Leptinito gneiss: The heritage stone of the old town, Rio de Janeiro, Brazil

This paper presents a significant heritage stone of Rio de Janeiro (Brazil), the Leptinito gneiss, the oldest masonry stone of the city, occurring close to the heart of the urban tissue. It is a Neoproterozoic high-grade metamorphic rock crystallized and deformed during the tectonic events of the Gondwana amalgamation. Light coloured, homogeneous and often dotted with garnet crystals, it was easier to work and carve than others that also occurred nearby. Colonial architectural remains, from the 17th century onwards, always have Leptinito gneiss door and window frames and cornerstones. Most of those buildings are part of Unesco's World Heritage of Rio de Janeiro. It was intensively quarried up to the first half of the 20th century. The environmental conditions of Rio, a tropical, humid coastal and polluted city, combined with the society's lack of knowledge about this significant building stone, have led to severe forms of degradation. Spalling, cracking, loss of mass and biological colonization are very frequent. In general, conservation practices regarding the Leptinito gneiss are based on pragmatical experience and have been unable to detain the progression of its decay. This paper aims to disseminate the geological characteristics of Leptinito gneiss at macro and micro-scale, its technological properties as a building material, the degradation patterns observed in many monuments and some insights into its behaviour against the external agents. Its sole outcrops stand on the Tijuca Nacional Park, and the only possibility to conserve the national heritage is to apply compatible conservation/restoration techniques and find compatible lithotypes. •Leptinito gneiss is the heritage stone of the oldest monuments in Rio de Janeiro, Brazil.•Rio's aggressive environment accelerates building stones degradation.•The main deterioration patterns are loss of mass, gypsum crusts, sub-efflorescences.•Science-based conservation strategy is needed to prevent cultural heritage loss.