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 Cu2+6A(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.
| Agardite-(Y), agardite-(Ce), agardite-(Nd), agardite-(La) | |
|---|---|
An example of agardite-(Ce) in the form of pistachio-green acicular crystals on contrasting matrix | |
| General | |
| Category | Arsenate minerals |
| Formula (repeating unit) | (REE,Ca)Cu6(AsO4)3(OH)6·3H2O |
| IMA symbol | Arg-Y, Arg-Ce. Agr-Nd, Arg-La |
| Strunz classification | 8.DL.15 |
| Dana classification | (Y): 42.05.01.02 (Ce): 42.05.01.02e (Nd): 42.05.01.02b (La): 42.05.01.02a |
| Crystal system | Hexagonal |
| Crystal class | Dipyramidal (6/m) H–M symbol: (6/m) |
| Space group | P63/m (agardite-(La): unknown space group) |
| Unit cell | a = 13.59, c = 5.89 [Å], Z = 2 |
| Identification | |
| Color | Yellow green |
| Crystal habit | Acicular |
| Cleavage | None |
| Fracture | Conchoidal |
| Mohs scale hardness | 3–4 |
| Luster | Vitreous |
| Streak | Greenish white |
| Specific gravity | 3.7 (measured), 3.775 (calculated) |
| Refractive index | nω = 1.725, nε = 1.81 |
| Birefringence | 0.085 |
| Pleochroism | yellowish green |
| References | |
Agardite-(Y) from the Bou Skour mine in Djebel Sarhro, Morocco was the first of the agardite-group minerals to be characterized. It was described by Dietrich in 1969 and was named after Jules Agard, a French geologist at the Bureau de Recherches Géologiques et Minières, Orléans, France. Agardite-group minerals have subsequently been found in Germany, Czech Republic, Greece, Italy, Japan, Namibia, Poland, Spain, Switzerland, the United Kingdom, and the United States.