| Agardite | |
|---|---|
A rich example of Agardite-(Ce) in the form of pistachio-green acicular crystals on contrasting matrix |
|
| General | |
| Category | Arsenate minerals |
| Chemical formula | (REE,Ca)Cu6(AsO4)3(OH)6·3H2O |
| Strunz classification | 08.DL.15 |
| Crystal symmetry | Hexagonal – dipyramidal, H-M symbol (6/m), space group P63/m |
| Unit cell | a = 13.59 Å, 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 | [1][2] |
Agardite is a mineral group consisting of agardite-(Ce), agardite-(Nd), agardite-(La), and agardite-(Y). They comprise a group of minerals that are hydrous hydrated arsenates of rare earth elements and copper, which contain variable amounts of calcium and sometimes lead. Yttrium, cerium, neodymium, lanthanum as well as trace to minor amounts of other rare earth elements are present in their structure. Agardite-(Y) is probably the most often found representative. The general formula for the group is (REE,Ca)Cu6(AsO4)3(OH)6·3H2O. They form needle-like yellow-green (variably hued) crystals in the hexagonal crystal system.
They were first described in 1970 in the Black Forest, Germany. They were named after Jules Agard, a French geologist.