The Volyn pegmatites from Volodarsk-Volynskyi in the Zhytomyr Oblast, NW Ukraine, are associated with granites genetically related to the Paleoproterozoic Korosten pluton. Their late-stage evolution is characterized by the formation of opal-cemented breccia. A polymineralic pseudomorph after beryl within the breccia includes bertrandite (±euclase) + Fmuscovite (with tobelite component) + buddingtonite + OM (organic matter) + opal (+ traces of K-feldspar, albite, columbite, FeS 2 , barite, REE-minerals). Sector zoned and platy to fibrous buddingtonite has variable (K+Na)-vs. NH 4-contents (electron microprobe analyses) and some H 2 O or H 3 O + , as indicated by microscope infrared spectroscopy. We suggest that ammonium was produced by decay of OM, which is partly preserved in the pseudomorph. Energy-dispersive electron microprobe data of the OM show with increasing O-decreasing C-N-content due to degassing; the OM contains the high-field strength elements Zr (≤7 atom%), Y (≤3 atom%), Sc (≤0.8 atom%), REE (≤0.3 atom%), Th (≤0.2 atom%) and U (≤1.25 atom%), which increase with increasing O-content. Transmission electron microscopy of the OM confirms the presence of N; Zr, Si, and O (with other HFSE) are concentrated in nanometersized areas and at the transition from OM to opal in nanometer-sized platy Zr-Si-O crystals. Crich areas are amorphous but show poorly developed lattice fringes. OM is present in the pseudomorph is available, textural arguments and phase equilibria indicate its formation in a late stage of the pegmatite evolution, at P-T-conditions below ~100 MPa/150°C. We favor a conceptual model for the formation of the Volyn buddingtonite in analogy to Phanerozoic occurrences of buddingtonite, where over and around the shallow anorthosite-granite Korosten pluton hydrothermal convection cells introduced N-bearing hydrocarbons and its precursors into the cooling igneous rocks. Due to the elevated temperature, the OM disintegrated into degassing volatile and non-volatile residual components analogous to petroleum maturation. Organic N, released as NH 4 , was then incorporated into buddingtonite.