On cooling it will begin crystallizing plagioclase with 85 percent An (point P on the solidus) at the liquidus temperature of about 1,470 °C. Starting with estimates from published literature it becomes obvious that at pressure, fayalite becomes important because the pressure effects on the melting temperatures are very different: large for albite, and small for anorthite. It represents a plagioclase with less than 10% anorthite content. Black Friday Sale! Consider a liquid of composition L (60 percent An + 40 percent Ab) which is at an initial temperature of 1,500 °C. As demonstrated above, complete reactions between early-formed minerals and the liquid depletes the supply of the liquid, thereby curtailing the progression down the series. 3c–f). The other end member, Anorthite, contains calcium and no sodium. The intermediary members are Oligoclase, Andesine, Labradorite, and Bytownite. These two examples illustrate two principal reactions that occur during crystallization of common magmas, one discontinuous (the olivine-liquid-pyroxene reaction) and the other continuous (the plagioclase-liquid reaction). This article was most recently revised and updated by, https://www.britannica.com/science/anorthite, The Mineral and Gemstone Kingdom - Anorthite, Amethyst Galleries - The Mineral Anorthite. High Albite: Stable above 800°C - usually formed in hydrothermal syntheses, has disordered Al-Si arrangement (in contrast to 'low albite'). At 1 atmosphere, anorthite melts at 1553 °C. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. A series between Albite and Anorthite Feldspar Group. Albite is one end member, containing sodium and no calcium. ; Tamil Nadu, India; Miyake, Japan; and Franklin, N.J. For detailed physical properties, see feldspar (table). A plagioclase feldspar with an albite : anorthite molar ratio ranging from 50 : 50 to 70 : 30. Any member of this series is called a plagioclase. The feldspar minerals are mixtures of sodium, potassium, and calcium aluminosilicates; they belong to a ternary system in which any mineral may be classed by its percentage of each of three pure compounds, called end-members: sodium aluminosilicate, potassium aluminosilicate, and calcium aluminosilicate. Starting with estimates from published literature it becomes obvious that at pressure, fayalite becomes important because the pressure effects on the melting temperatures are very different: large for albite, and small for anorthite. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. A basaltic liquid at the top of the Y can descend to the bottom of the series to crystallize quartz only if the earlier reactions are prevented. Our editors will review what you’ve submitted and determine whether to revise the article. Anorthite and the sodium-bearing end-member of the system, albite (NaAlSi3O8), form a solid-solution series in which the two intermingle; thus, in nature there is a continuous chemical variation between the two compounds. The phases that are crystallized first are the common minerals that compose basalt or gabbro, like bytownite or labradorite with pyroxene and minor amounts of olivine. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. It is a common constituent in felsic rocks. Now consider the case in which the liquid is prevented from reacting with the early-formed plagioclase. You can find the explanation on the plagioclase feldspar page. These include olivine, pyroxene, amphibole, biotite, and plagioclase feldspars.