Thermal Decomposition of Sjögrenite and Pyroaurite

Abstract

When sjögrenite or pyroaurite [Mg₆Fe₂(OH)₁₆(CO₃) ∙ 4 H₂O approx.] is heated, the molecular water is lost, reversibly, below 200°C. At 200 to 250°C on static, or 200 to 350°C on dynamic heating little further H₂O or CO₂ is lost, but there are changes in infrared spectrum and DTA effects are observed; X-ray evidence indicates gradual deterioration in crystallinity and gradual decrease of the layer thickness towards a value of about 5 Å. At 250 to 400°C (static) or 300 to 450°C (dynamic) these changes are completed, most of the remaining water and the CO₂ are lost, and MgO and MgFe₂O₄ begin to form. The crystallinity of these products gradually increases with rise in temperature up to about 750°C. Both are formed topotactically, that is, in definite crystallographic orientations relative to the starting material.