Cristobalite

Observe this: Cristobalite. A silica mineral, a mere polymorph of quartz, yet it demands attention. Not because it’s particularly exciting, but because it exists, stubbornly, at temperatures most things would simply cease to bother. The specimen you see, those modest spherulites within an obsidian matrix, are a product of devitrification—the slow, inevitable return to order from chaos, even if that order is a rather specific kind of chaos. This particular example hails from California, US, measuring a respectable 5.9 cm × 3.8 cm × 3.8 cm (2.3 in × 1.5 in × 1.5 in).

General Characteristics

• Category: A tectosilicate mineral, belonging to the rather extensive quartz group.
• Chemical Formula: Predictably, SiO 2. Because why complicate things?
• IMA Symbol: Crs [1] – a designation as arbitrary as most.
• Strunz Classification: 4.DA.15.
• Dana Classification: 75.1.1.1.
• Crystal System: Tetragonal, for the low-temperature form, α-cristobalite.
• Crystal Class: Trapezohedral (422).
• Space Group: P 4 1 2 1 2, P 4 3 2 1 2.
• Unit Cell (α polytype): a = 4.9709(1) Å, c = 6.9278(2) Å; Z = 4.
• Structure: For those who appreciate the intricacies, a Jmol (3D) Interactive image is available, if you're inclined to stare at atomic arrangements.

Identification

• Color: Typically colorless, or a rather uninspired white.
• Crystal Habit: Forms rather neat octahedra or those aforementioned spherulites, sometimes reaching several centimeters across.
• Twinning: Exhibited on {111}.
• Fracture: Conchoidal, like broken glass, which is fitting given its composition.
• Tenacity: Brittle, prone to shattering, much like certain illusions.
• Mohs Scale Hardness: A respectable 6–7, putting it firmly in the category of things that can scratch other things.
• Luster: Vitreous, again, like glass.
• Streak: White.
• Diaphaneity: Transparent, allowing you to see right through its simple existence.
• Specific Gravity: Ranging from 2.32 to 2.36.

Optical Properties

• Optical Properties: Uniaxial (−).
• Refractive Index: n ω = 1.487, n ε = 1.484.
• Birefringence: 0.003. A subtle difference, easily overlooked.
• Pleochroism: None. It makes no effort to change its appearance.

Thermal Properties

• Melting Point: 1,713 °C (3,115 °F) for the β-phase [2]. A temperature that speaks of deep Earth processes, or perhaps a particularly ambitious forge.