(of rocks) containing iron and magnesium, with little or no silica.
Also, ultramafic.

1890–95; ultra- + basic

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World English Dictionary
ultrabasic (ˌʌltrəˈbeɪsɪk)
(of such igneous rocks as peridotite) containing less than 45 per cent silica

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American Heritage
Science Dictionary
ultrabasic   (ŭl'trə-bā'sĭk)  Pronunciation Key 
Containing magnesium and iron and only a very small amount of silica. Used of igneous rock, and often used interchangeably with ultramafic. Peridotite is an ultrabasic rock.
The American Heritage® Science Dictionary
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