22P067-P
Synthesis of MgO-SiO2 Amorphous Silicate Nanoparticles
by Induction Thermal Plasmas
誘導結合型熱プラズマによるMgO-SiO2系非晶質ケイ酸塩ナノ粒子の合成
Masahito Yagishita, Sooseok Choi, Takayuki Watanabe, Junya Matsuno, Akira Tsuchiyama
八木下将史, 崔 秀錫，渡辺隆行，松野淳也，土`山 明
1) Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology
4259 G1-22, Nagatsuta, Yokohama, Kanagawa 226-8502, Japan
東京工業大学大学院総合理工学研究科 〒226-8502 横浜市緑区長津田町4259番地G1-222) Department of Earth and Space Science, Graduate School of Science, Osaka University
1-1 Machikaneyama, Toyonaka, Osaka 556-0043, Japan
大阪大学大学院理学研究科 〒556-0043 豊中市待兼山町1-1
Crystalline silicate found in circumstellar regions and in primitive materials of the solar system isbelieved to be crystallized from amorphous silicates like as GEMS (glass with embedded metal andsulfides). MgO-SiO2 amorphous silicate and iron embedded MgO-SiO2 amorphous silicate nanoparticles
was successfully prepared by induction thermal plasmas (ITP) in order to obtain GEMS analogue materials
for alteration experiment. The morphology of synthesized analogue particles is very similar to GEMS.
Si/Mg fractionation was found in the ITP process of synthesis of MgO-SiO2 silicate nanoparticles due tothe different nucleation temperatures of Mg and Si.

1. Introduction
Crystalline silicate in circumstellar regions andin primitive materials of the solar system isbelieved to be crystallized from amorphoussilicates. One candidate material of suchamorphous material is GEMS (glass withembedded metal and sulfide), which generally
exists in cometary dust [1,2]. According to recentwork, it is composed of amorphous silicate
including iron metal in the size of few tens ofnanometer. In order to reveal the evolution ofamorphous silicate in the early solar system, thealteration experiment is required.
In this study, we aim to synthesize analoguematerial of GEMS which is iron embedded
MgO-SiO2 amorphous silicate by ITP (induction
thermal plasmas). ITP provides high temperatureenvironment with rapid quenching. Therefore, allintroduced materials are immediately evaporated
and subsequently form metastable state
nanoparticles. Since synthesis of amorphoussilicate with two elements by ITP is not wellunderstood, preparation of MgO-SiO2 amorphoussilicate nanoparticles was carried out at first.
Then, synthesis of iron embedded sample on
GEMS mean composition [3] was followed.
2. Experimental
Fig.1 shows experimental setup composed of theRF torch and the synthesis chamber, used foramorphous silicate nanoparticles preparation.
Precursor is injected from the powder feeder by Ar

carrier gas. Its feeding rate was fixed at 250 mg/min.
SiO2 (quartz), MgO, and Fe powders in the average
size of 4μm, were physical mixed and used as theprecursor. Synthesized nanoparticles were collectedon the chamber wall after the plasma treatment.
Table 1 shows experimental condition for thesynthesis system. Ar-O2 (oxidative) sheath gas andAr-He (inert) sheath gas are used for MgO-SiO2
sample and iron embedded sample, respectively.

Fig.1. Experimental setup.
Table 1. Experimental conditions.
Sheath gas
Inner gas
Carrier gas
Composition
Starting materials
Average size
Feeding rate
Plasma power plate
Frequency
Pressure

Magnesia silicate
Ar-O2, 60+5 slpm

GEMS
Ar-He, 60+5 slpm

Ar, 5 slpm
Ar, 4-6.5 slpm
Mg/Si = 0.17-0.64
Mg/Si, Fe/Si =0.65, 0.56
SiO2 (quartz), MgO
Fe, SiO2 (quartz), MgO
4μm
250 mg/min
30 kW
4 MHz760 Torr

4. Conclusion
GEMS analogue material was obtained byusing ITP method which has high temperature andrapid quenching. Further investigation for thesynthetic mechanism for Mg and Si is required tounderstand the whole process of iron metal
embedded MgO-SiO2 silicate nanoparticles.
Table 2 Chemical analysis on precursor and final productStarting material
MgO [ wt. % ]Mg/Si [ - ]1020300.170.370.64

Synthesized powder
Mg/Si [ - ]0.140.220.30

MgO content of starting material
10 wt%

20 wt%

Intensity / a.u.

30 wt%

40 wt%
50 wt%

Forsterite(Mg2SiO4)

1020304050607080

2 / degree (CuK)

Fig.2. XRD patterns of synthesized MgO-SiO2
amorphous silicate powder.

-Iron (110)Intensity / a.u.

3. Results and discussion
Fig.2 shows XRD patterns of MgO-SiO2
amorphous silicate nanoparticles which weresynthesized under oxidative plasma sheath gas
condition. Although forsterite peaks are observed inthe high MgO contents of 40 wt.% and 50 wt.% inthe precursor, amorphous materials are successfully
synthesized in low MgO contents.
Table 2 shows MgO contents and Mg/Si ratio in
the precursor and in the final product measured by
ICP-AES. Reduced Mg content in final products
indicates the fractionation of Mg and Si in theplasma treatment. The fractionation is regarded tooccur in condensation stage due to the different
nucleation temperatures of Mg and Si.
Fig.3 shows XRD results of synthesized iron
metal embedded MgO-SiO2 silicate nanoparticlesunder inert sheath gas condition. In the Fig.3, peaks
corresponding to iron metal and halo pattern foramorphous phase are clearly measured while ironoxides and minerals are not observed. Iron exists asmetal during Mg and Si are oxidized due to its
higher Gibbs free energy of oxidation than Si andMg.
Fig.4 shows TEM images of (a) synthesized
MgO-SiO2 silicate particles (MgO 30 wt.%), (b)synthesized iron metal embedded MgO-SiO2
silicate particles, (c) magnification of the GEMS
analogue, and (d) GEMS grain in the interplanetary
dust particles [3]. From Fig.4 (a) it is found that thesynthesized MgO-SiO2 silicate particles (MgO 30wt.%) are amorphous with sub-micron size.
Morphology of the prepared iron embedded
nanoparticle in Fig.4 (b) and (c) is very similar toGEMS in sizes of amorphous particle and included
iron metal. EDS semi- quantitative analysis resultof the Mg/Si change from 0.56 to 0.43 is the same
trend with ICP-AES measurement. Still, further
investigations on reaction of raw material in ITP areneeded to understand the synthesis mechanism.

-Iron (200)10203040506070802 / degree (CuK)

Fig.3. XRD patterns of synthesized iron embedded
MgO-SiO2 amorphous silicate powder.

(a)

(b)

50.0 nm(c)(d)

50.0 nmFig.4. TEM image of synthesized materials andGEMS: (a) MgO-SiO2 (30 wt%), (b) GEMS
analogue, (c) magnification of analogue, and(d) GEMS in the interplanetary dust particles.
[3].
5. Reference[1] F. Kemper, W.J. Vriend, and A.G.G.M. Tielens:Astrophys. J. 609 (2004) 826.
[2] J.P. Bradley: Science 265 (1994) 925.
[3] L.P. Keller, S. Messenger: Geochim. Cosmochim.
Acta 75 (2011) 5336.