The research, led by Robert F. Martin of McGill University together with Dirk Schumann, Markku J. Lehtinen, and Sebastian Fuchs, was published in the European Journal of Mineralogy. The team examined five specimens of wollastonite ore from the Ihalainen open pit, a quarry 1.2 kilometers long and up to 120 meters deep, using large-area image mosaics from transmitted-light microscopy and scanning electron microscopy, combined with electron-microprobe analyses at the Federal Institute for Geosciences and Natural Resources in Hanover, Germany. Two of the specimens, cut through narrow aplitic dikelets that intrude the bluish marble, proved to be the key witnesses of an extraordinary magmatic event that had gone essentially unnoticed in the deposit’s long mining history.

The geological backdrop is essential to understanding what happened. The Ihalainen Formation consists of Paleoproterozoic calcitic and dolomitic marbles, originally limestones deposited about 1.9 billion years ago. During regional metamorphism at temperatures of 650 to 700 degrees Celsius and pressures around 500 megapascals, dated by radiometric analysis of titanite at 1.858 billion years, water infiltrated siliceous horizons in the limestone and produced the main wollastonite-rich zones. Roughly 300 million years later, the area was engulfed by the Wiborg batholith, a vast body of anorogenic rapakivi granite. The marble block, now largely surrounded by granite, is interpreted as a roof pendant, a floating remnant of older crust captured within the batholith. Crucially, the Wiborg granites are known to be strongly enriched in fluorine, and locally in uranium, thorium, beryllium, tin, and niobium.

What the team found in specimen MKL-1021 was unlike anything documented before in this setting. Narrow dikelets of aplitic material cut the marble, but the dikelets contain neither quartz nor feldspar in the studied sections. Instead, they consist of a texturally complex assemblage of forsterite, now altered to a lizardite-like mixed-layer mineral provisionally labeled “aliettite”, diopside, and a striking graphic intergrowth of wollastonite and fluorite, in which rods of the two minerals grew simultaneously and interfered with each other as they crystallized from a melt. The researchers interpret the material inside a ribbon-like envelope of diopside, wollastonite, or fluorite as a near-eutectic fluorosilicate melt that quenched on injection into the calcite host. The graphic intergrowth is the smoking gun: such textures form only when two minerals crystallize together from a common liquid, in this case a melt in the system CaO–MgO–SiO2–H2O–F.

The source of the fluorine, and the heat, was almost certainly the underlying Wiborg granite. Experiments show that the solubility of fluorite, CaF2, in water rises sharply with increasing pressure and temperature, so a fluorine-rich vapor phase emanating from the cooling pluton could have dissolved fluorite deep in the system and deposited it in the heated ore above. Because fluid advection is the most efficient mode of heat transfer available, the same vapor delivered both the flux and the thermal energy needed to melt the ore. Once fluorite and heat were added, the wollastonite assemblage crossed its melting threshold. The team estimates that in the pseudobinary system CaSiO3–CaF2–H2O, the relevant eutectic temperature would fall to roughly 800 degrees Celsius, well within reach of the granite’s thermal footprint.

Specimen ML-3A, collected closer to the granite contact, records an even more dramatic episode: true anatexis of the marble itself. Here the dikelet consists of an emulsion of two immiscible melts, a silicate liquid and a carbonate liquid, that mingled turbulently, exchanged components, and then froze. The incoming granitic melt was desilicated, stripped of its silica by the aggressively fluxing carbonate melt, until it became a feldspathic, effectively syenitic liquid. The carbonate melt, in turn, dissolved silicates and became silicocarbonatitic. The researchers visualize a vigorous, turbulent reaction zone in which globules of disaggregated granitic melt churned through the carbonate liquid, an intimate mingling that likely resembled processes documented in the Early Paleozoic Ol’khon collision system near Lake Baikal in Russia. Skeletal crystals of new wollastonite along the dikelet margins, and even an entrained fragment of what appears to be a wollastonite cumulate, testify to the speed and violence of the mixing.

The feldspathic melt crystallized as a single calcium-poor alkali feldspar, a sanidine solid solution, and its subsequent cooling history is preserved in one of the finest displays of spinodal decomposition ever described in natural feldspar. As the crystal cooled through the coherent solvus, sodium and potassium atoms interdiffused within the aluminosilicate framework, spontaneously organizing into sinusoidal, interpenetrating domains of albite and K-feldspar without any nucleation barrier. The resulting lamellae, less than a micrometer wide and not crystallographically aligned, form stripe and labyrinthine patterns oriented perpendicular to the enclosed blobs of calcite. Because the two feldspar domains remained structurally coherent, both were strained, and the process is considered rapid relative to ordinary strain-free exsolution. A later influx of aqueous fluid recrystallized the feldspars and relieved the strain, yet left the tell-tale spinodal texture intact, a billion-year-old fingerprint of a phase separation that occurred under non-equilibrium conditions at perhaps 550 to 500 degrees Celsius.

Perhaps the most tantalizing discovery lies in three minerals so unusual that they cannot yet be matched to any known species. Labeled UK1, UK2, and UK3, they concentrate high-field-strength elements transported by the fluorine-rich fluids. UK1 is a calcium-tin sulfide with an extraordinary sulfur content, found in pitted zones of the calcite matrix and unlike the only known comparable mineral, genplesite. UK2 is a hydrous calcium silicate of niobium and tantalum, with minor zirconium and titanium, coating cavity rims in calcite. UK3 is a hydroxyl-bearing calcium-niobium silicate found in a cavity within euhedral wollastonite, with proportions that differ from the only IMA-approved minerals containing those elements, komarovite and mongolite. Malayaite, the tin analog of titanite, and tiny grains of scheelite and cassiterite round out the exotic inventory, all consistent with vapor transport of tin, tungsten, niobium, and tantalum from the evolved granite into the marble.

The fluid history of the deposit reads like a chemical drama in acts. It began carbon dioxide-rich, favoring the dry conditions under which spinodal decomposition operated, then shifted to an aqueous regime as the system cooled, unleashing a bewildering cascade of secondary calcic minerals: pectolite replacing wollastonite, paragonite replacing albite, lawsonite pseudomorphs after plagioclase, holtstamite rims around calcite, prehnite, gonnardite, xonotlite, and fluorapophyllite-(K), which systematically replaced wollastonite at submagmatic temperatures. The circulating fluid was dominantly calcic, a consequence of the high solubility of both calcite and fluorite in hot water, which explains the sheer diversity of the alteration assemblage documented across dozens of element-distribution maps.

Beyond its scientific novelty, the study carries practical implications. The Ihalainen deposit is one of the world’s notable wollastonite sources, and the demonstration that fluorine-rich vapor can flux-melt and locally enrich such ores opens a new lens on skarn deposits worldwide, particularly those fringing evolved granites like the Kymi topaz granite cupola of the Wiborg batholith, which is enriched in fluorine, lithium, beryllium, rubidium, tin, and niobium. The authors note that larger crystals of the three unknown minerals will be needed before formal proposals to the International Mineralogical Association can proceed, but given the documented enrichment of the Wiborg batholith in high-field-strength elements, they expect more such discoveries at Ihalainen. A quarry that has been worked for over five centuries, it turns out, still guards secrets that only modern microscopy could unlock.

Subject of Research: Fluorine-induced melting and magma-carbonate interaction in the Ihalainen wollastonite marble deposit, Finland

Article Title: Fluorine-induced melting in the Ihalainen calcite-wollastonite marble deposit, Lappeenranta, Finland

Article References: Martin, R. F., Schumann, D., Lehtinen, M. J., & Fuchs, S. (2026). Fluorine-induced melting in the Ihalainen calcite-wollastonite marble deposit, Lappeenranta, Finland. European Journal of Mineralogy, 38(4), 431-448. https://doi.org/10.5194/ejm-38-431-2026

Image Credits: AI Generated

DOI: 10.5194/ejm-38-431-2026

Keywords: wollastonite, fluorine, marble, rapakivi granite, Wiborg batholith, anatexis, spinodal decomposition, carbonate melt, new minerals, niobium, tantalum, Finland

Bipolar plates are the workhorses of a PEMFC stack. They separate individual cells, distribute hydrogen and air across the electrode surfaces, carry current from cell to cell, and help manage water and heat. Traditionally, plates were machined from graphite, which is conductive and corrosion-resistant but brittle, thick, and costly to manufacture at volume. Metallic plates, by contrast, can be stamped into thin, intricate flow fields in seconds, making them far lighter and cheaper for automotive applications. The catch is that metals corrode in the acidic, humid environment inside a fuel cell, and the passive oxide films that form on their surfaces are poor conductors. That oxide layer is precisely what drives up interfacial contact resistance, or ICR, the area-specific resistance measured between the plate and the adjacent porous transport layer under the compressive load of stack assembly.

Researchers have explored a zoo of protective coatings to solve this, from physical vapour deposited nitrides to amorphous carbon films. But vapour-phase techniques require expensive vacuum equipment and struggle to coat complex, porous geometries uniformly. Sevinc and Hazar took a different route: a combined aqueous electrodeposition and electrophoretic deposition (EPD) process, carried out entirely in water-based baths. Electrodeposition builds the metallic matrix of the coating, while EPD uses an electric field to drive charged graphene particles toward the substrate, embedding the carbon sheets into the growing film. The approach is low-cost, scalable, and, crucially, compatible with porous structures that vacuum processes cannot easily penetrate.

The team applied their Ni–Cr–Ta–graphene coatings to an unusually broad set of substrates: dense aluminum, dense nickel, and AISI 316L stainless steel, the three most commonly discussed metallic bipolar plate materials, plus porous aluminum and porous nickel structures. Porous flow fields are an emerging design concept in which the conventional machined channels are replaced by an open-pore metal foam, giving vastly more contact area for gas distribution and water management. Coating such structures without clogging the pores is a major challenge, and demonstrating that an EPD-based coating preserves pore accessibility is one of the study’s most practically significant results.

The researchers systematically varied the deposition bath composition, designated S1 through S5, and the deposition time, then examined the resulting surfaces with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). These techniques revealed how uniformly the coating covered each substrate and how the graphene and metallic elements were distributed. The morphology proved highly sensitive to processing conditions: too short a deposition time left the surface incompletely covered, while excessive deposition produced thicker, less uniform layers. An optimized deposition time of 120 seconds yielded comparatively uniform coatings across the series and delivered the best electrical contact behavior within the investigated set of formulations.

The headline numbers come from the ICR measurements, taken under compressive pressures relevant to real PEMFC stack assembly, up to 140 N/cm². Among all tested formulations, the coating designated S4 achieved the lowest interfacial contact resistance, approximately 78 mΩ cm² at 140 N/cm² on the aluminum substrate. To appreciate how substantial that improvement is, consider the benchmark the team used for comparison: an aluminum specimen pre-coated with nickel alone, representing a conventional baseline, exhibited roughly 269 mΩ cm² under the same conditions. The multi-component graphene-containing coating therefore cut the contact resistance to less than a third of the baseline value, a reduction of nearly 70 percent achieved purely through smarter surface chemistry and processing.

The authors are refreshingly candid about the remaining gap. The U.S. Department of Energy has set a target of 10 mΩ cm² for bipolar plate contact resistance, a figure that reflects what is needed for maximum stack efficiency in mass-market vehicles. Even the best S4 coating remains approximately 7.8 times higher than that target. The study’s results, in other words, represent a relative improvement within the investigated coating series rather than a solved problem. Further optimization of bath chemistry, particle loading, deposition parameters, and possibly post-treatment steps will be required before these coatings can meet the stringent demands of commercial fuel cell stacks. This honesty is valuable in a field where headline numbers sometimes obscure how far laboratory results still sit from deployment thresholds.

Why does adding graphene to a nickel–chromium–tantalum matrix help so much? The answer lies in the complementary roles of each constituent. Nickel provides the conductive metallic backbone and serves as the electroplating medium from the aqueous bath. Chromium contributes corrosion resistance, helping to shield the underlying metal from the acidic PEMFC environment and suppress the growth of insulating oxide films. Tantalum is prized in the bipolar plate literature for its exceptional passivity and stability in simulated fuel cell conditions, and prior studies have shown tantalum-modified stainless steel plates resisting corrosion effectively. Graphene, meanwhile, brings its legendary in-plane electrical conductivity and chemical inertness; sheets distributed through the coating create conductive pathways and can act as a barrier to corrosive species. Together, the four constituents aim to satisfy the two demands that usually trade off against each other: protecting the metal from corrosion while keeping the surface electrically accessible.

The combined electrodeposition–EPD strategy also matters from a manufacturing standpoint. Metal matrix composite coatings produced by co-depositing particles into an electroplated layer are an established and diversifying technology, but systematic studies on Ni–Cr–Ta–graphene systems, particularly on both dense and porous substrates, had remained limited. By showing that the same water-based process works across aluminum, nickel, and stainless steel, and that it coats porous foams while keeping their pores open, the Firat University team has expanded the design space available to fuel cell engineers. Aqueous processing avoids the capital cost and geometric limitations of PVD and CVD chambers, and it integrates naturally with existing electroplating infrastructure in the automotive supply chain.

The work, which draws on the first author’s doctoral dissertation on metal-based flow plates for PEM fuel cells in electric vehicles and was funded by Firat University’s Scientific Research Projects Coordination Unit, arrives at a moment when hydrogen technology is scaling up globally. Every reduction in ohmic loss inside the stack translates directly into more kilometers per kilogram of hydrogen, and bipolar plate coatings are among the most cost-effective levers for achieving that. Sevinc and Hazar’s results demonstrate that aqueous EPD-based Ni–Cr–Ta–graphene coatings provide an effective route to improving electrical contact performance while preserving the pore accessibility that next-generation porous flow fields demand. The DOE target remains the finish line, but this study maps out a credible, low-cost path toward it, one graphene-laden layer at a time.

Subject of Research: Ni–Cr–Ta–graphene composite coatings for reducing interfacial contact resistance of metallic bipolar plates in proton exchange membrane fuel cells

Article Title: Microstructural and contact resistance behavior of Ni–Cr–Ta–graphene coatings on metallic bipolar plate substrates

Article References: Sevinc, H., & Hazar, H. (2026). Microstructural and contact resistance behavior of Ni–Cr–Ta–graphene coatings on metallic bipolar plate substrates. Ionics. https://doi.org/10.1007/s11581-026-07541-9

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07541-9

Keywords: PEMFC, bipolar plate, graphene, electrophoretic deposition, contact resistance, coatings, nickel, chromium, tantalum, corrosion, fuel cells, porous metal