A heritage stone of Rio de Janeiro (Brazil): The Facoidal gneiss

Abstract

This paper presents the Facoidal gneiss, the most significant and characteristic natural and heritage stone of Rio de Janeiro, Brazil, also known as 'the most carioca of rocks.' The metropolis that is a World Heritage Cultural Landscape was built on, over, around, and with local stones, mostly Neoproterozoic gneisses. Among them, Facoidal gneiss occurs both sides of the Guanabara Bay, shaping its natural monuments as the Corcovado and the Sugar Loaf. It is composed of K-feldspar megacrysts usually deformed, looking like eyes (augen), arranged in a quartz-feldspar and biotite rich matrix. Its colour varies from pink to greyish. This stone was substantially used as a building material in many heritage monuments and buildings, in frames of windows and doors in houses, in sidewalks and public infrastructures. The aggressive environment of Rio de Janeiro, a tropical coastal megalopolis, is reflected in decay patterns such as loss of matrix and spalling. There are no quarries in production as this rock outcrops stand inside nature conservation or urbanized areas. The current impossibility of extracting replacement material justifies calling attention to this stone for the better conservation of Rio de Janeiro's built heritage.