Cristobalite (inclusions in Obsidian)
Current inventory:  0 gems
 

  
Cristobalite is named after its first noted occurance (type locality) of Cerro San Cristóbal, Mun. de Pachuca, Hidalgo, Mexico.

Discovered in 1887; IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

SiO₂

Silicon Dioxide

Molecular Weight:

60.08 gm

Composition:

Silicon

46.74 %

Si

100.00 %

SiO₂

Oxygen

53.26 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Oxides

Strunz 8th Ed. ID:

4/D.01-30

Nickel-Strunz 10th Ed. ID:

4.DA.15

4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
D : Metal: Oxygen = 1:2 and similar
A : With small cations: Silica family

Related to:

Polymorph of Quartz, Tridymite, Coesite and Stishovite.

Varieties:

None

Synonyms:

Cosmic Obsidian, a-Cristobalite, b-Cristobalite, Low Cristobalite, Lussatine

Crystal Data

Crystallography:

Tetragonal - Trapezohedral

Crystal Habit:

As pseudo-octahedral crystals, to 4 mm, with {110} and {331}, rarely pseudocubic. Commonly dendritic to skeletal; as spherulites to several cm; fibrous or microcrystalline ("opal"), massive.

Twinning:

On {111}, common, interpenetrant, polysynthetic, repeated.

Physical Properties

Cleavage:

None

Fracture:

Irregular/Uneven

Tenacity:

Brittle

Moh's Hardness:

6.0 - 7.0

Density:

2.32 - 2.36 (g/cm³)

Luminescence:

Fluorescent; LW UV = Green Red; LW UV = Green Red

Radioactivity:

Not Radioactive

Other:

Thermal properties: inverts from high- or ß-Cristobalite at 286°C or below

Optical Properties

Color:

Colorless, White, milky White to Yellowish, Blue-Gray, Gray; Colorless in transmitted light.

Transparency:

Transparent to Translucent

Luster:

Vitreous

Refractive Index:

1.484 - 1.487  Uniaxial ( - )

Birefringence:

0.003

Dispersion:

None

Pleochroism:

None

Occurances

Geological 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.

Common Associations:

Alunite, Anorthoclase, Fayalite, Kaolinite, Magnetite, Opal, Quartz, Sanidine, Tridymite

Common Impurities:

Fe, Ca, Al, K, Na, Ti, Mn, Mg, P

Type Locality:

Cerro San Cristóbal, Mun. de Pachuca, Hidalgo, Mexico

Year Discovered:

1887

View mineral photos:

Cristobalite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Cristobalite is a polymorph of all of the members of the Quartz Group of minerals. These include Quartz, Coesite, Tridymite and Stishovite. A polymorph means that these minerals have the same chemistry, SiO2, but different structures. Cristobalite is common as microscopic crystals in volcanic rocks but larger, well formed crystals found in the vesicles of volcanic rocks are rare. Cristobalite itself could not be faceted, but bright white Cristobalite crystals captured in transparent black Obsidian (as pictured above) can make very attractive gems. These gems have been called Cosmic Obsidian. Material from Lipari Island, Messina Province, Sicily, Italy is the main source of this type of gem.

Notable occurrences include the type locality of Cerro San Cristóbal, Mun. de Pachuca, Hidalgo, Mexico; Eifel District, Germany; the Ellora Caves, Maharashtra, India; Lipari Island, Messina Province, Sicily, Italy; Goroyama, Japan; about 150 km north of Auckland, New Zealand; in the USA in the San Juan Mountains, San Juan County, Colorado; Coso Hot Springs, Inyo County, California; near Crater Lake, Klamath County, Oregon; and many other locations.
 

  
Cristobalite gems for sale:

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