CRISTOBALITE
Cristobalite – SiO2 – is a very rare tectosilicate that occurs mainly associated with intermediate and acidic volcanic rocks. It has no economic importance.
It is one of the polymorphs of SiO2, with quartz, β-quartz, tridymite, coesite and stishovite.
Cristobalite is high temperature and low pressure. It is metastable at room temperature: there is a paramorphic transition from high-temperature (cubic) cristobalite to low-temperature (tetragonal) cristobalite. It may be intergrown with tridymite and may contain varying amounts of Ca, Na, Fe, Mg, Al, K, Ti, Mn, and P.
1. Characteristics
Crystal system: Tetragonal trapezohedral, pseudocubic.
Color: Colorless, white, milky, yellowish.
Habit: Dendritic to skeletal, spherulites, fibrous, massive, pseudo-octahedrons, more rarely pseudocubes.
Cleavage: No.
Tenacity: Brittle.
Twinning: By common {111}, interpenetrating, polysynthetic, repeated.
Fracture: Conchoidal.
Mohs Hardness: 6 – 7
Parting: No.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 2.32 – 2.36
2. Geology and Deposits
Cristobalite occurs mainly in acidic and intermediate igneous rocks that have undergone a very rapid cooling process; may occupy cavities (vesicles or lithophysae) or fractures. It also forms as a product of hydrothermal alteration of these rocks.
It is a common constituent of certain types of opal (C-opal and CT-opal) in marine sediments rich in biogenic silica. It can be precipitated by hot springs.
Cristobalite also occurs in combustion buchitos, which are rare metamorphic rocks formed by the thermal impact of lava flows or scoriaceous eruptions on sedimentary rocks or soils. The fusion of these rocks generates a rock with more than 20% of volcanic glass, called buchito.
It forms in sandstones that have undergone contact metamorphism; can be generated during diagenesis or by recrystallization of siliceous sedimentary rocks. May occur in meteorites.
3. Mineral Associations
Cristobalite occur associated with volcanic glass (obsidian), tridymite, quartz (including amethyst variety), chalcedony, feldspar (sanidine, anorthoclase), fayalite (olivine), magnetite, kaolinite, hematite, andradite (garnet), topazolite, alunite and “opal”.
4. Transmitted Light Microscopy
Refraction indices: nω: 1.487 nε: 1.484
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless, easily goes unnoticed.
Relief: Very low.
Cleavage: No, but may have many fractures.
Habits: Euhedral crystals (pseudo-octahedrons) up to 4 mm, rarely pseudocubic. Commonly dendritic to skeletal, also occurs tabular or as fibrous spherulites with diameters of several centimeters. May be massive or microcrystalline (“opal”) May be intergrown with tridymite. The habit of cristobalite in lussatite (= opal CT) and lussatin (= cristobalite) resembles the habit of chalcedony.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Cristobalite is a pseudo-isotropic mineral. The maximum birefringence is 0.003: it corresponds to 1st order colors from black to very dark gray.
Extinction: Tends to be parallel.
Elongation sign: Lussatite = ES(+); lussatine = ES(-).
Twins: Lamellar twins according to {111} are common. They may be interpenetrating, polysynthetic or repeated twins.
Zoning: No.
CONVERGENT LIGHT
Character: U(-), but may be anomalous biaxial.
2V angle: when anomalous biaxial, 2V of 25º.
Alterations: does not alters; this is an important diagnostic aspect to differentiate from plagioclases (albite).
May be confused with: some other low-relief colorless minerals.
Quartz has another habit, it is U(-) and shows lower birefringence.
Tridymite is B(+) and often has cyclic twins, while cristobalite usually has lamellar twins.
Chalcedony has higher interference colors.
Chabazite (a zeolite) shows anomalous sub-grains, has a better developed cleavage and occurs in different paragenesis.
