Quartz - Polymorphs, Crystal Chemistry, CL and Provenance Potential

SiO₂ polymorphs

Polymorphs of SiO2 (simplified) Image source (Steve Dutch): http://www.uwgb.edu/dutchs/PETROLGY/Silica%20Poly.HTM

Polyhedral view of the beta quartz structure viewed along the c-axis. Several rotation axes are shown. Tetrahedra are viewed along twofold symmetry axes. Yellow, green and blue denote increasing distance from the plane of the diagram. Quartz has two polymorphs. Alpha quartz is trigonal and stable below 573 C. Above 573 C thermal agitation becomes vigorous enough to overcome the slight skewness of the chains and the structure inverts to beta quartz, which is hexagonal. The transformation involves no atomic rearrangement, and all quartz at surface conditions is alpha quartz.

CL of SiO₂ polymorphs

Inclusion of coesite (cs), quartz (qtz) and chalcedony (cha) in an inclusion within pyrope from Dora Maira Massif (Italy). This locality has reached ultrahigh P metamorphic conditions of >30 kb. CL image from a hot-cathode CL system   From Schertl et al. (2004) Eur. J. Mineral.

Textural information from CL

Example from Laubach et al. (2004) - crack seal in quartzites

Quartz CL response and chemistry

The actual local structure of quartz strongly influences the nature of the CL

• function of the type and frequency of lattice defects during crystallization
   

• influenced by post-crystallization effects e.g. metamorphic recrystallization and deformation
   

• lattice defects classed according to their size and structure i.e. point defects (key defect for CL), lattice translations, inclusions of paramagnetic minerals, and fluid inclusions.

Two main types of paramagnetic defect centers

• defects due to foreign ions (foreign ion centers or interstitial defects)

  • homovalent substitution (e.g. Ti⁴⁺ for Si⁴⁺)

  • heterovalent (coupled) substitution (e.g. Al³⁺ for Si⁴⁺ with interstitial Li⁺)

  • limited substitution is due to size and charge constraints i.e. Si⁴⁺ = 0.42A; Al³⁺ = 0.51A; Fe³⁺ = 0.64A; Ti⁴⁺ = 0.64A

  • interstitial cations are most commonly H⁺, Li⁺, Na⁺, K⁺, Cu⁺, Ag⁺

  • Al³⁺ is the most common substituent (up to 1000s of ppm), but Fe³⁺ and Ti⁴⁺ can be relatively high as well

   

• defects associated with vacant Si or O

Important emission bands:

• 390 nm (deep blue) - primarily due to [AlO₄/M⁺] center, and is sensitive to irradiation by electrons due to dissociation and electromigration of the charge-compensating interstitial cation (e.g. H⁺, Li⁺, Na⁺, K⁺)

• 500 nm (green-blue) - short-lived (30-60 seconds of e-bombardment) and primarily due to [AlO₄/M⁺] center, and is sensitive to irradiation by electrons due to dissociation and electromigration of the charge-compensating interstitial cation (e.g. H⁺, Li⁺, Na⁺, K⁺) - the highest Al yields the greatest CL intensity - also a stable 500 nm emission

• 580 nm (yellow) - associated with O vacancies - most common in agate and hydrothermal vein quartz - due to rapid growth

• 650 nm (red) - non-bridging O defects

• 705 nm (red) - due to Fe³⁺ substitution

Visible CL of natural quartz is mainly the result of emissions in the red and spectral regions and the resultant color depends on the relative intensity of each.

Hot cathode CL images of quartz from different sources. A. Blue-violet CL quartz from granite B,C. Brown CL polycrystalline quartz from schist   D. Oscillatory-zoned Blue-to-red CL quartz from rhyolite E. Blue-CL core and red-CL rim quartz from rhyolite F Broken blue-violet CL quartz with red-CL rim from rhyolite   G, H. Complexly-zoned hydrothermal quartz I, K Hydrothermal quartz with short-lived blue-CL (I) changing to brown-CL with continued electron bombardment   L. Red-orange CL quartz with snowball texture intergrown with albite (metamorphic?) M. Euhedral authigenic quartz in sulfate-facies evaporite with blue CL anhydrite inclusions N. Yellow-CL agate from granite

Hot cathode CL images of quartz from different sources. A. Red-CL quartz and albite with Kfs from fenite. Fe3+ is the cause of the CL. B,C. Blue-red CL core of quartz in rhyolite that is dissolved and with later hydrothermal growtht   D. Mixed CL (and source) in sandstone (blue-violet = igneous quartz; brown = metamorphic quartz; greenish-brown with zonation = hydrothermal quartz). Note authigenic overgrowths and radiation damage. E. Detrital quartz with radiation-damaged orange rims in a clastic U deposit.   F Silcrete with opal cement whose variation in blue are due to Al variability G Silicrete with detrital grains of multiple sources and brown-luminescent chalcedony cement H Silicified quartz sand sample with at least 3 generation of authigenic overgrowths   I,K. Sandstone with hydrothermal quartz veins L. Yellow-CL silicified wood with well-preserved cell structure and replacement quartz in blue-CL

Boggs attempt to use SEM-CL for quantifying provenance

SEM-CL

• attachment to SEM

• sequential RGB images

• post-acquisition processing

• stable beam and greater CL intensity

Some quantitative rigor   Volcanic generally bright blue   High grade metamorphic generally blue, but function of metamorphic grade   overlap of the fields limit the application to provenance

Walderhaug quartz orientation factor

A, B - CL and crossed polars view of polycrystalline quartz fragment from a 2500 m North Sea well. C, D - CL and crossed polars view of quartz from phyllite. E, F - CL and crossed polars view of quartz from migmatite. G, H - CL and crossed polars view of quartz from granodiorite. I, J - CL and crossed polars view of quartz from granite.

The promise of Ti in quartz as a geothermometer

Titaniq: Titanium-in-quartz thermometer (Spear, Wark and Watson, RPI)

Basis:

• Homovalent substitution of Ti⁴⁺ for Si⁴⁺ in quartz is a function of T

• The saturation level of Ti in quartz is fixed by the presence of rutile

• Ti is measurable on a EMP to concentration levels of 70-80 ppm Ti

• experimental calibration suggests the following relation:

T(K) = -4240/[log(Ti) - 6.15]

i.e. at 800 C the Ti should be 158 ppm, 750 C the Ti should be 100 ppm with uncertaintes of < +/- 5 C.

• at lower concentrations ( or T) it is likely necessary to use a SIMS