Respuesta :
Almost all rocks are minerals, but different rocks contain different mixtures of minerals. There are three main groups of rocks: sedimentary, igneous and metamorphic rocks.
Members of the plagioclase group are the most common rock-forming minerals. They are important for most of the dominant minerals in the igneous rocks of the earth's crust. They are the main components of many intrusive and extrusive igneous rocks, including granite, diorite, gabbro, rhyolite, andesite, and basalt. Rocks that contain many dark silicate materials and plagioclase in feldspar are basaltic in composition. Basalt rock contains minerals of the pyroxene group, which give the minerals their dark color.
It can be as light as dark green or as dark as black. Plagioclase (feldspar) is widespread and common in many igneous and metamorphic rocks, and to a much lesser extent in some immature sedimentary rocks. In general, the keys to identifying plagioclase are its absence (color), low birefringence, and twinning.
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