Crystal structure and Raman spectrum of a high-pressure Li-rich majoritic garnet, (Li2Mg)Si2(SiO4)3

Abstract

A Li-rich majoritic garnet (LiMGt), (Li 2 Mg)Si 2 (SiO 4 ) 3 , was synthesized at 15 GPa and 1500 °C and its structure studied with single-crystal X-ray diffraction and Raman spectroscopy. It is cubic with space group Ia3d and unit-cell parameters a = 11.2660(2) Å and V = 1429.91(1) Å 3 . The 8-, 6-, and 4-coordinated cation sites in LiMGt are occupied by (Li + + Mg 2+ ), Si 4+ , and Si 4+ , respectively. Whereas the SiO 6 octahedron is nearly regular, the XO 8 dodecahedron is the most distorted of all known silicate garnets in terms of the bond-length distortion index. All Raman peaks of LiMGt are broader than those of pyrope, due to the substitution of Li + for Mg 2+ at the dodecahedral site. Furthermore, both Si-O symmetric stretching (A 1g -ν 1 ) and O-Si-O symmetric bending (A 1g -ν 2 ) modes of LiMGt shift significantly to higher frequencies relative to the corresponding ones of pyrope. In contrast, the A 1g -(SiO 4 ) rotational mode of LiMGt displays a much lower frequency than that of pyrope. This study represents the first structural report on a garnet with an all-silicate framework and suggests that, like Na incorporation in garnets, the pressure-dependent coupled substitution of (Li + + Si 4+ ) for (Mg 2+ + Al 3+ ) is likely one of the primary mechanisms for Li enrichment in garnets in the mantle and the transition zone.

Mg and Li Stable Isotope Ratios of Rocks, Minerals, and Water in an Outlet Glacier of the Greenland Ice Sheet

“…Based on equilibrium fractionation models at a range of temperatures relevant to weathering and igneous processes (273-2,000 K), the δ 26 Mg and δ 7 Li values of garnet (CN = 8) are predicted to be lower (Schauble, 2011;Huang et al, 2013) compared to hornblende (CN = 6/5, Wunder et al, 2007) and for Mg isotopes this is what we observe (Figure 3). In contrast, the δ 7 Li value of garnet is not markedly lower than the other minerals as would be predicted if Li was in the dodecahedral site (e.g., Yang et al, 2009). Studies of lithium-rich synthetic garnets have demonstrated that Li can be contained in sites with CNs of 4 and 6 (Mazza, 1988;O'Callaghan et al, 2008;Cussen, 2010;Rettenwander et al, 2016).…”

Nomenclature of the garnet supergroup

“…An exception are the 259-1113 ppm Li in almandine from leucocratic granulite at Horní Bory, Czech Republic, corresponding to 0.019-0.079 Li pfu, determined by laser ablation-inductively coupled plasma-mass spectroscopy (Cempírek et al 2010 and unpublished data). According to Cempírek et al (2010) (Yang et al 2009) or sites occupied by Li in synthetic garnets. The majority of synthetic Li garnets are compounds of Li with REE, Ta, Nb, Te, Zr, and Ba that are valued for their high-ionic conductivity (e.g., Cussen 2006Cussen , 2010O'Callaghan and Cussen 2007;Wang and Lai 2012).…”

Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond

“…In the latter instance, the substitution of sodium into the structure is intimately tied to both Ti and/or Si substitution into the octahedral site: therefore, this component incorporates a coupling between the classic majorite substitution and sodium/alkali substitution into garnets. Indeed, while the majorite substitution is typically formulated as M 3 Al 2À2x M x Si x (Si 3 O 12 ) where M represents divalent cations and x is the mole fraction of majorite, garnets that have the formula A 2 1+ MSi 2 (Si 3 O 12 ) have been observed, where A is an alkali element (e.g., Pacalo et al, 1992;Bobrov et al, 2008;Yang et al, 2009). The resultant garnet can be termed hypersilicic and, as described below, we find the incorporation of a sodium/alkali component to be more compatible with the compositional patterns that we observe within suites of natural majoritic garnets.…”