Class : Sulphates, chromates, molybdates
    Subclass : Hydrated sulphates
    Crystal system : Trigonal
    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.

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).

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.