Effect of the Atmospheric Conditions on the Thermal Behaviour of the Sarkinite Mineral, Mn₂(OH)AsO₄

Abstract

The sarkinite, Mn 2 (OH)AsO 4 , mineral has been synthesized in laboratory as pure phase under mild hydrothermal conditions. The decomposition process of the hydroxiarsenate compound is strongly dependent on the atmospheric conditions. The results of the thermal treatment in air or argon are quite different. In this way, a new black phase appears in air atmosphere in the 400-630°C temperature range whereas under inert atmosphere the structure of Mn 2 (OH)AsO 4 at room temperature is maintained up to 560°C. The weight loss is attributed to the partial decomposition of * Dr. J. M. Rojo

Characterisation of the sintering behaviour of Waelz slag from electric arc furnace (EAF) dust recycling for use in the clay ceramics industry

“…The second step is from 200 to 900 °C, in which a weight gain of 3.8% is accompanied by exothermic effects that give rise to marked peaks at 600 and 740 °C. This step could be attributed to the oxidation of Fe(II) to Fe(III), which is in good agreement with the X-ray diffraction data, and to other compounds of Fe(II), for which a weight gain was noted (de Pedro et al, 2005 andBarthelet et al, 2003). At temperatures above 950 °C, an additional weight loss was observed, which was attributed to the emission of evolved gases.…”

A New Partially Deprotonated Mixed-Valence Manganese(II,III) Hydroxide–Arsenate with Electronic Conductivity: Magnetic Properties of High- and Room-Temperature Sarkinite

“…Taking into account the structural features of this mineral and the nature of the Mn ion ( S = 5 / 2 ), their magnetic properties are expected to be interesting. Moreover, the thermal behavior of this phase reveals a surprisingly large interval of stability from 300 to 650 °C, not observed in the rest of the compounds of the adamite-type family. This thermal stability, using X-ray thermodiffractometry, was associated with a thermostructural change.…”

“…The synthesis of compound 1 , Mn 2 (OH)AsO 4 , using hydrothermal techniques was previously reported . Room-temperature powder X-ray diffraction data of microcrystalline white samples were used to evaluate the purity of the product.…”

“…So, our interest focused on finding the optimum conditions to synthesize these compounds with high purity. Recently, the sarkinite mineral was laboratory-synthesized as a pure phase under a mild hydrothermal reaction starting from the manganese arsenate octahydrate . Taking into account the structural features of this mineral and the nature of the Mn ion ( S = 5 / 2 ), their magnetic properties are expected to be interesting.…”

“…The diffraction patterns showed, near to 300 °C, a rapid change of the crystallographic structure, stable up to 640 °C. A new adamite-type phase, compound 2 , is formed by heating (in air), which shows a transformation of color from white (compound 1 ) to black (compound 2 ) attributed to a change in the oxidation state of manganese Mn II –Mn III with a simultaneous loss of OH groups …”