Red Granite

Description

Red granite is a coarse-grained, intrusive igneous rock characterized by its prominent reddish hue. It is primarily composed of quartz, feldspar (predominantly potassium feldspar, which imparts the red color), and mica (biotite or muscovite), with minor amounts of amphibole or other accessory minerals. The interlocking crystalline texture is a hallmark of its slow cooling and solidification deep within the Earth's crust. The specific shade of red can vary from pale pink to deep brick red, depending on the concentration and type of iron-bearing minerals within the potassium feldspar, or the presence of red-stained quartz.

How to Identify

Color

Predominantly red, ranging from pale pink to deep brick red, often with white, gray, or black speckles from other minerals.

Luster

Vitreous (glassy) to dull, depending on the mineral components and weathering.

Texture

Phaneritic (coarse-grained), meaning individual mineral crystals are visible to the naked eye and are interlocking.

Crystal Form

Anhedral to subhedral crystals, forming an interlocking mosaic. Quartz typically anhedral, feldspars subhedral, micas euhedral to subhedral.

Cleavage

Feldspar minerals exhibit good cleavage (typically two directions at or near 90 degrees). Quartz has no cleavage. Mica has perfect basal cleavage.

Geological Environment

Forms in large intrusive bodies (plutons, batholiths, stocks) within continental crust, often associated with convergent plate boundaries, orogenic belts, and stable cratonic regions.

Key Facts

• Hardness: 6-7 on the Mohs scale (due to quartz and feldspar).
• Specific Gravity: 2.65-2.75 g/cm³.
• Crystal System: Monoclinic (for feldspar), Hexagonal (for quartz), Monoclinic (for biotite). The rock itself is an aggregate of these minerals.
• Color: Red, pink, reddish-brown, with varying amounts of white, gray, and black.
• Luster: Vitreous (glassy) for quartz and feldspar, pearly for mica.
• Transparency: Opaque (as a rock), individual quartz crystals can be translucent to transparent.
• Fracture: Conchoidal (quartz), uneven to splintery (feldspar).
• Cleavage: Good in feldspar (two directions at ~90°), perfect in mica (one basal direction), absent in quartz.
• Composition: Primarily quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase feldspar (0-65% of total feldspar), and micas (biotite, muscovite) or amphiboles. The red color is due to iron oxides within potassium feldspar or red-stained quartz.

Quick Check

• Color: Red, pink, or reddish-brown, often with white, gray, or black specks.
• Luster: Vitreous to dull.
• Streak: White (due to the dominant felsic minerals).

Physical Characteristics

• Crystal Habit: Granitic (equigranular, interlocking, anhedral to subhedral crystals).
• Cleavage Type: Good in feldspar (prismatic), perfect in mica (basal).
• Fracture Type: Conchoidal (quartz), uneven to splintery (feldspar).
• Tenacity: Brittle.
• Luster Type: Vitreous to dull.

Formation

Red granite forms from the slow crystallization of felsic magma deep beneath the Earth's surface. The magma, rich in silica, aluminum, and alkali metals (potassium and sodium), cools slowly over millions of years, allowing large, interlocking mineral crystals to grow. The characteristic red color is primarily due to the presence of potassium feldspar (orthoclase or microcline) that is rich in iron oxides (hematite) as inclusions or exsolution lamellae, or due to the presence of red-colored quartz.

Usage

Red granite is widely used as a dimension stone for construction (building facades, flooring, countertops, monuments, paving stones), and as crushed stone for road aggregate and railway ballast. Its durability, aesthetic appeal, and ability to take a high polish make it a popular choice.

Age Distribution

Granite forms throughout Earth's history, from the Archean Eon (over 2.5 billion years ago) to the Cenozoic Era (present day). Red granites are found in various ages, often associated with continental collision zones and magmatic arcs.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock (Plutonic)

Group

Felsic Intrusive Rocks

Crystal System

Polycrystalline aggregate; constituent minerals have their own crystal systems (e.g., monoclinic for feldspar, hexagonal for quartz).

Chemical Formula

Not applicable for a rock; it's a mixture of minerals. Dominant elements are Si, Al, K, Na, O.

Composition

Quartz (SiO₂), Potassium Feldspar (KAlSi₃O₈), Plagioclase Feldspar (NaAlSi₃O₈ - CaAl₂Si₂O₈), Biotite (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂), Muscovite (KAl₂(AlSi₃O₁₀)(OH)₂).

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