minerals Article The High-Pressure Structural Evolution of Olivine along the Forsterite–Fayalite Join Martha G. Pamato 1,* , Fabrizio Nestola 1 , Davide Novella 2, Joseph R. Smyth 3, Daria Pasqual 1, G. Diego Gatta 4 , Matteo Alvaro 2 and Luciano Secco 1 1 Department of Geosciences, University of Padova, Via G. Gradenigo 6, 35131 Padova, Italy; [email protected] (F.N.); [email protected] (D.P.); [email protected] (L.S.) 2 Department of Earth and Environmental Sciences, University of Pavia, Via A. Ferrata 1, 27100 Pavia, Italy; [email protected] (D.N.); [email protected] (M.A.) 3 Department of Geological Sciences, University of Colorado, 2200 Colorado Ave, Boulder, CO 80309, USA; [email protected] 4 Department of Earth Sciences, University of Milan, Via Botticelli 23, 20133 Milan, Italy; [email protected] * Correspondence: [email protected] Received: 7 November 2019; Accepted: 11 December 2019; Published: 14 December 2019 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. Keywords: olivine; forsterite; fayalite; crystal structure; high pressure; X-ray diffraction 1. Introduction In order to understand the formation and evolution of the solid Earth, a precise knowledge of the elastic properties (e.g., bulk and shear modulus) of minerals present in the Earth’s interior at high pressures and high temperatures is necessary. These data are the basis for interpreting seismological profiles needed to determine the structure and composition of the Earth’s inaccessible interior (e.g., [1]). Olivine is the most abundant mineral in the upper mantle (51–60%; [2]) extending from the base of the crust to approximately 410 km. In natural systems, olivine forms a solid solution between two Mg and Fe end members, namely Mg2SiO4 forsterite (Fo) and Fe2SiO4 fayalite (Fa). At depths greater than 410 km, olivine is no longer stable and transforms to its high-pressure polymorph wadsleyite (β-olivine). This transformation is the cause of the 410 km seismic discontinuity that separates the upper mantle from the deeper transition zone (e.g., [3]). The exact pressure of this transition is a function of the Fe content of the system at a particular temperature (see [4] and references therein). Therefore, to understand the structure and composition of the mantle, and consequently global dynamics, accurate knowledge of the evolution of the elastic properties of olivine as a function of pressure and chemical composition is required. The crystal structure of olivine (Pbnm space group) can be described as an expanded and distorted hexagonally close-packed (hcp) array of oxygen anions stacked along the a axis (e.g., [5,6]). Si cations Minerals 2019, 9, 790; doi:10.3390/min9120790 www.mdpi.com/journal/minerals Minerals 2019, 9, 790 2 of 11 Minerals 2019, 9, x FOR PEER REVIEW 2 of 11 are located at tetrahedral sites, whereas Mg and Fe are disordered in two distinct octahedral sites, with Si cations are located at tetrahedral sites, whereas Mg and Fe are disordered in two distinct octahedral the M1 polyhedron being smaller and more distorted than the M2 polyhedron (Figure1). sites, with the M1 polyhedron being smaller and more distorted than the M2 polyhedron (Figure 1). Figure 1. Crystal structure of olivine plotted down the a axis. The b axis is plotted vertically (positive Figuredirection 1. upwards),Crystal structure whereas of theolivinec axis plotted is plotted down horizontally the a axis. (positiveThe b axis direction is plotted towards vertically left). (positive Atomic directionsites M2, M1,upwards), and Si whereas are highlighted. the c axis The is plotted black rectangle horizontally represents (positive the direction unit cell. towards left). Atomic sites M2, M1, and Si are highlighted. The black rectangle represents the unit cell. Many studies have been conducted in the past four decades in order to determine the elastic propertiesMany ofstudies olivine have (e.g., been the bulkconducted modulus) in the as past a function four decades of pressure in order and compositionto determine (see the [elastic7–22]). propertiesHowever, theof olivine results (e.g., of these the studiesbulk modulus) have shown as a significantfunction of scattering. pressure and Nestola composition et al. [18 (see] conducted [7–22]). However,a systematic the study results of of the these elastic studies properties have shown of the si Mg-richgnificant side scattering. of the forsterite–fayalite Nestola et al. [18] join,conducted which ais systematic relevant for study mantle of the compositions. elastic properties Importantly, of the Mg-rich this studyside of illustrated the forsterite–fayalite that the isothermal join, which bulk is relevantmodulus for (KT0 mantle) and its compositions. first pressure Importantly, derivative (K 0this) do study not vary illustrated significantly that betweenthe isothermal Fo92 to Fobulk62, modulusthus a single (KT0 equation) and its first of state pressure was determinedderivative (K for′) do the not entire vary compositional significantly between range with Fo92K T0to =Fo124.7(9)62, thus aGPa single and equationK0 = 5.3(3). of state More was recently, determined Angel for et the al. [entire21] reviewed compositional all available range single-crystal with KT0 = 124.7(9) data fromGPa andthe literatureK′ = 5.3(3). and More constrained recently, the Angel elastic et al. properties [21] reviewed and pressure–volume–temperature all available single-crystal data equation from the of literaturestate (EoS) and of constrained mantle-composition the elastic olivine. properties For an and olivinepressure–volume–temperature with Fo92–90 composition, equation these authorsof state (EoS)provided of mantle-composition a KT0 = 126.3(2) GPa olivine. and KFor0 = an4.54(6). olivine On with the Fo Fe-rich92–90 composition, side of the these forsterite–fayalite authors provided join, aHazen KT0 = [126.3(2)7] determined GPa and a K′T0 = of4.54(6). 113 GPaOn the (K0 Fe-richfixed to side 4) forof the a synthetic forsterite–fayalite fayalite. join, More Hazen recently, [7] determinedSpeziale et al.a K [T016 of] used113 GPa Brillouin (K′ fixed scattering to 4) for toa synthetic determine fayalite. the single-crystal More recently, elastic Speziale constants et al. of[16] a usedFe-rich Brillouin olivine (Fescattering0.94Mn0.06 to)2 SiOdetermine4 and provided the single-crystal an adiabatic elastic bulk modulusconstants (K s0of) ofa 134Fe-rich GPa, olivine which (Fecorresponds0.94Mn0.06)2SiO to a4 KandT0 ofprovided about 135 an GPa.adiabatic The bulk modulus ( ofKs0 Speziale) of 134 GPa, et al. which [16] is corresponds considered to to be a KmoreT0 of accurateabout 135 than GPa. the The value bulk obtained modulus by of Hazen Speziale [7], et given al. [16] the is pioneering considered technology to be more used accurate more than the40years valueago obtained in the by latter Hazen study. [7], Ingiven fact, the a verypioneering recent worktechnology using used synchrotron more than light 40 inyears a multi-anvil ago in the latterapparatus study. [22 In] reported fact, a very bulk recent moduli work of 131.4 using GPa synchrotron for Fo55, 132.1 light for in Foa 36multi-anvil, 136.3 for Foapparatus18, and 134.8 [22] reportedfor Fo0, in bulk agreement moduli withof 131.4 Speziale GPa for et Fo al.55 [,16 132.1]. These for Fo data36, 136.3 confirmed for Fo18 that, and increasing 134.8 for Fo the0, fractionin agreement of Fe within olivine Speziale stiff enset al. its [16]. structure These anddata increases confirmed the that bulk in modulus,creasing the possibly fraction not of followingFe in olivine a linear stiffens trend. its structureHowever, and the increases reasons for the this bulk trend modulus, still require possibly further not following understanding. a linear trend. However, the reasons for thisCrystallographic trend still requir studiese further can provideunderstanding. insights into the nature of interatomic forces and compression mechanismsCrystallographic that control studies mineral can elasticity. provide This insights is especially into truethe withnature regards of interatomic to Mg-rich olivineforces that,and compressionas mentioned mechanisms above, displays that almost control no mineral bulk modulus elastici variationty. This is along especially a large tr compositionalue with regards variability to Mg- rich olivine that, as mentioned above, displays almost no bulk modulus variation along a large compositional variability from Fo100 to Fo62 [18]. Therefore, it is crucial to understand the factors controlling its compression mechanisms. Finkelstein et al. [19] and Zhang et al. [20] recently Minerals 2019, 9, 790 3 of 11 from Fo100 to Fo62 [18]. Therefore, it is crucial to understand the factors controlling its compression mechanisms. Finkelstein et al. [19] and Zhang et al. [20] recently investigated the crystal structure evolution of olivine end members Fo and Fa, respectively. However, a systematic study describing the structural evolution of olivine as a function of pressure and chemical composition is still lacking. This study reports structural refinements from single-crystal X-ray diffraction data obtained on olivines with composition Fo100 (synthetic), Fo80, and Fo62 (natural) collected at room temperature and high pressure up to approximately 8 GPa.