The transformation manganite-, pyrolusite

SUMMARY. During the transformation f manganite to pyrolusite the b-axis must contract about I5 To. There is evidence that the shrinkage takes place partly by contraction of the whole crystal, but electron-microscope investigations of natural pyrolusites and of manganite oxidized in the laboratory show that the reaction also involves the formation of lamellar pores about 85 A apart, parallel to (oIo) of the original manganite. Thus, although pyrolusite is tetragonal, the a- and b-axes are not micro-scopically identical, afact which could explain why, when natural pyrolusite is treated hydrothermally at 2oo and 3oo ~ and IOOO bars, only one orientation of manganite is formed (Dent Glasser and Smith, I968). The pores and the accompanying distortion of the structure act as a 'memory ' of the parent phase. Two forms of pyrolusite occur in nature. Rare, well-formed crystals with a hardness greater than quartz are considered to be of primary origin (Strunz, I943) but most pyrolusites are thought o be pseudomorphous after manganite and sometimes occur intergrown with it (Strunz, I943; Davis I967). Pseudomorphic pyrolusite usually occurs as a friable fibrous aggregate with relict (oIo) cleavage. It has a large surface area, which results in high chemical activity and absorption power