Cristobalita
Cristobalita is a mineral with formula SiO₂, in the Silicatos [Tectosilicatos (Grupo Cuarzo)] group. This specimen comes from Cougar Butte occurrence, Cascade Range, Siskiyou County, California, Estados Unidos and joined the Terrium collection in 2011.
Description
Small cream-coloured sphere, speckled with coffee-brown dots, embedded in black obsidian matrix. The obsidian shows sharp edges and a sail-like silhouette, with vitreous lustre and clean fractures.
History of this specimen
Almost all the cristobalite pieces from this locality usually come from Muyane's collection, assembled in the 1950s–1970s. Burton Ford bought this one from Bruce Wood Minerals at the 2011 Tucson show.
About Cristobalita
Cristobalite is a polymorph of quartz. Same silica content but a different structure and a different formation process. Cristobalite and quartz share the same chemical composition, SiO₂, but they do not arrange their atoms in the same way. That is why we speak of polymorphs: same chemistry, different crystal structure and, in general, different formation conditions. Cristobalite typically appears in high-temperature volcanic contexts, whereas quartz dominates under more stable conditions. Obsidian also derives from a silica-rich magma, but it does not crystallise: it cools so rapidly that the atoms do not have time to arrange and the material remains as glass. With time—through heat, fluids or internal rearrangement—parts of that glass can reorganise and pass from the vitreous to the crystalline state. That process is called devitrification. When devitrification starts at a point, microcrystals grow outwards and the growth front advances fairly uniformly in all directions. Such radial growth commonly generates rounded volumes, such as nodules or spherulites. The sphere does not "fall" into the obsidian—it forms within it, as a zone in which silica ceased to behave as glass and organised into a crystalline structure.
About the locality
It is a little-cited Californian occurrence for this type of specimen, which is why it is of interest: when it turns up, it turns up with a "signature" look. The typical piece combines jet-black, very lustrous obsidian with sharp edges with cream cristobalite nodules (eyes). Most material shows incomplete, ill-defined or broken eyes, and many spheres do not fully close. Hence a clean, centred and truly spherical "eye", like this, is very scarce even beyond the locality.
Technical data
- Catalogue No.
- 0285
- Composition
- SiO₂
- Name
- Cristobalita
- Group
- Silicatos [Tectosilicatos (Grupo Cuarzo)]
- Category
- Meritum Persé
- Matrix
- Obsidiana
- Associations
- fayalita
- Mine
- Cougar Butte occurrence
- District / Municipality
- Cascade Range
- Province
- Siskiyou County
- Region
- California
- Country
- Estados Unidos
- Size (cm)
- 2.7 x 8 x 4.8
- Weight
- 77.8 g
- Acquired
- 2011
- Ex-collection
- Burton Ford
- Etymology
- Named after Cerro San Cristóbal (Pachuca, Hidalgo, Mexico), its type locality.
- Quality
- Top
- Value trend
- Al alza
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