Rutile

Description

Rutile is a titanium dioxide mineral (TiO2) and is one of three naturally occurring polymorphs of TiO2, the others being anatase and brookite. It is characterized by its typically reddish-brown to black color, adamantine to submetallic luster, and high refractive index. Rutile often forms prismatic crystals, frequently twinned, and can occur as acicular (needle-like) inclusions in other minerals. Its exceptional chemical stability and hardness make it a common detrital mineral in placer deposits.

How to Identify

Color

Typically reddish-brown, dark red, yellowish-brown, black; rarely golden-yellow or violet. Transparent varieties can be golden-yellow.

Luster

Adamantine to submetallic, sometimes metallic on fracture surfaces.

Texture

Often occurs as prismatic crystals, sometimes acicular (needle-like) or massive. Can be granular.

Crystal Form

Tetragonal system, typically forming prismatic crystals with square cross-sections, often striated parallel to the c-axis. Characteristic twinning, especially geniculate (knee-shaped) twins, is common. Also found as acicular inclusions.

Cleavage

Distinct to good on {110} and {100}, but often imperfect or difficult to observe due to fracture.

Geological Environment

Found in igneous rocks (e.g., granites, syenites, kimberlites), metamorphic rocks (e.g., gneisses, schists, eclogites, granulites), hydrothermal veins, and as a heavy mineral in placer deposits and sedimentary rocks.

Key Facts

• Hardness: 6 - 6.5 on Mohs scale
• Specific Gravity: 4.23 - 4.30 g/cm3 (can vary slightly with impurities)
• Crystal System: Tetragonal
• Color: Reddish-brown, dark red, yellowish-brown, black; rarely golden-yellow or violet
• Luster: Adamantine to submetallic
• Transparency: Transparent to opaque
• Fracture: Subconchoidal to uneven
• Cleavage: Distinct to good on {110} and {100}, but often imperfect
• Composition: Titanium dioxide (TiO2)

Quick Check

• Color: Reddish-brown to black
• Luster: Adamantine to submetallic
• Streak: Pale brown to yellowish-brown

Physical Characteristics

• Crystal Habit: Prismatic, acicular, massive, granular; often twinned (geniculate twins common)
• Cleavage Type: Prismatic (distinct to good on {110} and {100})
• Fracture Type: Subconchoidal to uneven
• Tenacity: Brittle
• Luster Type: Adamantine to submetallic

Formation

Rutile forms under a wide range of pressure and temperature conditions. It is a common accessory mineral in igneous rocks (e.g., granites, syenites, kimberlites, and some mafic rocks), metamorphic rocks (e.g., gneisses, schists, eclogites, and granulites), and hydrothermal veins. It can also form as a detrital mineral in sedimentary rocks due to its high resistance to weathering. In metamorphic environments, rutile often forms from the breakdown of other titanium-bearing minerals like ilmenite or sphene (titanite) under increasing pressure and temperature.

Usage

Rutile is the most important ore mineral of titanium, which is used in lightweight alloys for aerospace, medical implants, and sporting goods. It is also the primary source for titanium dioxide pigment (TiO2), widely used in paints, plastics, paper, and cosmetics due to its high refractive index and opacity. Synthetic rutile is produced for these applications. Additionally, rutile is used in welding rod coatings, ceramics, and as a gemstone (especially as inclusions in quartz, known as 'rutilated quartz').

Age Distribution

Rutile is found in rocks of various ages, from Precambrian to Cenozoic, reflecting its stability and formation in diverse geological settings.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Oxides

Group

Rutile Group

Crystal System

Tetragonal

Chemical Formula

TiO2

Composition

Titanium (Ti) 59.93%, Oxygen (O) 40.07%

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