Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals

Khomenko, V. M.; Langer, Klaus; Beran, A.; Koch‐Müller, Monika; Fehr, Th.

doi:10.1007/bf00203218

Cited by 27 publications

(18 citation statements)

References 8 publications

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“…Incorporation of hydroxyl in the garnets described here would be consistent with the concept of precipitation of crack-healing garnet aided by an aqueous fl uid. Beran (1996) concluded that incorporation of defect hydroxyl in nominally anhydrous, close-packed silicate structures requires associated vacancies in the cation sites, for example vacancies at octahedral Cr or Al sites (Andrut et al 2002), or on tetrahedral sites associated with substitution of octahedral Ti for tetrahedral Si and octahedral Al (Khomenko et al 1994). Spatial variability of hydroxyl concentration within individual garnet grains has been resolved using FTIR spectroscopy by Khomenko et al (1994).…”

Section: Discussionmentioning

confidence: 97%

“…Beran (1996) concluded that incorporation of defect hydroxyl in nominally anhydrous, close-packed silicate structures requires associated vacancies in the cation sites, for example vacancies at octahedral Cr or Al sites (Andrut et al 2002), or on tetrahedral sites associated with substitution of octahedral Ti for tetrahedral Si and octahedral Al (Khomenko et al 1994). Spatial variability of hydroxyl concentration within individual garnet grains has been resolved using FTIR spectroscopy by Khomenko et al (1994). A similar type of coupled substitution in zircon in which Y and REE replace Zr, balanced by hydroxyl and vacancy defects, has been proposed as the source of cathodoluminescent emission (Ohnenstetter et al 1991).…”

Section: Discussionmentioning

confidence: 99%

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Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope

Cuthbert

Buckman

2005

American Mineralogist

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Gaseous secondary electron (GSE) imaging of eclogite garnets under the environmental scanning electron microscope (ESEM) at low chamber gas pressures reveals detailed image contrast patterns (charge-contrast images, CCI) that are not present in back-scattered or secondary electron images. Image intensity is a function of the amount of surface charge accumulation. Successful acquisition of CCI depends on frame size and beam scan rate at a given chamber gas pressure and beam current. Images are obtained in a few seconds, and are stable and reproducible. CCI patterns do not correlate with cracks or grain boundaries, but do correspond closely to variations in major-element composition, both in the form of concentric (growth) zoning, and branching, linear features interpreted as cracks that have been healed by new garnet growth. Causes of CCI are not yet well understood, but may be related to variations in lattice defect density and their infl uence on charge-trapping and dissipation. These in turn infl uence the rate of charge build-up at or very close to the specimen surface. One interesting possibility is that CCI images detect vacancies related to non-homovalent coupled substitutions involving, for example, REE and hydroxyl, so the method offers a way of imaging the distribution of these trace species in garnets. The CCI images are rich in microstructural detail and offer the potential for rapid, high-resolution, low-noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets.

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Section: Discussionmentioning

confidence: 97%

Section: Discussionmentioning

confidence: 99%

Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope

Cuthbert

Buckman

2005

American Mineralogist

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“…The concentration of Ti in aluminous garnet is generally higher at higher pressures (P) and/or higher temperatures (T), although there is considerable variation and Ti P-T relationships are complicated (e.g., Khomenko et al 1994;Ono 1998;Massonne and Brandelik 1998;; Kawasaki and Motoyoshi 2007;Hermann and Spandler 2008;Konzett and Frost 2009;Auzanneau et al 2010;Proyer et al 2013). Ti generally is thought to go into the octahedral site of garnet, but can also enter the tetrahedral site (e.g., Malitesta et al 1995;Massonne and Brandelik 1998;Ackerson et al 2013;Proyer et al 2013).…”

Section: Introductionmentioning

confidence: 99%

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

Axler

Ague

2015

American Mineralogist

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This study investigates distinctive oriented, needle-shaped multiphase inclusions found within cores of garnets from felsic granulites of the Upper Member of the Bigelow Brook Formation and the Brimfield Schist (northeastern Connecticut, U.S.A.). The rocks crop out in the southern end of the Central Maine Terrane and thus are a part of the Acadian/Neoacadian Orogen. The typical mineral assemblage is garnet + sillimanite + K-feldspar + plagioclase + quartz + cordierite + biotite ± spinel. The sillimanite is commonly pseudomorphous after kyanite.The multiphase needle inclusions and associated exsolved needles of rutile and ilmenite are mostly oriented parallel to <111> of garnet. The multiphase needles contain various combinations of quartz, micas, chlorite, rutile, graphite, a siliceous compositionally variable phase ("Phase-CV"), Zn-spinel, apatite, zircon, and rare ilmenite. We hypothesize that they represent inclusions of fluid that adhered to exsolving Ti±Fe oxide needles (mostly rutile) or extended along zones of weakness in garnet. This requires that multiphase needle formation occurred in response to cooling and/or decompression. The needles ultimately decrepitated during retrogression. We note that micaceous needle-shaped multiphase inclusions are rarely described; the closest analogs of which we are aware are found in UHP garnets of the Erzgebirge (Perchuk 2008).The Brimfield Schist in this area underwent ultrahigh-temperature metamorphism (UHTM) of ~1000 °C at a minimum pressure of ~1 GPa (Ague et al. 2013). Here we provide new temperature estimates for the adjacent Upper Member of the Bigelow Brook Formation. Ternary feldspar reintegration using the activity model of Benisek et al. (2004) and Zr-in-rutile thermometry (Tomkins et al. 2007) give average temperatures of ~990 and ~1010 °C, respectively, at 1 GPa for this unit. Therefore, the recently discovered UHT zone in the Brimfield Schist of northeastern Connecticut extends to the east to include the Upper Member of the Bigelow Brook Formation. Consequently, the first confirmed regional UHT locality in the United States is larger than initially recognized, and is at least 25 km long and 5-10 km wide. The oriented, elongate multiphase inclusions and petrographically obvious oriented Ti±Fe oxide needles may be useful indicators of extreme temperature and/or pressure rocks in other field areas.

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“…Principally, they should occur here as an overlap between the respective bands originating from Ti 3 § in the pyrope and rutile matrix as well. The increasing absorbance below 16000 cm -1 is typical of defect-bearing rutiles (Johnson et al 1968;Becker 1993, personal communication;Khomenko et al 1994). The most remarkable feature of the spectra of pyropes with rutile inclusions is the stepwise increase of absorbance in the UV, indicating the presence of two absorption edges (Fig.…”

Section: Optical Absorption Spectra and Their Interpretationmentioning

confidence: 95%

Single crystal absorption spectra of synthetic Ti, Fe-substituted pyropes

et al. 1994

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Abstract. Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by titanium or by titanium plus iron, were grown under defined conditions of P, T, x, fo2 in the presence of water using a piston-cylinder device. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by means of a microscope-spectrometer. Two absorption bands at 16100 and 22300 cm 1 in the spectra of pale-blue Fe-free Ti-bearing pyropes, grown under reduced conditions, were identified as originating from spin-allowed transitions, derived from 2T2g---+2Eg of octahedral Ti 3+ ions. The splitting value of the excited 2Eg state, 6200 cm -1, and the crystal field

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Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals

Cited by 27 publications

References 8 publications

Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope

Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

Single crystal absorption spectra of synthetic Ti, Fe-substituted pyropes

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