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.

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