Countertop Geology | 3 Popular Granite Countertops
Rock hounding and fossil hunting aren't really possible right now, and being indoors means staring at all the things that need to be fixed and dreaming of renovations. Top of everyone's list is a kitchen reno, and nothing completes a kitchen like a granite countertop! There are a huge number of countertop options and all tell a story. However, "granite" countertops are typically not granite in the geological sense (for an example check out the post on Black Marinace). "Granite", when used to define countertop material, refers to a hard stone that is polishable and requires harder tools to work it than are needed for marble (1). In geology, granite is a light coloured igneous rock that crystalizes underground from magma, and is made up of mainly quartz and feldspar and lesser amounts of mica and other minerals. The geological definition of granite is much more restrictive than that used for countertops. Countertop granite is mined around the world, with India (32%) and then Brazil (22%) being the two largest suppliers (1). The USA is third at 10% and Canada sits 8th at 4%. In India, most commercial granite is mined from the Dharwar Craton in southern India, and in Brazil most material comes from the Sao Francisco Craton in eastern Brazil (1). With that in mind, here is the geology of 3 very popular granite countertops in Canada.
Andromeda White Granite (AWG)
One of the most popular materials for kitchen countertops is Andromeda White Granite. This rock is sold just about everywhere and goes by many different names: Andromeda Granite, White Lanka Granite, Crystal Lanka Granite, White Galaxy Granite, and Colonial White Granite. It is super popular because of it’s light colour, and interesting patterns of gray and white with hints of red and black. But what is it exactly and where does it come from?
AWG is a granulite, meaning it is a high-grade metamorphic rock made up of small crystals of mainly quartz (translucent and gray) and feldspar (opaque and white) and potentially with small inclusions of garnet (red crystals) and biotite (black crystals). Muscovite mica (shiny white crystals) is also common. This rock forms under super high pressure and temperatures deep in the Earth, and so when it is exposed it is interesting to Geologists because it sheds light on metamorphic processes that occur deep in the Earth’s crust (2). The AWG used for countertops is from the PreCambrian. The original rock was formed about 1.95 billion years ago, and it was metamorphosed between 650 and 550 million years ago (4).
Andromeda White Granite is mined mainly in Sri Lanka near the capitol Colombo, but also in India (5). There are multiple quarries where the rock is mined, but unfortunately I could not find information on exact locations. Instead, the approximate location of most quarries is shown on the map above.
Black Galaxy Granite (BGG)
Black Galaxy Granite is another very popular choice for countertops because of it's consistent dark colour and sparkly appearance. The sparkles give the sense of the night sky, which is how it got its name. You can buy BGG under a variety of names and colours depending on the size of crystals and presence/absence of quartz veins (e.g. , Starbust Galaxy Granite; 6).
Black Galaxy Granite is mined mainly in Andhra Pradesh Province of India (2, 7), and there are over 100 quarries where it is mined (6). BGG is also not a true granite, but a gabbro (basalt that cooled underground) with bronzite (an iron-rich mineral; 1) that crystallized about 1.35 Billion years ago (8). It is intruded into (occurs within) rocks of the Dharwar Craton that are closer to 1.7 Billion years old.
Ubatuba Granite is a dark-greenish rock mined near Ubatuba, Brazil. It also is not a true granite, but is instead a charnockite (9) . A charnokite is a high-grade metamorphic rock that started out as an igneous rock that is part way between granite (light coloured) and gabbro (dark coloured) in mineral composition. Miners derive Ubatuba granite from the Ubatuba massif, which is basically a large block of very similar looking rock (10). These rocks are about 560 million years old (Ediacaran Period; 11), which means they crystalized during the time that complex life was flourishing in the world's oceans.
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1) Pivko, D. 2004. World's quarries of commercial granites - localization and geology. In: Dimension Stone 2004 (Prikryl, R. (Ed.)). Taylor & Francis. p. 147-152.
4) Baur, N., Kröner, A., Liew, T.C., Todt, W., Williams, I.S. and Hofmann, A.W. 1991. U-Pb Isotopic Systematics of Zircons from Prograde and Retrograde Transition Zones in High-Grade Orthogneisses, Sri Lanka. Journal of Geology, v. 99, p. 527-545.
7) Sarkar, T., Schenk, V., and Berndt, J.2015. Formation and evolution of a Proterozoic magmatic arc: geochemical and geochronological constraints from meta-igneous rocks of the Ongole domain, Eastern Ghats Belt, India: Contributions to Mineralogy and Petrology, v. 169, no. 1.
8) Vijayakumar, K., Frost, C., Frost, B., and Chamberlain, K., 2007, The Chimakurti, Errakonda, and Uppalapadu plutons, Eastern Ghats Belt, India: An unusual association of tholeiitic and alkaline magmatism: Lithos, v. 97, no. 1-2, p. 30-57.
10) Janasi, V. d. A., and Ulbrich, H. H. G. J., 1991, Late Proterozoic granitoid magmatism in the state of Sao Paulo, southeastern Brazil: PreCambrian Research, v. 51, p. 351-374.
11) Meira, V. T., Garcia-Casco, A., Juliani, C., and Schorscher, J. H. D., 2019, Late Tonian within-plate mafic magmatism and Ediacaran partial melting and magmatism in the Costeiro Domain, Central Ribeira Belt, Brazil: Precambrian Research, v. 334, no. 105440.