Growth twinning in johannsenite-hedenbergite clinopyroxenes

Johannsenite, along with hedenbergite and ilvaite, occurs in a skarn of the Campiglia Marittima Cu-Pb-Zn metasomatic ore deposit (central Italy). The skarn formed at low pressure, under decreasing temperature and in redox conditions which changed from oxidising to reducing. The clinopyroxenes are arranged in spheroidal bodies consisting of radiating crystals tens of centimetres long which formed under fast crystallisation rates. Optical and scanning electron microscopy (SEM) revealed two types of lamellar features. In one case the lamellae are parallel to [001], the morphological elongation of the crystal, but in other cases they are almost perpendicular. Single-crystal X-ray diffraction patterns of samples with lamellae parallel to [001] contain reflections forbidden for the C2/c space-group and compatible with either (100) or (10¯2) twin planes. High Resolution Transmission Electron Microscopy (HRTEM) confirms that (100) is the twin plane. HRTEM also shows the presence of a translational component necessary to achieve the twin structure. At the level of the crystal structure, the twin interface appears as an out of step boundary, with the two opposite portions related by a b-glide. The lamellar features almost perpendicular to [001] also derive from twinning, but in this case (001) is the twin plane. In contrast with the ubiquitous (100) twins, (001) twins are restricted to manganese-rich crystals with more equidimensional habit. The two twin types never occur together in the same crystal. Both (100) and (001) twins are obtained by reticular pseudo-merohedry (or alternatively, by twin-lattice quasi-symmetry, TLQS). They are interpreted as primary growth features.