Ettringite - Encyclopedia

    Class : Sulphates, chromates, molybdates
    Subclass : Hydrous sulphates
    Crystal system : Rhombohedral
    Chemistry : Ca6Al2(SO4)3(OH)12 . 26H2O
    Rarity : Rare

Ettringite is a highly hydrated sulphate of calcium and aluminum. It is a late-formation secondary carbonate environment mineral known from various geological contexts. Ettringite is often found in geodes within metamorphic limestone enclaves of nepheline lavas and metasomatised limestones. Some ettringites have a biomineral origin and can participate in the biohealing phenomena of some mortars due to a "cement bacteria". This mineral owes its name to its locality of discovery : Ettringen, near Mayen in Germany. It occurs in often unfinished prismatic crystals, in flattened dipyramids, sometimes in fine fibers. It is most often colorless to white, more rarely yellow. Because crystals are rare, they are sought after by collectors. It is used in the cement industry, its needle structure gives materials that contain primary ettringite interesting mechanical properties.

Ettringite from N'Chwaning Mines, Kuruman, South Africa
Ettringite from N'Chwaning Mines, Kuruman, South Africa
Ettringite from N'Chwaning Mines, Kuruman, South Africa
Ettringite from N'Chwaning Mines, Kuruman, South Africa

Ettringite in the World

Beautiful 20 cm crystals, the most famous known, come from the N'Chwaning manganese mine near Kuruman in South Africa. Ettringite is also present in the manganiferous complex near Franklin, New Jersey in small crystals associated with large crystals of clinohedrite, thomsonite and andratite. In very different contexts, ettringite occupies the metamorphic limestone cavities raised by tephrites in Ettringen, Eifel, Germany, the type- locality.

Ettringite in France

In France, ettringite is found in the form of coatings on limestone "cooked" by basalts of Boisséjour, near Clermont-Ferrand (Puy-de-Dôme, France). The crystals measure up to 1 cm in length, they are transparent and lemon yellow.

It is also found in white microcrystals in sheaves in the contact pyrometamorphism of Lapanouse-de-Sévérac, Aveyron (photo on the right taken by P. and D. Journet).

Twinning and special crystallizations

No twinning known for the species

Fakes and scams

No fake known for this species. It is a very difficult mineral to differentiate from sturmanite without chemical analysis, some specimens of ettringite can be labeled sturmanite and vice versa. Some crystals may exhibit mixtures and be zoned with parts of ettringite composition and other sturmanite.

Hardness : 2 to 2.5
Density : 1.77
Fracture : Undetermined
Streak : White

TP : Translucent to transparent
IR : 1.470 to 1.491
Birefringence : 0.021
Optical character : Uniaxial -
Pleochroism : Very weak
Fluorescence : None

Solubility : Hydrochloric or nitric acid

Magnetism : None
Radioactivity : None


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