Ettringite

Chemical formula: Ca₆Al₂(S⁶⁺O₄)₃(OH)₁₂·₂₇H₂O

Ettringite is a hydrated calcium aluminum sulfate, forming characteristic acicular or prismatic crystals with a vitreous luster, valued by collectors.

## Characteristics Ettringite is a mineral from the sulfate group, forming slender, hexagonal crystals with a prismatic or acicular habit. It typically occurs as radial or fibrous aggregates, and well-formed, single crystals are rare. Its appearance is very distinctive, often resembling bundles of needles or glass fibers. It is a brittle mineral. ## Physical Properties Ettringite is relatively soft, with a Mohs hardness of 2-2.5. It has a low specific gravity, around 1.77 g/cm³, which makes it light. The luster is typically vitreous, and the mineral can be transparent to translucent. Some specimens exhibit weak, reddish-purple fluorescence under ultraviolet light. ## Colors and Varieties Most often, it is colorless or white. However, it can take on delicate hues, such as pale yellow, greenish, or pink, depending on impurities. No named varieties are distinguished, and its collector value primarily depends on the crystal form and transparency. ## History and Name The name ettringite comes from its type locality, the Ettringen quarry near Mayen in the Eifel Mountains, Germany. The mineral was first described by J. Lehmann in 1874. It is also an important synthetic compound, formed during the hydration of Portland cement, where its formation can affect the durability of concrete. ## Uses Beyond its significance for mineral collectors, ettringite is the subject of intensive research in civil engineering and cement chemistry. Its controlled formation is crucial in the production of certain specialized mortars and concretes, while uncontrolled growth in hardened concrete can lead to cracking and deterioration.

Properties

Mohs hardness: 2-2.5
Color: Colourless; colourless in transmitted light; yellow
Luster: Vitreous
Streak: White
Density: 1.77
Cleavage: On <mi>{10_10}</mi>, perfect.
Fracture: Uneven
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Ettringite is most easily identified by its characteristic crystal form – long, slender, hexagonal prisms or needles, often gathered in radial aggregates. Low hardness (2-2.5), lightness (low density), and vitreous luster are also key features. It reacts with acids, dissolving in them. ## Distinguishing from Similar Minerals Ettringite is sometimes confused with other acicular minerals, such as natrolite, scolecite, or aragonite. From zeolites (natrolite, scolecite), it is distinguished by lower hardness and genesis. Aragonite is much harder (3.5-4) and denser, and also effervesces with cold hydrochloric acid. Gypsum (selenite) can also form acicular crystals, but it has excellent cleavage in one direction, which ettringite lacks. ## Crystal Forms Ettringite crystals are hexagonal (trigonal), forming six-sided columns and needles, often terminated by flat pinacoids or pyramids. It most often forms radial, stellate, or fibrous aggregates. Single, well-formed crystals are less common.

Geological environment

## Genesis Ettringite is a secondary mineral, forming at low temperatures. It forms as a result of the alteration of limestone and chalk rocks under the influence of sulfate solutions, often in contact zones with magmatic intrusions (contact metamorphism). It also forms in geodes and fissures in metamorphic rocks, such as calc-silicate xenoliths in volcanics. ## Mineral Associations It often co-occurs with other minerals formed under similar conditions, such as calcite, portlandite, afwillite, hydrocalumite, and thaumasite. In its type locality in Germany, it is accompanied by minerals such as mayenite, larnite, and brownmillerite. ## Localities The most important and classic ettringite localities worldwide are the quarries in the Eifel region of Germany (Ettringen, Bellerberg). Beautiful specimens also come from the N’Chwaning and Wessels mines in the Kuruman area, Republic of South Africa, where they occur in manganese deposits. It is also known from Crestmore, California (USA), Scawt Hill, Northern Ireland, and Hatrurim, Israel.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, transparent crystals of distinct prismatic or acicular habit. Aesthetic, radial aggregates are highly valued, especially those embedded in a contrasting rock matrix. Crystal size is an important factor – specimens with crystals exceeding a centimeter in length are rare and sought after. Color, though usually pale, can enhance attractiveness, especially in the case of yellow or pinkish specimens. ## Popular Localities By far the most desirable ettringites on the collector's market come from the Kalahari manganese mines (RSA), especially from the N’Chwaning mine. Specimens from there are famous for their large, often lemon-yellow crystals. Classic, though usually smaller, are specimens from the type locality in Ettringen, Germany.

Care and storage

## Cleaning Ettringite specimens are extremely delicate and brittle, so they should be cleaned with the utmost care. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, only distilled water and a very soft brush should be used, avoiding any pressure. After cleaning, the specimen should be immediately and thoroughly dried, for example, with a hairdryer set to cool air. ## What to Avoid Ettringite is sensitive to changes in humidity and temperature. It should not be heated, as it easily loses its water of crystallization, leading to its disintegration. Contact with acids and other chemicals should be avoided. Prolonged exposure to water can also damage the specimen. It should not be cleaned in ultrasonic cleaners. ## Storage It is recommended to store ettringite in stable conditions, in sealed containers or display cases that protect it from dust, mechanical damage, and sudden changes in humidity. Due to its brittleness, it should be mounted on a stable base and kept away from vibrations.