Gabbro (Gb)
Gabbro (Gb) is a igneous rock of plutonic depth class, the main rock-forming minerals of which are the main plagioclase (35-65%) and monoclinic pyroxene (35-65%). In secondary and accessory quantities, gabbro may also include olivine, hornblende, biotite, K-Na feldspar, apatite, sphen, ilmenite, magnetite, chromite, pyrite, pyrrhotite, chalcopyrite, pentlandite.
The name gabbro comes from the Italian “gabbro” – a place in northern Italy. The appearance of this rock significantly depends on the degree of development of low-temperature secondary changes. Fresh unchanged gabbro is a dark gray almost black rock with a clear crystalline medium-grained to coarse-grained structure. Since both plagioclase and pyroxene in unchanged gabbro have almost the same dark color, they can be distinguished from each other by translucency in a thin fracture.
On polished surfaces, plagioclase translucent to the depth, and pyroxene – no. With the development of secondary changes, plagioclase fades and brightens, becoming light gray, greenish-gray or yellowish-green color. Pyroxene remains dark and therefore differs well against plagioclase.
The texture of Gabbro (Gb) can be both massive and oriented, layered or spotted. Homogeneous uniformly-medium-grained varieties of gabbro with the darkest black-gray and black color are most valued as decorative stones.
Despite the fact that in the market of decorative stone gabbro is often incorrectly called “black granite”, these rocks have a number of fundamental differences in physical and mechanical properties. These differences are due to the different mineral composition of gabbro and granite. First of all, gabbro has a much higher content of mafic minerals and does not contain quartz. The hardness of the main rock-forming minerals of gabbro is more sustained – 6 on the Mohs scale. Neither plagioclase nor pyroxene are scraped with an ordinary knife and needle. For the processing of gabbro, as in the case of granite, use carbide or diamond tools and abrasives. However, due to the lack of gabbro quartz, the wear of these tools will be slower and abrasive costs lower. On the other hand, the abrasion index of gabbro is higher (0.1-0.7 g / m2) compared to granite.
Ukraine is famous for its significant reserves of gabbro. Gabroid massifs were found in the North-Western, Ingul and Priazovsky districts of the Ukrainian Shield. They are developed by many quarries in Zhytomyr, Cherkasy, Kirovohrad and Donetsk regions. The most famous are the Slipchytske, Bukynske and Torchynske gabbro deposits. In addition to Ukraine, ornamental gabbro is mined in China, India, the United States, South Africa and Zimbabwe