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Mineralpedia Details for Marecottite

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Marecottite

Marecottite

Occurs in weathered hydrothermal breccia veins at the contact between the pre-Variscan gneissic basement of the Aiguilles Rouges massif and the carbobiferous Vallorcine granite.

It is associated with gypsum, uraninite and secondary uranyl sulfates such as rabejacite, johannite and ktenasite at the type locality. Marecottite forms very brittle, elongated, diamond shaped platelets forming rosettes, is yellow-orange and is reported to be nonfluorescent at the type locality. It is a member of the Zippeite group with extensive solid solution in members rich in Mg, Ni, Ca, Fe, Mn, Zn. At the Rožná deposit in Moravia, Czech Republic, Marecottite occurs with Natrozippeite (fluorescent). Optically, it is biaxial with indices of refraction between 1.735 to 1.750 measured in the (011) face and fair pleochroism.

Originally found at La Creusaz uranium prospect near the village of Les Marécottes, Trient Valley, Valais in the Alps of Switzerland.

For more information see http://rruff.info/rruff_1.0/uploads/CM42_215.pdf .,
Also Roth, Philippe (year?), Minerals first discovered in Switzerland pg 106. An excellent photo here along with an SEM image.

Formula
Mg3O6(UO2)8(SO4)4(OH)2·28H2
Crystal System
Triclinic 
Crystal Habit
Elongated diamond-shped platelets in rosettes to 0.5mm 
Cleavage
Perfect, None, None 
Luster
Vitreous (Glassy) 
Color
yellow orange 
Streak
cololess 
Class
Triclinic - Pinacoidal 
Fracture
Brittle 
Hardness
WebMineral
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Mindat
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Marecottite from Bukov mine, Rožná deposit, Vysočina, Moravia, Czech Republic

Marecottite
            from Bukov mine, Rožná deposit, Vysočina, Moravia, Czech Republic
Special Info
Radioactive!!

Dark orange somewhat glassy Marecottite crusts and elongated crystals with steep terminations to about 0.5mm. On this specimen the Marecottite occurs with bright canary yellow Natrozippeite. Most of what you see is the Natrozippeite, which is on and presumably formed after the Marecottite. The zippeite is highly fluorescent yellow and is powdery. The Marecottite is a little fluorescent also, which is contrary ot the type locality where Marecottite is nonfluorescent. This may be due to solid solution since Marecottite is a member of the Zippeite group. Purchased from Dr. Jaroslav Hrysl. A good reference is Philippe Roth's "Minerals first discovered in Switzerland", where you will see a good photo of Marecottite and a SEM image.

Click thumbnail images for larger view.

Marecottite from Jachymov, Erzgegirge, Bohemia, Czech Republic

Marecottite
            from Jachymov, Erzgegirge, Bohemia, Czech Republic
Special Info
Radioactive!!

Yellow pulverulent masses.

Click thumbnail images for larger view.

Marecottite from La Creusaz prospect, La Creusaz, Les Marécottes Valais, Switzerland

Marecottite
            from La Creusaz prospect, La Creusaz, Les Marécottes Valais, Switzerland
Special Info
Radioactive!!

This specimen is loaded with orange crystal masses of Marecottite. The Jachmovite and Rabejacite is not seen in the photos as they are tiny, less than 0.5mm in size and are sparse. Under the microscope you can see light yellow puff-balls of Jachymovite and lemon yellow, smooth-looking balls of Rabejacite. I may have those switched around however. 

Click thumbnail images for larger view.

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