Povondraite

(repeating unit)NaFe3+3(Mg2Fe3+4)(Si6O18)(BO3)3(OH)3OIMA symbolPov[1]Strunz classification8/E.19-75Dana classification61.3.1.6Crystal systemHexagonal-RhomboidalCrystal classHemimorphic trigonalSpace groupR3m (No. 160)Unit cella=16.18Å, c=7.44ÅIdentificationFormula mass1156.50 gmColourDark brown to brown-black, blackCrystal habitPrismatic, striatedCleavageNone observedFractureIrregular, conchoidalTenacityBrittleMohs scale hardness7LusterResinousStreakBrownDiaphaneityTranslucentRefractive index1.74 - 1.82Birefringence0.069Common impuritiesK,Mg,Al,Cu,Pb,Sn,Ti,V,H2O

Povondraite is a rare silicate mineral from the tourmaline group with formula: NaFe3+3(Fe3+4,Mg2)(BO3)3Si6O18(OH)3O.[2] 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.[3][2]

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.[3]

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".[4] 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 (1924–2013),[5] a mineralogist at the Charles University in Prague, was approved by the International Mineralogical Association in the 1990s.[6]

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b "Povondraite" on Mindat.org
  3. ^ a b Handbook of Mineralogy
  4. ^ Kurt Valenta, Pete J. Dunn, "Ferridravite, a new mineral of the tourmaline group from Bolivia", American Mineralogist, Volume 64, pages 945-948, 1979
  5. ^ "Povondra, Pavel, 1924-2013 - Portaro - katalog knihovny". katalog.vsb.cz. Retrieved 2021-03-19.
  6. ^ Joel, D. Grice, T. Scott Encit, Frank C. Hawthorne,"Povondraite. a redefinition of the tourmaline ferridravite", American Mineralogist, Volume 78, pages 433-436, 1993