Chongite, Ca 3 Mg 2 (AsO 4 ) 2 (AsO 3 OH) 2 ·4H 2 O, a new arsenate member of the hureaulite group from the Torrecillas mine, Iquique Province, Chile

Abstract

The new mineral chongite (IMA2015–039), Ca 3 Mg 2 (AsO 4 ) 2 (AsO 3 OH) 2 .4H 2 O, was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with native arsenic, arsenolite, gajardoite, talmessite and torrecillasite. Chongite occurs as prismatic crystals up to ∼1 mm long grouped in tightly intergrown radial aggregates up to 2 mm in diameter. Crystals are transparent, with vitreous lustre and white streak. The Mohs hardness is ∼3½, tenacity is brittle and fracture is conchoidal. Cleavage is good on ﹛100﹜. The measured density is 3.09(2) g/cm 3 and the calculated density is 3.087 g/cm 3 . Optically, chongite is biaxial (-) with α = 1.612(1), β= 1.626(1), γ= 1.635(1) and 2V = 76.9(1)° (measured in white light). Dispersion is r 4σ F reflections) confirms that chongite is a member of the hureaulite group.