Plagioklassfeldspaadid

Plagioklassfeldspaadid are a group of tectosilicate minerals in the feldspar family. They form a solid solution series between the end-members albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). This means that plagioclase crystals can have any composition between pure albite and pure anorthite, with sodium and calcium substituting for each other in the crystal structure. The relative proportions of sodium and calcium determine the specific name of the plagioclase mineral. For example, a plagioclase with a high proportion of sodium is called albite, while one with a high proportion of calcium is called anorthite. Intermediate compositions have names like andesine, labradorite, and bytownite.

Plagioclase feldspars are very common in igneous rocks, particularly in basalt, andesite, and granite. They are also found in some metamorphic rocks and are a significant component of many sediments and soils. Their presence and composition are important indicators of the conditions under which rocks formed. Plagioclase typically appears as white or gray crystals, though some varieties can show iridescence. They have a Mohs hardness of 6 to 6.5 and exhibit two cleavage planes that are nearly at right angles to each other. The solid solution series of plagioclase feldspars is a fundamental concept in petrology, the study of rocks.