pla·​gio·​clase ˈplā-j(ē-)ə-ˌklās How to pronounce plagioclase (audio)
: a triclinic feldspar
especially : one having calcium or sodium in its composition

Examples of plagioclase in a Sentence

Recent Examples on the Web The Alaska Volcano Observatory has posted some great images of the the ash (see above) collected from the Veniaminof eruption, including some that show the glassy shards with blades of plagioclase (microlites) in the volcanic glass. Erik Klemetti, Discover Magazine, 23 Sep. 2013 Changing chemical composition of the magma can betray the rapid crystallization of the mineral plagioclase feldspar. Erik Klemetti, Discover Magazine, 14 Nov. 2018 Maaz, which is on top and thus probably younger, has a composition similar to most basaltic lava flows — full of minerals known as pyroxene and plagioclase but with little or no olivine. New York Times, 27 Apr. 2022 Basaltic lava has a low silica content and is composed predominantly of plagioclase, amphibole, and pyroxene. David Bressan, Forbes, 18 May 2021 The most common lava rock found at Mount St. Helens is indeed Dacite, composed predominantly of plagioclase feldspar and quartz (pure silicon-dioxide). David Bressan, Forbes, 18 May 2021 Here, partially melting of older voclanic and sedimenatyr rocks, together with water, forms a more differentiated rock type, composed of plagioclase, amphiboles, and micas; sometimes with minor amounts of pyroxenes and quartz. David Bressan, Forbes, 10 Mar. 2021 Basalt is a volcanic rock composed of augite and sometimes plagioclase and magnetite, which says nothing. Louise Erdrich, The New Yorker, 2 Sep. 2019 It’s mostly made of plagioclase, a rock formed out of molten magma. Brian Resnick, Vox, 15 July 2019

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'plagioclase.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


Greek plagios oblique + klasis breaking, from klan to break — more at clast

First Known Use

circa 1868, in the meaning defined above

Time Traveler
The first known use of plagioclase was circa 1868

Dictionary Entries Near plagioclase

Cite this Entry

“Plagioclase.” Dictionary, Merriam-Webster, Accessed 23 May. 2024.

More from Merriam-Webster on plagioclase

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!