Characterization of natural pyrolusite by electron microscopy

Abstract

A manganese-bearing mineral characterized by a black hair-like surface crystal growth, and somewhat unique to the Socorro County, New Mexico Area, has been referred to as psilomelane or pseudo-pyrolusite. X-ray diffraction measurements indicated the structure to be the tetragonal (a=4.42 Å, c=2.87Å) MnO₂ (pyrolusite) structure; with α-Mn enrichment. Detailed examinations of the mineral specimens made in the scanning and transmission electron microscopes showed the fiber crystals to be pseudo-fluted as a result of an apparent stacking of long needles to form continuous single-crystral bundles oriented along the x-axis These crystals were shown by selected-area electron diffraction to be composed of extended superstructure cells, and these to be commposed of as many as 10 MnO₂ unit cells stacked along the x-axis; interrupted by a stacking fault shear of the basic MnO₂ lattice resulting in stacking faults. Individual whisker crystals were observed to be terminated by crystallographic facets which is indicative of solution growth expected on considering their geological and mineralogical environment.