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Summary Article: Gabbro
from Guide to Minerals, Rocks and Fossils

Colour Grey, dark grey, black; may have a bluish or greenish tone. Colour index 30 to 90; with a decrease of coloured minerals gabbro grades into anorthosite, and with an increase it grades into pyroxenite and peridotite. Grain size Coarse; can be pegmatitic. Texture Granular; porphyritic texture rare. Ophitic texture is common. Structure Layering, defined by alternating layers of light and dark coloured minerals, often occurs. Individual layers vary from several metres to a centimetre or two in thickness. Often contain pegmatitic veins or segregations. Mineralogy Essentially plagioclase (labradorite or bytownite) and pyroxene; quartz (quartz gabbro), olivine (olivine gabbro) or hornblende may occur, and iron oxides, chromite and serpentine are common accessories. Field relations Stocks, sills and dykes. Individual intrusions may be of considerable size (a few kilometres is usual). Rare, very large sheet-like intrusions, called lopoliths, have diameters of hundreds of kilometres, but within these intrusions other rocks such as pyroxenite and anorthosite are also important. Layering is common and field observation indicates that the layers are often arranged like a stack of saucers. If the intrusion has been affected by earth movements such as folding or faulting the layering may be steeply inclined.

© 2003 Philip's

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