Mineral-Rich Carbon (MRC): Diverse Material Systems

• MRC is a relational term for diverse carbon–mineral systems characterized by the intimate coupling of carbon with mineral phases across disciplines.
• In water treatment, MRC (and its enhanced form TMRC) is engineered for fluoride adsorption through modeled ion exchange and chemisorption processes.
• In geological storage and planetary science, MRC describes both natural and synthetic carbon-bearing systems critical for reactive transport analyses and planetary formation studies.

Mineral-Rich Carbon (MRC) is a context-dependent designation applied to several distinct classes of carbon-bearing mineral systems rather than to a single standardized material. In water-treatment research, it denotes carbonised mammalian or avian bone meal used as a fluoride adsorbent (Auton et al., 2023). In geological carbon storage, it refers implicitly to carbonate-rich solid products formed during CO$_2$-rock reaction and to the mechanically altered reactive zones generated by mineral precipitation in porous rock (Nooraiepour et al., 2024, Aminzadeh et al., 4 Aug 2025). In planetary science, it denotes carbon-rich disk reservoirs, carbon-rich rocky interiors, and alteration assemblages in which carbon is tightly coupled to minerals, dust, or rock-forming phases (Cugno et al., 18 Sep 2025, Volz et al., 11 Nov 2025, Madhusudhan et al., 2012, Brown et al., 2019, Bergin et al., 10 Feb 2026). The breadth of this usage is itself a defining feature of the term.

1. Terminological scope and disciplinary usage

Across the current literature, MRC functions as a family resemblance term: the common element is not a single composition, but the co-occurrence of carbon with mineral phases in a chemically or structurally significant way. This includes adsorbent solids, secondary carbonates, carbon-rich disk material, and carbon-bearing planetary mineralogies.

Domain	Meaning of MRC	Representative sources
Water treatment	Carbonised bone-derived adsorbent, hydroxyapatite-rich	(Auton et al., 2023, Auton et al., 16 Jul 2025)
Geological storage	Carbonated solid products and carbonate-rich precipitates from reactive transport	(Nooraiepour et al., 2024, Aminzadeh et al., 4 Aug 2025)
Planet formation and planetary materials	Carbon-rich disk reservoirs, interiors, and alteration assemblages	(Cugno et al., 18 Sep 2025, Volz et al., 11 Nov 2025, Madhusudhan et al., 2012, Brown et al., 2019, Kaluna et al., 2016, Bergin et al., 10 Feb 2026)

A common misconception is that MRC names a single material class. The literature instead uses it for chemically distinct systems whose shared feature is a strong coupling between carbon and mineral matter. This suggests that MRC is best understood as a relational concept whose meaning depends on the operative process: adsorption, mineral carbonation, disk chemistry, planetary differentiation, or aqueous alteration.

2. Hydroxyapatite-rich MRC in water treatment chemistry

In fluoride-removal studies, MRC is untreated carbonised bone meal, chemically described as a hydroxyapatite-based material containing calcium phosphate mineral phases with hydroxyl groups. A typical MRC molecule is stated to consist of ten Ca$^{2+}$ ions, six PO$_4^{3-}$ groups, and two OH$^-$ groups. MRC grains are about $0.4$–$0.6$ mm, and its fluoride removal capacity is about $14\ \mathrm{mg/g}$. TMRC is MRC that has been ground and coated with aluminium hydroxide, $\mathrm{Al(OH)}_3$, producing grains about $0.1$–$0.3$ mm and a much larger adsorption capacity around $^{2+}$0, but at higher cost and with greater clogging risk (Auton et al., 2023, Auton et al., 16 Jul 2025).

The central modeling claim in this literature is that Langmuir’s one-step adsorption law is not chemically faithful enough for MRC, especially for kinetics. Chemically based models therefore derive the governing reactions from the inferred surface composition. For MRC, one formulation resolves fluoride uptake into chemical ion exchange on hydroxylated hydroxyapatite sites,

$^{2+}$1

and physical adsorption or electrostatic association at calcium-associated sites,

$^{2+}$2

A later validation paper adopts an equivalent two-pathway interpretation under $^{2+}$3 and fluoride concentrations up to $^{2+}$4, retaining chemisorption or ion exchange together with weaker physisorption via hydrogen bonding and excluding dissolution–precipitation pathways under those conditions (Auton et al., 2023, Auton et al., 16 Jul 2025).

For TMRC, the dominant retained mechanism is fluoride exchange on the $^{2+}$5 coating,

$^{2+}$6

or, in the later notation,

$^{2+}$7

The papers emphasize that these are not merely phenomenological curve fits: they are derived from the chemical composition and likely reaction pathways of the adsorbents. For MRC, that means hydroxyapatite-like POH and PCa sites; for TMRC, surface $^{2+}$8 chemistry (Auton et al., 2023, Auton et al., 16 Jul 2025).

This chemically explicit formulation matters because it explains why MRC and TMRC behave differently despite shared ancestry in bone char. MRC is mechanistically heterogeneous and comparatively weak, whereas TMRC is dominated by a high-affinity aluminium pathway. A plausible implication is that the acronym MRC, in this domain, encodes not only composition but also a specific hierarchy of surface reactions.

3. Reactive-transport models for MRC-TMRC filter systems

The mathematically explicit treatment of MRC and TMRC extends from batch tests to fixed-bed columns. In the earlier model, the authors validate chemically grounded batch isotherm and kinetic equations for both materials and then embed them in a homogeneous porous-medium column model with axial advection and diffusion. The packed bed is a $^{2+}$9 MRC:TMRC mixture by mass, and outlet fluoride concentration is predicted through coupled transport and surface-reaction equations. The principal conclusion is that the chemically based MRC-TMRC model reproduces breakthrough curves much better than Langmuir, even with comparable fitting flexibility (Auton et al., 2023).

The later validation paper makes this transport picture considerably more quantitative. The full column model is tested against six breakthrough curves: three with varying inlet fluoride concentration,

$_4^{3-}$0

at fixed flow rate $_4^{3-}$1, and three with varying flow rate,

$_4^{3-}$2

at approximately $_4^{3-}$3 inlet fluoride. The bed height is about $_4^{3-}$4, the mixture remains approximately $_4^{3-}$5 by mass, and the full model achieves $_4^{3-}$6 for all six curves with $_4^{3-}$7. Individual values span $_4^{3-}$8 to $_4^{3-}$9 and $^-$0 to $^-$1 (Auton et al., 16 Jul 2025).

The same paper identifies why TMRC dominates overall filter performance despite being the minor component by mass. Its adsorption rate constant satisfies

$^-$2

and its maximum capacity satisfies

$^-$3

The outlet evolution therefore contains two timescales: a fast TMRC-dominated front and a much slower MRC chemisorption front. MRC contributes at early and late times, but TMRC controls operational breakthrough. The paper quantifies the slowness of MRC chemisorption by noting that at $^-$4 the chemisorption channel is only about $^-$5 occupied and that $^-$6 saturation is not reached until about $^-$7 (Auton et al., 16 Jul 2025).

Because MRC has such a small effect on breakthrough, the authors also propose a reduced TMRC-only model that sets the MRC source terms to zero. This reduced model retains only one breakthrough fitting parameter, still gives $^-$8 for all six curves, and yields $^-$9 from about $0.4$0 to $0.4$1. The full model remains more accurate, but the reduced model is presented as a mechanistically justified simplification rather than a purely empirical shortcut (Auton et al., 16 Jul 2025).

This literature establishes a notable contrast. MRC, although the bulk component in the filter bed, is not the dominant kinetic actor. That result is chemically intuitive given the aluminium coating on TMRC, but the value of the models is that they reconcile batch and column behavior with physically interpretable parameters rather than treating breakthrough as a black-box transport curve.

4. Mineralized carbon in porous rock and the mechanics of reactive transport

In subsurface carbon storage research, MRC refers implicitly to the solid mineral carbonation inventory generated by CO$0.4$2-rock interaction. In basaltic systems, injected CO$0.4$3-acidified water enhances dissolution of basalt glass and basaltic minerals, releasing Ca$0.4$4, Mg$0.4$5, and Fe$0.4$6/Fe$0.4$7, after which carbonate minerals precipitate once saturation is reached. Basalt is attractive because it contains abundant Ca, Mg, and Fe silicate minerals and often has useful porosity, permeability, and injectivity. The process is described as an intricately linked dissolution–precipitation sequence rather than a simple equilibrium conversion (Nooraiepour et al., 2024).

A central result of the basalt study is that carbonate precipitation is spatially localized and nucleation-controlled. Columnar flow and batch surface-growth experiments showed spontaneous formation of a limited number of large crystals at various locations, appearing as isolated pockets rather than uniform coatings. Numerical models predicted precipitation near the calcite–basalt transition and, at $0.4$8 and $0.4$9, suggested possible formation of MgFeCa-carbonates such as ankerite, siderite, and magnesite. Experimentally, however, the dominant products were calcium carbonate polymorphs, specifically calcite and aragonite. The paper attributes this discrepancy most likely to concurrent smectite formation, which can deplete Mg and Fe, inhibit nucleation through a honeycomb-like coating, and reduce flow by clogging pore throats and narrowing fractures (Nooraiepour et al., 2024).

The sensitivity to solution chemistry is explicit. Batch experiments at $0.6$0, $0.6$1, and $0.6$2 over initial pH values from about $0.6$3 to $0.6$4 showed that at model-estimated initial pHs of $0.6$5, $0.6$6, and $0.6$7, basaltic glass surfaces were covered almost entirely by smectite overgrowths and no visible carbonate crystals formed. At $0.6$8 and $0.6$9, localized calcium carbonate crystals appeared alongside smectite. This makes clear that MRC formation in basalt is controlled by the competition between carbonation and clay alteration, not by cation supply alone (Nooraiepour et al., 2024).

A complementary study addresses how such reactive transport might be monitored mechanically. Using Dunnville sandstone rods about $14\ \mathrm{mg/g}$0 long and $14\ \mathrm{mg/g}$1 in diameter, a $14\ \mathrm{mg/g}$2 ultrasonic transducer, and a 3D Scanning Laser Doppler Vibrometer, the authors performed full-field monitoring of one-dimensional axial waves during reactive transport driven by NaCl-rich fluid, capillary rise, and evaporation-induced supersaturation. The measured axial particle velocity $14\ \mathrm{mg/g}$3 was converted in the frequency domain according to

$14\ \mathrm{mg/g}$4

and multiple line scans were averaged. Three frequencies, $14\ \mathrm{mg/g}$5, $14\ \mathrm{mg/g}$6, and $14\ \mathrm{mg/g}$7, were chosen so that wavelengths were much larger than the rod diameter, supporting a quasi-1D assumption (Aminzadeh et al., 4 Aug 2025).

Back-analysis used the modified-error-in-constitutive-relation (MECR) elastography framework to reconstruct a spatially varying complex Young’s modulus $14\ \mathrm{mg/g}$8. Maxwell and Kelvin–Voigt constitutive forms were compared, and the Maxwell model fit better: $14\ \mathrm{mg/g}$9 The reconstructed fields showed degradation of dynamic Young’s modulus and increased attenuation after mineralization. At $\mathrm{Al(OH)}_3$0, intact rock had $\mathrm{Al(OH)}_3$1 about $\mathrm{Al(OH)}_3$2, whereas reacted specimens dropped to about $\mathrm{Al(OH)}_3$3; the paper reports roughly $\mathrm{Al(OH)}_3$4–$\mathrm{Al(OH)}_3$5 local decrease and an overall about $\mathrm{Al(OH)}_3$6 drop, together with about a $\mathrm{Al(OH)}_3$7-fold increase in attenuation. Spatially, the modulus damage increased with distance from the feed point (Aminzadeh et al., 4 Aug 2025).

Microstructural observations linked these macroscopic changes to grain-scale processes. X-ray micro-computed tomography at $\mathrm{Al(OH)}_3$8 voxel size did not resolve major porosity differences, but SEM revealed halite precipitates, powder-like precipitate matter, and damaged underlying structure after washing. The interpretation is that microcracking is predominantly responsible for stiffness loss, while pore filling is partly responsible and may account for slight regional modulus recovery. Although this experiment used NaCl precipitation rather than direct carbon mineralization, it is presented as a monitoring concept for mineral carbon storage: reactive transport and mineral precipitation imprint a spatially heterogeneous mechanical fingerprint that can in principle be sensed remotely (Aminzadeh et al., 4 Aug 2025).

5. Carbon-rich disk reservoirs in planet-forming environments

In disk studies, MRC refers to carbon-rich gas and dust reservoirs in which carbon-bearing molecules dominate the observable chemistry. A striking case is the circumplanetary disk around CT Cha b, observed with JWST/MIRI Medium-Resolution Spectrograph. Over $\mathrm{Al(OH)}_3$9 to $0.1$0, and within a full $0.1$1–$0.1$2 MIRI dataset, the disk shows emission from C$0.1$3H$0.1$4, C$0.1$5H$0.1$6, HCN, C$0.1$7H$0.1$8, CO$0.1$9, C$0.3$0H$0.3$1, C$0.3$2H$0.3$3, and the isotopologue $0.3$4CCH$0.3$5. Spectral cross-correlation was used to locate the companion in the cube, and zero-dimensional LTE slab models were fit with column density $0.3$6, temperature $0.3$7, and emitting area $0.3$8. The inferred gas-phase carbon-to-oxygen ratio is $0.3$9, derived from the column density ratio of C$^{2+}$00H$^{2+}$01 to CO$^{2+}$02, implying a genuinely carbon-rich disk atmosphere (Cugno et al., 18 Sep 2025).

The contrast with the host disk is chemically sharp. In the same dataset, CT Cha A shows H$^{2+}$03O and OH but no carbon-bearing molecules. The difference is interpreted as rapid, divergent chemical evolution on $^{2+}$04million-year timescales within a system about $^{2+}$05 old. CT Cha b is therefore treated as the first direct mid-infrared molecular characterization of material in a circumplanetary disk and as a direct example of a carbon-rich disk reservoir (Cugno et al., 18 Sep 2025).

A related result appears in the transitional disk RX J1615.3-3255. JWST-MIRI MRS observations over $^{2+}$06–$^{2+}$07 show strong emission from H$^{2+}$08O, HCN, C$^{2+}$09H$^{2+}$10, $^{2+}$11CO$^{2+}$12, $^{2+}$13CO$^{2+}$14, OH, and $^{2+}$15C$^{2+}$16CH$^{2+}$17, whereas the comparable disk GM Aur shows only H$^{2+}$18O and OH in that range. J1615 has an exceptionally high

$^{2+}$19

far above the median Spitzer T Tauri value of $^{2+}$20. The system also shows larger dust grains, a higher crystalline silicate fraction, stronger settling with $^{2+}$21 versus $^{2+}$22 for GM Aur, and a lower accretion rate, $^{2+}$23 versus $^{2+}$24 (Volz et al., 11 Nov 2025).

The interpretation is that carbon-rich gas can persist or become more observable when accretion is weaker and dust is more processed. Larger grains reduce MIR opacity and move the $^{2+}$25 surface deeper, crystalline silicates absorb UV more effectively than amorphous grains, and strong settling can expand the intermediate-temperature molecular layer. This does not imply that all transitional disks are carbon-rich; rather, it identifies one pathway by which carbon-rich gas and mineralogically evolved dust can coexist (Volz et al., 11 Nov 2025).

A broader review places these cases in a disk-evolution framework. Inner-disk planetary carbon compositions are said to depend strongly on supply and survival of carbonaceous solids. The soot line is assigned a temperature of about $^{2+}$26, evolving from roughly $^{2+}$27 to $^{2+}$28 as disks cool. Without barriers, a few tens to over a hundred Earth masses of carbon- and water-bearing solids can drift into the inner disk over the disk lifetime. A strong pressure bump inside $^{2+}$29 must form within $^{2+}$30 to meaningfully reduce that inward flux. Within this framework, carbon-rich rocky worlds and carbon-rich sub-Neptunes may be common, especially in systems lacking giant planets that suppress inward transport (Bergin et al., 10 Feb 2026).

6. Carbon-rich rocky interiors and mass-radius degeneracy

MRC in exoplanet interior studies denotes carbon-rich refractory mineralogies rather than adsorbents or disk gas. The canonical case is 55 Cancri e, whose measured properties,

$^{2+}$31

with an alternative gray radius

$^{2+}$32

can be fit by standard Earth-centric oxygen-rich models only if a substantial volatile envelope is added, previously estimated as $^{2+}$33 by mass of supercritical H$^{2+}$34O. The paper shows that the same mass-radius data can also be matched by a carbon-rich solid interior composed of Fe, C, SiC, and/or silicates, without any volatile envelope (Madhusudhan et al., 2012).

The interior models assume spherical symmetry and up to three chemically distinct layers, with room-temperature equations of state for Fe, MgSiO$^{2+}$35, SiC, H$^{2+}$36O, graphite, and diamond. Two ternary families are explored: Fe–SiC–C and Fe–MgSiO$^{2+}$37–C. Using the visible radius, Fe mass fraction is allowed up to $^{2+}$38 in the Fe–SiC–C case, and an Earth-like Fe core of $^{2+}$39 permits essentially any mixture of SiC and C, including $^{2+}$40 and $^{2+}$41. Using the gray radius, Fe is more tightly limited to a maximum of $^{2+}$42, and for $^{2+}$43 the model requires roughly C $^{2+}$44 and SiC $^{2+}$45. In Fe–MgSiO$^{2+}$46–C models, an Earth-like Fe fraction of $^{2+}$47 requires more than $^{2+}$48 C and allows at most about $^{2+}$49 MgSiO$^{2+}$50 (Madhusudhan et al., 2012).

These carbon fractions are extreme relative to Earth, which has $^{2+}$51 C by mass. For 55 Cnc e, allowed values span $^{2+}$52–$^{2+}$53 C, two orders of magnitude higher. The significance is not that the planet is proven to be carbon-rich, but that mass-radius data alone do not uniquely imply an oxygen-rich interior plus water layer. Different mineralogies can produce similar bulk densities, so the composition problem is fundamentally degenerate (Madhusudhan et al., 2012).

The formation argument rests on the host star’s reported abundances,

$^{2+}$54

If the protoplanetary disk resembled the star, condensation calculations suggest that above $^{2+}$55 SiC and Fe dominate, from $^{2+}$56–$^{2+}$57 graphite becomes dominant followed by Fe and silicates, and below $^{2+}$58 Mg$^{2+}$59SiO$^{2+}$60 dominates even in a carbon-rich environment. The present orbit at $^{2+}$61 then implies probable inward migration from a zone where carbon-rich refractory accretion was possible (Madhusudhan et al., 2012).

This use of MRC is geophysically consequential. Carbon and SiC have $^{2+}$62 higher thermal conductivity than silicates and oxides, so a carbon-rich rocky planet would occupy a geochemical and geophysical regime distinct from terrestrial planets in the Solar System. The principal controversy is not whether such materials can exist, but whether present observations can discriminate them from volatile-rich oxygen-dominated alternatives.

7. Altered planetary surfaces and remote mineralogical fingerprints

On airless bodies and ancient terrains, MRC appears as a surface-mineralogical and spectral problem. Laboratory space-weathering experiments on aqueously altered minerals show that pulsed laser irradiation of an Fe-rich assemblage composed of cronstedtite, pyrite, and siderite produces strong darkening, band-depth suppression, initial reddening, and then spectral bluing with increasing irradiation time. Scanning and transmission electron microscopy reveal micron-sized carbon-rich particles containing notable fractions of nitrogen and oxygen. Radiative transfer modeling attributes the initial reddening to $^{2+}$63 and the later bluing to increasing production of micron-sized carbon particles, $^{2+}$64pC. The experiments therefore indicate that irradiation can synthesize carbon-rich phases in situ rather than merely modifying inherited carbonaceous material (Kaluna et al., 2016).

The compositional dependence is central. Mg-rich lizardite shows slight reddening, slight darkening, and pronounced reduction in band depths, whereas the Fe-rich assemblage evolves along a non-monotonic redden $^{2+}$65 blue trajectory. The authors suggest that $^{2+}$66 and possibly cronstedtite promote synthesis of organic-like compounds, and they motivate this with the Fischer-Tropsch-type relation

$^{2+}$67

This suggests that MRC-like surface materials on volatile-rich asteroids may record both aqueous history and irradiation-driven carbon synthesis (Kaluna et al., 2016).

On Mars, remote sensing identifies a different manifestation: the Jezero Crater olivine-carbonate lithology. CRISM mapping shows that carbonates occur only in association with olivine whose $^{2+}$68 absorption band is shifted to longer wavelengths. The olivine band is fit with an asymmetric Gaussian, and the mapped population is isolated using

$^{2+}$69

The centroid threshold $^{2+}$70 is especially important because it excludes highly Mg-rich or very fine-grained olivine and isolates the long-wavelength-shifted lithology consistently associated with carbonates (Brown et al., 2019).

The interpretation is that the long-wavelength shift reflects more Fe-rich olivine and/or larger grain size, with lower grain-size bounds of $^{2+}$71 and upper bounds around $^{2+}$72 in the strongest cases. Yet not all long-wavelength olivine is carbonated, and the most extremely shifted olivine does not always correspond to carbonate. THEMIS tests show no consistent correlation between thermal inertia and the long-wavelength olivine signature, indicating that present-day thermophysics is not a simple proxy for the VNIR mineralogical signal. The favored geological interpretation is partial aqueous alteration of an olivine-rich protolith, with serpentinization on early Mars treated as a leading formation mechanism (Brown et al., 2019).

Taken together, these planetary-surface studies show that MRC can be a dynamic spectral product rather than a static composition. On asteroids it may emerge through irradiation-driven synthesis of carbon-rich particles; on Mars it may mark incomplete carbonation and hydrothermal alteration in olivine-rich crust. In both cases, the critical signal is not carbon abundance alone, but the mineralogical context in which carbon is hosted, transformed, or detected.