Felsite (or felstone) contains a very high 68-75% SiO2 depending on the amount of quartz in it. It is a very fine grained light colored felsic rock, usually white through light gray, or reddish to tan. The mass of the rock consists of a matrix of particularly quartz, sodium and potassium feldspar, often with phenocrysts of quartz. This rock is typically of extrusive origin, formed by compaction of fine volcanic ash, and may be found in association with obsidian and rhyolite. Image: Dendritic pyrolusite formations on felsite, from Elba, Italy. (Photograph taken at the Natural History Museum, London by Aram Dulyan)
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