Granite Composition and Colors

Granite is an igneous rock with a fascinating composition and a wide range of colors. It's primarily composed of quartz, feldspar, and mica minerals. Quartz gives granite its light color and durability, while feldspar contributes to the pink, red, gray, or white hues. 

Granite Key Features:

• Category: Intrusive igneous rock
• Color: Granite comes in a variety of colors, including pink, gray, red, and black.
• Texture: Granite has a coarse-grained, phaneritic texture.
• Chemical Composition: Felsic
• Hardness: Very hard rock, ranking 6-7 on Mohs scale of hardness. 

Mineral composition of granite rock with visible grains of feldspar, quartz, and mica

Granite Chemical Composition

Chemically, granite is rich in silica (SiO₂), typically containing 70-77% by weight. It also contains significant amounts of alumina (Al₂O₃), as well as varying percentages of potassium, sodium, and calcium oxides.

The mineral composition translates into a specific chemical makeup for granite.

• 70-77% Silica (SiO₂): This is the main component of quartz and gives granite its hardness and resistance to weathering.
• 11-13% Alumina (Al₂O₃): This comes from the feldspars and contributes to the rock's strength.
• 3-5% Potassium Oxide (K₂O): Primarily from alkali feldspar.
• 3-5% Soda (Na₂O): Associated with plagioclase feldspar.
• 1% Lime (CaO): Often from minerals like sphene or apatite.
• 2-3% Total Iron (FeO + Fe₂O₃): This can be present in various minerals like biotite and hornblende, influencing the granite's color.
• Less than 1% Magnesia (MgO) and Titania (TiO₂): These are minor components from trace minerals.

Granite Mineral Composition

Granite is an intrusive igneous rock that forms deep within the Earth's crust. Its composition is characterized by the following key elements:

Quartz: 20-60% of granite's composition. It's a hard, transparent to translucent mineral composed of silicon dioxide (SiO2).

Feldspar: 65-90% of the rock's total composition, divided into two main types:

Mica: 5-15% of the total composition:  Mica is another common mineral in granite, occurring as thin, sheet-like crystals.

• Muscovite: Light-colored mica, giving granite a sparkly appearance.
• Biotite: Darker mica, often black, responsible for the speckled look in some granites.

Amphiboles: Usually less than 10%, with hornblende being the most common type in granite.

Accessory minerals: These make up a small percentage (typically less than 1%) of the rock but contribute to its overall characteristics: a) Zircon b) Apatite c) Magnetite d) Ilmenite e) Sphene (titanite).

Granite variety of colors due to mineral composition.

Granite Colors

The color of granite depends on its mineral composition:

White Granite: High quartz and white feldspar content with minimal amounts of dark minerals like biotite or hornblende. Popular varieties include Kashmir White and Alaska White.

Pink and Red Granite: These colors are usually due to high concentrations of orthoclase feldspar. Examples include Rosy Pink and Red Multi.

Black Granite: Although true granite is rarely black, dark-colored granite-like stones such as gabbro are often marketed as black granite. Popular varieties include Black Galaxy and Absolute Black.

Gray Granite: It gets its color from a variety of minerals, including plagioclase feldspar, biotite, and amphibole.

Green Granite: Green hues often come from a variety of minerals including amazonite feldspar, epidote, or serpentine. Varieties include Verde Butterfly and Tropical Green.

Brown and Tan Granite: These colors are typically due to a combination of feldspar, biotite, and other minerals. Popular examples include Baltic Brown and Tan Brown.

Close-up of granite showing its mineral composition, visible crystals of quartz, feldspar, amphibole, and mica.