₂ stored [15,32]. Plagioclase exhibits a lower potential CO₂ uptake with only 16% [15,32,57]. The mineralized products are formed as carbonates with relevant amounts of the feed minerals.

The mineralogical variation and mineral content of rocks affect the differences in total CO₂ uptake estimation. The most abundant mineral formed in the rock appears to predict the capacity for CO₂ uptake for the whole rock. Based on the modal mineral content obtained by petrographic analysis, the wehrlite has a potential CO₂ uptake of 52.07%, accounting for the completion of carbon mineralization. This calculated number indicates that every ton of CO₂ sequestered requires 1.9 tons of the wehrlite. However, the wehrlite mineralogy obtained from XRD analysis confirms that half the amount of the olivine phase has been altered to serpentine and chlorite. A decreasing amount of olivine and increasing amounts of serpentine cause a reduction in the uptake potential to 41.95%. To store a ton of CO₂, around 2.4 tons of the altered wehrlite are required. The required quantities of the ultramafic wehrlite for CO₂ sequestration are much lower than the average value of mafic rocks, especially basalts. Approximately 5 tons of basalts are required to trap one ton of CO₂ [17,18,66]. It is suggested that the wehrlite has the potential for CO₂ uptake using mineral traps and is a suitable rock for further development in the enhanced weathering approach.

Besides the CO₂ uptake potential, the availability of rock resources significantly affects the sufficiency of reactive minerals. For instance, sequestering 100 Mt CO₂ by carbon mineralization through the ERW technology requires 190–240 Mt of the wehrlite. Exploration of the ultramafic rock deposits and geological surveys are important to further augment the precision of resource estimation for providing a sufficient amount of a mineral target.

This study strongly suggests that it is crucial to conduct a primary assessment of the CO₂ uptake potential of the rocks based on their mineral contents at any scale. The analytical techniques referred to are straightforward and help in making a basic decision for suitable rock selection. Furthermore, it should be noted that more advanced methods for quantifying the carbon uptake of other minerals and a better understanding of their reaction kinetics, efficiencies, and behaviors, are strongly recommended.

6. Conclusions

The batch experiments involving CO₂-dissolved water and various-sized ultramafic wehrlite performed under ambient conditions resembling natural rock weathering processes provide several types of evidence for the occurrence of CO₂-rock interaction in the presence of water. Firstly, the increasing weight of all products indicates an increase in the density of the initial rock caused by changes in the mineral assemblage and possibly the molecular weight of CO₂ turning into solid phases. Secondly, the mineralogical changes observed in the products revealed by the X-ray diffraction patterns serve as major evidence of the reaction of rock-forming minerals with CO₂. However, this analytical method is used solely to provide comparative geochemical data on all products to better understand the relationships between grain sizes and the relative degrees of interaction. Lastly, the existence of calcite formation is key in providing further proof of emerging carbon mineralization. New phase materials exhibiting carbonate formation potential form as crustal layers exhibiting irregular surfaces along the interfaces between rock particles and solutions. The amount of calcite and weight gain vary in different grain sizes of the products due to the differences in surface areas. The most reactive sizes shown in this work consistently align with the average sizes of olivine or other CO₂-reactive minerals formed in the rock. This study shows that the optimal grain sizes used for ERW relate to the average size of the most CO₂-reactive minerals abundant in the rocks.

Estimating the potential CO₂ uptake through theoretical calculation based on mineralogical observation offers new insights into the basic evaluation of suitable rocks for carbon sequestration. The wehrlite in this study, for example, shows a CO₂ uptake range from 41.95% to 52.57%. A higher uptake value likely indicates the fresh rock condition, whereas a lower uptake value indicates an altered version of the rock. Although most ultramafic rocks seem to be promising candidates for carbon sequestration by CO₂ mineralization, an understanding of their mineralogy and microtextural characteristics is necessary to increase the estimated CO₂ uptake potential leading to suitable rock selection. To enable the future development of enhanced weathering strategies for this rock type, more advanced analytical methods tested under broader condition regimes in connection with an assessment of resource availability are highly recommended.