Cronstedtite - Encyclopedia
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 in the World
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