The two most common silicate minerals are orthoclase and plagioclase. Compared to orthoclase, plagioclase is more abundant in the Earth's crust. Plagioclase is more commonly found in igneous and metamorphic rocks while orthoclase is more common in sedimentary rocks and sand due to its capability of withstanding weather changes.
Both orthoclase and plagioclase are feldspars, a part of the silicate family. The former has a monoclinic shape, while the latter has a triclinic shape. The color of orthoclase ranges from cream to pink, while plagioclase's color ranges from white to gray. Both are pearly with a whitish streak. The best way to differentiate between the two is to look at the crystal surface. If it has striations or thin parallel grooves, it is plagioclase.