Crystal orientation plays an important role in mineral amorphization during solar wind implantation. To discuss these effects, ion implantation experiments were carried out to irradiate natural olivine grains by 1 × 1017 cm−2 50 keV He+. Based on the olivine grains irradiated in our experiment, residual crystal planes have been identified by reference to the crystal plane's spacing shown in diffraction images. It is found that He+ ions injected along  damages the olivine structure more effectively than with other orientations and that this possibly relates to the higher atomic density and the vertical impact of the flux on MO6 (where M commonly represents Fe2+ and Mg2+) octahedra chains. Crystal planes perpendicular or approximately perpendicular to  may be destroyed easily during the early stages of irradiation, particularly for (040). However, crystal planes, such as (041), (021), (022), (120), and (140), parallel to  or  may survive until the final stages of olivine amorphization. These different characteristics affected by crystal orientation in ion implantation might help researchers to better understand the process of solar wind weathering and in dating the exposure time of lunar and asteroidal soil grains as well as interplanetary dust particles affected by the solar wind.
 Amorphization is a remarkable change within minerals as the result of prolonged space weathering. This process was verified during the period of Apollo exploration that returned material in which the majority of sample grains exhibited unusual rims, those in which the mineral of interest had changed to an amorphous material [Bibring et al., 1972, 1974; Keller and Mckay, 1997; Noble et al., 2005]. Such rims were also found in interplanetary dust particles (IDPs) and dust particles as well as meteorites that originate from asteroids collected in later years [Bradley and Brownlee, 1986; Noble et al., 2011; Noguchi et al., 2011, 2013]. Without the protection of a thick atmosphere, mineral grains on the surface of airless planetary bodies (such as the Moon, Mercury, or an asteroid) and IDPs endured the alteration of space weathering during their exposure history. Sample analysis shows that processes of space weathering mainly include solar wind implantation and micrometeorite bombardment, which have produced amorphous rims on the surface of lunar soil grains [Keller and Mckay, 1997]. Amorphous rims on IDPs and lunar soil grains (a lunar soil amorphous rim is a type of amorphous structure on the grain surface which had been classified by Keller and Mckay ) share similar characteristics, such as a range of thicknesses from ~20 to 100 nm, noncrystalline, cation depletion, and oxygen superstochiometry. On the other hand, rims produced by micrometeorite bombardment share the given characteristics, such as thickness exceeding 100 nm, different chemical composition compared with the host grain (especially enriched in S), inclusions (Fe-metal, ilmenite, Fe-sulfides) either randomly dispersed throughout the thickness of the rim or occurring as discrete layers of inclusion [Keller and Mckay, 1993; Keller and Mckay, 1997]. It is generally thought that amorphization of the outermost layer of IDPs, the inner layer of Itokawa dust particle rims, and the origin of the lunar soil amorphous rim is related to the crystalline modification produced by solar wind implantation, not the vapor deposition produced by micrometeorite bombardment [Bradley, 1994; Keller and Mckay, 1997].
 Micrometeorite bombardment induced melting and vaporization of the mineral grains, leaving no crystal structures that survive in the melt or vapor deposition. However, part of the crystal structure may survive during the irradiation of solar wind if the total dose is lower than the mineral's critical amorphous dose. Partially amorphized crystal structure is the specific characteristic of mineral grains that have been altered by solar wind particles. Partial amorphization has never been identified in lunar soil, IDPs, and asteroid breccia's grains in previous works. The newly finished research work by Cs-corrected Scanning Transmission Electron Microscope (STEM) demonstrated that the inner layer of Itokawa dust rims is partially amorphized. Its origination is considered to be controlled by the irradiation of solar wind [Noguchi et al., 2011, 2013]. Therefore, partially amorphized structure of lunar soil grains and IDPs may be used to identify its solar wind origination. However, none of the relationships between crystal orientation and residual crystal planes has been identified.
 To verify this conjecture, experiments have been performed to investigate the effect of implanted dose, ion type, and mineral type on amorphization of mineral grains. Previous studies have shown that amorphization will occur when the implanted ions reach a critical dose [Jäger et al., 2003]. Studies of the crystalline-to-amorphous transition of oxidiferous minerals triggered by ion irradiation have been carried out by various authors. Bibring et al.  studied the behavior of feldspar, olivine, ilmenite, and magnetite under the bombardment of low-energy helium ions and found that the critical amorphous doses are 1016, 3 × 1016, 2 × 1017, and 5 × 1017 ions/cm2, respectively. The different critical doses were believed to be related to the various mineral species. In addition, critical amorphous dose has been proved to be also related to the different chemical compositions of olivine. The behavior of forsterite and five further members of the olivine series have already been performed by Wang and Ewing  and Wang et al.  for 1.5 MeV krypton ions. It is demonstrated that the critical amorphous dose increases rapidly with an increase in the Mg/Fe ratio. The different critical amorphous doses in the amorphization process are believed to be related to the melting temperature and bond ionicity in different olivine species. To discuss the relationship between the amorphization and solar wind implantation of interstellar dust, a series of radiation experiments concerning enstatite amorphization were performed by Schrempel et al.  and Jäger et al. . In these studies, the ion radiation dose was emphasized, and an accumulated dose suffered by interstellar dust was estimated to determine whether the exposed time is long enough to damage the mineral microstructure. Through the analysis of high-resolution transmission electron miscroscopy (HR-TEM) images, they found that while the (001) plane had been damaged, the (011) and (221) planes had not, in the examined grains. They concluded, therefore, that irradiation perpendicular or oriented at an acute angle to the SiO4 chain, which consists of pure, covalent bonds, is more efficient than irradiation of other surfaces [Schrempel et al., 2002; Jäger et al., 2003]. In addition, Futagami et al.  also proved that the diffusion rate of implanted Ne in the  direction of olivine is 1 to 2 orders of magnitude higher than in the  direction. That is, the mineral structure might play an important role in amorphization during the space weathering process, which, however, is far from being understood. The study of crystal orientation effects on the amorphization of olivine might be helpful in understanding the amorphous transformation process within mineral grains present on the surfaces of airless planets and especially for the origin of amorphous rims on lunar or asteroidal soil grains and IDPs.
 In order to investigate the structural effects on the amorphization of silicate minerals under ion irradiation, an experimental procedure was designed whereby micron-scale olivine grains could be irradiated by 1 × 1017 cm−2 50 keV He+. In this paper, the effects of crystal orientation in amorphization of olivine during solar wind implantation are discussed.
2 Crystal Structure of Olivine
 Olivine is a common mineral constituent within lunar and asteroidal materials and of IDPs. This mineral is an orthosilicate consisting of isolated SiO4 tetrahedrons that are joined laterally through edge-sharing MO6 octahedra cations (where M commonly is occupied by Fe2+ and Mg2+). Chains of octahedra cations run parallel to the crystallographic c axis (or ), as is shown in Figure 1. The octahedra chains are cross-linked with SiO4 tetrahedron by point sharing. The major cations (Fe2+ and Mg2+) in the octahedra sites distribute with a high degree of disorder (randomness) over the octahedra sites (see Figure 1).
 Previous studies have shown that the susceptibility of pure covalent bonds to amorphization is higher than in the case of higher ionicity in the bond [Naguib and Kelly, 1975]. As the SiO4 tetrahedra are constructed by pure covalent bonds, its strength is weaker than that of the MO6 octahedra which are constructed through ionic bonding. Additionally, SiO4 tetrahedra are isolated from one another being only connected through the MO6 octahedra by point-sharing ionic bonds. The strength of this point-sharing connection is weaker than that of the MO6 chains which are formed through edge-sharing octahedra (see Figure 1). The effect of damage on the anisodesmic olivine crystal structure under ion bombardment, therefore, is mainly dependent upon the edge-sharing MO6 octahedras rather than the isolate, SiO4 tetrahedra. Therefore, the action of MO6 octahedra chains during this ion implantation experiment was considered seriously.
3 Amorphization of Olivine Grains During He Implantation
 Natural, crystalline olivine (with a composition of Mg1.84Fe0.16SiO4) used in this experiment was crushed in a planetary ball mill. The final grain size was less than 20 µm. He+ implantation was performed utilizing a 200 kV ion implanter housed in the School of Physics and Technology, Wuhan University, China. The crystal orientation of olivine grains are random and irradiated by He+ randomly in this experiment. This is similar to the irradiation of dust particles on the surface of the Moon and asteroids. The experiment was carried out at room temperature, in a vacuum chamber with a residual pressure of 1.0 × 10−3 Pa when the helium gas was introduced. In order to prevent the charging of the sample during implantation, olivine powders were compacted on the double-sided conductive tape and pasted to the metallic target. During implantation the electric charge was conducted away by the tape. The accelerating voltage of 4He+ was 50 kV, and the injected target mineral was oriented at normal incidence. The ion flux was 3.5 μA, and the implantation dose was 1 × 1017 cm−2. Implanted grains were embedded in low-viscosity epoxy, and ultrathin sections eventually were prepared using an ultramicrotome. These sections were analyzed utilizing a 200 kV, FEI Tecnai F20 transmission electron microscope (TEM) housed at the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. The accelerating voltage, during the experiment, was reduced to 120 kV in order to protect the specimens from destruction by electron beam bombardment.
 At each impact, He+ transfers energy to the target via both electronic and nuclear interactions [Mark, 1994]. An injected ion collides with nuclei of the lattice atoms, transfers its kinetic energy to the lattice atoms, creates vacancy and interstitial pair, and finally causes crystal structure damage. If ions graze the lattice atoms, they interact with the lattice atom's electrons and not the positive core. The collision between injected ion and electrons of the lattice atoms experiences drag due to “free” or polarizable electrons. Energy transfer of low-energy ion's electronic collision is very small, and crystal structure damage is negligible. Therefore, olivine's amorphization under the irradiation of either 4 or 50 keV He+ is controlled by the nuclear collision, not the electric collision. In addition, higher energy of injected He+ makes the irradiation damage much more remarkable. Although there are some differences between 50 and 4 keV He+, we believe that it still worked to simulate the irradiation damage of solar wind particles by 50 keV He+.
 As show in Figure 2, energy loss of nuclear collision increased with the reduction of kinetic energy of injected He+ and reaches its peak value at 0.7 keV. This proves that the irradiation damage made by nuclear collision mainly takes place at the end of the injection range. However, the penetration path of He+ is not a straight line. Injected He+ as well as lattice vacancy mostly distributed at the mean value of the projected range, as shown in Figure 3.
 TEM and HR-TEM images of specimens are presented in Figures 4a, 4b, and 5a–5h and are discussed herein. Although the average thickness of the ultrathin sections is 50 nm, the edges of some of the grains were less than 10 nm and, as such, were suitable for HR-TEM observation. The width of these ultrathin edges ranged from several to tens of nanometers. Diffraction characteristics were subsequently obtained by Fourier transformation of the suite of HR-TEM image using Digital Micrograph software.
 Ultrathin sections from a total of 39 olivine grains were analyzed by TEM, and amorphous stages were primarily evaluated. Focal image series were taken to make sure the microstructure of irradiated olivine specimens were accurately displayed. Based on these TEM images, all the grains were classified into four stages according to their amorphous transformation characteristics: mild amorphization, moderate amorphization, severe amorphization, and complete amorphization, respectively. The classification criteria of various amorphization stages are listed in Table 1. Most of these investigated olivine grains exhibited incomplete amorphization, falling within the first three categories (see Table 1).
Table 1. Crystal Planes in Original and Irradiated Olivine Grains
Classification criteria of amorphization stages of irradiated olivine grains are listed as follows: Stage I: The orientation of most of the residual crystal planes is random. Only part of the crystal planes disappeared and cannot be identified in the diffract spots lattice. Stage II: Orientation of the residual crystal planes is still random. But the residual lattice fringes and diffraction spots were more indistinct and difficult to be identified than olivine grains of amorphization stage I. Stage III: The number of residual crystal planes is less than those in olivine grains of amorphization stages I and II. Nearly all of the residual crystal planes are parallel to  or  crystal axis. Stage IV: None of the lattice fringes and diffraction spots can be recognized in the HR-TEM and FFT images.
Ol-3.1 has not been irradiated by accelerated ions; its amorphization stage is “N/A,” in which the crystal structure is original and without any irradiation damage.
 Figures 4a and 4b are, respectively, the HR-TEM and TEM images of Ol-3.1, which is an unirradiated olivine grain. The image on the top right corner is the selected area electron diffraction (SAED) result. Lattice fringes in the HR-TEM image are complete and regular. At least 30 crystal planes can be recognized in the diffraction lattice of the SAED. Ol-3.1 was used to explain the structural differences between original and irradiated olivine grains.
 Figures 5a and 5b are, respectively, the TEM and HR-TEM images of Ol-2.7, which is a grain, typically of mild amorphization. Fourier transformation of the HR-TEM image indicates that the diffraction peak of (040) is absent in the diffraction lattice of the fast Fourier transform (FFT) (see Figure 5b; a white cross represents its position). This has been produced by He+ which has been injected along the  axis and vertically impacts the MO6 octahedra chains. Only part of the crystal planes parallel to the MO6 octahedra chains, such as (040), is destroyed seriously. Most of the MO6 octahedra chains and SiO4 tetrahedra have been completely saved. From the HR-TEM image an amorphous rim-like edge is also found (see Figure 5b). It is possible that this might not represent a real amorphous rim, being only the result of over focus in TEM analysis, as the edge of the section is higher than the interior. Figures 5c and 5d are, respectively, the HR-TEM and TEM images of Ol-2.5, which is a typical moderately amorphous specimen. The remaining lattice fringes are dim and difficult to be recognized. The diffraction spots of this grain are disordered and indistinct. This indicates that part of the lattice elements of both the SiO4 tetrahedra and MO6 octahedra have been substituted by He+ or knocked out to form lattice defects or interstitial atoms. The crystalline structure of olivine grain has been significantly destroyed. Figures 5e and 5f are, respectively, the TEM and HR-TEM images of Ol-1.25, which is an example of severe amorphization. Its crystalline structure was heavily damaged, and the remaining lattice fringes are difficult to distinguish. Only the diffraction spots of (140) and (041) can be distinguished in the FFT image. The interface between diffraction spots and the amorphous halo is not clearly observed. The diffraction spots of (140) and (041) prove that only the crystal planes that are parallel to the  or  crystal axis were saved during the radiation. Not only Ol-1.25 but also the residual crystal planes in nearly all of the olivine grains of severe amorphization are parallel to  or  crystal axis, except (112) in Ol-1.6 and (211) in Ol-2.46 (see Table 1). This proves that crystal frameworks parallel or approximately parallel to the  and  axes were more difficult to be destroyed than those parallel or approximate parallel to the  axis. A large part of the lattice elements of SiO4 tetrahedra and MO6 octahedra chains have been substituted by He+ or knocked out to form lattice defects or interstitial atoms. The crystalline structure of olivine grain has been seriously destroyed. Figures 5g and 5h are, respectively, the TEM and HR-TEM images of Ol-2.28 which has underwent complete transformation to an amorphous state. None of the remaining lattice fringes can be distinguished from the HR-TEM images (see Figure 5h). No diffraction lattice is shown in the FFT images. Focal image series were taken to make sure that there is no crystal plane survival during the irradiation of 50 keV He+. The results prove that both the MO6 octahedra chains and the SiO4 tetrahedras have been completely destroyed while preserving the disordered elemental tetrahedral and octahedral lattice elements.
 The edge of this grain in Figure 5h is rougher on the nanometer scale compared with the mild amorphous grain in Figure 5b. Formation of this irregular edge might relate to the erosion of implanted ions. Increased morphological surface roughness has previously been proven through atomic force microscopy testing of olivine irradiated by He+ [Davoisne et al., 2008].
4 Residual Crystal Planes
 Residual crystal planes have been identified by use of the crystal plane's spacing of lattice fringes which can be measured in the HR-TEM and diffraction images. After the distance between diffraction spots and transmission spot in FFT image had been measured, the remaining crystal planes were determined through comparison of the observed distance to the crystal plane data for olivine in various X-ray diffraction databases of MDI jade 5.0. Residual crystal lattice planes identified in all of the examined olivine grains are listed in Table 1, sorted by amorphization stage.
 As shown in Table 1, there are several obvious characteristics of the residual crystal lattice planes. Only the diffraction spot of (040) which is perpendicular to  cannot be investigated in the diffraction pattern of Ol-2.7 (see Figure 5b). Some crystal planes, such as (131) and (132), are nearly perpendicular to  and can only be distinguished in the olivine grains which are mildly to moderately amorphized. In contrast, crystal planes, such as (140) and (041), that occur parallel to  or  can be found in all of the incompletely amorphized olivine grains, and their percentage in the remaining crystal planes is in direct proportion to the stage of amorphization. This may imply that crystal planes perpendicular to  are more easily destroyed, and crystal planes perpendicular to  or  are more difficult to be destroyed, surviving until the mineral grain is completely amorphized. These characteristics might indicate that the crystal structure is more prone to damage along the direction  as compared to direction  or  during solar wind particle implantation.
 How is it possible that crystal structures along  or  could have survived ion bombardment, whereas those along  are much easier to be destroyed? The different microstructures present along the crystal axes and a channeling effect might be the important causes.
 As shown in Figure 1, the atom density along the  direction is higher than that of  or . MO6 octahedra consist of serrated chains which lie parallel to . In the olivine crystal structure, the SiO4 tetrahedra are isolated, and as a result their susceptibility to amorphization is greater than that for the cations occupying the MO6 octahedra. Hence, the strength of olivine's crystal structure is mainly dependent upon the MO6 octahedra chains rather than SiO4 tetrahedra. Because of the higher atom density and vertical impact of MO6 octahedra chains, higher intensity collision might take place, and the crystal structure might be destroyed more effectively when ions are injected along . This means that a large part of the lattice elements of SiO4 tetrahedra and MO6 octahedra chains have been knocked out to form lattice defects and interstitial atoms or substituted to form substitution atoms. The crystalline structure of olivine grain has been seriously destroyed.
 For olivine grains in the experiment that were spread out randomly, the probability of ions injected directly along the three axes is approximately equal. Because the collision intensity between injected ions and crystal atoms is greatest during the vertical impact of MO6 octahedra chains, the crystal structure of olivine grains might be damaged more easily than in other orientations when they are irradiated along . In the experiment, samples including Ol-1.13, Ol-1.19, Ol-1.21, Ol-2.23, Ol-2.24, Ol-2.25, Ol-2.28, Ol-2.40, Ol-2.42, and Ol-2.50 show complete amorphization characteristics in observed HR-TEM images. This may indicate that these grains were irradiated parallel or nearly parallel to .
 When ions are injected into the olivine crystal structure along , they also impact the MO6 octahedra chains perpendicularly. However, the damage along  is less effective compared to that along . The reason is that the atom density in this direction is lower than for , and channeling effect might be more dramatic. It is verified in our experiment that part of the crystal planes parallel to  survived after a dose of 1 × 1017 cm−2 He+ implantation. With HR-TEM analysis of olivine grains, (041), (021), and (022) are commonly found in most of the grains which are moderately to severely amorphized.
 When energetic ions are injected into the crystal structure along , the ions move along MO6 octahedra chains. Similar to , the destruction injected along  is less effective compared to that of ; again, this is thought to relate to their lower atom density in that direction. Additionally, because ions penetrate in the crystal interior along MO6 octahedra chains, the destruction might be of lowest effect in all three axis directions. Hence, the crystal structure is difficult to destroy and survives with only slight damage in this case. In the experiment, samples of Ol-1.31, Ol-2.2, Ol-2.3, Ol-2.7, and Ol-2.49 are slightly amorphous, and most of the crystal planes, regardless of crystal orientation, survived the irradiation relatively unharmed. This suggests that these grains might have been injected along .
 Actually, the direction of ions injected into the crystal structure is random and might not coincide with the three crystal axes exactly in the experiment. But the effect of crystal orientations on the amorphization could still be identified. Crystal planes oriented at an acute angle to , such as (131) and (132), can only be recognized in the olivine grains which are mildly and moderately amorphous. In addition, the percentage of crystal planes that are parallel or approximately parallel to  or  increases with the degree of amorphization.
 The effects of crystal orientation on the amorphization of olivine are obvious in this experiment. With homogenization composition and irradiation dose, the difficulty of the crystal structure's destruction might depend on its crystal orientation. These different characteristics affected by crystal orientation in ion implantation might help researchers to better understand the process of solar wind weathering and in dating the exposure time of lunar and asteroidal soil grains as well as interplanetary dust particles (IDPs) affected by the solar wind.
 The research results of this study can be used to identify the relative exposure history of olivine grains to solar wind irradiation. The amorphous stages of olivine grains on the airless planetary bodies, which increase proportionately with its exposure time, can be classified based on the residual crystal planes in its rims. Then the exposure history of olivine grains can be identified according to its critical dose of specific crystal axis and amorphous stage.
 Mineral amorphization is a common result of space weathering occurring on the surface of airless planetary bodies. As a major mineral constituent, crystal structure modification of olivine as the result of solar wind space weathering is affected by the crystal orientation independent of the chemical composition and/or implantation dose. In the crystal structure of olivine, the susceptibility of SiO4 tetrahedral to amorphization is higher than for MO6 octahedra and the strength of olivine's crystal structure is mainly dependent upon cations residing in MO6 octahedra chains during the irradiation process. Ions injected along high atom density crystal axes and perpendicular or oriented at an acute angle to the MO6 octahedra chains might most effectively destroy the crystal structure and induce amorphization. Compared to the three crystal axes of olivine, the most effective destruction is found through injection along . In all of the examined crystal planes of olivine grains, (040) is most easily destroyed, disappearing first under He+ implantation, while (041), (021), (022), (120), and (140) survived until the crystal structure is completely destroyed. MO6 octahedra chains in the olivine grains with amorphization stages I and II maybe still be integrated and with a small part of the octahedral lattice elements be substituted by injected He+ or knocked out to form displaced atoms and crystal defects. However, SiO4 tetrahedral maybe destructed seriously. The MO6 octahedra chains in olivine grains with amorphization stage III may be seriously destructed, only leaving the crystal planes that are parallel to  or  axis to survive, such as (041), (021), (022), (120), and (140). Both of the MO6 octahedra chains and SiO4 tetrahedral in olivine grain of amorphization stage IV have been completely destructed, leaving no crystal framework that survived.
 These different characteristics affected by crystal orientation in ion implantation might help researchers to better understand the process of solar wind weathering and in dating the exposure time of airless bodies' dust particles affected by the solar wind. But olivine's critical amorphization dose of each crystal axis should be made sure first.
 We would like to thank Liping Guo (Wuhan University), Kang Tsang (Hong Kong University of Science & Technology), and Jinping Zhang (Suzhou Institute of Nano-Tech and Nano-Bionics, CAS) for their kind help in this study. This study was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Lunar Program of Geochemical Institute), China's Lunar Exploration Program (Grant TY3Q20110029), and National Natural Science Foundation of China (Grants 41003027, 41273080, 40803019, and 41373067).