Studies of natural and synthetic agardites

Agardite of formula [(Al, Nd, REE)Cu₆(AsO₄)₃(OH)₆ · 3H₂O] has been discovered at Cobar, New South Wales, Australia. A series of synthetic agardites were analysed by X-ray diffraction and a correlation exists between the effective ionic radius of the REE³⁺ in the M site and the unit cell size for each respective agardite mineral. No value for the effective ionic radius of 9-coordinate Bi³⁺ has been reported but a value of approximately 115.5 pm is estimated from this correlation. The results of the TGA analyses show that the synthetic agardites are all fully hydrated, i.e., n = 3. Near infrared spectroscopy and mid- infrared spectroscopy has been used to characterise a group of synthetic agardites of formula ACu₆(AsO₄)₂(OH)₆ · 3H₂O where A is a rare earth element. The hydroxyl stretching region is characterised by four bands observed at around 3568, 3489, 3382 and 3290 cm⁻¹. The first two bands are attributed to the stretching mode of hydroxyl units and the last two bands to water stretching vibrations. The position of these bands indicates strongly hydrogen bonded water. The water in agardites is zeolitic type water. Near-IR spectroscopy shows a series of bands at 7242, 7007, 6809, 6770 and 6579 cm⁻¹ attributed to the first overtones of the hydroxyl fundamentals. The NIR spectrum of agardite (Sm) is different and may be affected by electronic bands. Combination bands are observed at around 4404, 4343, 4340, 4294 and 4263 cm⁻¹. Bands attributed to water combination modes are found at around 5200, 5173, 5082 and 4837 cm⁻¹. Agardites are a group of minerals known for their REE content and have been rarely studied. NIR spectroscopy is an excellent technique for the characterisation and ready identification of these minerals