Wangdaodeite, the LiNbO₃‐structured high‐pressure polymorph of ilmenite, a new mineral from the Suizhou L6 chondrite

Abstract

Wangdaodeite, the shock-induced lithium niobate-structured polymorph of ilmenite, was found in the Suizhou L6 chondrite. It occurs as small irregular particles (2-20 lm in size) inside or adjacent to the shock melt veins. Wangdaodeite coexists in veins with ringwoodite, majorite, and xieite. The chemical formula of wangdaodeite is FeTiO 3. The empirical formula is: (Fe 0.85 Mg 0.10 Mn 0.05) Σ1.00 Ti 0.99 O 3 , which is similar to that of its host ilmenite. The Raman spectra of wangdaodeite display the bands at 174-179, 273-277, 560-567, 738-743 cm À1 , which are different to those for ilmenite. TEM images show that ilmenite is composed of polysynthetic-twinned crystals while wangdaodeite is composed of random-oriented nanometric domains sized 20-50 nm. Electron diffraction established wangdaodeite to be trigonal with the lithium niobate structure. Cell parameters are: a = 5.13(1) A, c = 13.78(1) A; c/a = 2.69; space group R3c; calculated density = 4.72 g cm À3. The P-T conditions for formation of wangdaodeite were estimated to be 20-24 GPa and >1200°C.

Structure Analysis of Natural Wangdaodeite—LiNbO3-Type FeTiO3

“…Wangdaodeite and recrystallized ilmenite replace deformed ilmenite single-crystal clasts within or at the border of the shock melt vein. The crystallites of both phases of FeTiO 3 assume size mostly between 200 to 600 nm in diameter (Figure 1b,c), which is noticeably smaller than that reported for wangdaodeite type material (of 2-20 µm in diameter [7]). No indication of pressure-induced decomposition of FeTiO 3 into TiO 2 and FeTi 2 O 4 post-rutile and post-spinel phases [17] was observed.…”

“…Xie et al [6,7] reported the same phase from the Suizhou impact structure. Based on Xie et al's findings, LiNbO 3 -type FeTiO 3 was approved as a mineral with the name "wangdaodeite" (IMA 2016-007 [6,7]).…”

“…FeTiO 3 is trimorph with ilmenite stable at ambient pressure; liuite [1], a GdFeO 3 -type perovskite, stable above~15-20 GPa [2][3][4][5]; and wangdaodeite, a LiNbO 3 -type perovskite as intermediary phase [3,4,6,7]. The high-pressure phase transition of ilmenite to an intermediate structure prior to the formation of the GdFeO 3 -type perovskite was first reported by Syono et al [8] and Ito and Matsui [9].…”

“…In 2009, the natural occurrence of this phase in shock-metamorphized gneiss from the Ries impact crater was announced in a conference abstract by Dubrovinsky et al [12]. Xie et al [6,7] reported the same phase from the Suizhou impact structure. Based on Xie et al's findings, LiNbO 3 -type FeTiO 3 was approved as a mineral with the name "wangdaodeite" (IMA 2016-007 [6,7]).…”

“…Neither Dubrovinsky et al [12] nor Xie et al [6,7] presented structure analyses of natural LiNbO 3 -type FeTiO 3 . Xie et al 2020 [7] showed by means of TEM-SAED (transmission electron microscopy-selected area electron diffraction) that the new mineral species is consistent with the LiNbO 3 -structure. Here we present the first structure analysis of natural wangdaodeite based on synchrotron X-ray diffraction data and Rietveld refinement (Tables 1-3 and Supplementary Cif).…”