Defect structure and luminescence behaviour of agate — results of electron paramagnetic resonance (EPR) and cathodoluminescence (CL) studies

Abstract

Generally, the abundance of 03 centres (silicon vacancy) and E'I centres (oxygen vacancy) in agate is remarkably higher than in quartz. The high defect density in agates points to rapid growth of silica from a strongly supersaturated solution probably with a noncrystalline precursor.CL microscopy reveals internal structures and zoning in agates and quartz incrustations which clearly differ from those discernible by conventional polarizing microscopy. The CL spectra of agates differ from those of quartz from crystalline rocks. At least three broad emission bands were detected in the CL spectra: a blue band of low intensity, a yellow band at about 580 nm, and an intense red band at 650 nm. The CL emission at 650 nm shows some relations to the hydroxyl or alkali content and the abundance of O 3-centres and E' 1 centres. The emission intensity increases during electron bombardement due to the conversion of different precursors (e.g. -Si-O-H, --Si-O-Na groups) into hole centres. Another conspicuous feature in the CL spectra of agates is the existence of a yellow emission band centred at around 580 nm. The predominance of the yellow CL emission band and the high concentration of E' 1 centres are typical for agates of acidic volcanics and are indicative of a close relationship between the two.

Cathodoluminescence characterization of enstatite

“…A red emission at around 670 nm in the synthetic enstatite is assigned to an impurity center derived from activated Mn 2+ substituted for Mg as an electron transition of 4 T 1g (G) → 6 A 1g (S) (Catalano et al, 2014). This electron transition similar to those in olivine and calcite is affected by the crystal field of the ligant oxygens coordi- nated to the host Mn ion (Marfunin, 1979;Götze et al, 1999). Lofgren and DeHart (1992) confirmed that synthetic enstatite without impurities of Mn and Cr emits blue CL, suggesting other emission centers not related to impurity centers.…”

Cathodoluminescence characteristics of Archean volcanogenic massive sulfide related quartz: Noranda, Ben Nevis and Matagami districts, Abitibi Subprovince, Canada

“…Lasting less than 30 s, the yellow CL shifts to reddish violet, similar to the final color observed in the quartz grains. Götze et al [1999] detected yellow (580 nm) CL emission in agates and hydrothermal vein quartz, which they associated with high concentrations of lattice defects, primarily E 1 centers (oxygen vacancies). Lattice defects are extremely common in these types of quartz, a result of rapid silica growth from strongly supersaturated solutions, in some cases with a noncrystalline precursor [Götze et al, 1999].…”

“…Götze et al [1999] detected yellow (580 nm) CL emission in agates and hydrothermal vein quartz, which they associated with high concentrations of lattice defects, primarily E 1 centers (oxygen vacancies). Lattice defects are extremely common in these types of quartz, a result of rapid silica growth from strongly supersaturated solutions, in some cases with a noncrystalline precursor [Götze et al, 1999]. The transient bright yellow CL associated with quartz microcracks and grain boundary fillings clearly documents secondary hydrothermal infiltration through the original massive sulfide mounds.…”

“…These structures and their host quartz are characterized by a very unstable (t < 20 s) bright yellow CL indicating a high concentration of E 1 center (oxygen vacancy) lattice defects [Götze et al, 1999]. As noted above, E 1 center lattice defects are common in agate and hydrothermal quartz as a result of rapid silica growth from a strongly supersaturated solution and noncrystalline/chalcedonic precursor [Götze et al, 1999]. The suggestion that quartz may form from a noncrystalline precursor, such as opal via chalcedony, is not new [e.g., White et al, 1956;Fournier, 1985;Alt et al, 1987].…”

Imaging and visualising nanometre scale surface geometry of a crystalline mineral (SiO2) in monochromatic spectra

“…With the recent development of scientific tools many advanced researches are conducted to understand the physical natures of quartz. Agrosì et al (1992), Rink et al (1993), Onasch and Vennemann (1995), Stevens Kalceff and Phillips (1995), Carpenter et al (1998a), Götze et al (2001), Botis et al (2005) and Götze et al (2005) focused on different physical characteristics of quartz and its importance as a natural compound. Considering the physical importance of quartz in material and earth sciences we decided to perform an experiment on imaging for visualising nanomorphological characteristics or surface geometry this mineral by digital imaging.…”