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Silicates (several polymorphs) SiO 2 Presented by Paul Sandlin.

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Presentation on theme: "Silicates (several polymorphs) SiO 2 Presented by Paul Sandlin."— Presentation transcript:

1 Silicates (several polymorphs) SiO 2 Presented by Paul Sandlin

2 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


4 Alpha-Quartz

5 Beta-Quartz

6 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

7 Quartz Occurrences Common and abundant Igneous, metamorphic, sedimentary, pegmatite veins, deposited on sea floor Mechanically and chemically stable

8 Orthorhombic-Tridymite

9 Hexagonal Tridymite

10 Tridymite monoclinic


12 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 SiO 2, but small amounts of Na and Al may be in solid solution Stable from 870°C to 1470°C

13 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

14 Alpha- Cristobalite

15 Beta-Cristobalite


17 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)

18 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

19 Coesite

20 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)

21 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

22 Stishovite

23 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

24 Stishovite occurrences High pressure environments - meteoritic impacts


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