Are Black Granites Truly Granite?

Frederick M. Hueston   StoneForensics.com

Black granite can refer to different types of rocks, and its classification depends on the specific rock in question. Some black granites are true granites, while others may be classified as basalt.

True granite is an intrusive igneous rock composed of quartz, feldspar, and mica. It forms from the slow cooling and solidification of molten magma deep within the Earth’s crust. Black granites that meet the criteria of true granite contain these minerals and exhibit the granular texture typical of granite.

On the other hand, basalt is a fine-grained extrusive igneous rock that forms from rapidly cooling lava on the Earth’s surface. Basalt typically lacks the visible mineral grains found in granite. It is composed mainly of minerals such as pyroxene and plagioclase feldspar.

There are black granites that have a high mineral content of mafic minerals, such as biotite or hornblende, which can give them a dark coloration. These black granites may have a composition that overlaps with basalts. However, they are still classified as granites if they meet the criteria of true granite composition and texture.

It’s important to note that the term “black granite” is used widely in the industry and can encompass a range of rocks, including true granites and rocks that are more similar to basalts. Therefore, it is necessary to examine the specific mineral composition and texture of a black granite to determine if it is a true granite or a basalt.

Here are examples of black granites that can fall into both categories:

Absolute Black Granite: This is a widely known and popular black granite. While it is commonly classified as a true granite, it may contain high concentrations of mafic minerals like biotite, giving it a deep black color. Some variations of Absolute Black Granite can have a composition that overlaps with basalts.

Black Galaxy Granite: Another widely recognized black granite, Black Galaxy Granite is predominantly composed of quartz and feldspar, meeting the criteria of true granite. However, it may contain small amounts of mafic minerals such as biotite, which contribute to its distinctive speckled appearance. Some Black Galaxy Granite varieties can have similarities to basalts due to the presence of these mafic minerals.

Black Pearl Granite: Black Pearl Granite is predominantly composed of minerals like quartz and feldspar, meeting the criteria for true granite. However, it can contain minor amounts of mafic minerals such as biotite or hornblende, which contribute to its dark black coloration.

Zimbabwe Black Granite: Zimbabwe Black Granite is considered a true granite as it is composed of quartz, feldspar, and mica. However, it often contains high concentrations of mafic minerals like biotite or hornblende, which give it a deep black color.

Impala Black Granite: Impala Black Granite is primarily composed of quartz, feldspar, and mica, meeting the criteria for true granite. However, it can also contain small amounts of mafic minerals such as pyroxene, which contribute to its dark appearance.

Nero Absolute Granite: Nero Absolute Granite is a black granite that is primarily composed of quartz, feldspar, and mica, making it a true granite. It may contain varying amounts of mafic minerals like biotite or hornblende, which contribute to its deep black color.

These examples demonstrate how certain black granites can have mineral compositions that align with both true granite and basalts, resulting in overlaps between the two categories. It’s important to note that the specific mineral composition and texture of each rock may vary, and individual samples should be analyzed to determine their exact classification.