Original paper

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

Chukanov, Nikita V.; Aksenov, Sergey M.; Rastsvetaeva, Ramiza K.; Möhn, Gerhard; Rusakov, Vyacheslav S.; Pekov, Igor V.; Scholz, Ricardo; Eremina, Tatiana A.; Belakovskiy, Dmitriy I.; Lorenz, Joachim A.

European Journal of Mineralogy Volume 28 Number 5 (2016), p. 1005 - 1017

published: Dec 31, 2016

DOI: 10.1127/ejm/2016/0028-2565

BibTeX file

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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/cm3, D calc = 2.506 g/cm3. Magnesiovoltaite is optically anomalously anisotropic, uniaxial with ε = 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 Fe3+ in accordance with Mössbauer data, H2O by gas chromatography of ignition products, wt.&percnt): Na2O 0.13, K2O 4.64, MgO 9.13, MnO 1.73, ZnO 0.84, Al2O3 2.47, Fe2O3 13.36, SO3 50.83, H2O 17.6, total 100.73. The empirical formula based on 66 O atoms per formula unit (apfu) is (K1.85Na0.08)(Mg4.25Mn0.46Zn0.14)Fe3+ 3.14Al0.91(SO4)11.91(H2O)18.325O0.035. The simplified formula is K2Mg5Fe3+ 3Al(SO4)12·18H2O. The crystal structure has been refined to R = 3.2% using 1147 unique reflections with I > 2σ(I). Magnesiovoltaite is cubic, Fd–3c, a = 27.161(1) Å, V = 20038(2) Å3, and Z = 16. Magnesiovoltaite is isostructural with other cubic voltaite-group minerals. The strongest lines of the powder X-ray diffraction pattern [d, Å (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.

Keywords

alcaparrosa minemagnesiovoltaitechilesulfatevoltaite groupnew mineraloxidation zonecrystal structure