Etymology
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igneous (adj.)
1660s, "pertaining to or resembling fire," from Latin igneus "of fire, fiery; on fire; burning hot," figuratively "ardent, vehement," from ignis "fire, a fire," extended to "brightness, splendor, glow;" figuratively "rage, fury, passion," from PIE root *egni- "fire" (source also of Sanskrit agnih "fire, sacrificial fire," Old Church Slavonic ogni, Lithuanian ugnis "fire"). Geological meaning "produced by volcanic forces" is from 1791, originally in distinction from aqueous. Earlier in the sense "fiery" were ignean (1630s), ignic (1610s).
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igneo- 
word-forming element meaning "of fire; of fire and; of igneous origin," from Latin igneus (see igneous).
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ignivomous (adj.)
"vomiting fire," c. 1600, from Late Latin ignivomous, from Latin ignis "fire" (see igneous) + vomere "to vomit" (see vomit (n.)).
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ignis fatuus (n.)
"will o' the wisp, jack-o-lantern," 1560s, Medieval Latin, literally "foolish fire;" see igneous + fatuous. "It seems to have been formerly a common phenomenon; but is now exceedingly rare" [OED].
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ignite (v.)

1660s (trans.), "kindle or set on fire, cause to burn," from Latin ignitus, past participle of ignire "set on fire, make red hot," from ignis "fire" (see igneous). Attested earlier as an adjective (1550s). Intransitive sense of "catch fire, begin to burn" is from 1818. Related: Ignited; igniting.

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ignition (n.)

1610s, "act of heating to the point of combustion," from French ignition or directly from Medieval Latin ignitionem (nominative ignitio), noun of action from past-participle stem of Latin ignire "set on fire," from ignis "fire" (see igneous). Meaning "means of sparking a fire" (originally in a gun) is from 1881; meaning "means of sparking an internal combustion engine" is from 1906.

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pluton (n.)

in geology, "large, intrusive body of igneous rock formed beneath the earth's surface," 1936, Modern Latin, from the geological sense of plutonic (q.v.).

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traps (n.1)
"expanse of dark igneous rock," 1794, from Swedish trapp (Torbern Bergman, 1766), from trappa "stair," related to Middle Low German trappe "staircase" (see trap (n.)). So called from the step-like appearance of the rock.
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gabbro (n.)
type of igneous rock, 1823, introduced in geology 1809 by German geologist Christian Leopold von Buch (1774-1853), from Italian (Tuscan) gabbro, a word among the marble-workers, of obscure origin; perhaps from Latin glaber "bare, smooth, bald" (see glad). Related: Gabbroic.
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pyroxene (n.)

type of mineral, 1800; from Greek pyr "fire" (see pyro-) + xenos "stranger" (see xeno-). According to OED, so named in 1796 by Abbé Haüy, French mineralogist, "because he thought it 'a stranger in the domain of fire' or alien to igneous rocks." Related: Pyroxenic.

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