Gabbro
Gabbro is a dense, mafic intrusive rock. It generally occurs as batholiths and laccoliths and is often found along mid-ocean ridges or in ancient mountains composed of compressed and uplifted oceanic crust. Gabbro is the plutonic equivalent of basalt.
gabbro |
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Colour - dark grey to black.
Texture - phaneritic (medium to coarse grained).
Mineral content - predominantly plagioclase and pyroxene ( augite) with lesser olivine.
Silica (SiO 2) content - 45%-52%.
Uses - as aggregate, fill etc. in the construction and roading industries; cut and polished for dimension stone (called black granite) for building facings, foyers etc.
New Zealand occurrences - Northland, sporadic occurrences in the northwest Nelson area, Marlborough and Southland.