ANKERITE

    Class : Carbonates, Nitrates, Borates
    Subclass : Anhydrous carbonates
    Crystal system : Trigonal
    Chemistry : CaFe(CO3)2

    Rarity : Very common

Ankerite is a common carbonate of iron and calcium which forms a series with dolomite. It is a fairly common matrix mineral in medium and low temperature hydrothermal veins, associated with dolomite, calcite, siderite and various sulphides. It is the dominant carbonate in the Precambrian iron formations ("algoma" type ores). It is also known in saliferous marls, associated with dolomite. Its name was given to it in honor of the Austrian mineralogist Mathias Joseph Anker. Ankerite generally occurs in often striated rhombohedral crystals, more rarely in prisms, in tabular or pseudo-octahedral crystals, never in scalenohedra; sometimes in lamellar aggregates with curved faces called "saddle", facies also very common for dolomite. Ankerite is also massive. Transparent to translucent, it has a vitreous luster, and is usually yellowish to brown, sometimes salmon-coloured, which changes superficially to blackish-brown by atmospheric weathering into goethite. Ankerite benefits from restricted use in the construction industry, where it is used in the manufacture of certain cements and refractories. It has constituted a poor quality iron ore in countries lacking this metal or during difficult periods in history.

Main photo : Ankerite and quartz from Spruce Ridge, Middle Fork of the Snoqualmie River, Snoqualmie Mining District, King Co., Washington, USA © Rob Lavinsky

Ankerite from Grands Caou, Var, France - © Pierre Clolus
Ankerite from Brownley Hill Mine, Cumbria, England - © Rruff Project
Ankerite & calcite from Obergesteln, Valais, Switzerland - © Christian Bracke
Ankerite from Cuasso al Monte, Lombardie, Italie - © Peter Neschen

Ankerite in the World

Beautiful ankerite crystals are rare, often present in the same deposits as dolomite. The best known specimens come from the salt marls of Asturreta, near Eugui, and Ojos Negros, near Monreal del Campo (Spain), Brosso and Traversella (Piedmont, Italy), the Czech veins of Pribam, as well as American iron deposits (Missouri, Washington, New York...).

 
Ankerite from France

In France, the alpine veins of La Taillat and Pessey-Nancroix also gave superb centimetric crystals.

Twinning ans special crystallizations

Twins are numerous and common for ankerite, especially on {0001}, on {1010} and on {1120}.

Fakes and treatments

No fake identified for this mineral species but very difficult to differentiate from dolomite and siderite without chemical analysis.



Hardness : 3.5 to 4
Density : 2.9 to 3.1
Fracture : Sub-conchoidal
Trace : White




TP : Transparent to translucent
RI : 1.510 to 1.750
Birefringence :  0.180 to 0.202
Optical character : Uniaxial -
Pleochroism : Very weak
Fluorescence : Orange


Solubility : Acids

Magnetism : None
Radioactivity : None