Mineral Information and Data:

Cristobalite
SiO2 [CNMNC approved formula]

Named after its locality: Cerro San Cristobal, Mexico.

IMA status : Valid species (Pre-IMA 1887) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
75.01.01.01  
  (75) Tectosilicate Si Tetrahedral Frameworks
  (75.01) SiO2 with [4] coordinated Si
  (75.01.01)
  
Nickel-Strunz
(10th edition) :
04.DA.15
  (04) Oxides and Hydroxides
  (04.D) Metal:Oxygen = 1:2 and similar
  (04.DA) With small cations: Silica family
  
Crystal system: Tetragonal System
Point group (H-M): 422 — trapezoidal
Unit cell: a = 4.9709 Å, c = 6.9278 Å
Crystal Habit: Crystals pseudo-octahedral or rarely pseudo-cubic, small; massive; stalactitic, spherulitic, crusts or botryoidal
Twinning: Common on {111}, penetration or polysynthetic
  
Color: White, gray, bluish-gray, yellowish, brownish
Diaphaneity: Transparent
Luster: Vitreous
Hardness (Mohs): 6.5
Measured Density: 2.33 g/cm3
Tenacity: Brittle
Streak: White
  
Polymorphism and Series: Polymorphic with quartz, tridymite, coesite, and stishovite; inverts from high- or β-cristobalite at 268 °C or below.
Geologic Setting: In vesicles and lithophysae; a late-crystallizing phase in basaltic to rhyolitic volcanic rocks; from acid-sulfate-type hydrothermal alteration of volcanic rocks; precipitated by hot springs. By contact metamorphism of sandstone; developed during diagenesis, recrystallized from siliceous sedimentary rocks.
Mineral Association: Tridymite, quartz, sanidine, anorthoclase, fayalite, magnetite, kaolinite, alunite,
"opal"
  
Synonyms/varieties: Lussatine

 

Cristobalite
Cristobalite on ferrohornblende
Origin: Bellerberg quarry, Laacher See, Rheinland-Palatinate, Gernany
Sample size: Microscopic image
Photo and Owner: Lou Perloff

 

Cristobalite
Cristobalite in obsidian (volcanic glass)
Origin: Cerro San Cristobal, Pachuca, Hidalgo, Mexico
Sample size: Microscopic image
Photo and Owner: Lou Perloff

 

Cristobalite
Cristobalite with fayalite (brown)
Origin: Coso Hot Springs, Inyo Co., California, U.S.A.
Microscopic image, 2.9 x
Photo and Owner: Milton Speckels

 

Cristobalite
Chalcedony-like parts of cristobalite variety lussatite on a bitumen-rich matrix
Origin: Puy de Dome, France
Sample size: 5 x 6 cm
Photo: Diederik Visser

 

Cristobalite
(zoom-in of above)

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