Dacite
Dacite is a felsic extrusive rock, intermediate in composition between andesite and rhyolite. It is often found associated with andesite, and forms lava flows, dikes, and, in some cases, massive intrusions in the centres of old volcanoes. Dacite is the volcanic equivalent of granodiorite.
Colour - variable, but generally bluish-grey or pale grey.
Texture - generally porphyritic.
Mineral content - groundmass generally of plagioclase with amphibole ( hornblende), biotite, pyroxene ( augite), quartz, and glass; phenocrysts of plagioclase, amphibole and often quartz.
Silica (SiO 2) content - 63%-69%.
Uses - can be used as aggregate, fill etc. in the construction and roading industries (often not ideal for concrete aggregate because of high silica content).
New Zealand occurrences - Sporadic occurrences in Northland, Western Bay of Plenty (Kaimai Range), Rotorua-Taupo area (Taupo Volcanic Zone).