Ralstonite

(n.)
A fluoride of alumina and soda occurring with the Greenland cryolite in octahedral crystals.
  

General Information:
 Chemical Formula:
NaxMgxAl2-x(F,OH)6·(H2O)
 Environment:
Alteration product of cryolite.
 Locality:
Ivigtut, Greenland.
Physical Properties:
 Cleavage:
[???] Perfect, [???] Perfect, [???] Perfect
 Color:
colorless or white.
 Density:
2.55
 Diaphaniety:
Transparent to translucent
 Hardness:
4.5 - Between Fluorite and Apatite
 Luminescence:
Non-fluorescent.
 Luster:
Vitreous - Greasy
 Streak:
white
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