Over fifteen centuries of weathering: mineralogical identification and decay assessment of the Tāq-e Bostān Sasanian rock reliefs

Abstract Tāq-e Bostān in northwestern Iran is a monumental Sasanian complex carved into a karstic cliff that exhibits progressive deterioration—including detachment, cracking, delamination, and surface crusts—threatening both the legibility and durability of the reliefs. This study combines petrographic thin sections with XRF, XRD, and SEM–EDS analyses to characterize lithology and alteration products. The limestone is fine-grained and low-porosity, with microfractures, and secondary calcite infill that govern moisture movement and increase susceptibility to weathering. Analytical data show that twentieth-century gypsum–lime and cementitious restoration mortars have intensified decay by promoting calcium leaching, sulfate mobilization, and the formation of gypsum crusts and soil-rich accretions, whereas orange oxalate patinas represent stable biomineral films. These findings demonstrate the interaction of intrinsic stone properties, hydrological dynamics, and incompatible repairs, underscoring the need for minimal and evidence-based conservation, including removal of incompatible mortars, selective crust reduction, and use of stone-compatible materials.