HIGH-TEMPERATURE SYNCHROTRON XRD STUDIES OF ZN/MN-OXIDES: A COMPARISON OF DEHYDRATION MECHANISMS IN CHALCOPHANITE AND WOODRUFFITE

Chalcophanite (ZnMn₃O₇ ^. 3H₂O) and woodruffite (Zn_0.7Mn_3.45O₇ ^. 2.4H₂O) form in oxidizing zones of Zn- and Mn-bearing deposits. Chalcophanite is constructed of sheets of Mn⁴⁺-O octahedra with Zn and H₂O between the layers. One of seven octahedral sites in the Mn-O sheet is vacant, and the Zn cations sit above and below the vacancies (Post and Appleman 1988). Among the birnessite-like layer Mn oxides, chalcophanite is unique in its tendency to form crystals suitable for detailed structural analysis, thereby providing special insight into this environmentally important mineral group. Post et al (2003) recently determined that the woodruffite structure consists of large tunnels that measure 3 and 4 octahedra on a side, the largest of any yet reported in natural or synthetic Mn oxides. The woodruffite structure may serve as a model for a new class of octahedral molecular sieves with enhanced capabilities as catalysts and selective cation-exchange agents.

We have used in situ, temperature-resolved synchrotron powder X-ray diffraction to investigate the thermal stability and dehydration behaviors of chalcophanite and woodruffite. The data were collected at the National Synchrotron Light Source using an imaging plate. Samples of chalcophanite and woodruffite were each heated in glass capillaries in air from 26 to 800 ^oC. Our results revealed that as chalcophanite loses interlayer H₂O between ~175 and 200 ^oC, the interlayer spacing collapses from 7.3 to 4.8 , forming the anhydrous phase ZnMn₃O_7. The interlayer Zn changes from octahedral coordination (to 3O and 3H₂O) in chalcophanite to tetrahedral coordination in the anhydrous phase. Above ~500 ^oC, the Mn-O distances increase and exhibit Jahn-Teller type distortions indicating reduction of some Mn⁴⁺ to Mn³⁺ on the octahedral sites. Also above ~500 ^oC, the anhydrous chalcophanite phase starts transforming into the Zn/Mn spinel, hetearolite [(Zn_.75Mn_.25)Mn₂O₄]. By ~700 ^oC, this conversion is complete. Dehydration in woodruffite also leads to a spinel-like phase, but the reaction sequence is similar to that of todorokite, which has a 3x3 tunnel topology. The collapse of the woodruffite structure and loss of H₂O at ~300 ^oC is triggered by the loss of O atoms from the octahedral framework and the consequent reduction of Mn, followed by the appearance of a Zn/Mn spinel.