Magnesiovoltaite, K2Mg5Fe3+ 3Al(SO4)12·18H2O, a new mineral from the Alcaparrosa mine, Antofagasta region, Chile

Abstract

Magnesiovoltaite, a new voltaite-group mineral, was discovered in the Alcaparrosa mine, Cerro Alcaparrosa, El Loa province, Antofagasta region, Chile, in two associations, one of which includes coquimbite, tamarugite, alum-(Na), rhomboclase, yavapaiite, voltaite and opal, and the other one is botryogen, opal, tamarugite, alum-(K), pickeringite, magnesiocopiapite, and jarosite. Magnesiovoltaite forms yellow, brownish-yellow or pale yellowish-greenish isometric crystals up to 2 mm across. The main crystal forms are {111} and {100}; the subordinate forms are {110} and {211}. The new mineral is brittle, with subconchoidal fracture; cleavage is not observed. Mohs’ hardness is 2½. D meas = 2.51(2) g/cm 3 , D calc = 2.506 g/cm 3 . Magnesiovoltaite is optically anomalously anisotropic, uniaxial with e = 1.584(2) and ω = 1.588(2), or biaxial (−) with α = 1.584(2), β = 1.587(2), and γ = 1.588 (2). Possible causes of the optical anomalies are discussed. The infrared spectrum is given. The chemical composition is (EDS-mode electron microprobe, all iron is considered as Fe 3+ in accordance with Mossbauer data, H 2 O by gas chromatography of ignition products, wt. %): Na 2 O 0.13, K 2 O 4.64, MgO 9.13, MnO 1.73, ZnO 0.84, Al 2 O 3 2.47, Fe 2 O 3 13.36, SO 3 50.83, H 2 O 17.6, total 100.73. The empirical formula based on 66 O atoms per formula unit ( apfu ) is (K 1.85 Na 0.08 )(Mg 4.25 Mn 0.46 Zn 0.14 )Fe 3+ 3.14 Al 0.91 (SO 4 ) 11.91 (H 2 O) 18.325 O 0.035 . The simplified formula is K 2 Mg 5 Fe 3+ 3 Al(SO 4 ) 12 ·18H 2 O. The crystal structure has been refined to R = 3.2% using 1147 unique reflections with I > 2σ( I ). Magnesiovoltaite is cubic, Fd –3 c, a = 27.161(1) A, V = 20038(2) A 3 , and Z = 16. Magnesiovoltaite is isostructural with other cubic voltaite-group minerals. The strongest lines of the powder X-ray diffraction pattern [ d , A ( I , %) ( hkl )] are: 9.56 (29) (022), 6.77 (37) (004), 5.53 (61) (224), 3.532 (68) (137), 3.392 (100) (008), 3.034 (45) (048), 2.845 (30) (139). The type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia.