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Ettringite from the N'Chwaning Mine II near Kuruman , Northern Cape Province, South Africa (size: 4 cm × 3 cm × 1.7 cm)
General and classification
chemical formula Ca 6 Al 2 [(OH) 12 | (SO 4 ) 3 ] • 26 H 2 O
Mineral class
(and possibly department)
hydrous sulphates with foreign anions
System no. to Strunz
and to Dana
7.DG.15 ( 8th edition : VI / D.13)
Similar minerals Storm manite
Crystallographic Data
Crystal system trigonal
Crystal class ; symbol ditrigonal-pyramidal; 3 m
Space group P 31 c (No. 159)Template: room group / 159
Lattice parameters a  = 11.26  Å ; c  = 21.48 Å
Formula units Z  = 2
Physical Properties
Mohs hardness 2 to 2.5
Density (g / cm 3 ) 1.8
Cleavage perfectly
Break ; Tenacity uneven
colour yellowish, white, colorless
Line color White
transparency transparent
shine Glass gloss, silk gloss
Crystal optics
Refractive indices n ω  = 1.491
n ε  = 1.470
Birefringence δ = 0.021
Optical character uniaxial negative

Ettringite (also Woodfordite ) is a rarely occurring mineral from the mineral class of water-containing sulfates with foreign anions. It crystallizes in the trigonal crystal system with the chemical composition Ca 6 Al 2 [(OH) 12 (SO 4 ) 3 ] · 26 H 2 O, with some 24 mol of water of crystallization being given. It usually develops well-defined, prismatic or needle-like, pseudo-hexagonal crystals , which can be from yellowish to almost greenish in color, but also very white.

The synthetically precipitated product is calcium aluminate sulfate , as a suspension and coating color it is known as satin white and "casul". According to the more common notation in construction chemistry , the oxidic  empirical formula is 3CaO · Al 2 O 3 · 3CaSO 4  · 32H 2 O.

Etymology and history

Volcanic rock with enclosed Ettringite, found in Ettringen / Eifel

Ettringit was named after its first location in Ettringen in the Eifel . It was described in 1874 by J. Lehmann in his book "About Ettringite, a new mineral in lime inclusions in the lava of Ettringen (Laacher area)".

In 1890, Candlot and Michaelis first identified the mineral as tricalcium aluminate trisulfate hydrate.


In the old (8th edition) and new systematics of minerals according to Strunz (9th edition) , the ettringite belongs to the division of "Water-containing sulfates with foreign anions ". The new Strunz'sche mineral classification, however, subdivides here more precisely according to the size of the cations involved and the mineral is therefore now in the sub-section “With large to medium-sized cations; with NO 3 , CO 3 , B (OH) 4 , SiO 4 or IO 3 ', where it meets Bentorit , Birunit , Buryatit , Carrarait , Charlesit , Jouravskit , Korkinoit , Sturmanit and Thaumasit the group 7.DG.15 forms.

The systematics of minerals according to Dana , which is common in the English-speaking world , also assigns the ettringite to the sulphate class, but in the section of "Hydrated sulphates with hydroxyl or halogen" and the subsection " Various hydrous sulphates with hydroxyl or halogen ", where it is included Bentorite and Buryatite form the unnamed group 10/31/2 .

Crystal structure

Ettringite crystals (x 12,000), SEM image

Ettringite crystallizes trigonal in the space group P 31 c (space group no. 159) with the lattice parameters a  = 11.26  Å and c  = 21.48 Å as well as 2 formula units per unit cell . The aluminum ions are coordinated octahedrally by six hydroxide ions , and the calcium ions eightfold by four hydroxide ions and four water molecules each. The coordination environments are linked in such a way that chains are created along the crystal, with one aluminum ion alternating with three calcium ions. The sulfate ions and the other water molecules are embedded between these columns and form hydrogen bonds . However, since there are only weak interactions, these are disordered and not necessarily in a fixed relationship . Therefore the ratio formula Ca 6 [Al (OH) 6 ] 2 (SO 4 ) 3 · 25.7 H 2 O was established, which reflects the bonding situation in the crystal and the uncertainty in the proportion of water of crystallization. Template: room group / 159


Chemical properties

Ettringite is one of the minerals with the highest water content of crystalline water and is therefore relatively voluminous and light. A part of the crystal water escapes from a temperature of 80 ° C. At a temperature of 250 ° C, extensive dehydration (calcination) is achieved, from 500 ° C, complete. Ettringite breaks down during calcination into calcium aluminate , calcium oxide and calcium sulfate anhydrite . In connection with water, the dehydrates form again ettringite.

Ettringite reacts sensitively with acids and already decomposes at a pH value of under 9. Basically, ettringite can only form in a strongly alkaline environment (pH value approx. 12).

Physical Properties

Ettringite is one of the soft minerals with a Mohs hardness of 2 to 2.5; it can still be scratched with a fingernail. The crystals show a glass luster, while fibrous aggregates tend to have a silk luster. The molecular weight of ettringite is 1255.11.

Education and Locations

Ettringite with hematite on calcite and Gaudefroyite from the N'Chwaning Mine II, South Africa (size: 10 cm × 8 cm × 4 cm)
Needle-like ettringite from the Concordia slag heaps near Eschweiler , North Rhine-Westphalia, Germany (field of view 8 mm)

Ettringite is produced either through precipitation of a hydrothermal solution or through precipitation from aluminum sulfate solution with hydrated lime or through precipitation / recrystallization from calcium aluminate hydrate and gypsum , as well as through metamorphosis together with the very similar Sturmanite . Well known - and important for the hardening of cements - is the formation of ettringite during the hydration of cement from the components tricalcium aluminate , gypsum and water . In addition to afwillite , gypsum, hydrocalumite , mayenite and portlandite , accompanying minerals include calcite , brucite , hematite , hausmannite , manganite and oyelite .

In addition to the volcanic deposits of the Eifel, Ettringite has so far been found at around 60 locations worldwide, including Limburg and Liège in Belgium; Gera , Maroldsweisach , Oberwolfach and Richelsdorf Mountains in Germany; Clermont-Ferrand in France, Klöch and Brixlegg in Austria, Kuruman in South Africa, and Franklin / New Jersey and Crestmore / Riverside Counties in the USA.

In the concrete structure, monosulphate usually forms first, which then recrystallizes to ettringite. This process is associated with a three-fold increase in volume and is known as ettringite drifting or sulphate drifting . This is often triggered by the penetration of sulphate-containing water into concrete structures. Ettringite can also be produced synthetically as calcium aluminate sulfate . Impure ettringite is produced from wastewater containing sulphate by adding aluminates and lime. That is why sulphates can be removed from wastewater by adding cement slurries ("ettringite precipitation").


Ettringite is a technically important phase in the strength development of cement . Sulphates are added to the cement when it is mixed. When setting ettringite, which delays the process of solidification occurs. Sulphate-free cement, on the other hand, hardens immediately.

Ettringit slurry (Satin White / Casul) is preferably used as a white pigment for coating paper (coated papers). The papers coated with Satin White / Casul are characterized above all by high whiteness, opacity and gloss. They are used as art paper with an excellent print image and for high-quality food packaging. Another application is in emulsion paints and liquid plasters .

See also


  • Petr Korbel, Milan Novák: Encyclopedia of Minerals . Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 150 ( Dörfler Natur ).

Web links

Commons : Ettringite  - collection of images, videos and audio files

Individual evidence

  1. a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  410 .
  2. a b c Ettringite at (English) .
  3. a b A. E. Moore, HFW Taylor: Crystal structure of ettringite . In: Acta Crystallographica, Section B . 26, 1970, pp. 386-393. doi : 10.1107 / S0567740870002443 .
  4. G: Römpp Lexikon Chemie, 10th edition, 1996-1999 Volume 2: Cm - G . Georg Thieme Verlag, 2014, ISBN 3-13-199971-3 , p. 1245 ( limited preview in Google Book search).
  5. H. Eick: About the calcium aluminate sulfate hydrates , In: Zement - Kalk - Gips Volume 17, No. 5, 1964, pp. 169-174.
  6. Ettringite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 63.9 kB ).
  7. Find location list for ettringite in the Mineralienatlas and Mindat .
  8. Conversion of dissolved sulfates from process and waste water to secondary raw materials ; WLB 6/1996.
  9. ^ Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp. 597 .