Hubbry Logo
DuniteDuniteMain
Open search
Dunite
Community hub
Dunite
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Dunite
Dunite
Dunite
View on Wikipedia
from Wikipedia
Small volcanic bomb of (black) basanite with (green) dunite

Dunite (/ˈduːnaɪt, ˈdʌnaɪt/), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite, magnetite, and pyrope. Dunite is the olivine-rich endmember of the peridotite group of mantle-derived rocks.

Dunite and other peridotite rocks are considered the major constituents of the Earth's mantle above a depth of about 400 km (250 mi). Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base of ophiolite sequences where slabs of mantle rock from a subduction zone have been thrust onto continental crust by obduction during continental or island arc collisions (orogeny). It is also found in alpine peridotite massifs that represent slivers of sub-continental mantle exposed during collisional orogeny. Dunite typically undergoes retrograde metamorphism in near-surface environments and is altered to serpentinite and soapstone.

The dunite field is highlighted in green.

The type of dunite found in the lowermost parts of ophiolites, alpine peridotite massifs, and xenoliths may represent the refractory residue left after the extraction of basaltic magmas in the upper mantle. However, a more likely method of dunite formation in mantle sections is by interaction between lherzolite or harzburgite and percolating silicate melts, which dissolve orthopyroxene from the surrounding rock, leaving a progressively olivine-enriched residue.

Dunite may also form by the accumulation of olivine crystals on the floor of large basaltic or picritic magma chambers. These "cumulate" dunites typically occur in thick layers in layered intrusions, associated with cumulate layers of wehrlite, olivine pyroxenite, harzburgite, and even chromitite (a cumulate rock consisting largely of chromite). Small layered intrusions may be of any geologic age, for example, the Triassic Palisades Sill in New York and the larger Eocene Skaergaard complex in Greenland. The largest layered mafic intrusions are tens of kilometers in size and almost all are Proterozoic in age, e.g. the Stillwater igneous complex (Montana), the Muskox intrusion (Canada), and the Great Dyke (Zimbabwe). Cumulate dunite may also be found in ophiolite complexes, associated with layers of wehrlite, pyroxenite, and gabbro.

Dunite was named by the Austrian geologist Ferdinand von Hochstetter in 1859, after Dun Mountain near Nelson, New Zealand.[1] Dun Mountain was given its name because of the dun colour of the underlying ultramafic rocks. This color results from surface weathering that oxidizes the iron in olivine in temperate climates (weathering in tropical climates creates a deep red soil). The dunite from Dun Mountain is part of the ultramafic section of the Dun Mountain Ophiolite Belt.

A massive exposure of dunite in the United States can be found as Twin Sisters Mountain, near Mount Baker in the northern Cascade Range of Washington. In Europe it occurs in the Troodos Mountains of Cyprus. In southern British Columbia, Canada dunite rocks form the core of an ultramafic rock complex located near the small community of Tulameen. The rocks are locally enriched in platinum group metals, chromite and magnetite.

Carbon sequestration potential

[edit]
See also: Carbon dioxide removal and Enhanced weathering

Dunite could be used to sequester CO2 and help mitigate global climate change via accelerated chemical rock weathering. This would involve the mining of dunite rocks in quarries followed by crushing and grinding to create fine ground rock that would react with the atmospheric carbon dioxide. The resulting products are magnesite and silica which could be commercialized.[2][3]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Dunite

View on Grokipedia
from Grokipedia
Dunite is a coarse-grained ultramafic plutonic composed almost exclusively of , with the constituting more than 90% of its volume, and minor amounts of accessory such as , , or . This rock type exhibits a phaneritic texture and typically displays a light greenish hue due to the dominance of forsteritic . Dunite primarily forms in the through processes such as of , leaving residual olivine-enriched material, or via reactive infiltration where migrating melts dissolve , concentrating . It occurs in geological settings including complexes, which represent obducted and , pipes, and layered intrusions, providing key insights into mantle dynamics, melt migration pathways, and lithologies resistant to further . Economically, dunite serves as a host rock for deposits and is mined as a during magnesite extraction, with applications in , for sequestration via olivine , and as a in iron production or foundry . Its high magnesium content and reactivity make it valuable for and , though large-scale remains limited compared to other ultramafics.

Etymology and History

Discovery and Naming

Dunite was named in 1859 by Austrian geologist Ferdinand von Hochstetter following his examination of ultramafic rocks during the Austrian Novara expedition's geological survey of New Zealand. He designated the term for the olivine-dominated rock observed at its type locality on Dun Mountain (also known as Maungatapu), a peak near Nelson in New Zealand's South Island. This naming reflected the mountain's distinctive dun (dull brownish) coloration, resulting from surface weathering that oxidizes iron in the olivine to produce a yellowish-orange rind over the fresh green interior. The Dun Mountain occurrence forms part of a larger Permian-age sequence, where dunite represents a mantle-derived body intruded or tectonically emplaced amid and sedimentary rocks. Hochstetter's description established dunite as a distinct , characterized by over 90% modal (forsterite-rich Mg-endmember), distinguishing it from broader . Early recognition of such rocks predated formal naming, with similar olivine-rich masses noted in European localities, but Dun Mountain provided the eponymous prototype due to its accessibility and purity. Subsequent mining for in the area further highlighted the deposit's economic potential, though initial surveys focused on petrological classification.

Early Petrological Studies

The petrological study of dunite began with Ferdinand von Hochstetter's fieldwork in during the Austrian Novara Expedition in 1859, where he examined exposures at Dun Mountain near Nelson. Hochstetter identified the rock as consisting predominantly of crystals, distinguishing it from the surrounding serpentinized ultramafics by its fresh, coarse-grained texture and lack of pervasive alteration. He described it as a novel type, nearly pure forming an extensive mass, and formally named it "dunite" based on the mountain's characteristic dun-colored weathering. Hochstetter's initial analysis relied on hand-specimen examination and early microscopic techniques, noting the grains' granular fabric and accessory , while tentatively proposing an eruptive origin emplaced during within a broader ophiolitic sequence. His observations, communicated in letters to colleague Julius von Haast, emphasized the rock's uniformity and rarity as a "pure " lithology, unprecedented in European at the time. Following Hochstetter's departure in 1860, von Haast extended mapping efforts along the Dun Mountain belt, corroborating the dominance through field descriptions and integrating dunite into regional stratigraphic interpretations as an ultrabasic intrusive body associated with serpentinites. By the early , petrological examinations incorporated more systematic modal analyses, as in the 1911 survey by Bell, Clarke, and Marshall, which detailed the Dun Mountain dunite's textural variations but lacked chemical compositions, relying instead on qualitative abundance estimates exceeding 90 volume percent. These studies affirmed dunite's ultramafic nature and igneous character, though debates persisted on its precise emplacement mechanism, with some attributing it to differentiation from basaltic magmas rather than primary mantle material. Early interpretations generally viewed dunite as a fractionated end-member of suites, setting the stage for later geochemical validations.

Petrological Characteristics

Mineral Composition

Dunite consists predominantly of the mineral , which typically comprises more than 90% of its volume, distinguishing it from other peridotites like that contain significant orthopyroxene. The olivine is magnesium-rich, often with contents (Fo, defined as molar Mg/(Mg+Fe)) ranging from Fo90 to Fo93, as observed in alpine-type peridotites. This high Fo value reflects derivation from mantle sources with elevated Mg/Fe ratios, minimizing iron enrichment during crystallization. Accessory minerals, constituting less than 10% of the rock, include orthopyroxene (primarily ), clinopyroxene (such as chromian ), and members of the like . occurs as disseminated euhedral to subhedral grains, often with elevated Cr/(Cr+Al+Fe3+) ratios (0.30–0.75), influencing the rock's potential for chromite enrichment in certain deposits. Rare phases such as or may appear but are typically absent or secondary, resulting from alteration rather than primary igneous processes. The monomineralic nature arises from extreme fractional crystallization or melt-rock reaction in ultramafic systems, concentrating olivine while depleting other silicates.
MineralTypical Modal AbundanceKey Characteristics
Olivine>90 vol.%Forsterite-rich (Fo90–Fo93), coarse-grained, equant to anhedral
Orthopyroxene ()<5 vol.%Accessory, often altered to serpentine
Clinopyroxene (chromediopside)<5 vol.%Trace, Cr-bearing
Chromite/spinel<1–2 vol.%Euhedral grains, high Cr#

Textural and Structural Features

Dunite primarily exhibits a phaneritic texture, consisting of coarse-grained, interlocking olivine crystals typically ranging from 1 mm to several centimeters in diameter, visible to the naked eye without magnification. This granular fabric reflects slow crystallization from mantle-derived melts under plutonic conditions, often resulting in an equigranular or subequigranular arrangement of euhedral to subhedral olivine grains. In cumulate varieties, which predominate in layered intrusions, the texture is characterized by adcumulate or mesocumulate features, where primocrysts of olivine show overgrowths from interstitial liquids, indicating fractional crystallization and crystal settling processes. Structurally, dunite bodies are frequently massive and homogeneous on a hand-specimen scale, lacking pronounced foliation or schistosity in their primary igneous form, though crude modal layering may occur in cumulate sequences due to rhythmic deposition of olivine-rich slurries. In mantle peridotite contexts, such as ophiolite complexes, dunite lenses or veins can display aligned olivine megacrysts with preferred crystallographic orientations, reflecting reactive infiltration or shear deformation during ascent. Porphyroclastic textures, featuring large relict olivine porphyroclasts amid finer recrystallized matrix grains (0.5–10 mm), are common in tectonized examples, signaling post-emplacement dynamic recrystallization under high strain. These features distinguish primary cumulate dunites from residual or replacive types formed by melt-rock reaction in highly depleted peridotites.

Physical and Chemical Properties

Dunite exhibits a coarse- to medium-grained phaneritic texture, with individual mineral grains typically visible to the naked eye. Fresh exposures display an olive-green to straw-green color derived from the content, though weathering oxidizes iron to produce surface alterations ranging from dull brown in temperate regions to deep red in tropical environments. The rock possesses a Mohs hardness of 6.5 to 7, consistent with its dominant olivine component, and a specific gravity of 3.2 to 3.3, reflecting the high concentrations of dense magnesium- and iron-bearing silicates. It lacks prominent cleavage, appearing massive, with a white streak and subvitreous to greasy luster on fresh fracture surfaces. Chemically, dunite is an ultramafic rock dominated by olivine ((Mg,Fe)₂SiO₄) exceeding 90 vol.%, yielding bulk oxide compositions typically featuring 36–42% MgO and 36–39% SiO₂, alongside minor FeO (up to ~10%), Al₂O₃, and trace elements from accessory phases like chromite and pyroxenes. This low-silica profile (<45% SiO₂ total) distinguishes it from more evolved igneous rocks, with the forsterite-rich olivine (often Fo₈₆–₉₃) indicating derivation from mantle sources with limited fractional crystallization.

Formation and Geological Context

Mantle Origin and Igneous Processes

Dunite forms in the upper mantle primarily through reactive processes during melt extraction, where basaltic liquids derived from partial melting of peridotite interact with surrounding harzburgite, dissolving orthopyroxene and precipitating additional olivine to produce dunite bodies. This reactive porous flow occurs under conditions of mantle upwelling, such as at mid-ocean ridges, enhancing melt connectivity and focusing ascent through high-permeability channels up to several meters wide. Observations from ocean drilling and ophiolites indicate that these dunites constitute 10-20% of mantle sequences in some settings, with olivine compositions typically reaching forsterite contents (Fo) of 90-93, reflecting refractory residues modified by melt infiltration rather than simple partial melting residues. Igneous cumulation represents another key mantle-linked process, where olivine crystals settle from primitive, high-MgO magmas (e.g., picrites or ) generated by deep mantle melting, accumulating as monomineralic layers in subcrustal chambers or along conduit walls. Tabular or pod-like dunite bodies, often 10-100 meters thick, result from this fractional crystallization, with clinopyroxene oikocrysts occasionally enclosing olivine chadacrysts, as documented in layered intrusions and ophiolitic mantle sections. This mechanism dominates in suprasubduction settings, where fluxed melting elevates temperatures to 1300-1400°C, promoting early olivine saturation and gravitational segregation before magma reaches crustal levels. Distinguishing these origins relies on textural and geochemical signatures: reactive dunites show equilibrated, interstitial textures with pyroxene dissolution fronts, while cumulates exhibit adcumulate fabrics from crystal packing and compaction. Both processes underscore dunite's role in mantle convection and melt segregation, with numerical models indicating that dunite channel formation can increase melt flux by orders of magnitude compared to homogeneous porous flow. Isotopic data, such as elevated 87Sr/86Sr in some cumulates, further trace mantle heterogeneity and recharge events during prolonged igneous activity.

Associated Alterations and Serpentinization

Dunite, composed predominantly of olivine, undergoes serpentinization as its primary alteration process, a low-temperature metamorphic hydration reaction where olivine (Mg,Fe)₂SiO₄ interacts with aqueous fluids to form serpentine-group minerals such as lizardite, chrysotile, or antigorite, along with brucite Mg(OH)₂ and magnetite Fe₃O₄. This process typically occurs under hydrothermal conditions at temperatures of 100–400 °C and pressures corresponding to upper mantle to crustal depths, often facilitated by fluid infiltration in tectonically exhumed peridotite bodies or ophiolite complexes. The reaction releases hydrogen gas (H₂) and heat, with the iron oxidation step driving magnetite precipitation: for example, 18 Fe₂SiO₄ (fayalite component) + 26 H₂O → 13 Fe₃O₄ + 9 SiO₂ (as serpentine) + 32 H₂. In nearly monomineralic dunite lacking orthopyroxene, serpentinization proceeds via olivine replacement without silica buffering, initially forming brucite-rich assemblages in fractures or mesh textures, where fine-grained lizardite networks enclose magnetite grains and residual olivine cores. Progression involves multistage veining, as observed in New Caledonian dunites, with early narrow veins (50–100 μm) of serpentine-dominated mesh followed by wider bastite veins and late cross-cutting chrysotile fibers, accompanied by progressive iron partitioning into magnetite that depletes serpentine and brucite of FeO. Accessory chromite in dunite alters concurrently, typically to Cr-bearing magnetite via ferritchromization, occurring above serpentine stability but below chlorite, without significant volume change. Associated alterations beyond core serpentinization include localized talc formation through silica metasomatism, yielding soapstone (talc + carbonate) at contacts with country rocks, and potential carbonation where CO₂-bearing fluids react with residual brucite or olivine to form magnesite MgCO₃. These processes enhance rock plasticity, reducing shear strength by up to 50% at 300–520 °C and 30–40 kbar, influencing fault mechanics in subduction zones. Full serpentinization can increase rock volume by 30–50% due to hydration, fracturing the host and promoting fluid pathways.

Global Occurrences

Type Locality and New Zealand Deposits

The type locality of dunite is Dun Mountain, situated southeast of Nelson in the northern South Island of New Zealand, where the rock type was first identified and described. Austrian geologist Ferdinand von Hochstetter named the rock dunite in 1859, deriving the term from the mountain's name, which reflects the dull brownish ("dun") hue resulting from supergene oxidation of iron in the weathered ultramafic outcrops. The Dun Mountain ultramafic massif, comprising the type dunite, forms a key segment of the Permian-aged (ca. 275–285 Ma) Dun Mountain ophiolite belt, a tectonically emplaced sequence of mantle peridotites, gabbros, and overlying sediments spanning over 300 km along the island's margin. At this locality, the dunite exhibits >90% modal (typically forsteritic, Fo90–92) with minor and orthopyroxene, often variably serpentinized due to hydrothermal alteration, confirming its mantle-derived origin via residues or cumulates in an oceanic crust-forming environment. New Zealand hosts several other notable dunite deposits, primarily within ophiolitic and ultramafic complexes of the , reflecting and subduction-related magmatism. The Greenhills Complex, near Bluff on the Southland coast (approximately 30 km south of ), represents a major with thick dunite zones, hosting economic and resources derived from in-situ alteration; its indicates fractional crystallization from mantle-derived melts around 500–600 Ma. Further south, contains dunite lenses within massifs of the Darran Complex, associated with arc-related plutonism dated to the (ca. 170–180 Ma). Smaller occurrences appear in Northland at North Cape, linked to obducted fragments of age. These deposits, while not as extensively mined as those in other countries, support niche extractions for materials and fertilizers, with reserves estimated in the tens of millions of tonnes across sites, though precise volumes vary by local and accessibility.

Major Worldwide Deposits

The Twin Sisters dunite massif, located in the northern Cascade Range of Washington State, United States, constitutes one of the largest known dunite bodies globally, spanning approximately 36 square miles (93 km²) along a northwest-trending fault zone and serving as a primary source for olivine extraction. This deposit, part of an ultramafic complex emplaced during the Tertiary period, has supported industrial-scale mining operations, highlighting its economic significance for refractory materials. In , major dunite-hosted deposits occur within complexes such as the Vourinos, Pindos, and Othrys massifs in , where podiform ores are embedded in dunite lenses, forming some of the largest metallurgical-grade resources in . The Xerolivado-Skoumtsa mine in this region exemplifies high-volume extraction, with reserves estimated at 6 million tons of ore grading 22% Cr₂O₃. Commercial operations, including those by Grecian Magnesite, produce over 1 million tons per annum of MgO-rich dunite (40-45% MgO) for industrial applications like slag conditioning. Norway's Åheim deposit in the Sunnmøre region represents a key European source of high-purity dunite, with estimated reserves exceeding 200 million tons, historically mined for used in sands and refractories. This body, part of a layered ultramafic intrusion, underscores the region's role in global supply despite limited current production. Other significant occurrences include the Troodos in , featuring extensive dunite within the mantle sequence and associated pods, though primarily exploited for rather than bulk dunite.

Economic Extraction and Uses

Mining Operations

Dunite extraction primarily employs open-pit quarrying techniques, leveraging the rock's occurrence in extensive, near-surface massive bodies that facilitate large-scale mechanical excavation. Operations typically commence with blasting to fracture the hard, olivine-rich , followed by loading with excavators and haul trucks, then primary crushing at the site to reduce material size for transport or further processing into aggregates, powders, or concentrates. These methods are standard for ultramafic rocks due to their low resistance and uniform composition, minimizing the need for underground . Major commercial operations often yield dunite as a co-product or byproduct during the mining of associated minerals such as or . In , Grecian Magnesite extracts around 500,000 tonnes of dunite annually alongside from deposits in the region, utilizing integrated quarrying and beneficiation facilities to separate and process the material for and environmental applications. Similarly, in , dunite output totaled 86,495 tonnes in the reviewed by the Indian Bureau of Mines, reflecting a 123% year-over-year increase, predominantly as or waste from operations in states like and . Spain hosts dedicated dunite mining by Pasek Minerales, operational since 1972 near Yerro, where quarry extraction and on-site milling produce processed dunite for export via local ports, emphasizing its olivine content for industrial fillers and abrasives. In , the type locality at Dun Mountain has seen historical chromite extraction but limited modern commercial quarrying; smaller active sites, such as the Dunite Quarry at Greenhills near and operations along Omaui Road in Southland, focus on localized excavation for research or niche uses like soil amendment, with minimal large-scale production. Elsewhere, dunite mining supports extraction in ultramafic-hosted deposits, as seen in open-pit operations within Brazilian dunite bodies rich in lateritic ores, where selective blasting and target disseminated sulfides amid the matrix. Global production remains modest compared to other aggregates, constrained by niche demand, with total incidental yields from co-mining exceeding direct extraction volumes.

Industrial Applications

Dunite is principally utilized as a refractory material in metallurgical and high-temperature processes, owing to its composition, which confers a above 1700°C and a low, uniform coefficient of . Calcined dunite, processed in rotary kilns at approximately 1650°C, produces forsterite-bonded bricks suitable for linings in furnaces, ladles, blast furnaces, kilns, and up to 1600°C. These refractories exhibit resistance to and chemical attack from slags, making dunite a cost-effective alternative to in . In steel production, dunite functions as a metallurgical flux, supplying (MgO) for slag formation and desulfurization in blast furnaces and sintering operations. Application rates typically range from 15 to 45 kg per ton of , substituting for dolomite to improve slag fluidity and reduce content. Its high MgO content, often exceeding 40%, enhances process efficiency in ironmaking. Ground or crushed dunite serves in applications, including resin-bonded sands for molds and cores, where its angular grains provide good permeability and collapsibility. It is also employed as an abrasive for and , capitalizing on olivine's hardness (6.5-7 on ) and low friability. In construction, processed dunite aggregates contribute to road bases, railway ballast, and concrete foundations, valued for their compressive strength exceeding 100 MPa and resistance to weathering. Limited use extends to ceramics and glass manufacturing as a magnesium silicate flux, though this is secondary to its refractory and metallurgical roles.

Resource Potential and Reserves

Dunite serves as a primary source of olivine for industrial applications, with global reserves of unaltered olivine estimated at approximately 200 gigatons, though much of this occurs in dispersed mantle-derived bodies rather than concentrated economic deposits. Economic extraction is constrained by factors such as degree of serpentinization, which reduces olivine purity and reactivity; accessibility; and proximity to markets, limiting active mining to select high-grade occurrences. Resource potential extends beyond traditional uses like refractories and foundry sands to magnesium production and enhanced weathering for carbon dioxide removal, but scalability depends on technological advances in processing low-grade or altered material. Norway hosts the most significant commercial reserves, producing over 2.5 million tonnes of annually from dunite-hosted deposits and supplying roughly half of global demand. The Åheim (Gusdal) operation, operated by Sibelco, represents the world's largest commercial mine, with supporting production for at least 150 years at rates exceeding 2 million tonnes per year. These deposits, part of the Almklovdalen massif, yield high-purity forsteritic (typically >90% Mg-rich ) suitable for slag conditioning and abrasive applications. In the United States, the Twin Sisters dunite body in , stands as one of the largest known deposits globally, with estimated resources on the order of 1.8 billion tonnes, though production has historically been modest due to market fluctuations and environmental regulations. Further south, ultramafic belts in the Appalachian region offer substantial potential; combined reserves in and Georgia exceed 1.17 billion tonnes of dunite, including over 233 million tonnes of high-grade (48% MgO) unaltered material amenable to quarrying at costs of $0.60–$1.50 per tonne. Key areas like Webster-Balsam (456 million tonnes) and Buck Creek (385 million tonnes) in contain partially serpentinized dunite averaging >40% MgO, viable for refractory-grade but requiring beneficiation to mitigate hydration effects. Other notable reserves include smaller deposits in New Zealand's Dun Mountain belt and India's incidental dunite output from mining (86,000 tonnes in recent years), but these lack quantified large-scale economic viability comparable to or the U.S. Overall, while dunite's abundance underscores long-term resource security, current reserves support sustained production only where and align, with untapped potential in ophiolitic complexes worldwide pending improved and reduced serpentinization impacts.

Environmental Applications and Debates

Carbon Sequestration Mechanisms

Dunite facilitates carbon sequestration primarily through the mineral carbonation of its dominant mineral, olivine (forsterite, Mg₂SiO₄), which reacts with carbon dioxide to form stable magnesium carbonate minerals such as magnesite (MgCO₃) and amorphous silica (SiO₂). The fundamental exothermic reaction is Mg₂SiO₄ + 2CO₂ → 2MgCO₃ + SiO₂, releasing approximately 1.25 tons of CO₂ sequestered per ton of olivine reacted under ideal conditions. ![{\displaystyle {\ce {Mg2SiO4olivineolivine + 2CO2 -> 2MgCO3magnesitemagnesite + SiO2silicasilica}}}}[center] This process mimics natural silicate weathering but is accelerated in engineered settings to enhance CO₂ drawdown. The mechanism proceeds in two main stages: dissolution of , where Mg²⁺ and SiO₄⁴⁻ ions are released into an aqueous medium under acidic conditions from dissolved CO₂ (forming , H₂CO₃), followed by precipitation of carbonates as rises due to (HCO₃⁻) formation and subsequent . Olivine dissolution is rate-limited by the formation of a passivating silica-rich layer on grain surfaces, which can be mitigated by mechanical grinding to increase surface area or chemical additives to enhance reactivity. efficiency peaks at intermediate temperatures of 185–200 °C, where a shift from dissolution-dominated to precipitation-dominated kinetics occurs, with near-complete conversion possible under supercritical CO₂ conditions. In enhanced rock weathering applications, crushed dunite is deployed on land or in marine environments, where rainwater or seawater provides the aqueous medium, promoting alkalinity generation and CO₂ uptake via bicarbonate export to the ocean. Marine settings accelerate dissolution 8–19 times compared to static conditions due to constant grain agitation, though silica gel formation still poses kinetic barriers. For ex-situ processes, direct aqueous carbonation of dunite involves pretreatment (e.g., heat activation at 630 °C) followed by reaction in NaHCO₃ solutions at 185 °C and 130 bar, achieving significant magnesite formation. In-situ mineralization in dunite formations, such as peridotite reservoirs, enables rapid carbonate precipitation upon CO₂ injection, with up to 88% mineralization within 45 days via reactions involving residual brucite (Mg(OH)₂) and serpentine phases. These mechanisms ensure long-term stability, as formed carbonates resist reversal under geological conditions, unlike biological or ocean-based storage.

Soil Remediation and Agricultural Uses

Dunite, primarily composed of , serves as a soil amendment in due to its high magnesium (Mg) and (Si) content, which can enhance nutrient availability and correct acidity. When applied as a , dunite promotes physiological changes in crops such as increased Mg , higher Si uptake, and improved grain yield in , with rates up to 4 tons per showing positive effects on growth parameters. It also elevates foliar , reducing sugars, and levels while supplying essential micronutrients through gradual . However, its efficacy varies with and environmental conditions; finer grains (e.g., <0.5 mm) dissolve faster, releasing nutrients more readily under higher rainfall, though excessive application risks elevating nickel (Ni) and chromium (Cr) levels, potentially exceeding safe thresholds for edible crops like barley and wheat. In soil remediation, dunite mining wastes and tailings are utilized for stabilizing heavy metal(loid)s such as arsenic (As), copper (Cu), lead (Pb), and zinc (Zn) in contaminated sites, leveraging the rock's alkaline nature and high cation exchange capacity (CEC) to immobilize pollutants via adsorption, precipitation, or co-precipitation. Combined with compost or nano-zero-valent iron (nZVI), these amendments reduce bioavailable metal concentrations in polluted soils by up to 50-70% in roots and shoots of test plants, while improving overall soil fertility. At ultramafic mining sites, dunite-derived materials support phytoremediation strategies, aiding vegetation establishment on Ni-rich tailings by enhancing soil pH and nutrient retention, though long-term monitoring is required to mitigate secondary leaching of trace elements. Despite these benefits, applications must account for dunite's inherent trace metal content, with studies recommending site-specific dosing to avoid exacerbating contamination.

Feasibility, Limitations, and Criticisms

The feasibility of dunite-based carbon sequestration relies on the exothermic carbonation of its dominant mineral, forsterite olivine (Mg₂SiO₄), which reacts with CO₂ to form stable magnesite (MgCO₃) and silica, offering potential for permanent storage without reverting under geological timescales. Enhanced rock weathering (ERW) deployments of ground dunite on croplands could theoretically remove 1–4 t CO₂ ha⁻¹ yr⁻¹ at application rates of 10–50 t ha⁻¹, with economic viability projected at around 60 USD t⁻¹ CO₂ for optimized dunite use due to its high magnesium content and reactivity relative to basalts. In-situ mineralization in dunite outcrops, such as ophiolite-hosted reservoirs, has shown progress in pilot scales, with reaction fronts advancing centimeters per year under injected CO₂ conditions. For soil remediation, dunite's magnesium release supports pH stabilization and nutrient replenishment in acidic or Mg-deficient soils, potentially boosting crop yields by 10–20% in magnesium-limited regions without lime equivalents. Limitations arise primarily from kinetic barriers, with olivine dissolution rates averaging 10⁻¹³ mol m⁻² s⁻¹ under field conditions, slowed further by passivating silica gels, incongruent weathering (high Mg/Si ratios >50), and reduced water-rock contact from preferential flow or drying, capping global ERW potential at ~0.07 Gt CO₂ yr⁻¹ even with aggressive scaling. Pretreatments like fine grinding (<150 μm) or activation enhance rates but demand 50–200 kWh t⁻¹ , plus and emissions that can offset 20–50% of gross sequestration. In agricultural applications, heavy metal leaching—particularly at 20–340 nmol L⁻¹ in solutions, often exceeding stringent environmental thresholds, and minor chromium mobility—poses contamination risks, necessitating site-specific avoidance of Ni-Cr-enriched deposits. Monitoring net removal is hindered by open-system losses, such as elevated or carbonate re-dissolution in acidic soils (pH <6), which can release up to 30% of stored CO₂. Criticisms highlight systematic overestimation of fluxes in models and lab trials, where initial rapid dissolution of fresh surfaces (e.g., Mg release dropping from 60 to 2 mg L⁻¹ within hours) inflates long-term projections by 1–3 orders of magnitude, ignoring passivation and accessory interference that attributes non-silicate CO₂ uptake to ERW. Field-scale verification challenges persist, with DIC flux measurements failing to distinguish silicate-derived sequestration from biotic or other sources, complicating certification for carbon markets. Environmental trade-offs, including disruption from large-scale dust application and , have prompted calls to favor lower-risk basalts over dunite, potentially halving reactivity advantages. Proponents counter that tailored sourcing and hybrid monitoring (e.g., tracing) mitigate risks, but skeptics note insufficient multi-year trials to validate gigatonne-scale claims amid logistical hurdles for distributing billions of tonnes annually.

References

  1. https://sandatlas.org/dunite/
  2. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/dunite
  3. https://www.alexstrekeisen.it/english/pluto/dun.php
  4. https://academic.oup.com/petrology/article/50/7/1205/1510665
  5. https://www.geologyin.com/2023/12/dunite.html
  6. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001gc000277
  7. https://www.sciencedirect.com/science/article/abs/pii/S1342937X15001823
  8. https://www.frontiersin.org/journals/environmental-science/articles/10.3389/fenvs.2023.1172621/full
  9. https://www.mdpi.com/2075-163X/9/8/481
  10. https://pasek.es/wp-content/uploads/2023/10/dunita-aplicaciones-generales.pdf
  11. https://www.merriam-webster.com/dictionary/dunite
  12. http://geologylearn.blogspot.com/2015/03/dunite.html
  13. https://www.alexstrekeisen.it/english/pluto/dunite.php
  14. https://www.tandfonline.com/doi/abs/10.1080/00288306.1965.10426418
  15. https://teara.govt.nz/en/photograph/28804/dun-mountain-and-dunite
  16. https://pubs.geoscienceworld.org/gsa/gsabulletin/article/95/5/551/202941/Structure-and-origin-of-the-Dun-Mountain
  17. https://www.theprow.org.nz/people/ferdinand-hochstetter-1829-188/
  18. https://pubs.geoscienceworld.org/gsl/books/book/1657/chapter/107478598/Nineteenth-century-observations-of-the-Dun
  19. https://gsnz.org.nz/assets/Uploads/Shop/Products/MP133/MP133K.pdf
  20. https://www.tandfonline.com/doi/pdf/10.1080/00288306.1959.10422757
  21. https://www.osti.gov/servlets/purl/938528
  22. https://pubs.usgs.gov/publication/70227162
  23. https://www.dnr.wa.gov/publications/ger_reprint8_twin_sisters_dunite.pdf
  24. https://dggs.alaska.gov/webpubs/dggs/gr/text/gr018.pdf
  25. https://pubs.usgs.gov/bul/0947d/report.pdf
  26. https://academic.oup.com/petrology/article/43/3/423/1383621
  27. https://pubs.usgs.gov/bul/1662/report.pdf
  28. https://www.researchgate.net/figure/Texture-of-Isua-dunite-peridotite-a-Picture-of-anhydrous-peridotite-with-olivine_fig4_294721357
  29. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GC011913
  30. https://rocksminerals.flexiblelearning.auckland.ac.nz/rocks/peridotite.html
  31. https://geologybase.com/dunite/
  32. https://www.bharanibms.com/dunite-1
  33. https://www.ibm.gov.in/writereaddata/files/08172015132039D%2520and%2520P.pdf
  34. https://www.sciencedirect.com/science/article/abs/pii/S0024493724004535
  35. http://www-odp.tamu.edu/publications/209_IR/chap_03/c3_2.htm
  36. https://www.ldeo.columbia.edu/sites/default/files/uploaded/file/2002%2520Braun%2520&%2520Kelemen.pdf
  37. https://www.ldeo.columbia.edu/sites/default/files/uploaded/file/1995%2520Kelemen%2520et%2520al%2520Nature.pdf
  38. http://ui.adsabs.harvard.edu/abs/1982CoMP...78..413Q/abstract
  39. https://www.sciencedirect.com/science/article/pii/026437079190039H
  40. https://pubs.geoscienceworld.org/msa/ammin/article/99/5-6/1035/46394/Role-of-silica-for-the-progress-of
  41. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006GL025681
  42. https://academic.oup.com/petrology/article/57/1/3/2375384
  43. https://www.sciencedirect.com/science/article/abs/pii/S0024493713000339
  44. https://pubs.geoscienceworld.org/msa/ammin/article/59/5-6/608/541085/Alteration-of-Chromite-from-the-Twin-Sisters
  45. https://www.mdpi.com/2075-163X/13/2/159
  46. https://www.science.org/doi/10.1126/science.153.3741.1285
  47. https://www.mindat.org/min-48408.html
  48. https://theses.otagogeology.org.nz/items/show/535
  49. https://www.stuff.co.nz/environment/climate-news/129893155/what-is-dunite-the-cup-of-tea-that-led-to-a-nzfirst-experiment
  50. https://www.nzpam.govt.nz/assets/Uploads/the-mineral-potential-of-new-zealand-part-1-overview-of-new-zealands-mineral-deposits-and-their-resources.pdf
  51. https://pubs.usgs.gov/ds/414/downloads/DS414_text_508.pdf
  52. https://ui.adsabs.harvard.edu/abs/2018EGUGA..2012043T/abstract
  53. https://www.grecianmagnesite.com/node/235
  54. https://everettrockclub.com/Dunite
  55. https://www.sciencedirect.com/science/article/pii/S2772443325000352
  56. https://www.grecianmagnesite.com/sites/default/files/2023-11/Dunite%2520-%2520For%2520Web%2520site.Final_.pdf
  57. https://www.mindat.org/loc-192318.html
  58. https://www.thepress.co.nz/nz-news/360639111/about-south-quarries-quakes-and-dock-bay
  59. https://www.sciencedirect.com/science/article/pii/S0013935125007157
  60. https://cordis.europa.eu/project/id/BRE20344
  61. https://mineravi.com/en-il/collections/state-assembly
  62. https://www.sibelco.com/en/150-years/aheim
  63. https://www.mindat.org/loc-49916.html
  64. https://epd.georgia.gov/sites/epd.georgia.gov/files/related_files/site_page/B-47.pdf
  65. https://pubs.acs.org/doi/10.1021/acs.estlett.9b00301
  66. https://austinvernon.site/blog/rockweathering.html
  67. https://pubs.acs.org/doi/10.1021/acs.energyfuels.4c04120
  68. https://www.sciencedirect.com/science/article/pii/S2666821125001991
  69. https://www.sciencedirect.com/science/article/pii/S0016703723004349
  70. https://www.sciencedirect.com/science/article/pii/S2405844023090047
  71. https://www.nature.com/articles/s43247-025-02509-5
  72. https://aiche.onlinelibrary.wiley.com/doi/10.1002/ep.13075
  73. https://www.researchgate.net/publication/350469955_Dunite_solubilization_kinetics_in_silicon-magnesium_fertilization
  74. https://cmagfert.com.br/wp-content/uploads/2019/10/Can-Dunite-Promote-Physiological-Changes-Magnesium-Nutrition-and-Increased-Corn-Grain-Yield.pdf
  75. https://www.tandfonline.com/doi/abs/10.1080/00103624.2019.1635143
  76. https://www.researchgate.net/publication/373210298_The_effects_of_dunite_fertilization_on_growth_and_elemental_composition_of_barley_and_wheat_differ_with_dunite_grain_size_and_rainfall_regimes
  77. https://www.sciencedirect.com/science/article/pii/S2773050625000254
  78. https://link.springer.com/article/10.1007/s11104-023-05879-x
  79. https://www.researchgate.net/publication/335054688_Reuse_of_Dunite_Mining_Waste_and_Subproducts_for_the_Stabilization_of_Metaloids_in_Polluted_Soils
  80. https://www.frontiersin.org/journals/climate/articles/10.3389/fclim.2024.1510747/full
  81. https://d-nb.info/1170873804/34
  82. https://iopscience.iop.org/article/10.1088/1748-9326/aaa9c4
Add your contribution
Related Hubs
User Avatar
No comments yet.