The magnetite colloidal crystal in themeteorite formed 4.6 billion years ago〇Jun ozawa, Katsuo Tsukamoto, Kenta Yamada
Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

nozawa@ganko.tohoku.ac.jp
1. Introduction
polarized image

2. Observationsand Discussion

Meteorite parentbody (～km size)

BSEevaporation

2-1 Occurrence of Magnetite
Fig. 3LiquidRockWater

RockchondruleFe2+

matrixFe3+
matrix500µm

magnetite

20µm
Fig.1 polarized image and BSE image of the thin section of meteorite

Meteorite matrix have preserved information on the early solar system (Fig. 1). To
date, most of researches investigating the meteorites have been focused onmineralogy, measuring the chemical compositions, isotopic ratios and so on.
In the present work, detailed observation was carried out on the magnetite withhighly ordered three dimensional, namely colloidal crystals. As is the case withcrystal surface or morphology, these aggregates structures also reflectedenvironment in which they formed in the parentbody. Investigating these objects
from the view point of the colloidal science and crystal growth, we will provide new
valuable information to the planetary science.

Magnetite (Fe3O4)
Fig. 2 The model of aqueous alteration on themeteorite parentbody. (Rosenberg et al., 2001)

Fig. 2 shows model of aqueous alteration process in meteorite
parentbody (Rosenberg et al., 2001). Radiogenic heat melted ice
to water, in which nucleation and aggregation of fine magnetite
particles took place.

phyrosilicate

2-2 Lattice structure of colloidal crystals
Fig. 41µm

Tagish Lake carbonaceous chondrite has porous
structure and magnetite (Fe3O4) fine particles arefound frequently in those interstices of matrix (Fig. 3).
These magnetite particles were formed when aqueousalteration took place. Such as olivine and pyroxene
were altered to phyrosilicates.

Fig. 5

Fig. 6{111}{311}

Fig. 7{110}{110}morphologyof particles

diameter of lattice structure ofparticles [nm] colloidal crystal

{100}

{311}

f.c.c. / h.c.p.

b.c.c.

500nm

f.c.c.

1µm

Although magnetite particles usually appear randomly in the matrix (e.g. Fig. 3), there are areas where the size ofparticles is uniform. Furthermore, they are aligned in a regular periodic 3D structure, and form colloidal crystals.
We could observe several types of colloidal crystals in this meteorite. Fig. 4 is one with b.c.c. (body centeredcubic) lattice structure comprised by 150nm particles. Fig. 5 is one with f.c.c (face centered cubic) lattice structure
comprised by 200nm particles.

Fig. 6 is the colloidal crystal with f.c.c. andh.c.p.(hexagonal close packing) lattice structure
comprised by 600nm particles. The f.c.c and h.c.p.
structure could not distinguish by the appliedobservation method.

Potential energy (kT)(c)

Fig. 12
attraction
Colloidal solution

Electrical repulsion

86DLVO curve420-2050

10050nm

200

250

300

Particle separation
distance (nm)-6200

Energy barrier
van der Waals attractive force

1020

30

40

50

-400

Colloidal Water
particles-600

repulsion

-800
-1000

Colloidal
crystallization

The particles of the colloidal crystal of Fig. 5 is only formed by {110} faces, soit forms rhombic dodecahedron. Fig. 8 indicates why the colloidal crystal canonly form f.c.c. structure. It is interesting to note that this particle has thedepressed area on certain edge (arrows in Fig. 9 (a),(b)). HR-TEM
investigations of the depressed edge exposed mismatch of lattice, which
could result in the boundaries of the crystals (c). These morphologies andstructures are similar to those of multiple twin particles.

2-7 The Greigite occurs together
with magnetite
greigite (040)
0.2470nm

mag-1200-1400

Magnetic force included

-1600

2nm

greigite (311)0.2980 nm{110}

207 ± 3

FCC657 ± 20

FCC{110}{100}

Fig. 7 shows the relationship of the lattice
structure of the colloidal crystal and themorphologies and size of the particles. Itwas found that the size or morphology of theparticles determine the lattice structure ofcolloidal crystal.

Magnetic force are not included

0-200 0(b)150-4-8TEM imageBCC

DLVO curve・・・
sum of the vander Waals
attraction andthe electrostaticrepulsion.
V = VA + VR

10-10

DisorderedInteraction force aggregate

50nm

182 ± 6

2-4 How to form the 2-5 DLVO curve of magnetite 2-6 MFM Measurement ofmagnetite colloidal crystals
colloidal crystals?

2-3 Characteristics of Rhombic
dodecahedron particles(a)1µm{111}

One of the characteristics of the colloidal
crystal is the interaction force between
the particles are repulsion. If theinteraction force are attractive, particles
form the flocculation (Fig. 12).

colloidal solution

Magnetite

Greigite

Crystal system

Cubic

Cubic

Space group

Fd3m

Fd3m

Lattice parameter

8.391

9.868

replacement
72.5°°

2 nm

greigite (311)0.2980nm

Colloidal
crystallization

Magnetite has magnetic moment when its
diameter is bigger than few nm. Magnetic
Force Microscopy (MFM) measurement tothe magnetite colloidal crystals was carriedout. The orientation of the magnetic
moment of the magnetite particles found to
be random. If the colloidal crystal had
formed by magnetic force in the solution,
each magnetic orientation must be align,and north pole of one particles contact withsouth pole of neighbor particles.

3. Conclusions
Well arranged colloidal crystals of magnetite were found in
Tagish Lake meteorites.
These colloidal crystals were classified into several types
by it’s lattice structure, b.c.c., f.c.c., and h.c.p.. Thesestructures found to be depending on the morphology of theconstituent particles.
The morphology bounded only by {110} face was notexpected considering its crystallographic structure. HRTEM observation revealed these are multiple-twin particles.

Weak magneticforceSFe

EDS measurement to magnetite colloidal crystal detects not only Fe and O
but also S. Analyzing the high resolution TEM image, these were identified
to greigite which is composed by few nm sizes crystalline. The magnetite
particles accompany with the greigite in most cases, which indicate thegreigite have deep relation in generation of magnetite.

size = 200nm(most commonsize in thismeteorite)
Debye lengthκ-1= 10nm(Stumm andMorgan, 1981)
Zeta potential
ζ= -40～40mV(Zhong-Xi etal., 1998)

DLVO curve of the magnetite in the pure water
indicate always large attractive force within the100nm of particle distance. However, it requires
more than 5-10 kT of the energy barrier to keep
the particles dispersed, magnetic force enable toachieve this value.

2-8 Possible formation mechanismof magnetite colloidal crystal
Greigite60OFe

Schematic diagram of the possible formation mechanism of magnetite
colloidal crystal. At first, greigite which has weak magnetic force hasformed. Then, they dispersed in the solution, and form colloidal crystals.
Finally, replacement of S to O makes magnetite particles.

To form magnetite colloidal crystals in the meteorite othermaterials which has not or no magnetic force, such asgreigite, must nucleate firstly.

4. References
Peter G. Brown et al., Science 290, 320-324,2000
Zolensky M. E. et al. Meteorit. Planet. Sci. 35, A178-A179,2000
Sandra Pizzarello et al., Science 293, 2236,2001