Bournonite with Freibergite

The Viboras mine at Machacamarca in the Potosí Department of Bolivia is most famous for its well-crystallised Bournonite. This small miniature features several excellent cog-wheel crystals and for added interest these are entwined with stalactitic rod-like overgrowths of the mineral Freibergite. It should be pointed out that Freibergite is now of a somewhat uncertain status and currently goes under the collective Freibergite Subgroup. The Bournonite crystals are developed up to 1.9 cm across and have a brilliant steel-grey metallic lustre. By contrast the micro-crystallised Freibergite tends to be a sooty matt black.