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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 98, Issue 10

Issues

Carlfrancisite: Mn3 2+(Mn2+,Mg,Fe3+,Al)42(As3+O3)2(As5+O4)4[(Si,As5+)O4]6[(As5+,Si)O4]2(OH)42, a new arseno-silicate mineral from the Kombat mine, Otavi Valley, Namibia

Frank C. Hawthorne / Yassir A. Abdu / Neil A. Ball / William W. Pinch
Published Online: 2015-03-07 | DOI: https://doi.org/10.2138/am.2013.4426

Abstract

Carlfrancisite, Mn32+(Mn2+,Mg,Fe3+,Al)42(As3+O3)2(As5+O4)4[(Si,As5+)O4]6[(As5+,Si)O4]2(OH)42, is a new mineral from the Kombat mine, Otavi Valley, Namibia, and occurs as curved platy aggregates ~2 cm across on a matrix of Mn arsenates and oxides. It is yellowy orange to pale yellow with a very pale-yellow streak, translucent with a vitreous to opalescent luster, and does not fluoresce under ultraviolet light. Cleavage is micaceous on {001}, and no parting or twinning was observed. Mohs hardness is 3, and carlfrancisite is brittle with a hackly fracture. The calculated density is 3.620 g/ cm3. Optical properties were measured with a Bloss spindle stage for the wavelength 590 nm using a gel filter. The indices of refraction are e = 1.756, w = 1.758, and it is non-pleochroic. Carlfrancisite is trigonal, space group R3̄c, a = 8.2238(2), c = 205.113(6) Å, V = 12 013.5(4) Å3, Z = 6, c:a = 1:24.941. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I, (hkl): 2.826, 100, (2̄ 2 44); 2.371, 88, (2̄ 3 40, 1̄ 3 41); 1.552, 84, (1̄ 5 0); 2.676, 63, (2̄ 3 7); 3.243, 54, (0 1 56, 1̄ 2 39); 4.107, 48, (1̄ 2 0); 2.918, 47, (0 2 40). Chemical analysis by electron microprobe and crystalstructure refinement gave As2O5 13.07, As2O3 3.18, P2O5 0.50, V2O5 0.74, SiO2 8.96, Al2O3 0.78, FeO 0.22, MnO 53.25, MgO 9.37, H2O(calc) 8.42, sum 98.49 wt%. The H2O content and the valence states of As were determined by crystal-structure analysis. The empirical formula is Mn2+ 33.55Mg10.39Fe2+0.14Al0.68 As3+1.44(Si6.67P0.32V5+ 0.37As5+5.08)O54 (OH)42 on the basis of 96 anions with (OH) = 42 apfu. The structure of carlfrancisite is closely related to that of mcgovernite and turtmannite.

Keywords : Carlfrancisite; new mineral species; arseno-silicate; Tsumeb mine; Otavi Valley; Namibia; electron microprobe analysis; optical properties; chemical analysis; mcgovernite; turtmannite

About the article

Received: 2012-12-02

Accepted: 2013-05-17

Published Online: 2015-03-07

Published in Print: 2013-10-01


Citation Information: American Mineralogist, Volume 98, Issue 10, Pages 1693–1696, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2013.4426.

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© 2015 by Walter de Gruyter Berlin/Boston.

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