Chongite, Ca₃Mg₂(AsO₄)₂(AsO₃OH)₂·4H₂O, a new arsenate member of the hureaulite group from the Torrecillas mine, Iquique Province, Chile

Abstract

The new mineral chongite (IMA2015–039), Ca3Mg2(AsO4)2(AsO3OH)2.4H2O, 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/cm3and the calculated density is 3.087 g/cm3. 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 isr < v,distinct. The optical orientation isX= b;Z^a =27° in obtuse angle β. The mineral is slowly soluble in dilute HCl at room temperature. The empirical formula, determined from electron-microprobe analyses, is (Ca2.90Mg1.93Mn0.14)Σ4.97As4O20H10.07. Chongite is monoclinic,die, a =18.5879(6),b =9.3660(3),c =9.9622(7) Å, β = 96.916(7)°,V=1721.75(14) Å3and Z=4. The eight strongest powder X-ray diffraction lines are[dobsÅ(I)(hkl)]: 8.35(29)(110), 4.644(62) (3ˉ11,020,400,2̄02), 4.396(26)(311), 3.372(62)(022,312,5̄11), 3.275(100)(420,22ˉ2,421), 3.113(57)(222), 2.384(30)(711,530,7̄12) and 1.7990(22)(9̄13,334,5̄34). The structure determination(R1= 1.56% for 1849 Fo> 4σFreflections) confirms that chongite is a member of the hureaulite group.

Molecular structure of the arsenate mineral chongite from Jáchymov – a vibrational spectroscopy study

“…The refined unit-cell parameters of chongite (Tab. 3) are close to the data published for this mineral from original locality (Kampf et al 2016). Similarly, as found Meisser et al (2019) for Mn-Ca members of this group, an increasing portion of Ca 2+ entering the structure of Mg-Ca members leads to an increasing of unit-cell volume.…”

“…In comparison with the type material (Kampf et al 2016), chongite from Jáchymov is practically Mnfree and at the M2 site contains significant Ca besides dominant Mg (Fig. 4).…”

“…Positions and intensities of reflections were found and refined using the PearsonVII profileshape function with the ZDS program package (Ondruš 1993) and the unit-cell parameters were refined by the least-squares algorithm implemented by Burnham (1962). The experimental pow-der pattern was indexed in line with the calculated intensities obtained by Lazy Pulverix program (Yvon et al 1977) from the crystal structure of chongite (Kampf et al 2016).…”

“…In the asymmetric part of the monoclinic (space group C2/c, Z = 4) chongite unit cell (Kampf et al 2016), there are three symmetrically distinct M cations (two Ca and one Mg + Ca), one (AsO 4 ) 3and one (AsO 3 OH) 2group, and two independent H 2 O molecules. The crystal structure of chongite, as same as other members of hureaulite group (Kampf et al 2016), is based on an octahedral edge-sharing pentamer formed by M1 (Ca), M2 (Mg + Ca) and M3 (Ca) octahedra. Pentamers are linked into a loose framework by sharing corners with octahedra in adjacent pentamers and are further linked via AsO 4 and AsO 3 OH tetrahedra (Kampf et al 2016).…”

“…According to Kampf (2009), vacancy may also occur at this site. The M3 site is then occupied by larger cations as Ca 2+ , Mn 2+ or Zn 2+ (Elliot et al 2009;Kampf 2009;Kampf et al 2016;Meisser et al 2019). Arsenate members of this mineral group have the tetrahedrally coordinated T1 and T2 sites occupied by (AsO 4 ) 3and (AsO 3 OH) 2groups with only minor documented contents of the phosphate component.…”