Decoding Yellow Tuff in Pompeian Architecture: key mineralogical and geochemical constraints for a (re-)evaluation of its geological origin

Tuff was one of the most widely used construction materials in central and southern Italy, valued since antiquity for its workability, lightness, and insulating properties, despite its intrinsic fragility and limited mechanical strength. In the Roman world, the selection of building materials depended on economic resources, craftsmanship, and structural requirements, as well as, in some cases, urgent rebuilding conditions, such as the post-earthquake reconstruction of Pompeii. Within this context, the recurrent use of yellow tuff in Pompeii raises important questions, foremost its geological attribution. The nearby Campi Flegrei volcanic field emplaced the two deposits most likely to be its sources: the Campanian Ignimbrite (CI, ~40 ka), to which it is attributed the Lithified Yellow Tuff (LYT), a welded and zeolite-rich facies, and the Neapolitan Yellow Tuff (NYT, ~15 ka), which generated the massive yellow tuff widely used in Naples. Distinguishing between these deposits is notoriously challenging due to their internal variability and partial overlap in mineralogy and geochemical composition. This distinction is, therefore, crucial for reconstructing provenance, chronology, and construction choices in Pompeian architecture, particularly during the intense reconstruction and restoration of buildings after the 62/63 CE earthquake. To address this issue, 49 yellow-tuff specimens were collected from 40 Pompeian contexts, focusing on key architectural elements and classifying them as pre- or post-seismic relative to the 62/63 CE earthquake. A multi-analytical approach was applied, combining textural and spectroscopic techniques, quantitative mineralogical analyses, and microanalysis of juvenile matrix glass, which provided the most diagnostic provenance constraints. The samples exhibit the typical mineralogical assemblage of Phlegraean trachytic tuffs, dominated by phillipsite and chabazite in varying proportions, together with analcime, albite, biotite, clinopyroxene, hematite, calcite, and gypsum in minor or trace amounts. Lower sanidine abundances are also observed, except for three sanidine-rich specimens correlated with the Welded Grey Ignimbrite (WGI) facies of the CI, a distinction also supported by spectroscopic data. Quantitative mineralogical analyses revealed variable amounts of amorphous material, likely related to low-ordered and/or amorphous phases such as juvenile clasts, whose composition ranges from tephriphonolite to latite and is essentially homogeneous across pre- and post-seismic contexts. The microanalysis of juvenile matrix glass proved particularly revealing, consistently falling within the compositional field of NYT Member B, clearly distinct from the more evolved silica–alkali-rich signatures typical of CI-derived WGI and LYT. This provides decisive provenance evidence: mineralogical, spectroscopic, and especially juvenile matrix glass data all converge on a predominantly NYT origin. These results refine long-standing hypotheses on material sourcing, construction practices, and post-seismic rebuilding in the ancient Pompeii.