Plagioclase feldspars are aluminum silicate minerals typically found in igneous environments, metamorphic rocks, and less commonly in sedimentary rocks. Plagioclase is one of two feldspar mineral groups; the other being potassium feldspar. These two mineral groups differ in that K-feldspar consists of three mineral polymorphs that all share the same potassium ion-bearing chemistry, while plagioclase is subdivided into six classifications with chemical compositions varying in calcium and sodium content. Bowen's Reaction Series explains that with increasing magma cooling and crystallization, plagioclase feldspars evolves from being Ca-rich in composition to more Na-rich, making these six comprise of: anorthite, bytownite, labradorite, andesine, oligoclase, and albite; based on the relative abundance either anorthite (An, Ca-rich end member) or albite (Ab, Na-rich end member). This sample is an anorthite plagioclase, but this level of classification requires further chemical testing and can typically be simply referred to as plagioclase. Although its chemical composition can vary, plagioclase still exhibits fundamental feldspar properties, such as a blocky crystal habit, a hardness of 6-6.5 (greater than glass), a chalky to dull to porcelaneous lustre, and two well-developed cleavage planes meeting at ~90�, as indicated in A-1' and A-2'. Twinning is also used as a diagnostic property of plagioclase feldspars, especially in differentiating it apart from K-feldspar. K-feldspars exhibit simple Carlsbad twinning which can be best observed in thin section, but its diagnostic physical property is its perthitic texture (see Potassium Feldspar samples), whereas both polysynthetic and simple twinning can occur alongside one another in plagioclase and can be observed in both hand sample and thin section. As seen in A-3 to A-6, these twinning features occur as fine parallel striations on rock surfaces that can reflect light at slight angles, revealing prominent alternating light and dark bands of albite twinning.