Thermal Decomposition of a Natural Manganese Dioxide A Laboratory Experiment for Undergraduate Students M. C. Puerta and P. Valergal Facultad de Ciencias de Cadiz, Apartado 40, Puerto Real 11510, Spain The system of oxides of manaanese is a complex one. The most important of these compoinds is the dioxide hecause it is the main constituent in the ore from which manganese is derived (1). It is also important in many other applications. Primarily manganese is used in the production of steel and ferromanganese. The greatest nonmetallurgical use of MnO, is in the manufacture of dry cell batteries. In Leclanche-type drv cells maneanese dioxide acts a denolarizer.. . orohahlv th;ough the reaction MnO,
+ Ht+ e-
-
MnO(0H)
Maneanese dioxide has been used to decolorize commercial glass ("glassmaker's soap"). I t is a n active catalyst for the oxidation of carbon monoxide. and it is also em~lovedin the brick industry and in the production of ferritesfo;electrical devices. Manganese dioxide has several polymorphs (Z),two of which are the main mineral forms: pyrolusite or &Mn02 (tetragonal rutile-type lattice with a = 4.398, b = 2.873 A) and ramsdellite (with similar framework hut with a n orthorhombic unit cell with a = 4.53. b = 9.27. c = 2.86 A). (3). . ~ Other forms of manganese dioxide have hken descrihed: arM n 0 2 (a group of manganese dioxides that always contain a foreign ion), y-MnOz (usually a poorly crystalline material that exists in a ramsdellite matrix with statisticallv distrihuted microdomains of pyrolusite), 6-Mn0, (also a poorly crystalline material), n-MnO2 (probably 7-MnO, with different crysmllite size and concentration of micrudomains), and r-MnOr. Many of the various . oolvmorohs . . are prohahlv impure forms. Heating of manganese dioxide leads first to improve crystallinity and then causes it to lose oxygen giving Mnp03and finally hausmannite, Mn30a (4,5). There are two forms of the black Mnv02 - " (6): . . a-MnqO?. havine the C sesouioxide structure related to that of ~ i F z & o v i i g onequarter of the anions (Fie. 1) and the rearraneine the atoms somewhat. and r-bln.,O
