Cation mixing in natural MgAl2O4spinel; a high-temperature27Al NMR study

Maekawa, Hideki; Kato, Satoshi; Kawamura, Katsuyuki; Yokokawa, Toshio

doi:10.2138/am-1997-11-1210

Cited by 81 publications

(94 citation statements)

References 16 publications

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“…Notice that the inversion parameter strongly depends on the applied method. ESR (Schmocker and Waldner 1976) and 27Al NMR (Maekawa et al 1997) usually yield lower inversion parameters (0.05 and below) of gemstone spinels of similar formation conditions as Burmese spinel.…”

Section: Discussionmentioning

confidence: 99%

Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study

Widmer

Malsy²,

Armbruster

2014

Phys Chem Minerals

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688 nm), and N3 (689 nm). N lines are caused by different Mg, Al environments of Cr 3+ . With increasing inversion parameter (x), the R line decreases in intensity and the N lines become prominent leading to strongly broadened peaks with a maximum shifted toward higher wave lengths (687.5 nm at 1,100 °C). Criteria for the detection of heat treatment on gemstone spinel applicable to gemological routine examination are provided. Extrapolation of u, a, and bond lengths from heat-treated Burma spinel toward the natural crystal suggests a retrograde "closing temperature" of ca. 400 ± 100 °C at which Mg, Al disorder was frozen.

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Section: Discussionmentioning

confidence: 99%

Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study

Widmer

Malsy²,

Armbruster

2014

Phys Chem Minerals

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“…Since larger ions prefer the T-site of the 2-3 Sp, the Si-free MgAl2O4-Sp should generally adopt a normal Sp structure, as verified by some studies on natural Sp with compositions close to the MgAl2O4 formula (x = ~0.02-0.04 in Schmocker & Waldner [47]; x = 0.05 in Maekawa et al [51]). By the same token, Si in the MgAl2O4-Sp should occupy the M-site.…”

Section: Synthetic Mgal2o4-sp and Its Sio2mentioning

confidence: 85%

“…There has been excellent agreement on the T effect on the Mg-Al disorder process of the MgAl2O4-Sp at ambient P: x increases as T increases [42,47,48,50,51,[53][54][55][56][57]. As to the P effect at ambient T, discrepancy presumably exists because the order-disorder reaction could not be readily activated and did not adequately approach its equilibrium during the course of a conventional high-P study [52,90,91].…”

Section: Mg-al Order-disorder State Of Synthetic Mgal2o4-spmentioning

confidence: 99%

“…With a relatively small size difference between the Mg and Al cations in the MgAl2O4-Sp, the cation disorder achieved under high P-T conditions can be partially preserved [42,45,48,50,51,53]. In contrast, the relatively large size difference between the Mg and Si cations in the Rw may strengthen this size-dependent site preference and accelerate the cation-redistribution process during cooling, so that the cation disorder attained under high P-T conditions can be easily lost, leading to null signals for cation disorder, as observed experimentally [61,64].…”

Section: Introductionmentioning

confidence: 99%

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Si-Disordering in MgAl2O4-Spinel under High P-T Conditions, with Implications for Si-Mg Disorder in Mg2SiO4-Ringwoodite

Liu

Bao

et al. 2018

Minerals

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A series of Si-bearing MgAl 2 O 4 -spinels were synthesized at 1500-1650 • C and 3-6 GPa. These spinels had SiO 2 contents of up to~1.03 wt % and showed a substitution mechanism of Si 4+ + Mg 2+ = 2Al 3+ . Unpolarized Raman spectra were collected from polished single grains, and displayed a set of well-defined Raman peaks at~610, 823, 856 and 968 cm −1 that had not been observed before. Aided by the Raman features of natural Si-free MgAl 2 O 4 -spinel, synthetic Si-free MgAl 2 O 4 -spinel, natural low quartz, synthetic coesite, synthetic stishovite and synthetic forsterite, we infer that these Raman peaks should belong to the SiO 4 groups. The relations between the Raman intensities and SiO 2 contents of the Si-bearing MgAl 2 O 4 -spinels suggest that under some P-T conditions, some Si must adopt the M-site. Unlike the SiO 4 groups with very intense Raman signals, the SiO 6 groups are largely Raman-inactive. We further found that the Si cations primarily appear on the T-site at P-T conditions ≤~3-4 GPa and 1500 • C, but attain a random distribution between the T-site and M-site at P-T conditions ≥~5-6 GPa and 1630-1650 • C. This Si-disordering process observed for the Si-bearing MgAl 2 O 4 -spinels suggests that similar Si-disordering might happen to the (Mg,Fe) 2 SiO 4 -spinels (ringwoodite), the major phase in the lower part of the mantle transition zone of the Earth and the benchmark mineral for the very strong shock stage experienced by extraterrestrial materials. The likely consequences have been explored.

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“…The degree of cation distribution in spinel has been determined by various methods such as IR (infrared spectroscopy) [4] and Raman spectroscopy [5][6][7], NMR (nuclear magnetic resonance) [3,[8][9][10], calorimetric measurements [11,12], neutron diffraction [13,14], and ALCHEMI (atom location by channeling enhanced microanalysis) [15].…”

Section: Introductionmentioning

confidence: 99%

Neutron irradiation effect on site distribution of cations in non-stoichiometric magnesium aluminate spinel

Sawabe

Yano

2008

Journal of Nuclear Materials

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Cation mixing in natural MgAl₂O₄spinel; a high-temperature²⁷Al NMR study

Cited by 81 publications

References 16 publications

Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study

Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study

Si-Disordering in MgAl2O4-Spinel under High P-T Conditions, with Implications for Si-Mg Disorder in Mg2SiO4-Ringwoodite

Neutron irradiation effect on site distribution of cations in non-stoichiometric magnesium aluminate spinel

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