Water incorporation in synthetic and natural MgAl2O4 spinel

Abstract

The solubility and incorporation mechanisms of water in synthetic and natural MgAl 2 O 4 spinel have been investigated in a series of high-pressure/temperature annealing experiments. In contrast to most other nominally anhydrous minerals, natural spinel appears to be completely anhydrous. On the other hand, non-stoichiometric Al-rich synthetic (defect) spinel can accommodate several hundred ppm water in the form of structurally-incorporated hydrogen. Infrared (IR) spectra of hydrated defect spinel contain one main O-H stretching band at 3343-3352 cm À1 and a doublet consisting of two distinct O-H bands at 3505-3517 cm À1 and 3557-3566 cm À1 . IR spectra and structural refinements based on single-crystal X-ray data are consistent with hydrogen incorporation in defect spinel onto both octahedral and tetrahedral O-O edges. Fine structure of O-H bands in IR spectra can be explained by partial coupling of interstitial hydrogen with cation vacancies, or by the effects of Mg-Al disorder on the tetrahedral site. The concentration of cation vacancies in defect spinel is a major control on hydrogen affinity. The commercial availability of large single crystals of defect spinel coupled with high water solubility and similarities in water incorporation mechanisms between hydrous defect spinel and hydrous ringwoodite (Mg 2 SiO 4 ) suggests that synthetic defect spinel may be a useful low-pressure analogue material for investigating the causes and consequences of water incorporation in the lower part of Earth's mantle transition zone.

Hydrogen and deuterium diffusion in non-stoichiometric spinel

“…The starting material for this study was a non-stoichiometric spinel from the same batch used in hydration experiments by Bromiley et al [10] (their sample 'synthetic spinel 1'), synthesised at ambient pressure and high temperature from spec-pure reagents using the Verneuil method. Pieces of this material were prepared as 1 × 1 × 2 mm polished rectangular prisms, cut using a diamond wire saw and polished gently using up to 1 µm diamond polishing suspensions.…”

“…However, Bromiley et al [10] demonstrated that synthetic, non-stoichiometric, Al-rich spinel can incorporate significant quantities of hydroxyl under high pressure/temperature (HPT) (P-T) mantle conditions, and may provide a useful, low-pressure analogue for detailed investigations of H incorporation in NAMs. In particular, large gem-quality crystals of non-stoichiometric spinel can be readily synthesised, enabling detailed studies of H + incorporation mechanisms and defect coupling using multiple techniques, something which is challenging to perform on run products from high PT studies of mantle minerals.…”

“…In fully ordered, stoichiometric spinel, MgAl 2 O 4 ('ideal spinel'), Mg fully occupies T sites and Al fully occupies M sites. Bromiley et al [10] demonstrated that non-stoichiometric spinel contains a significant proportion of both Al-occupied T sites and vacant M sites, resulting in a structure with an H + solubility several orders of magnitude higher than ideal spinel. However, although Bromiley et al [10] performed a series of high PT annealing experiments on non-stoichiometric spinel, problems with recrystallisation (due to the inherent instability of non-stoichiometric spinel at high temperature) prevented them from extracting key data on H diffusivity during annealing.…”

“…Bromiley et al [10] demonstrated that non-stoichiometric spinel contains a significant proportion of both Al-occupied T sites and vacant M sites, resulting in a structure with an H + solubility several orders of magnitude higher than ideal spinel. However, although Bromiley et al [10] performed a series of high PT annealing experiments on non-stoichiometric spinel, problems with recrystallisation (due to the inherent instability of non-stoichiometric spinel at high temperature) prevented them from extracting key data on H diffusivity during annealing. Assessing H mobility in NAMs is essential in constraining the importance of H + incorporation on mantle properties and processes within the deep Earth, and in providing additional insight into H + incorporation mechanisms, for example [18][19][20][21].…”

The mineral tooeleite Fe6(AsO3)4SO4(OH)4⋅4H2O – An infrared and Raman spectroscopic study-environmental implications for arsenic remediation

“…The valance of 7 arsenic in Tooeleite was corrected from +5 to +3 recently, as determined by Morin and 8 Nishimura et al, [1,3]. The mineral is typically cadmium orange to brownish or 9 yellowish in colour [5] and is triclinic as listed by the IMA (International 10 Mineralogical Association). The crystal data was corrected from orthorhombic to 11 monoclinic structure, space group C2/m [3], and the cell dimensions are a = 8.9575 Ǻ, 12 b = 6.4238 Ǻ, c = 9.7912Ǻ, β = 96.032°, unit-cell volume (V) = 560.27 Ǻ 3 .…”