The High-Pressure Structural Evolution of Olivine along the Forsterite–Fayalite Join

Abstract

Structural refinements from single-crystal X-ray diffraction data are reported for olivine with a composition of Fo100 (forsterite Mg2SiO4, synthetic), Fo80 and Fo62 (~Mg1.6Fe0.4SiO4 and ~Mg1.24Fe0.76SiO4, both natural) at room temperature and high pressure to ~8 GPa. The new results, along with data from the literature on Fo0 (fayalite Fe2SiO4), were used to investigate the previously reported structural mechanisms which caused small variations of olivine bulk modulus with increasing Fe content. For all the investigated compositions, the M2 crystallographic site, with its bonding configuration and its larger polyhedral volume, was observed to control the compression mechanisms in olivine. From Fo100 to Fo0, the compression rates for M2–O and M1–O bond lengths were observed to control the relative polyhedral volumes, resulting in a less-compressible M1O6 polyhedral volume, likely causing the slight increase in bulk modulus with increasing Fe content.

Wardite (NaAl3(PO4)2(OH)4·2H2O) at High Pressure: Compressional Behavior and Structure Evolution

“…The (isothermal) bulk modulus of wardite, derived by a second-order BM-EoS fit to the P-V data (K V0 = 85.8(4) GPa), is significantly high, if compared to other crustal materials. For comparison, the bulk modulus of quartz is~37 GPa [18], feldspars~50-60 GPa (e.g., [19] and references therein), micas~40-60 GPa (e.g., [20,21] and references therein), pyroxenes~100-120 GPa (e.g., [22] and references therein), and olivines 120-130 GPa (e.g., [23] and reference therein). The bulk modulus of wardite reflects the calculated compressibilities of the building units: the bulk moduli of the P-tetrahedron, Al-octahedra and Na-polyhedron are, respectively, 175(33), 145 (25) and 61(5) GPa (Table S4).…”

Natural Forsterite Strongly Enriched in Boron: Crystal Structure and Spectroscopy

“…A growing body of crystal structure and spectroscopy investigations have been carried out on both synthetic and natural olivine over the past few years. The majority of the previous studies have primarily focused on crystal structure refinement [3,4,8,9], the effect of temperature and pressure on the crystal structure [10][11][12][13][14], the chemical composition of the forsterite-fayalite series determination [15,16], residual pressure calculation [17], and water content estimation [18]. Although extensive studies have been conducted on olivine over a range of compositions, temperatures, and pressures using a variety of methods, more research is needed on several aspects of the properties of B-rich olivine.…”

Equation of state and sound wave velocities of fayalite at high pressures and temperatures: implications for the seismic properties of the martian mantle

“…6 in Stixrude and Lithgow-Bertelloni, 2005). As recently proposed, such a relation may not work for all olivine compositions (Béjina et al, 2019;Pamato et al, 2019). Consequently it is important to measure the elastic properties across the entire solid solution in order to clearly define a compositional trend.…”

“…Firstprinciples calculations by Stackhouse et al (2010) predicted an even larger bulk modulus for fayalite, above 139 GPa. Therefore, a Fe-compositional trend of the bulk modulus for Fe-rich olivines is difficult to define and may even be nonlinear (Béjina et al, 2019;Pamato et al, 2019).…”