2018 Volume 113 Issue 5 Pages 251-262
The crystal chemistry of poppiite, V–analogue of pumpellyite–group mineral, from the Komatsu mine, Saitama Prefecture, Japan and from the Gambatesa mine, Genova, Italy (type locality) were studied using electron microprobe (EMPA) and X–ray single–crystal diffraction methods. Both samples are characterized by very high V content, and the average V2O3 content is 31.1 wt% for the Komatsu poppiite and 25.4 wt% for the Gambatesa specimen. The Komatsu specimen in this study is the V–richest poppiite ever reported. Structure refinements converged to R1 values of 5.01% for Komatsu and 3.74% for Gambatesa. The determined structural formula (Z = 4) of poppiite from the Komatsu and Gambatesa mines is
(V3+1.35Fe3+0.21Al0.44)Si3.00O10.64(OH)3.36, respectively. Both specimens are classified as poppiite–(V3+) because the X and Y octahedral sites are predominantly occupied by V3+. The Me2+:Me3+ ratio at X in this study is 0.41:0.59 for the Komatsu poppiite, and 0.36:0.64 for the Gambatesa one. The former has the larger unit–cell volume (1033.2 Å3) than the latter (1026.6 Å3). Both charge distribution analysis and located hydrogen sites indicate that O5, O7, O10, and O11 positions host hydroxyl groups. The <Y–O> distance and cell–dimensions positively correlate with mean ionic radius at Y. On the other hand, the a– and c–dimensions are shorter than the values expected by the regression lines estimated by approximately Me2+:Me3+ ≈ 0.5: 0.5 because of the high proportion of Me3+ at X. The studied poppiite crystals are characterized by small β angles [97.283(2)° for Komatsu, and 97.343(3)° for Gambatesa]. These values are considered to be related to distortional variations of the YO6 octahedra caused by the different characteristics of octahedral V3+ ions compared to Fe3+ and Cr3+.