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povondraite
Povondraite is a rare silicate mineral from the tourmaline group with formula: NaFe3+3(Fe3+4,Mg2)(BO3)3Si6O18(OH)3O.〔("Povondraite" on Mindat.org )〕 It is a dark brown to black nearly opaque mineral with a resinous to splendent luster. It crystallizes in the trigonal crystal system as equant, distorted prisms with trigonal pyramid terminations.〔(Handbook of Mineralogy )〕〔
It occurs as rare fracture and cavity encrustations within schists derived from sedimentary rocks. Associated minerals include quartz, potassium feldspar, muscovite, schorl, riebeckite and magnesite.〔
Discovered at the San Francisco mine, near Villa Tunari (in Alto Chapare), Bolivia, in 1976, originally it was called ferridravite, for the composition and the assumed relationship to dravite, i.e., "ferric dravite" .〔Kurt Valenta, Pete J. Dunn, "(Ferridravite, a new mineral of the tourmaline group from Bolivia )", American Mineralogist, Volume 64, pages 945-948, 1979〕 However later investigations yielded a new empirical formula which had no relation to the dravite. This called for renaming, and the new name, after Pavel Povondra, a mineralogist at the Charles University in Prague, was approved by the International Mineralogical Association. 〔Joel, D. Grice, T. Scott Encit, Frank C. Hawthorne,("Povondraite. a redefinition of the tourmaline ferridravite" ), American Mineralogist, Volume 78, pages 433-436, 1993 〕
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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