Silicates (several polymorphs)

SiO2

Presented by Paul Sandlin

• 3 principal crystalline forms

- Quartz, tridymite, and cristobalite

• Sluggish transformation, so high temp forms (cristobalite and tridymite) can exist metastably below their inversion temps

• Each has low and high temp modification designated α and β respectively

Alpha-Quartz

Beta-Quartz

Quartz

• Quartz is most to a pure compound

• Bachheimer (1980) found evidence for 1st-order transition from α-quartz to intermediate phase at 573°C and 2nd-order transition to β-quartz at 574°C

- micro twinning upon cooling high quartz

• Only minor atomic adjustments without breaking of Si-O bonds

Quartz Occurrences

• Common and abundant

• Igneous, metamorphic, sedimentary, pegmatite veins, deposited on sea floor

• Mechanically and chemically stable

Orthorhombic-Tridymite

Hexagonal Tridymite

Tridymite monoclinic

Tridymite

• When pure quartz is heated, it bypasses tridymite and transforms directly to cristobalite at ~ 1050°C (Mosesman and Pitzer, (1941)

- “Mineralizing agent” needed for tridymite formation

• Several low-temp polymorphs

• Ideally SiO2, but small amounts of Na and Al may be in solid solution

• Stable from 870°C to 1470°C

Tridymite occurrences

• Typical occurrence is in acid volcanic rocks such as rhyolite, obsidian, trachyte, andesite and dacite.

- Often found in cavities of such rocks

• ? If it occurs magmatically (“metamorphic”)

- pneumatolytic metamorphism

• 6 months after Mt. Pelée eruptions

Alpha- Cristobalite

Beta-Cristobalite

Cristobalite

• Contains some Na and Al

• Low cristobalite structure is tetragonal, whereas high cristobalite is isometric.

• Stable from 1470°C to 1728°C (melting point)

Cristobalite occurrences

• Typically a mineral of volcanic rocks

- may occur in cavities, often in association (metastable) with tridymite

• Found in obsidian, rhyolite, trachyte, andesite, dacite, and olivine basalt.

• Often a late product of crystallization

• Due to the ability to occur as an unstable form outside equilibrium field, time of crystallization is difficult to pinpoint

Coesite

Coesite

• Composed of four-membered rings of Si tetrahedra linked at corners to form chains parallel to c.

• One Si-O-Si angle constrained to be 180° because this O1 site is located on a center of symmetry

• Slight distortion occurs with pressure, and Si2-O2-Si2 angle decreasing from 142.7° to 136.4° at 5.19 GPa (Levien and Prewitt, 1981)

Coesite occurrences

• Recently discovered in sheared porous sandstones at Meteor Crater, Arizona

• Granite and pumaceous tuff near the rim of the Rieskessel crater, Bavaria

- developed by the shock wave generated by meteoritic impact

Stishovite

Stishovite

• Prototype phase having octahedrally coordinated silicon

• Structural properties at high pressure are highly sensitive to stress (Ross et al., 1990)

• More compressible in the a direction than the c direction due to significant Si-Si repulsion across the shared edges of octahedra that form chains in the c direction (Ross et al., 1990)

• At ambient conditions, O-O distance of 2.29Å is one of the shortest found in any oxide not containing hydrogen

Stishovite occurrences

• High pressure environments

- meteoritic impacts