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sillar A local Peruvian name for ignimbrites that have been pervasively altered by vapour-phase crystallization. The alteration of original pumice and glass shards involves deposition of tridymite, cristobalite, and alkali feldspar that occurs as drusy infills of the matrix and pumice cavities in the upper parts of ignimbrite cooling units. The gas is derived by diffusion from juvenile glassy fragments within the flow and from heated groundwater percolating through the flow. The resulting material makes excellent, lightweight, easily worked building stone, usually white. The city of Arequipa, southern Peru, is renowned for its buildings of sillar.