Electron microscope study of the humite minerals: II. Mn-rich specimens

The results of an electron-microscope study of the alleghanyite (Mn-humite) minerals differ in some respects from those previously reported for the Mg-rich humites. They again reveal coherent intergrowths of the group members, but in some cases with very dense lamellar faulting, especially in alleghanyite specimens from the Benallt mine, Wales. Some of these faults were higher members of the series which occurred as very thin lamellae one or two unit cells thick. Superstructures (ordered intergrowths at the unit-cell level) were very rare; one, which was an ordered intergrowth of alternating bands of manganhumite and sonolite (Mn-clinohumite), was observed. Complex intergrowths at this level, but without translational periodicity, were more common. Such fragments were often twinned polysynthetically. Many specimens yielded relatively large crystal fragments of more than one member of the series (but without coherent boundaries). In some cases leucophoenicite was also present.