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Antofagastaite, Na2Ca(SO4)2·1.5H2O, a new mineral related to syngenite

Published online by Cambridge University Press:  12 April 2019

Igor V. Pekov*
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991Moscow, Russia
Vadim M. Kovrugin
Department of Crystallography, St Petersburg State University, University Embankment 7/9, 199034St Petersburg, Russia Laboratoire de Réactivité et Chimie des Solides, UMR 7314 CNRS, Université de Picardie Jules Verne, 33 rue St Leu, 80039Amiens, France
Oleg I. Siidra
Department of Crystallography, St Petersburg State University, University Embankment 7/9, 199034St Petersburg, Russia Nanomaterials Research Center, Kola Science Center, Russian Academy of Sciences, 184200Apatity, Russia
Nikita V. Chukanov
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow region, Russia
Dmitry I. Belakovskiy
Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninsky Prospekt 18-2, 119071Moscow, Russia
Natalia N. Koshlyakova
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991Moscow, Russia
Vasiliy O. Yapaskurt
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991Moscow, Russia
Anna G. Turchkova
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991Moscow, Russia
Gerhard Möhn
Dr.-J.-Wittemannstrasse 5, 65527Niedernhausen, Germany
*Author for correspondence: Igor V. Pekov, Email:


The new mineral antofagastaite, ideally Na2Ca(SO4)2·1.5H2O, was found in the oxidation zone of sulfide–quartz veins at the abandoned Coronel Manuel Rodríguez mine, Mejillones, Antofagasta Province, Antofagasta Region, Chile. It is associated with sideronatrite, metasideronatrite, aubertite, gypsum, ferrinatrite, glauberite, amarillite and an unidentified Fe phosphate. Antofagastaite occurs as prismatic crystals up to 0.5 mm × 1 mm × 5 mm, elongated along [010], typically combined in open-work aggregates up to 1 cm across. Antofagastaite is transparent and colourless, with vitreous lustre. It is brittle; the Mohs’ hardness is ca 3. Cleavage is distinct on (001). Dmeas. is 2.42(1) and Dcalc. is 2.465 g cm−3. Antofagastaite is optically biaxial (–), α = 1.489(2), β = 1.508(2), γ = 1.510(2) and 2Vmeas. = 40(10)°. The IR spectrum is reported. Chemical composition (wt.%, electron microprobe, H2O determined by gas chromatography) is: Na2O 20.85, CaO 17.42, SO3 52.56, H2O 7.93, total 98.76. The empirical formula (based on 8 O atoms belonging to sulfate anions per formula unit with all H belonging to H2O molecules) is Na2.06Ca0.95S2.01O8·1.35H2O. Antofagastaite is monoclinic, P21/m, a = 6.4596(4), b = 6.8703(5), c = 9.4685(7) Å, β = 104.580(4)°, V = 406.67(5) Å3 and Z = 2. The strongest reflections of the powder XRD pattern [d, Å (I, %) (hkl)] are: 9.17 (100) (001), 5.501 (57) (011), 3.437 (59) (020), 3.058 (43) (003), 2.918 (50) (2¯11), 2.795 (35) (013) and 2.753 (50) (121, 201). The crystal structure was solved based on single-crystal X-ray diffraction data, R1 = 5.71%. The structure of antofagastaite consists of ordered and disordered blocks and is related to syngenite K2Ca(SO4)2·H2O. Incorporation of additional H2O molecules in the syngenite-type structure results in disorder of the one of the two tetrahedral sulfate groups occurring in antofagastaite. In addition to the above-reported type material, antofagastaite together with syngenite and blödite occurs in the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia.

Copyright © Mineralogical Society of Great Britain and Ireland 2019

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Associate Editor: František Laufek


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