Blatter, Dawnika Berkeley Geochronology Center, Berkeley, California.
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An igneous volcanic rock that is the extrusive equivalent of granodiorite, according to the IUGS (International Union of Geological Sciences) classification scheme. The overall color of dacite lava is generally light to medium gray but can appear pinkish or tan when oxidized or weathered. The color index of dacite averages 12–14%, denoting that mafic or dark minerals (hornblende, biotite, hypersthene, augite, titanomagnetite, and ilmenite) make up 12–14% of the rock. As an extrusive igneous rock, dacite is usually porphyritic in texture, meaning it contains 0–50% phenocrysts (visible crystals), in a fine-grained or glassy matrix called the groundmass (Fig. 1). The phenocryst minerals typically include some combination of plagioclase, hypersthene, augite, hornblende, biotite, quartz, potassium feldspar, titanomagnetite, and ilmenite, depending on chemical composition and equilibration conditions (pressure, temperature, and oxygen fugacity) of the magma prior to eruption. Dacite may also occur as aphyric (no visible crystals) lava, composed entirely of microcrystalline matrix minerals or glass (vitrophyric). See also: Granodiorite; Igneous rocks; Lava; Magma; Phenocryst
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