Rhombohedral K₂Ca₂(CO₃)₃, a new pyro-biophase formed in the ash of the desert spoon (Dasylirion wheeleri) from the Sonoran Desert

Terrestrial vascular plants affect Earth’s long-term geological processes, contributing to carbon cycling, chemical weathering and soil formation. Plants transport elements from the soil to their above-ground structures, accumulating a range of macroelements including Na, K, Mg, Ca, Si, S, P and Cl. Wildfire combustion concentrates these macroelements into inorganic ash. This ash is dominated by oxides, carbonates, halides, sulfates and phosphates of Na, K, Mg and Ca. This work describes K₂Ca₂(CO₃)₃ which occurs abundantlyin the ash of the desert spoon (Dasylirion wheeleri), a plant native to the Sonoran Desert. Electron microprobe analysis, powder X-ray diffraction Rietveld refinement and Raman spectroscopy confirm that this phase matches synthetic rhombohedral (R3) K_2Ca2(CO3)3. This phase forms during the smouldering combustion of D. wheeleri trunks, producing friable, decimetre-sized, porous, ash lumps that pseudomorphically preserve the plant’s fibrous structure. This ash occurs as glassy, sintered, porous aggregates, dominated by K_2Ca2(CO3)3 with sylvite, calcite, fairchildite, arcanite and minor hydroxyapatite and periclase. Several double K-Ca carbonates form under surficial pressures and temperatures below ~800°C, including K_2Ca2(CO3)3 and bütschliite (K_2Ca(CO3)2) and its dimorph, fairchildite. The occurrence of rhombohedral K_2Ca2(CO3)3 and fairchildite are consistent with smouldering between 518 and 780°C. Upon exposure to water, K_2Ca2(CO3)3 rapidly decomposes, leaving calcite. The occurrence of K_2Ca2(CO3)3 as a major phase in the plant ash expands our understanding of Earth’s mineral diversity, provides new insights into the widespread geological process of wildfire ash formation and highlights the role that these fires play in forming mineral phases that are rare in other geological settings. Though K_2Ca2(CO3)3 was first identified in Dasylirion wheeleri, this phase probably forms in other fire-adapted plant species. The occurrence of K_2Ca2(CO3)3 in plant ash is an example of an inorganic phase that bridges the gap between biomineralisation and geological mineral formation.