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Saccoite, Ca2Mn+32F(OH)8⋅0.5(SO4), a new, microporous mineral from the Kalahari Manganese Field, South Africa

Published online by Cambridge University Press:  13 July 2022

Gerald Giester Open the ORCID record for Gerald Giester [Opens in a new window] ,
Christian L. Lengauer ,
Chutimun Chanmuang N. ,
Dan Topa ,
Jens Gutzmer  and
Karl-Ludwig von Bezing
Gerald Giester*
Affiliation:
Institut für Mineralogie und Kristallographie, Universität Wien - Geozentrum, Josef-Holaubek-Platz 2, 1090 Wien, Austria
Christian L. Lengauer
Affiliation:
Institut für Mineralogie und Kristallographie, Universität Wien - Geozentrum, Josef-Holaubek-Platz 2, 1090 Wien, Austria
Chutimun Chanmuang N.
Affiliation:
Institut für Mineralogie und Kristallographie, Universität Wien - Geozentrum, Josef-Holaubek-Platz 2, 1090 Wien, Austria
Dan Topa
Affiliation:
Naturhistorisches Museum Wien, Burgring 7, 1010 Vienna, Austria
Jens Gutzmer
Affiliation:
Helmholtz-Zentrum Dresden-Rossendorf, Helmholtz Institute Freiberg for Resource Technology, Chemnitzer Str. 40, 09599 Freiberg, Germany
Karl-Ludwig von Bezing
Affiliation:
Independent Researcher, Kimberley 8301, RSA
*
*Author for correspondence: Gerald Giester, Email: [email protected]
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Abstract

Saccoite, Ca2Mn3+2F(OH)8⋅0.5(SO4), is a new mineral found at the N'Chwaning III mine, Kalahari Manganese Field, Northern Cape Province, Republic of South Africa. It occurs as fillings of voids in hydrothermally altered manganese ore (comprising mostly of bixbyite and baryte). Further associated minor minerals are braunite, gypsum, chlorite, sturmanite and ettringite. Saccoite forms small needles, felted crystal masses or crusts. The new mineral is olive green, transparent, with white streak and vitreous lustre. No luminescence is observed. Saccoite is uniaxial (–) with refractive indices at 589(1) nm of ω = 1.705(5) and ɛ = 1.684(2). Pleochroism is distinct, i.e. bluish green (ω) and yellowish green (ɛ). The chemical composition was studied by means of an electron probe micro-analyser (EPMA) using wavelength-dispersive X-ray spectrometry (WDS). The empirical mineral formula is Ca2.06Mn3+1.78Cu0.10Mg0.07F0.97(OH)8.02(SO4)0.39. The unit-cell dimensions of saccoite (space group P4/ncc) are a = 12.834(3) Å, c = 5.622(2) Å, V = 926.0(4) Å3), and the calculated mass density is 2.73 g⋅cm–3. Saccoite exhibits a heteropolyhedral framework structure that is composed of edge- and corner sharing CaF2(OH)6 and M(OH)6 polyhedra (M = Mn3+ and Cu2+) with large channels along [001], which host disordered and only partially occupied groups, especially SO42–. The hydrogen atoms of the OH groups point into the channel to form hydrogen bonds with the channel anions. Ca–F distances are ~2.3 Å, the Ca–OH distances in the range of 2.44–2.58 Ǻ, and the M(OH)6 octahedron is strongly 4+2 Jahn-Teller distorted (4 × ~1.92 Å, 2 × 2.27 Å). The F atom is tetrahedrally coordinated to calcium atoms. The strongest lines in the powder X-ray diffraction pattern [d in Å (relative intensity) (hkl)] are: 9.0735 (35) (110), 4.5370 (95) (220), 4.0644 (20) (310), 3.0105 (100) (321), 2.8117 (20) (002), 2.7242 (75) (411), 1.9755 (35) (611), and 1.8142 (20) (550).

Keywords

saccoiteCa2Mn+32F(OH)8⋅0.5(SO4)new mineralmicroporous structureKalahari Manganese FieldSouth Africa
Type
Article
Information
Mineralogical Magazine , Volume 86 , Issue 5 , October 2022 , pp. 814 - 820
Copyright
Copyright © The Author(s), 2022. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland

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Footnotes

Associate Editor: Sergey V Krivovichev

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