Compressibility of Natural Olivine Single-Crystals
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摘要: 采用金刚石对顶砧高压装置和同步辐射X射线衍射技术，在常温下对两种天然橄榄石单晶(含铁的铁镁橄榄石(Mg1.83Fe0.17)SiO4和不含铁的纯镁橄榄石Mg2SiO4)进行了高压结构行为研究。实验结果表明，两种天然橄榄石单晶在最高压力为9.9GPa的条件下没有发生相变。采用二阶Birch-Murnaghan状态方程描述压力-晶胞体积(p-V)关系，获得了铁镁橄榄石和纯镁橄榄石在零压下的晶胞体积(0.2929(3)、0.2899(1)nm3)和体积模量(140(3)、151(2)GPa)。两种镁橄榄石呈高度各向异性，轴压缩性表现为：b轴的压缩率最大，c轴次之，a轴的压缩率最小。随着铁组分的加入，橄榄石的体波速减小，两种橄榄石的体波速相差5.2%，该结果对于研究上地幔的物质构成和弹性波速具有一定的意义。Abstract: In situ X-ray diffraction measurements for natural Fe-bearing and Fe-free Mg2SiO4 forsterite single-crystals ((Mg1.83Fe0.17)SiO4 and Mg2SiO4) were performed with diamond anvil cell high-pressure device and synchrotron radiation X-ray diffraction at room temperature.The experimental results showed that no phase transition of two natural olivine single-crystals was observed within the upper highest pressure of 9.9GPa.The isothermal relationship of pressure and volume in the two kinds of crystals is well described by the second-order Birch-Murnaghan equation of state with the zero-pressure unit cell volume and zero-pressure bulk modulus of 0.2929(3)nm3 and 140(3)GPa for Fe-bearing forsterite and 0.2899(1)nm3 and 151(2)GPa for Fe-free forsterite, respectively.The linear compressibilities along a, b and c axes (ka, kb and kc) are elastically anisotropic in the two kinds of forsterite and the anisotropy can be described as kb>kc>ka.With the addition of iron into the crystals, the bulk velocity of olivine decreases and the difference between the two kinds of forsterite single-crystals is 5.2%.This result will be significant for the study of the composition of the earth's upper mantle and its elastic wave velocities.
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Key words:
- olivine /
- compressibility /
- anisotropy /
- bulk velocity
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图 1 纯镁橄榄石和铁镁橄榄石单晶的XRD谱(方框所示为实验所得的单晶衍射点，数字表示对应的指标化结果；“D”表示金刚石的衍射点，衍射环为压标Au的衍射环)
Figure 1. Single-crystal XRD patterns of Fe-free forsterite and Fe-bearing forsterite (Squares show single-crystal diffraction peaksand figures indicate indexed results.'D' represents diamond reflections, while the diffraction rings in the patterns are from Au)
图 2 铁镁橄榄石和纯镁橄榄石在常温下的p-V关系
Figure 2. p-V relationship for Fe-bearing forsteriteand Fe-free forsterite at 300K
图 3 晶胞参数(a、b和c)随压力的变化
Figure 3. Lattice parameters a, b and c as a function of pressure
表 1 不同压力下铁镁橄榄石和纯镁橄榄石的晶胞参数
Table 1. Unit-cell parameters of Fe-bearing and Fe-free forsterite at various pressures
Material p/(GPa) a/(nm) b/(nm) c/(nm) V/(nm3) Fe-free forsterite 0.0001 0.47382(6) 1.0218(2) 0.59817(5) 0.28961(6) 0.5 0.47283(4) 1.0236(1) 0.59743(4) 0.28917(3) 1.2 0.47279(4) 1.0206(2) 0.59637(4) 0.28776(6) 2.9 0.47124(4) 1.0168(2) 0.59438(4) 0.28481(6) 4.0 0.47015(4) 1.0149(2) 0.59275(4) 0.28284(6) 5.3 0.46926(4) 1.0126(2) 0.59106(4) 0.28060(6) 6.2 0.46872(4) 1.0089(2) 0.58968(4) 0.27885(6) 7.0 0.46810(4) 1.0085(2) 0.58839(4) 0.27777(6) 7.7 0.46761(4) 1.0077(2) 0.58746(4) 0.27667(6) 8.7 0.46677(4) 1.0043(2) 0.58692(4) 0.27514(6) 9.9 0.46609(4) 1.0013(2) 0.58564(4) 0.27240(5) Fe-bearing forsterite 0.0001 0.47832(2) 1.0150(1) 0.60201(5) 0.29230(4) 2.6 0.47847(7) 1.0114(1) 0.59514(3) 0.28801(3) 4.1 0.47840(7) 1.0070(2) 0.59284(2) 0.28561(3) 4.8 0.47682(7) 1.0058(2) 0.59165(3) 0.28376(3) 5.8 0.47538(8) 1.0047(2) 0.59022(2) 0.28200(3) 7.0 0.47413(8) 1.0023(2) 0.58876(3) 0.27979(4) 7.6 0.47407(8) 1.0000(2) 0.58805(3) 0.27880(3) 8.4 0.47306(8) 0.9996(2) 0.58687(2) 0.27753(3) 9.1 0.47199(9) 0.9984(3) 0.58630(2) 0.27628(4) 9.7 0.47122(8) 0.9982(3) 0.58553(4) 0.27542(3) 
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表 2 橄榄石的状态方程参数
Table 2. Equation of state parameters for olivines
Composition V0/(nm3) K0/(GPa) pmax/(GPa) Pressurecalibration Pressuremedium Method Reference Mg2SiO4 289.9(1) 151(2)a 9.9 Au ME XRD(SC) This study Mg2SiO4 289.7(3) 132(6)a 5.0 Olivine ─ XRD(SC) Ref.[11] Mg2SiO4 255 128(8)a 41.8 Ruby ─ XRD(PD) Ref.[21] Mg2SiO4 290.14(9) 125(2)a 17.2 Ruby ME XRD(SC) Ref.[20] Mg1.84Fe0.16SiO4 292.5(1) 128.8(5)a 8.0 Quartz MEW XRD(SC) Ref.[22] Mg1.83Fe0.17SiO4 292.9(3) 140(3)a 9.7 Au ME XRD(SC) This study Mg1.8Fe0.2SiO4 290.4(5) 131(1)a 9.7 Ruby ME XRD(SC) Ref.[23] Mg1.66Fe0.34SiO4 269 134(10)a 34 Ruby ─ XRD(PD) Ref.[21] Mg1.6Fe0.4SiO4 294.6(1) 129.4(5)a 7.4 Quartz MEW XRD(SC) Ref.[4] Mg1.42Fe0.58SiO4 296.6(2) 129.4(5)a 8.2 Quartz MEW XRD(SC) Ref.[4] Mg2SiO4 128.8(5)b 16.2 Ruby Ar/He/ME BS Ref.[8] Mg1.8Fe0.2SiO4 292.0(1) 131(2)c 32.4 Quartz ME BS Ref.[24] Note:(1) Superscript letter a represents refitting K0 with fixed K′0=4;
(2) Superscript letter b represents that K0 was based on Brillouin scattering, and K′0 was 4.2(2);
(3) Superscript letter c represents that K0 was based on Brillouin scattering, and K′0 was 3.8(2);
(4) ME:4:1 methanol-ethanol mixture; MEW:16:3:1 methanol-ethanol-water mixture; SC:single crystal; PD:powder; BS:Brillouin scattering.
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