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Mineralogical ClassificationIMA 2013-089 = chromo-alumino-povondraite

16th Aug 2014 10:00 UTCMarco E. Ciriotti Manager

Reference:

▪ Reznitskii, L., Clark, C.M., Hawthorne, F.C., Grice, J.D., Skogby, H., Hålenius,U., Bosi, F. (2014): Chromo-alumino-povondraite, NaCr3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup. American Mineralogist, 99, 1767-1773.


Abstract:

Chromo-alumino-povondraite, NaCr3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with dravite, oxy-chromium-dravite, oxy-dravite, quartz, calcite, chromphyllite, eskolaite, chromite, uvarovite, chromian phlogopite, and pyroxenes of the diopside-kosmochlor series, Cr-bearing tremolite, Cr-bearing titanite, Cr-bearing rutile, and pyrite.

Crystals are green and transparent with a vitreous luster, and exhibit a pale-green streak and conchoidal fracture. Chromo-alumino-povondraite has a Mohs hardness of approximately 7½, and a calculated density of 3.227 g/cm3. In plane-polarized light, chromo-alumino-povondraite is pleochroic (O = emerald green and E = pale yellowish green) and uniaxial negative: ω = 1.745(5), ɛ = 1.685(5). Chromo-alumino-povondraite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.0277(2), c = 7.3085(1) Å, V = 1625.93(5) Å3, Z = 3.

The crystal structure of chromo-alumino-povondraite was refined to an R1 index of 1.68% using 1803 unique reflections collected with MoKα X-radiation. Ideally, chromo-alumino-povondraite is related to oxy-dravite and oxy-chromium-dravite by the homovalent substitution Cr3+ ↔ Al3+. Tourmaline with chemical compositions classified as chromo-alumino-povondraite can be either Al-dominant or Cr-dominant as a result of the compositional boundaries along the solid solution between Al and Cr3+ that are determined at Y+Z(Cr1.5Al5.5), corresponding to NaY(Cr1.5Al1.5)Z(Al4Mg2)Si6O18(BO3)3(OH)3O, and Y+Z(Cr5Al2), corresponding to NaY(Cr3)Z(Cr2Al2Mg2)Si6O18(BO3)3(OH)3O.
 
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