A TEM investigation of natural metamict zircons: structure and recovery of amorphous domains

Capitani, Giancarlo; Leroux, Hugues; Doukhan, Jean-Claude; Rı́os, Susana; Zhang, Ming; Salje, Ekhard K. H.

doi:10.1007/s002690000100

Cited by 75 publications

(52 citation statements)

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“…Considering the cracked or porous structure seen in Figure 1p, a possible explanation for the low analytical totals is that some molecular water was retained within pores or microcracks. However, though conceivable, it appears to be more likely that most of the water, as OH (formed by hydrolysis of the amorphous silica network) and possibly as H 2 O, was trapped and dissolved within interstitial amorphous remnants, which were found to still occur in partially annealed metamict zircon (e.g., Capitani et al 2000). However, the exact water speciation as well as the its location, which relates to the question as to whether water is a true catalyst in a precise definition of the term, remains speculative and is a challenge for ongoing research.…”

Section: Hydrothermal Alteration Of Partially Metamict Zirconmentioning

confidence: 97%

“…Also, the amorphous phase becomes increasingly polymerized during progressive self-irradiation (Farnan and Salje 2001). Short-time laboratory heating of a partially metamict zircon to temperatures up to about 700 ∞C induces only the recovery of the disordered crystalline remnants, i.e., the removal of defects inside the crystalline material, whereas at higher temperatures or longer annealing times, epitaxial recrystallization of the amorphous phase takes place (e.g., Colombo et al 1999;Capitani et al 2000;Zhang et al 2000aZhang et al , 2000bGeisler et al 2001a;Geisler 2002). In heavily metamict crystals, the amorphous phase decomposes to ZrO 2 and SiO 2 before new ZrSiO 4 crystallizes (e.g., Capitani et al 2000;Zhang et al 2000aZhang et al , 2000b.…”

Section: Introductionmentioning

confidence: 99%

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Experimental hydrothermal alteration of partially metamict zircon

Geisler

Pidgeon

Kurtz

et al. 2003

American Mineralogist

272

140

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We present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from Sri Lanka in the temperature range 350 to 650 ∞C and with different solutions (2 M AlCl 3 , 2 M CaCl 2 , pure H 2 O, and a multi-cation solution). Under these conditions, sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems. The reaction zones are characterized by a marked increase in the cathodoluminescence intensity, a decrease of the back-scattered electron emission, and an increased degree of structural order, as revealed by micro-Raman and infrared spectroscopy. Sensitive high-resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations (e.g., Ca 2+ , Ba 2+ , Mg 2+ , Al 3+ ) from the solution and lost variable amounts of Zr, Si, Hf, the REE, U, Th as well as radiogenic Pb. A comparison between "dry" and "hydrothermal" annealing trends shows that the kinetics of structural recovery, including recrystallization of the amorphous phase in metamict zircon, is strongly enhanced under hydrothermal conditions. This finding suggests that water "catalyzes" structural recovery processes in metamict zircon. We found that the structure of the reacted areas does not resemble that of crystalline zircon, i.e., is still characterized by a temperature-dependent degree of disorder, which would not be expected if the reaction is controlled by a coupled dissolution and re-precipitation process. Instead, the alteration process can be described best by a diffusion-reaction-recrystallization model. In this model, it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts. We found that the rate and the extent of solid-state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements. This interpretation is indicated by the observation that trace elements, including U and Th, were preferentially lost during the reaction with a fluid at low temperatures, where recrystallization of the amorphous material was slow or not activated at all. The observed chemical alteration patterns are believed to reflect a competition between the kinetics of long-range diffusion and ion exchange and the kinetics of the short-range diffusion necessary for the recrystallization process.

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Section: Hydrothermal Alteration Of Partially Metamict Zirconmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Experimental hydrothermal alteration of partially metamict zircon

Geisler

Pidgeon

Kurtz

et al. 2003

American Mineralogist

272

140

View full text Add to dashboard Cite

show abstract

“…Distinct steps of thermal recovery of amorphous zircon are known from annealing experiments between ∼1100-1450°C that result in distinct granular textures (Weber 1990;McLaren et al 1994;Capitani et al 2000;Zhang et al 2000a;Ewing et al 2003). Granular textured zircon with dark colors and reduced birefringence may therefore represent less intense thermal annealing at lower temperatures, while the polycrystalline type probably experienced more prolonged annealing at higher temperatures.…”

Section: Granular Textured Zirconmentioning

confidence: 99%

Shock‐metamorphosed zircon in terrestrial impact craters

Wittmann

Kenkmann

Schmitt

et al. 2006

Meteorit & Planetary Scien

156

201

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Shock-metamorphosed zircon in terrestrial impact cratersAbstract-To ascertain the progressive stages of shock metamorphism of zircon, samples from three well-studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well-preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock-metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high-pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO 2 only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post-shock temperatures in excess of ∼1700 °C but could also be shock pressure-induced, which is indicated by possible relics of a high-pressure polymorph of ZrO 2 . However, ZrO 2 was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO 4 and reidite that reverted to zircon. This requires annealing temperatures >1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post-shock temperature history of suevite-like rocks until cooling below ∼600 °C.

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“…It has been shown that damaged zircon contains a random network of crystalline and amorphized phases (Murakami et al, 1991;Salje et al, 1999;Capitani et al, 2000). The radiation-damaged zircon can be treated as a mixture of amorphous and damaged crystalline zircons in a first approximation -ignoring the interaction between amorphous areas and damaged crystalline areas.…”

Section: Effective Medium Approach and The Fraction Of The Amorphizedmentioning

confidence: 97%

“…Extensive investigations were performed to gain understanding of what happens at the atomic level in metamict materials during high-temperature annealing (Weber, 1991;Ellsworth et al, 1994;Farges, 1994;Colombo and Chrosch, 1998;Meldrum et al, 1998;Capitani et al, 2000;Zhang et al, 2000a,b;Geisler et al, 2001b). However, controversies remain regarding the recrystallization path and activation energy.…”

Section: Recrystallization and Structural Recovery At High Temperaturesmentioning

confidence: 98%