Medium or coarse grained rocks, gabbros Dark green pyroxene in principle (augite and smaller orthopyroxene amounts plus white or green colored plagioclase and black, millimeter sized grains of magnetite and / or ilmenite. There is a gabbro Intermediate or low silica content and rarely contains quartz. Gabbro is actually intrusive basalt equivalent, but unlike basalt gabbro it has a variable variable mineral content. Generally stratification of light and dark minerals (layered gabbro), significant amounts of olivine (olivine gabbro) or high percentage of coarse crystals of plagioclase feldspar (löcogabbro).
Group – Plutonic Igneous Rock
Colour – Dark grey to black.
Texture – phaneritic (medium to coarse grained).
Mineral content – predominantly plagioclaseand pyroxene ( augite) with lesser olivine.
Silica (SiO 2) content – 45%-52%.
Composition and Classification
The most important minerals in the gabbron are plagioclase and pyroxene. plagioclase often appear more. It is a small amount of olivine and amphibole. Plagioclase is sodium-calcium feldspar. Gabrobro contains more calcium than sodium. If there is more sodium in the plagioclase, then the rock type is called diorite. Gabbro is greenish and dark.
Diorite, gabbro, anorthosite — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anorthosite (M < 10%), diorite (M > 10%, plagioclase An0
– An50), gabbro (M > 10%, plagioclase An50 –An100). Gabbros may be further subdivided. Either of the two synonymous terms dolerite or diabase may be used for medium-grained gabbros rather than the term microgabbro, if required.
Gabbroic rocks— the gabbros (sensu lato) of QAPF field 10, may be further subdivided according to the relative abundances of their orthopyroxene, clinopyroxene, olivine and hornblende as shown in Fig. Some of the special terms used are:
Gabbro (sensu stricto) = plagioclase and clinopyroxene
Norite = plagioclase and orthopyroxene
Troctolite = plagioclase and olivine
Gabbronorite = plagioclase with almost equal amounts of clinopyroxene and orthopyroxene
Orthopyroxene gabbro = plagioclase and clinopyroxene with minor amounts of orthopyroxene
Clinopyroxene norite = plagioclase and orthopyroxene with minor amounts of clinopyroxene
Hornblende gabbro = plagioclase and hornblende with pyroxene < 5%.
Gabbro and Basalt Difference
The rocks are very common in the world. The reason why they are so widespread is that they are formed of magma or lava, which are cooled directly as they approach the surface of the earth. Although both are magmatic rocks with almost the same composition, the main difference between these two rocks is the formation process or the cooling rate of the liquid rock that they initiate.
Gabbro occurs when the liquid rock is cooled very slowly over a long period of time in the underground. This is an intrusive magmatic rock and is close to the very hot core of the world, which is why it takes longer to cool, and therefore the rock is visibly very different from the opposite of the basalt. The crystals are very large and clear to the naked eye and the texture is very coarse. This coarse grained texture can be labeled porphyric or a mixture of large and finer grained crystals, which are very large crystals. The time required for the liquid rock to cool will determine the texture.
On the other hand, the basalt is an extrusive magmatic rock. The surface is much closer to the surface of the earth and therefore the cooling process is much faster than the gabbone. The fact that the cooling rate is much higher gives the basal what it calls aphanitic texture, or it is so finely grained that it is not seen by the helpless human eye.
Uses of Gabbro
Gabbro can be polished to a bright black glow. Bright polished gabbro cemetery signs are used in kitchen stalls, floor tiles, facade stone and other size stone products. It is a highly desirable rock based on weather and wear. In the stone industry size gabbro is sold under the name “black granite”. Gabbro is also used to make a large number of rough cut products, such as borders, stones, paving stones and other products. The most common use of gabbro is like crushed stone or aggregate. The crushed gabbro is used as a basic material in construction projects, as a crushed stone for road construction, as a railway ballast and as a filler where a resistant crushed stone is needed.
- Flexiblelearning.auckland.ac.nz. (2019). Geology – rocks and minerals. [online] Available at: https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/gabbro.html [Accessed 2 Mar. 2019].
- Sandatlas.org. (2019). Gabbro – Igneous rocks. [online] Available at: http://www.sandatlas.org/gabbro/ [Accessed 2 Mar. 2019].
- Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
- Ronald Louis Bonewitz, (2012) NATURE GUIDE AND MINERALS, Smithsonian NATURE GUIDE, LONDON, NEW YORK, MELBOURNE, MUNICH, AND DELHI