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Containing magnesium and iron and only a very small amount of silica. Used of igneous rock.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(Geological Science) (of such igneous rocks as peridotite) containing less than 45 per cent silica
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(ˌʌl trəˈbeɪ sɪk)

(of rocks) containing iron and magnesium, with little or no silica.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
References in periodicals archive ?
The vegetation of siliceous and ultramafic rock crevices at low altitudes of the temperate and boreal Europe has been in the past mostly classified within the Androsacetalia vandellii (e.g.
Geological formations which are observed in the study area include (Figure 2) schists, limestones, volcano-sedimentary formations, chromiferous iron ore occurrences, ultramafic rock masses, flysch, conglomerates, marly limestones with travertines, marly formations with lignite intercalations, and alluvial deposits [23-25].
The Yakuno ophiolite in the Oshima peninsula and Ayabe areas (Figure 1) consists of ultramafic rock of upper mantle origin and mafic rocks of OIA and BAB origins.
Hole NAD11-236 was drilled 100 metres down-dip of hole NAD11-223 and did not intersect significant widths of ultramafic rock or mineralization.
The test results over a large area of ultramafic rock at Sundsberget indicate that the area hosts a substantial tonnage of mineralised serpentinite.
Rubicon is also starting a 5,000-metre drill program on its wholly-owned McFinley project, targeting a four-kilometre-long gold-bearing ultramafic rock under Red Lake.
Drilling has intersected 66m grading 0.52% Ni within sulphide bearing ultramafic rock. This intercept includes 1.69% Ni over 1.5m.
Ultramafic rock is traversed by joints trending in different directions such as N30[degrees], N170[degrees] and N110[degrees].
The surface expression of the deposit is characterized by rubbley outcrops of fresh to slightly weathered metabasalt and more strongly weathered ultramafic rock. Weathering of the metabasalt is typically best developed along joint planes where it persists to depths of up to 30 meters.
The serpentinization of ultramafic rock is a common process in which the original rock changes its mineralogical composition owing to hydration of the original phases (O'HANLEY, 1996).