Carbonate polymorph evolution in historic lime–clay geomaterials from Lamayuru Monastery, Ladakh: evidence from a cold-arid Himalayan environment

Abstract

Lime–clay plasters exposed to cold–arid Himalayan environments experience low temperatures, pronounced thermal variability, intermittent moisture availability, freeze–thaw cycling, and reduced atmospheric CO₂ partial pressure, conditions capable of influencing carbonate phase evolution in historic lime-based materials. This study aims to characterize the mineralogical and microstructural composition of historic lime–clay plasters from Lamayuru Monastery (Ladakh, India), with particular emphasis on the occurrence of CaCO₃ polymorphs and their variability within the plaster matrix, using integrated optical, mineralogical, thermal, and microstructural characterization. Optical microscopy reveals a stratified plaster system comprising distinct lime–clay and clay-rich layers with preserved interfacial continuity. X-ray diffraction and FTIR analyses identify coexistence of calcite, aragonite, and vaterite, indicating persistence of multiple CaCO₃ polymorphs within the plaster matrix. Variability in relative peak prominence among samples suggests heterogeneous carbonate distribution associated with compositionally variable microenvironments. Thermogravimetric analysis identifies a principal carbonate decomposition event near ~ 710 °C, lower than temperatures commonly reported for well-crystallised geological calcite, consistent with heterogeneous carbonate-bearing domains within clay-rich and organic-containing microenvironments. SEM–EDS observations demonstrate a heterogeneous lime–clay–silicate architecture comprising distributed carbonate phases, aluminosilicate materials, and pore-rich microstructural domains. The combined observations indicate that the Lamayuru plasters represent compositionally heterogeneous geomaterials shaped by interaction among carbonate phases, clay minerals, organic constituents, and prolonged environmental exposure. The study provides integrated archaeometric data on carbonate polymorphism and microstructural variability in historic Himalayan lime-based plasters and contributes to a broader understanding of environmentally exposed lime-based geomaterials from cold-region environments worldwide.