Ettringite Mineral Overview

Mineral name: Ettringite
Mineral class: Sulfate

Known Facts

• Chemical formula: Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O
• Crystal system: Trigonal
• Mohs hardness: 2–2.5
• Color range: Colorless, white, pale yellow
• Luster: Vitreous to silky
• Streak: White
• Cleavage: Poor
• Fracture: Uneven
• Specific gravity: Approximately 1.7–1.8
• Common locations: Germany, France, United States, Italy, cement environments worldwide
• Uses: Cement chemistry studies, collector specimens
• Similar minerals: Thaumasite, gypsum, halotrichite

Physical and Optical Characteristics

Ettringite typically forms:

• Slender acicular (needle-like) crystals
• Radiating sprays or fibrous bundles
• Crusts or coatings in cavities

Crystals are usually transparent to translucent and often delicate.

Color is typically colorless or white, sometimes with a faint yellow tint. The luster is vitreous on crystal faces and may appear silky in fibrous aggregates.

With a hardness of 2 to 2.5, Ettringite is soft and fragile, and crystals can be easily damaged.

Chemical Composition and Structure

Ettringite is a hydrated calcium aluminum sulfate, with the formula:

• Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O

Key features include:

• Calcium (Ca) and aluminum (Al) as major cations
• Sulfate groups (SO₄²⁻)
• Hydroxyl (OH) groups
• A very high water content (26 H₂O molecules)

Its structure consists of:

• Columns of calcium and aluminum polyhedra
• Channels containing sulfate ions and water molecules

The high water content contributes to:

• Low density
• Instability under changing environmental conditions

Formation and Geological Occurrence

Ettringite forms in low-temperature, alkaline environments, particularly:

Natural settings

• Rarely in metamorphic or hydrothermal environments
• Occurs in altered rocks and cavities

Artificial environments

• Commonly forms in cement and concrete systems
• Develops during hydration of Portland cement

Formation involves:

• Reaction of calcium, aluminum, and sulfate-rich solutions
• Precipitation under highly alkaline conditions

It is often associated with:

• Gypsum
• Thaumasite
• Other sulfate minerals

Common Locations

Natural occurrences of Ettringite are relatively uncommon but include:

• Germany: Type locality (Ettringen, Eifel region)
• France and Italy: Volcanic and altered rock environments
• United States: Various localities

It is also widely found in:

• Concrete and cement structures worldwide

Uses and Practical Significance

Ettringite is important primarily in:

Cement and materials science

• Plays a key role in:
  • Cement hydration processes
  • Concrete durability and expansion

Excessive formation can lead to:

• Expansion and cracking in concrete (delayed ettringite formation)

Mineral collecting

• Collected for its delicate needle-like crystals

It has no traditional industrial use as a mined mineral.

Similar and Related Minerals

Ettringite may be confused with:

• Thaumasite: Similar habit but contains carbonate
• Gypsum: Softer and typically forms larger crystals
• Halotrichite: Fibrous sulfate mineral with different composition

Distinguishing features include:

• Needle-like crystal sprays
• Very low density
• Occurrence in alkaline or cement environments

Identification Notes for Collectors

• Look for white, needle-like crystal sprays in cavities or altered material
• Note softness and fragility
• Consider association with cement or sulfate-rich environments
• Observe silky luster in fibrous aggregates
• Use analytical methods for confirmation, as similar sulfates may appear alike

Ettringite is a hydrated calcium aluminum sulfate, recognized for its needle-like crystals and importance in cement chemistry, as well as its occurrence in specialized low-temperature environments.