CRONSTEDTITE

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Trigonal
    Chemistry : Fe3(SiFe)O5(OH)4
    Rarity : Uncommon

Cronstedtite is a phyllosilicate of the kaolinite-serpentine group which has at least eight polytypes. It is an hydrothermal mineral forming at low temperature in mineralized veins from high to low temperatures, and which accompanies various iron-bearing minerals (siderite, chlorite, pyrite, etc...). It was named in honor of the Swedish chemist and mineralogist Axel Frederick Cronstedt. This mineral forms divergent groups or hemispheres of spindle-shaped crystals, with a trigonal or hexagonal habit, barely reaching 1 cm, as well as botryoidal to cylindrical masses. Twins are common. The color is variable : black to dark greenish-brown, passing to dark emerald green on the fine lamellae.

Main photo : Cronstedtite from Pribram, Czech Republic © Eugene & Sharon Cisneros

Cronstedtite from Herja Mine, Baia Mare, Maramures, Romania © Christian Auer
Cronstedtite from Brunita open pit, La Unión, Murcia, Spain © Enrico Bonacina
Cronstedtite from Brunita open pit, La Unión, Murcia, Spain © Elmar Lackner
Cronstedtite from Salsigne, Aude, France © Elise Chaigneau and Eric Penet

Cronstedtite in the World

Exceptional centimeter-sized groups of crystals come from the Herja mine, near Baia Mare (Romania). Superb groups have also been discovered in the Czech Republic (Pribram, Kutna Hora...). The silver and tin veins of Bolivia (Huanuni, Llallagua) and Brazil (Congonhas do Campo, Minas Gerais) also provided very aesthetic specimens. Cronstedtite is also known from other deposits, including the veins of English Cornwall (Saint-Just, Kea, etc...) and those of Aquiles Serdan (Chihuahua, Mexico).

Cronstedtite in France

In France, cronstedtite is known at the Salsigne gold mine (Aude) as well as at Ste-Marie-aux-Mines (Haut-Rhin).

Twinning

Twins are frequent but not identifiable by eye.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 3.5
Density : 3.34 to 3.35
Fracture : Micaceous
Streak : Dark green

TP : Translucent to opaque
RI : 1,720 to 1,800
Birefringence : 0.080
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Acids


Magnetism : None
Radioactivity : None