Description and synthesis of the selenide minerals^*

A study of natural and artificial selenides, mainly by microscopic and x-ray methods, has led to revised descriptions of the selenide minerals. The mineral descriptions contain some new measurements of specific gravity, new unit cell dimensions (notably for eucairite, crookesite, and umangite), together with x-ray powder data for all the species and reproductions of the patterns. The classification of the selenides given in Dana (1944) is modified by the addition of a tetragonal group to the A₂X type, and the recognition of an A_2−xX type.