measured carefully, this also gives a useful composition estimate. Plagioclase almost universally shows a characteristic polysynthetic twinning that produces twinning striations on [010]. These striations allow plagioclase to be distinguished from alkali feldspar. Plagioclase often also displays Carlsbad, Baveno, and Manebach Law twinning. Plagioclase series members The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages. The distinction between these minerals cannot easily be made in the field. The composition can be roughly determined by specific gravity, but accurate measurement requires chemical or optical tests. The composition in a crushed grain mount can be obtained by the Tsuboi method, which yields an accurate measurement of the minimum refractive index that in turn gives an accurate composition. In thin section, the composition can be determined by either the Michel Lévy or Carlsbad-albite methods. The former relies on accurate measure of minimum index of refraction, while the latter relies on measuring the extinction angle under a polarizing microscope. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). Endmembers Anorthite was named by Gustav Rose in 1823 from Greek ('not') + ('straight'), literally 'oblique', referring to its triclinic crystallization. Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites. Albite is named from the Latin , in reference to its unusually pure white color. The name was first applied by Johan Gottlieb Gahn and Jöns Jacob Berzelius in 1815. It is a relatively common and important rock-making mineral associated with the more silica-rich rock types, in hydrothermal veins, with greenschist facies metamorphic rocks, and in pegmatite dikes, often as the variety cleavelandite and associated with rarer minerals like tourmaline and beryl. Intermediate members The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member (albite 2.62) increases 0.02 per 10% increase in anorthite (2.75). Bytownite, named after the former name for Ottawa, Ontario, Canada—Bytown— is a rare mineral occasionally found in more basic rocks. Labradorite is the characteristic feldspar of the more basic rock types such as gabbro or basalt. Labradorite frequently shows an