plagioklassid

Plagioklassid are a group of feldspar minerals that form a solid solution series. This means that they have a variable chemical composition that can change gradually between two end-members. The plagioclase series ranges from albite, which is a sodium-rich feldspar, to anorthite, which is a calcium-rich feldspar. Their chemical formula can be generalized as (Na,Ca)(Al,Si)4O8, with the ratio of sodium to calcium varying along the series. Plagioclase feldspars are a major constituent of many igneous rocks, particularly those in the basalt and granite families. They are also found in metamorphic and some sedimentary rocks. The color of plagioclase can vary widely, from colorless and white to gray, and sometimes with subtle hues of pink or green, depending on impurities and their chemical composition. They typically exhibit cleavage in two directions, which are roughly at right angles to each other. A characteristic optical property of many plagioclases is the presence of lamellar twinning, visible under a microscope as parallel striations on crystal faces. The specific composition within the plagioclase series is often determined by geological context and the overall chemistry of the rock in which it is found.