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. Crystal-chemical analysis resulted in the empirical structural formula:  
(Na0.87Ca0.070.04K0.02)XΣ1.00(Cr2.293+Mg0.71)YΣ3.00(Al3.04Mg1.54Cr1.183+V0.223+Fe0.013+)ZΣ6.00[(Si5.96Al0.04)TO18](BBO3)3(OH)V3[O0.73F0.25(OH)0.02]WΣ1.00

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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