High temperature stability of natural maghemite: a magnetic and spectroscopic study

Abstract

S U M M A R YA combined magneto-mineralogical approach is used to diagnose maghemitization in magnetic grains of basaltic rock fragments from sand dunes in the Namibian desert in SW Africa. Data were obtained from static magnetic analysis, ferromagnetic resonance (FMR) spectroscopy, micro-Raman spectroscopy and electron microscopy. Micro-Raman spectroscopy showed that the magnetic grains in the lithic fragments form oxidative solid solution series with magnetite and maghemite as end-members. The five active Raman modes at 712, 665, 507, 380 and 344 cm −1 indicate that maghemite in the magnetic grains has well-defined structural properties. The FMR spectral analysis provides evidence for long-range dipolar coupling, which suggests intergrowth of the magnetic phases of the oxidative solid solution series. Thermomagnetic experiments and hysteresis measurements reveal a Curie temperature of about 890 K for this maghemite. Upon heating to 970 K part of the maghemite is altered to thermodynamically more stable hematite. After selective thermal decomposition of the maghemite in a protected atmosphere, the remaining magnetic phase has a Curie temperature of 850 K, characteristic for magnetite. The unique thermal stability of this natural maghemite above its Curie temperature is explained by the well-defined mineral structure, which formed during slow oxidative alteration of magnetite under arid climate conditions.

Evidence for controlled insertion of Fe ions in the framework of clinoptilolite natural zeolites

“…As the temperature decreases, the corresponding g eff value and linewidth increase, reaching maximum values 77 K, with an overall reduction of the intensity. A similar trend with temperature was ascribed to dispersed and randomly oriented iron oxides magnetite-maghemite (Fe 3 O 4 , c-Fe 2 O 3 ) nanoparticles [29,30].…”

High-temperature thermomagnetic properties of vivianite nodules, Lake El'gygytgyn, Northeast Russia

“…Continued cycling of samples from the first type shows in the second run drop of MS at ∼ 620 • C. On the third run the temperature has increased to 650 • C. If this is indicative of a Curie temperature (630-650 • C), it is lower than that of hematite and closer to that of maghemite (Özdemir, 1990;Gehring et al, 2009). According to J rs /J s and B cr /B c ratios pseudo-single-domain particles were formed during the first heating (Day et al, 1977).…”

Supergrupo Minas na Serra de Bom Sucesso, extremo sudoeste do Quadrilátero Ferrífero - MG: petrografia, geoquímica e isótopos de U-Pb

“…Segundo Dunlop e Özdemir (1997), a transição de Verwey e Curie ocorre na magnetita a -153 e 580°C e, na hematita, a -15 e 680°C. A maghemita possui temperaturas A granulação é variável, de muito fina a média (< 0,01 a 1,20 mm), e as curvas termomagnéticas de 10 amostras de FFB mostram que são desmagnetizados em torno de 620 e -138°C (Figuras 4C e D), indicando o mineral maghemita (Gehring et al, 2009). Localmente, em análi-ses de lâminas delgadas polidas, foi observada a presença de relictos de magnetita, que se distingue da maghemita pela cor cinza escura ou ocorrendo como cristais subédri-cos a euédricos (Figura 4E).…”