Agardite

Agardite is a mineral group consisting of agardite-(Y), agardite-(Ce), agardite-(Nd), and agardite-(La). They comprise a group of minerals that are hydrous hydrated arsenates of rare earth elements (REE) and copper, with the general chemical formula (REE,Ca)Cu₆(AsO₄)₃(OH)₆·3H₂O. Yttrium, cerium, neodymium, lanthanum as well as trace to minor amounts of other REEs are present in their structure. Agardite-(Y) is probably the most often found representative. They form needle-like yellow-green (variably hued) crystals in the hexagonal crystal system. Agardite minerals are a member of the mixite structure group, which has the general chemical formula Cu₆A(TO₄)₃(OH)₆•3H₂O, where A is a REE, Al, Ca, Pb, or Bi, and T is P or As. In addition to the four agardite minerals, the other members of the mixite mineral group are calciopetersite, goudeyite, mixite, petersite-(Ce), petersite-(Y), plumboagardite, and zálesíite.