Mineral description : Cristobalite

Mineral class Silicates : Quartz group.
Chemical formula SiO2
Crystal system Tetragonal.
Habitus Spherical. Pseudo-isometric microscopic crystals. Pseudomorphs of beta-cristobalite occurs as pseudo-octahedrons.
Cleavage None, fracture brittle.
Hardness 6.5
Density 2.32
Color Colorless to white.
Streak White.
Luster Vitreous.
Description Christobalite is a polymorph of quartz. It is believed to form under high temperature in volcanic rocks. There is a high temperature phase called beta-cristobalite that crystallises in the isometric crystal system, creating octahedral crystals. When the temperature goes down the internal structure transforms into cristobalite but the crystal form is not changed, therefore is the most cristobalite crystals pseudomorphs of beta-cristobalite.
Cristobalite is crystallized from molten rock at temperatures above 1470 degrees Celsius. If the temperature is lower, silica crystallizes as tridymite (870-1470 degrees Celcius) or beta-quartz (below 870 degrees Celsius).
Occurance In volcanic rocks. Crystals in crevices and cavities.
Associates  
Notes Refractive index : 1.48
Type locality : Cerro San Cristobal, Pachuca, Estado Hidalgo, Mexico.
Ref : MinMag 46, pg 187
Locations A very common mineral
  • Duobblon, somewhere in Sweden. Send me a mail if you know where it is.

To silicate index.
Mineral group index.
Main index.

If you have some questions, suggestions or comments you are welcome to write me a line or two.


axelsson@acc.umu.se
To my homepage.
Last changed : 1999-03-09