serpentinitePronunciation: /ˈsəːp(ə)ntɪˌnʌɪt/Definition of serpentinite noun [mass noun] Geology a dark, typically greenish metamorphic rock, consisting largely of serpentine or related minerals, formed when mafic igneous rocks are altered by water. Origin: 1930s: from serpentine + -ite1serpentinite in other Oxford dictionaries Definition of serpentinite in the US English dictionary