Gabbro-Norite

Some of the text and one photo is from http://granites.uk/ More specifically: http://granites.uk/impala_black.html This rock, popularly known as Impala Black, is a coarsely crystalline igneous rock known as a Norite, a type of Gabbro which is part of the Precambrian Bushveld Igneous Complex intruded about 2 billion years ago. It is quarried in the Bosspruit region, Rustenburg, Northwest, South Africa. The minerals, ranging in colour from greenish to brown, are orthopyroxenes (either MgSiO3 - or FeSiO3). The colourless mineral is plagioclase feldspar ranging from sodic to calcic, viz. NaAlSi2O8 to CaAl2Si2O8
Gabbro: Any of several medium or coarse-grained intrusive igneous rocks that consist primarily of plagioclase feldspar and pyroxene. Essentially, gabbro is the intrusive (plutonic) equivalent of basalt, but whereas basalt is often remarkably homogeneous in mineralogy and composition, gabbros are exceedingly variable. Gabbros are found widely on the Earth and on the Moon as well.
Norite: A mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. It is essentially indistinguishable from gabbro without thin section study under the petrographic microscope. The principal difference between norite and gabbro, however, is the type of pyroxene of which it is composed; norite is predominately composed of orthopyroxenes, largely high magnesian enstatites, whereas the principal pyroxenes in gabbro are clinopyroxenes, generally medially iron-rich augites.
Above:Thin section photo, plane polarised light, of “Impala Black”. The minerals, ranging in colour from greenish to brown, are orthopyroxenes (either MgSiO3 - or FeSiO3). The colourless mineral is plagioclase feldspar ranging from sodic to calcic, viz. NaAlSi2O8 to CaAl2Si2O8
The same photo as above, but under crossed polars. The orthopyroxenes show bright interference colours, e.g. red and blue while the feldspars show grey to white interference colours. The minerals which appear to be opaque are orientated to the “extinction” position.
	Polished slab of Impala Black. Lighter coloured minerals are feldspars, dark colours are mainly orthopyroxenes. Back to Thumbnails