Sphalerite, niccolite and ullmannite. Settlingstones Mine, North Pennines, Britain
250µm
Sphalerite (grey, bottom right) is intergrown with, and encloses, radiating niccolite (pink-brown, centre) which shows faint reflection pleochroism, and with euhedral ullmannite (white, centre right), which shows triangular pits (black, centre right). Although the optical properties of ullmannite are similar to those of galena, it has too high a reflectance and is harder. Quartz is dark grey (left centre), as is carbonate (top right).
Polished block, plane polarized light, x 80, air
