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 2ndorder 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