Granite colours range from white to black to pink, but what causes such variation in a single rock type? We'll learn what gives each granite its unique hue, as well as what that informs us about its mineralogy and source. You could be a novice geologist, simply curious, or seeking for new granite countertops. Whatever your purpose, you'll be astounded by the huge array of distinct granites.
Granite is one of the most well-known forms of rock, with applications ranging from construction to sculpture. It has been used for thousands of years as a symbol of status, strength, and durability.
What is Granite?
Granite is an intrusive igneous rock that has big grains (minerals) that can be seen with the naked eye. Pink, white, grey, and black are the most frequent granite colours. It is crucial to note, however, that certain stones advertised as black 'granite' are most likely gabbro, as granite must include at least 20% quartz within a rock to be considered granite.
Let's take a closer look at what an intrusive igneous rock is:
An intrusive rock is one that cooled within the crust rather than being released as molten rock. The prolonged cooling of molten rock is required to form the massive crystals of a single mineral found in granites. Differential lithification or solidification of molten rock occurs over time depending on chemical makeup, allowing different types of minerals to form at different times and vary the final producing granite. As a result, the size of each grains is related to the rate at which the molten rock was cooled. Extrusive rocks cool during a volcanic eruption and do not provide time for mineral orientation, resulting in a homogeneous-looking rock with no identifiable grains.
An igneous rock is one that formed from molten rock. In comparison, sedimentary and metamorphic rocks are the two other major forms of rock.
What Determines Granite Colors?
Granite is a mineral and rock conglomerate composed mostly of quartz, potassium feldspar, mica, amphiboles, and trace minerals. Granite is commonly composed of 20-60% quartz, 10-65 percent feldspar, and 5-15 percent micas (biotite or muscovite). The minerals that make up granite give it the distinct colours that we observe in various types of granite.
The original source of molten rock that cooled to form the granite is mostly responsible for the relative quantity of different coloured minerals in a granite. If the molten rock has a lot of potassium feldspar, the granite will turn salmon pink. On the other hand, if the molten rock has a lot of quartz and the minerals that make up amphibole, you'll probably get a black and white speckled granite.
Quartz - often a milky white colour
Feldspar - typically an off-white colour
Potassium Feldspar has a salmon pink tint.
Biotite is usually black or dark brown in hue.
Muscovite - usually shiny gold or yellow in colour
Amphibole - usually black or dark green in colour
The mix of the minerals listed above provides for the majority of the hues found in granite. Now, let's go through the different types of granite and a quick summary of what gives them their colour.
White granite is a type of granite that is predominantly formed of the minerals quartz (milky white) and feldspar (opaque white). Microscopic amphibole grains are most likely responsible for the small black flecks in the granite above. This might be owing to a lack of chemical components required to produce amphiboles, or it could be because the cooling process was not conducive to the development of amphiboles.
If you see a rock that is completely white, it is most likely not granite, but rather a man-made rock designed to seem like granite or a quartz (quartzite) countertop.
The term "black granite" is often used in commercial rock, however it is not granite. As previously stated, granite must include at least 20% quartz, implying that an all-black rock is not a granite. Black granite is most typically gabbro, a mafic intrusive igneous rock comparable to basalt. Gabbro is mostly made up of the minerals pyroxene and plagioclase, with trace amounts of olivine (dark green) and amphibole.
Pink granite is caused by the accumulation of potassium feldspar in the granite. Small specks of milky semi-transparent quartz, dark brown/black amphibole, and opaque white feldspar can be seen. The predominant mineral in a granite like the one above, on the other hand, is potassium feldspar.
Red granite is a type of pink potassium feldspar rich granite in which the k-feldspar is redder rather than pinker. Red colouring can also be obtained from iron oxide in hematite grains or inclusions inside feldspar, which is essentially the same mechanism that produces rusted metal ruby red.
Blue granite countertops may be advertised, but they are almost probably not granite. One possibility is that the rock is Larvikite, an igneous variant of monzonite that is commonly referred to as "blue granite" despite the fact that it is not granite. Anorthosite, a rock that includes a lot of blue labradorite and is occasionally advertised as blue granite, is another popular option.
Green Granite
When marketed as green granite, the stone is frequently a green variant of marble that obtains its green colouring from serpentine impurities. It could potentially be a green variant of soapstone that has been mislabeled as granite. Green coloured minerals are not plentiful in granites, although green minerals are abundant in a range of other rock types. The incorporation of amazonite, a green type of feldspar, is one unique approach to achieve a green tint.
This concludes our guide to granite colours, which should have taught you a lot about the many types of granites.
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