Granite Rock Type, Definitions, Composition, Formation, & Uses

Granite is one of the most famous rocks in the world. It is a light-colored, coarse-grained intrusive igneous rock. The grains on granite can be seen with the human eye. The mineral composition usually gives granite red, pink, gray, or white color. The mineral grains are visible throughout the rock.

Granite is also the most studied and well-known rock on earth. Granite is easily recognized because it is one of the most common rocks on the earth’s surface. Moreover, its frequent use has made it popular among consumers.

Granite’s sturdiness, durability, and beauty increase its demand. It is more costly than marble and man-made options.

Table of Contents

Granite Definitions

Granite is defined in three ways based on your interest level.

Introductory Definition for Students: It is a coarse-grained and light-colored igneous rock composed of feldspars, quartz, and trace amounts of mica and amphibole minerals.

Petrological Definition: It is a plutonic rock with 10-50% quartz and 65-90% feldspar content. Petrologists describe granite as alkali granites, granodiorites, pegmatites, or aplites, based on mineral composition and grain size.

Commercial Definition: It explains granite as a stone with visible and interlocked grains and a harder, durable, and attractive rock than marble. Based on this definition, different rocks like basalt, gabbro, pegmatite, schist, diorite, and diabase are called granite. Their applications are almost the same as well.

Rock Composition

Granite consists of feldspar minerals in abundance, usually white, pink, gray, or red. The grains usually exhibit two-direction cleavages that intersect at right angles. You can observe these patterns with a hand lens.

Quartz is also present in granite as a transparent mineral with colorless or gray color. Quartz grains exhibit a conchoidal fracture. The mica minerals in granite are muscovite and biotite. Mica minerals give a highly reflective vitreous luster.

QAPF diagram with granite field highlighted in yellow

Granite composition is often studied with respect to physical and chemical properties.

Physical Properties

Density: 2.65 – 2.75 g/cm3
Compressive strength: 200 MPa
Viscosity: 3–6·1020 Pa·s
Melting temperature: 1215 – 1260°C (2219 – 2300°F), reduces to 650°C in the presence of water.
Permeability: Primary: poor, secondary: high (through cracks and fractures)

Chemical Properties

SiO2 (Silica): 72.04%
Al203 (Alumina): 14.42%
K2O: 4.12%
Na2O: 3.69%
CaO: 1.82%
FeO: 1.68%
Fe2O3: 1.22%
MgO: 0.71%
TiO2: 0.30%
P2O5: 0.12%
MnO: 0.05%

Granite Rock Formation

Granite rock is formed by the melting of continental rocks. The continental rocks melt for two reasons: adding heat and adding volatiles Granite solidifies slowly at deeply buried locations because it has large mineral grains.

Where is Granite Found?

Granite is found in large plutons on the continents and in areas where the earth’s crust has been eroded. The rock is found in small stock masses around 100 square kilometers. It is also found in batholiths in orogenic mountain ranges. Granite is found in laccolith, trenches, and thresholds as well.

Massachusetts, Georgia, New Hampshire, South Dakota, and Idaho have high-quality large granite deposits. Most granite used in the US comes from these states.

There are huge reservoirs of granite deposits in Brazil, India, China, South Africa, Finland, and North America. China, India, and Brazil are the leading granite manufacturers worldwide. These countries also produce the most exotic colors of granite.

Granite Uses

Ancient

Granite has been used for centuries for making monuments and sculptures. It was used in Egyptian pyramids. Ancient Egyptians also used it in columns, door lintels, sills, walls, and moldings. Brihadeeswara Temple in Tamil Nadu, India, was built in the 11th century AD. It is the world’s first temple that was completely made of granite. Roman empire used granite as a building material and architectural stone.

Mount Rushmore in the Black Hills, South Dakota, depicting United States presidents George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln, is sculpted from a granite outcrop. It is one of the most famous monumental sculptors ever made from granite.