plagioclase
noun
pla·gio·clase ˈplā-j(ē-)ə-ˌklās 
ˈpla-,
-ˌklāz
: a triclinic feldspar
especially : one having calcium or sodium in its composition
Example Sentences
Recent Examples on the Web 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 Apollo 11 samples showed that the lunar highlands (bright, high-standing regions as opposed to the dark lunar maria in low-lying areas) contain high concentrations of the mineral plagioclase. Erica Jawin, Scientific American, 2 July 2019 Your average Hood plagioclase crystal that formed 21,000 years ago might have spent only 1-12% of its entire history, a few hundred to a few thousand years, in conditions that would have allowed eruption of the magma (think: hot). Erik Klemetti, WIRED, 17 Feb. 2014 See More
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 Apollo 11 samples showed that the lunar highlands (bright, high-standing regions as opposed to the dark lunar maria in low-lying areas) contain high concentrations of the mineral plagioclase. Erica Jawin, Scientific American, 2 July 2019 Your average Hood plagioclase crystal that formed 21,000 years ago might have spent only 1-12% of its entire history, a few hundred to a few thousand years, in conditions that would have allowed eruption of the magma (think: hot). Erik Klemetti, WIRED, 17 Feb. 2014 See More Word History
Etymology
Greek plagios oblique + klasis breaking, from klan to break — more at clast
First Known Use
circa 1868, in the meaning defined above