This mineral was found upon several specimens of argentite from Chañarcillo which were acquired for the British Museum in 1886.

The specimens consist of argentite in large drusy octahedra implanted on quartz or calcite, and associated with proustite and a little asbestos.

The proustite is mostly in the form of small brilliant prisms with sealenohedral terminations {201̅}, capped by the rhombohedron {110}, which are grouped upon the argentite. But dispersed on the argentite, together with these crystals of proustite, are a number of fine glittering scales, which present at first sight precisely the appearance of the pyrrhosiderite or göthite of Siegen, having the same bronzy red colour, and a lustre resembling that of earthy haematite when seen on the matrix, that is to say partly by reflected and partly by transmitted light. Under the microscope the scales are easily distinguished from göthite ; they are without striations, remain dark when rotated between crossed nicols, and appear of a nearly blood-red colour by transmitted light ; whereas scales of göthito are striated in one direction, and exhibit marked differences of absorption when rotated above the polariser.