Kanoite, donpeacorite and tirodite: Mn-Mg-silicates from a manganiferous quartzite in the United Arab Emirates
Gnos, Edwin; Armbruster, Thomas; Nyfeler, Daniel
European Journal of Mineralogy Volume 8 Number 2 (1996), p. 251 - 262
published: Apr 26, 1996
manuscript accepted: Oct 24, 1995
manuscript received: Dec 9, 1994
ArtNo. ESP147050802011, Price: 29.00 €
Abstract Kanoite, ideally MnMgSi2O6, donpeacorite (Mg,Mn)MgSi2O6, and tirodite Mn2Mg5Si8O22(OH)2, form augen-shaped grains or clusters in an oxidized quartzite (quartz + hematite + pyrophanite + ferrianbraunite), which is the granulite-facies equivalent of a chert-hosted sedimentary manganese deposit. Mn-bearing minerals were studied by electron microprobe analysis and single-crystal X-ray methods. Pyroxenes from these rocks vary in composition between Mn0.89Mg0.89Fe3+1.10Ca0.07Na0.05Si2O6 and Mn0.60Mg1.30Fe3+0.02 Ca0.05Na0.01Si2O6, with the Mn-rich compositions comprising clinopyroxenes (kanoite) and the Mg-rich compositions comprising orthopyroxenes (donpeacorite). Clino- and orthopyroxenes were distinguished by X-ray single-crystal methods. Two kanoite single crystals were employed for X-ray crystal-structure refinement (a = 9.725(2), b = 8.876(2), c = 5.263(1) Å, ß = 108.53(2), Z = 4, space group P21 /c) leading to the crystal chemical formula (Mn0.02Mg0.98)(Mn0.78Mg0.15Ca0.07)Si2O6 which confirms that the studied samples are highly Mn, Mg ordered clinopyroxenes. Tirodite, with the average composition Na0.27Ca0.15Mn2.12Mg4.37Fe3+0.19Al0.10Si8.07O22(OH)2, is a retrogressive alteration product replacing kanoite and donpeacorite either along their rims or even completely in some cases.