Ask the ChatbotGames & QuizzesHistory & SocietyScience & TechBiographiesAnimals & NatureGeography & TravelArts & CultureProConMoneyVideos
plagioclase
Table of Contents
Introduction References & Edit History Related Topics
Images
plagioclase Moon rock crystals
Quizzes
Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
(Bed) Rocks and (Flint) Stones
Discover
earthquake. Heavily damaged school in the town of Yingxiu after a major earthquake struck China's Sichuan Province on May 12, 2008.
The 6 Deadliest Earthquakes Since 1950
The Great Depression Unemployed men queued outside a soup kitchen opened in Chicago by Al Capone The storefront sign reads 'Free Soup
5 of the World’s Most Devastating Financial Crises
The Lost Colony of Roanoke, North Carolina, where 115 people mysteriously disappeared c. 1590. John White discovers the word Croatoan carved onto a tree upon his return to the deserted Roanoke Colony in 1590
The Lost Colony of Roanoke
The Castillo, a Toltec-style pyramid, rises 79 feet (24 meters) above the plaza at Chichen Itza in Yucatan state, Mexico. The pyramid was built after invaders conquered the ancient Maya city in the tenth century.
What’s Inside the Pyramid at Chichén Itzá?
Clouds of smoke billow up from controlled burns taking place in the Gulf of Mexico May 19, 2010. The controlled burns were set to reduce the amount of oil in the water following the Deepwater Horizon oil spill. BP spill
The Perils of Industry: 10 Notable Accidents and Catastrophes
Close-up of crocodile eye; location unknown (rainforest, reptile).
What’s the Difference Between Alligators and Crocodiles?
The Colosseum, Rome, Italy. Giant amphitheatre built in Rome under the Flavian emperors. (ancient architecture; architectural ruins)
New Seven Wonders of the World
plagioclase
plagioclase

plagioclase

mineral
Written and fact-checked by
The Editors of Encyclopaedia Britannica
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. They write new content and verify and edit content received from contributors.
The Editors of Encyclopaedia Britannica
Article History

plagioclase, any member of the series of abundant feldspar minerals usually occurring as light-coloured, glassy, transparent to translucent, brittle crystals. Plagioclase is a mixture of albite (Ab), or sodium aluminosilicate (NaAlSi3O8), and anorthite (An), or calcium aluminosilicate (CaAl2Si2O8); the two intermingle and form a continuous chemical range (called a solid-solution series) between albite and anorthite (the end-members of the series). Intermediate members are homogeneous mixtures of the pure end-members.

Moon rock crystals
Moon rock crystalsA scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission.

Most of the crystals of the members of this series exhibit microscopic, repeated lamellar albite twinning; this produces the fine parallel striations observed on some surfaces. The twinning sometimes causes an iridescence, usually blue or green, that may arise either from reflection or diffraction at the edges of the lamellae, or from diffusion by adjoining areas with different optical properties.

Plagioclase is used in the manufacture of glass and ceramics; the iridescent varieties peristerite and labradorite are valued as gemstones or ornamental material. The primary importance of plagioclase, however, derives from its role in rock formation. Oligoclase, the most common plagioclase, occurs in granite, diorite, and other felsic igneous rocks and in some metamorphic rocks; notable occurrences are at Aust-Agder, Norway, and Fine, New York, United States. Andesine, less common, occurs in many granular and volcanic rocks with intermediate silica content, as in Marmato, Colombia, and Bodenmais, Bavaria, Germany. The rarest plagioclase is bytownite, which occurs in basic igneous rocks and in stony meteorites.

Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
Britannica Quiz
(Bed) Rocks and (Flint) Stones
The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn.
Britannica Chatbot logo

Britannica Chatbot

Chatbot answers are created from Britannica articles using AI. This is a beta feature. AI answers may contain errors. Please verify important information using Britannica articles. About Britannica AI.
feldspar
Table of Contents
References & Edit History Related Topics
Images
Figure 1: Schematic diagram showing ordered (left) and disordered (right) arrays within a structure having two kinds of sites (type 1 and type 2) and two types of occupants (x atoms and y atoms). In the ordered structure all x atoms are distributed uniformly in the spaces between the y atoms, whereas in the disordered structure no regular arrangement obtains. Figure 2: Twinning in feldspars. alkalic syenite granite Figure 42: Granitic rock consisting of reddish orange feldspar, nearly white quartz, and green epidote that appears to have been introduced into an original quartz-feldspar granite. This rock, sometimes called unakite, is used widely in jewelry. Bowen's reaction series
For Students
feldspar summary
Quizzes
Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
(Bed) Rocks and (Flint) Stones
Discover
Orange basketball on black background and with low key lighting. Homepage 2010, arts and entertainment, history and society
The 10 Greatest Basketball Players of All Time
VAN, TURKEY - OCTOBER 25: A building and car ruined during the earthquake of Van-Ercis on October 25, 2011 in Van, Turkey. It is 604 killed and 4152 injured in Van-Ercis Earthquake.
What Is an Aftershock?
Clouds of smoke billow up from controlled burns taking place in the Gulf of Mexico May 19, 2010. The controlled burns were set to reduce the amount of oil in the water following the Deepwater Horizon oil spill. BP spill
The Perils of Industry: 10 Notable Accidents and Catastrophes
Close-up of crocodile eye; location unknown (rainforest, reptile).
What’s the Difference Between Alligators and Crocodiles?
Charles Darwin, carbon print photograph by Julia Margaret Cameron, 1868.
What Darwin Got Right (and Wrong) About Evolution
African Americans demonstrating for voting rights in front of the White House as police and others watch, March 12, 1965. One sign reads, "We demand the right to vote everywhere." Voting Rights Act, civil rights.
Timeline of the American Civil Rights Movement
The Castillo, a Toltec-style pyramid, rises 79 feet (24 meters) above the plaza at Chichen Itza in Yucatan state, Mexico. The pyramid was built after invaders conquered the ancient Maya city in the tenth century.
What’s Inside the Pyramid at Chichén Itzá?

feldspar

mineral
Written by
R.V. Dietrich
Emeritus Professor of Geology, Central Michigan University, Mount Pleasant. Author of Stones; coauthor of Gems, Gravels and Granites.
R.V. Dietrich
Fact-checked by
The Editors of Encyclopaedia Britannica
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. They write new content and verify and edit content received from contributors.
The Editors of Encyclopaedia Britannica
Last Updated: • Article History

feldspar, any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium. Feldspars make up more than half of Earth’s crust, and professional literature about them constitutes a large percentage of the literature of mineralogy.

Of the more than 3,000 known mineral species, less than 0.1 percent make up the bulk of Earth’s crust and mantle. These and an additional score of minerals serve as the basis for naming most of the rocks exposed on Earth’s surface.

Each of the common rock-forming minerals can be identified on the basis of its chemical composition and its crystal structure (i.e., the arrangement of its constituent atoms and ions). The nonopaque minerals can also be identified by their optical properties. Fairly expensive equipment and sophisticated procedures, however, are required for such determinations. Therefore, it is fortunate that macroscopic examination, along with one or more tests, are sufficient to identify these minerals as they occur in most rocks. The following descriptions include basic chemical and structural data and the properties used in macroscopically based identifications. Optical data, which are not included in these descriptions, are available in mineralogy books.

Two important rock-forming materials that are not minerals are major components of a few rocks. These are glass and macerals. Glass forms when magma (molten rock material) is quenched—i.e., cooled so rapidly that the constituent atoms do not have time to arrange themselves into the regular arrays characteristic of minerals. Natural glass is the major constituent of a few volcanic rocks—e.g., obsidian. Macerals are macerated bits of organic matter, primarily plant materials; one or more of the macerals are the chief original constituents of all the diverse coals and several other organic-rich rocks such as oil shales.

In the classification of igneous rocks of the International Union of Geological Sciences (IUGS), the feldspars are treated as two groups: the alkali feldspars and the plagioclase feldspars. The alkali feldspars include orthoclase, microcline, sanidine, anorthoclase, and the two-phase intermixtures called perthite. The plagioclase feldspars include members of the albite-anorthite solid-solution series. Strictly speaking, however, albite is an alkali feldspar as well as a plagioclase feldspar.

Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
Britannica Quiz
(Bed) Rocks and (Flint) Stones
Britannica Chatbot logo

Britannica Chatbot

Chatbot answers are created from Britannica articles using AI. This is a beta feature. AI answers may contain errors. Please verify important information using Britannica articles. About Britannica AI.