Publication Date:
2012-12-06
Description:
Greenwoodite, ideal formula Ba 2– x (V 3+ OH) x V 9 (Fe 3+ ,Fe 2+ ) 2 Si 2 O 22 , space group P m1, a 5.750(1) Å, c 14.459(1) Å, V 414.00(8) Å 3 , Z = 1, is a new mineral from the Wigwam deposit of Southeastern British Columbia, Canada. It is a metamorphic mineral formed under greenschist-facies conditions as part of an assemblage that includes quartz, celsian, apatite, sphalerite, pyrrhotite, galena, pyrite, zoltaiite, and batisivite. Greenwoodite has a Mohs hardness of 5, one perfect cleavage, a semi-prismatic to tabular habit, and a calculated density of 4.81 g/cm 3 . It is opaque with reflectance and color similar to those of sphalerite. The strongest eight lines of the calculated X-ray powder diffraction pattern [ d in Å ( I ) ( h k l )] are 2.925(100)(0 1 4), 2.875(38)(1 1 0), 2.672(23)(1 1 2), 2.469(35)(1 1 3), 2.354(28)(2 0 2), 2.212(28)(2 0 3), 1.669(26)(1 2 4), and 1.438(35)(2 2 0). The empirical formula, derived from microprobe analysis and the crystal structure, is Ba 0.60 (V 3+ OH) 0.40 (V 3+ 8.33 ,Cr 0.33 ,Ti 0.13 ,Al 0.13 ,Mn 3+ 0.02 ) 9 (Fe 3+ 1.08 ,Fe 2+ 0.60 ,Zn 0.22 ,Al 0.06 ,Mg 0.04 ) 2 (Si 1.72 ,Fe 3+ 0.28 ) 2 O 22 . The crystal structure was solved using direct methods for the location of Ba, V, and Fe and using Patterson synthesis and Fourier maps for the positions of the Si and O. The final R factor was 1.9% using 475 unique observed reflections. The structure is based on hexagonal closest packing (hcp) of oxygen atoms with 4 distinct cation layers designated BV, V1, V2, and V3 having a stacking sequence BV-V1-V2-V3-V2-V1-BV-V1- . . . The core layers (all layers except BV) contain 3 distinct V-dominated sites, one tetrahedral Si site and one Fe-dominated tetrahedral site containing a disordered mixture of equal amounts of trivalent and divalent cations. The BV layer is a disordered mixture of 2 endmember layer structures related by the coupled substitution 2Ba 2+ = 2O 2– + 2V 3+ + 1 + 2H + . In the Ba endmember the Ba is at a cubo-octahedral site formed by a hole in one of the hcp oxygen layers. In the VOH endmember the Ba is replaced by an O atom which forms the shared corner of a triplet of octahedral sites. Only 2/3 of these sites are occupied, predominantly by V. Charge balance is achieved by bonding H to this O atom. The composition of the type grain of greenwoodite is 60% toward the Ba endmember of this coupled-substitution solid solution.
Print ISSN:
0008-4476
Topics:
Geosciences
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