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Chemical Formula | SiO2 |
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Composition | Silicon dioxide |
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Color | White, light yellow, light gray, light brown, blue-gray |
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Streak | White |
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Hardness | 6 - 7 |
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Crystal Forms and Aggregates |
(Tetragonal) Cristobalite is a always a pseudomorph after Beta Cristobalite, which
crystallizes in the isometric system, and thus
Cristobalite occurs in isometric shaped crystals. (See The crystal structure of SiO2) Crystals are usually in microscopic grains, and small octahedral crystals can sometimes be detected. The octahedral crystals may be twinned with Spinel twinning. Cristobalite is much more common in globular groups and as rounded balls. It also occurs massive, platy, crusty, and stalactitic. Crystals are occasionally cubic. It is interesting to note that the white "snowflakes" of Snowflake Obsidian are actually tiny clusters of white Cristobalite crystals. |
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Transparency | Translucent |
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Specific Gravity | 2.3 |
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Luster | Vitreous |
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Cleavage | None |
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Fracture | Conchoidal |
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Tenacity | Brittle |
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Other ID Marks | 1) Crystals exhibit a strong double refraction 2) Sometimes fluorescent |
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Other Names |  Lussatite, Lussatine |
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Polymorphs | See Polymorphs of SiO2 |
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In Group | Silicates ; Tectosilicates ; Silica group May be classified as an oxide by a few abstract references (Oxide ; Simple oxides) |
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Uses | Cristobalite is used in scientific study. Its crystals provide key information to how crystals form, and how they change over in different environments. |
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Striking Features | Mode of occurrence and crystal habits |
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Complex Tests | When heated to 350ºF (180ºC), Cristobalite becomes clear and transparent. However, when it cools down afterwards, it returns to its translucent state and becomes whitish again. |
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Popularity (1-4) | 3 |
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Prevalence (1-3) | 2 (However, very rare in visible crystals.) |
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Demand (1-3) | 2 |
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Distinguishing Similar Minerals |
There are many minerals similar to Cristobalite, but its mode of occurrence can distinguish it from all other minerals. |
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Commonly Occurs With |
Tridymite, Feldspar, Olivine, Hornblende |
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Noteworthy Localities |
Cristobalite occurs in igneous rocks located in areas of volcanic
activity. It rarely occurs in detectable samples. Some
localities where noticeable material has come from are: Cerro San Cristobal, Pachua, Mexico (the origination of its name); Little Lake, Coso Hot Springs, Inyo Co., California; the San Juan Mountains in Colorado; Obsidian Cliff in Yellowstone National Park, Wyoming; Mt. Lassen in Lassen Volcanic National Park in California; and Crater Lake National Park, Klamath Co., Oregon. |
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