Gypsum, mirabilite, and thenardite efflorescences of tuff stone in the underground environment

Abstract

This study deals with stone deterioration in underground cultural heritage and geoheritage, correlating the origin, compositional features, and space–time changes of salt weathering with the rock properties (petrography and geochemistry) and environmental setting (microclimate and chemistry of rainwater and groundwater). The material of study is Oya-ishi, one of the best-known building stones of Japan; it is a porous and soft acid tuff with a long tradition of use in historical and modern architecture. A number of underground sites of cultural or commercial significance was investigated in central Japan, where Oya stone appears extensively affected by efflorescence growth. The efflorescences are composed of mixed sulfates, including gypsum, mirabilite, and thenardite, whose development depends on the stone–environment interaction. The underground environment is extremely humid and water is the main driving force of salt weathering, having a twofold function. First, it conveys the alteration of the rock components, in particular pyrite, glass (pumice, fiamme, and shards), zeolites (clinoptilolite), clay minerals (smectites), and feldspars, which release SO42−, Na+, and Ca2+ ions. Second, water controls the salt stability and cycles of crystallization/deliquescence and hydration/dehydration, depending on the site depth and seasonal microclimate fluctuations; in this regard, knowing the specific thermodynamic behavior of the phases involved can help predicting where and when salt weathering and the resulting stone damage are most severe.