Agardite

Agardite is a mineral group consisting of agardite-(Y),^[4][5] agardite-(Ce),^[6] agardite-(Nd),^[7] and agardite-(La).^[8] 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.^{[citation needed]} 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,^[9] goudeyite,^[10] mixite,^[11] petersite-(Ce),^[12] petersite-(Y),^[13][14] plumboagardite,^[15] and zálesíite.^[16]

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	(REE,Ca)Cu6(AsO4)3(OH)6·3H2O
IMA symbol	Arg-Y, Arg-Ce. Agr-Nd, Arg-La[1]
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	[2][3]

Agardite-(Y) from the Bou Skour mine in Djebel Sarhro, Morocco was the first of the agardite-group minerals to be characterized.^[17] 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.^[18] Agardite-group minerals have subsequently been found in Germany,^[19] Czech Republic,^[20] Greece,^[21] Italy,^[22] Japan,^[23] Namibia,^[24] Poland,^[25] Spain,^[26] Switzerland,^[27] the United Kingdom,^[28] and the United States.^[29]

See also

• List of minerals