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
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.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Review article. The crystal structure and vibrational spectroscopy of jarosite and alunite minerals
- Versatile monazite: Resolving geological records and solving challenges in materials science. Petrogenesis of the Kulyk Lake monazite-apatite-Fe(Ti)-oxide occurrence revealed using in-situ LA-(MC)-ICP-MS trace element mapping, U-Pb dating, and Sm-Nd isotope systematics on monazite
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- Unlocking the secrets of Al-tobermorite in Roman seawater concrete
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- Controls of P-T path and element mobility on the formation of corundum pseudomorphs in Paleoproterozoic high-pressure anorthosite from Sittampundi, Tamil Nadu, India
- Aluminum speeds up the hydrothermal alteration of olivine
- Iron pairs in beryl: New insights from electron paramagnetic resonance, synchrotron X-ray absorption spectroscopy, and ab initio calculations
- Effects of fluid and melt density and structure on high-pressure and high-temperature experimental studies of hydrogen isotope partitioning between coexisting melt and aqueous fluid
- DFT simulation of the occurrences and correlation of gold and arsenic in pyrite
- Crystal structure and hydration/dehydration behavior of Na2Mg(SO4)2·16H2O: A new hydrate phase observed under Mars-relevant conditions
- The diffusion behavior of hydrogen in plagioclase feldspar at 800–1000 °C: Implications for re-equilibration of hydroxyl in volcanic phenocrysts
- Quantification of dissolved CO2 in silicate glasses using micro-Raman spectroscopy
- Spin transition of Fe2+ in ringwoodite (Mg,Fe)SiO4 at high pressures
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