Mimetite is a relatively common lead arsenate mineral found in the oxidised zone of lead ore deposits. It forms hexagonal prisms and comes in a range of colours including white, cream, yellow, green, orange, red and brown.
A rarer form is campylite, a name sometimes used for mimetite that occurs as barrel-shaped prisms. The most famous occurrence of this is at Dry Gill, Caldbeck Fells, Cumbria, England.
The ones photographed are from Broken Hill, New South Wales, and individual crystals are about 3mm long. These ones are actually cream in colour, having been coated with brown iron hydroxide.