ARTICLES

Sci. Rep. Kanazawa Univ.
Vol. 65, pp. 17-30, 2021

Amorphization of wollastonite and diopsideby ball milling
Yoshitaka MUROUCHI,1,* Masayuki OKUNO,1 and Naoya SASAKI 2
1

Department of Earth Science, Graduate School of Science and Technology, Kanazawa University,
Kanazawa, 920-1192, Japan
2
Kutani Ware Research Center, Industrial Research Institute of Ishikawa,
Komatsu, 923-0151, Japan(Received December 22, 2020 and accepted in revised form February 3, 2021)

Abstract The amorphization of wollastonite (CaSiO3) and diopside (CaMgSi2O6) crystals by
conventional ball milling under the dry condition and the atomic-scale structures of their amorphous
phases were investigated by powder X-ray diffraction (XRD) analysis and Fourier transformed infrared(FT-IR) spectroscopy. The obtained results indicate that both wollastonite and diopside crystals were
almost amorphized by ball milling. Moreover, the wollastonite crystal amorphized faster than thediopside crystal. The atomic-scale structures of the amorphous phases in the milled wollastonite anddiopside obtained by X-ray radial distribution analyses were approximately similar to those in the fusedglasses of wollastonite and diopside, respectively. However, FT-IR data may indicate that the single
chains of SiO4 tetrahedra remained as the dominant species even in the wollastonite and diopside
subjected to prolonged ball milling.
Keywords.

Ball milling, Amorphization, Inosilicate, Wollastonite, Diopside

1 Introduction
Wollastonite (CaSiO3) is one of the pyroxenoid-group minerals, and diopside (CaMgSi2O6) is aclinopyroxene mineral. They are major rock-forming minerals and inosilicates, with single chains of
SiO4 tetrahedra. The wollastonite has excellent biological activity forming hydroxyapatite(Ca10(PO4)6(OH)2) and binding to bones. However, its degradation rate is too fast, and its mechanicalproperties are inferior. By contrast, the diopside has higher mechanical properties given its slow
degradation rate. Garcia et al. [1] reported that sprayed amorphous wollastonite and wollastonitediopside compositions are useful new bioactive coating materials for metal implants to bones. Recently,
artificial bones could be easily made with a three-dimensional (3D) printer. Especially for a selective
laser sintering printer, ceramics and metals with a high melting point are suitable for powder source
materials. Hence, wollastonite, diopside, their mixed crystals, and their glass powders have received
much attention as bioceramics and bioglasses [2–8].
On the other hand, numerous milling experiments have been conducted on various minerals suchas quartz, kaolinite, talc, and feldspar [9–11]. Disordered surface and hydrated phases are formed on thediopside by ball milling and dissolution [12,13]. Kalinkina et al. [14,15] and Kalinkin et al. [16] reported* Corresponding author Email: murouchiy@yahoo.co.jp

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Amorphization of wollastonite and diopside by ballthat both wollastonite and diopside absorbed atmospheric water vapor and carbon dioxide, and α-quartz
crystallized during milling by agate (SiO2) pestle-and-mortar under open atmospheric conditions.
However, they did not describe the amorphization of the wollastonite and diopside by mechanicalmilling and the atomic-scale structures of their amorphous phases. Their atomic-scale structures may be
helpful for bioceramic design. Therefore, our significant aims in this study are to precisely investigate
the amorphization processes of the wollastonite and diopside milled under dry conditions and analyzed
the atomic-scale structures of their amorphous phases. Obtained atomic-scale structures are comparedwith those in their fused glasses.

2 Experimental
Specimens and chemical analysis
Polycrystalline white wollastonite was obtained from the Kiku mine at Fukuoka, Japan, andmonocrystalline light-green diopside with a particle size of 10–20 mm was obtained from block D at
Merelani, Tanzania. The densities of the original specimens were measured at room temperature by abuoyancy method as a reference. The observed wollastonite and diopside densities were ρ = 2.85 and3.25 g/cm3, respectively.
Their chemical compositions were evaluated by electron microprobe analysis (EPMA：JXA-8800;
JEOL, Japan) with the ZAF correction procedure. The measurement conditions consisted of anacceleration voltage of 20 kV, a beam current of 50 nA, and a probe diameter of 3 µm. Standard materials
adopted natural wollastonite for Ca and Si, natural jadeite for Na, synthetic samples of Al2O3, MgO,
KTiPO4, Fe2O3, Cr2O3, MnO, and NiO for Al, Mg, K and Ti, Fe, Cr, Mn, and Ni, respectively.

Preparation of milled samples of wollastonite and diopside crystals
We prepared milled samples of wollastonite and diopside crystals using the following procedure. The
original specimens were roughly ground using a tungsten carbide cylinder mortar and then passed
through a 30 mesh (595 µm) sieve. These samples are referred to hereafter as the “initial” samples.
Approximately 9.5 g of each initial sample and 40 ceramic balls 20 mm in diameter (94.92 mass% of
ZrO2 + HfO2 and 4.81 mass% of Y2O3) were placed in a 2000-mL alumina vessel (93.0 mass% of Al2O3
and 7.0 mass% of SiO2). A desktop pot mill (PM-001; AS ONE, Japan) operated at a rotational speed of
100 rpm was used in all batches at room temperature. The vessel was sealed during milling. The milled
samples were collected in 1.0 g batches after 2, 16, 67, and 317 h of milling and after 512 h of millingin the diopside case. These samples are identified as “wollastonite-Th” and “diopside-Th,” where T
denotes the milling time. The milled samples remaining in the vessel were dried in an oven at 80 °C for
10 min after each sampling to avoid atmospheric water vapor absorption.

Preparation of fused glasses for wollastonite and diopside
Approximately 0.50 g of each wollastonite and diopside crystal was separately placed in Pt crucibles
and was melted at 1650 °C and 1500 °C, respectively, for 3 h in an electric furnace. These temperatures
were higher than the melting points of pure wollastonite (1540 °C) and diopside (1342 °C) by 110 and
158 °C, respectively. The melts were rapidly quenched in the water to obtain fused glasses. The fusedglass of wollastonite (wollastonite glass) was colorless and transparent; the fused glass of diopside(diopside glass) was light-green.

Yoshitaka MUROUCHI, Masayuki OKUNO, and Naoya SASAKI

19

Particle size and its distribution measurements
All particle size distributions were measured using a laser scattering particle distribution analyzer (LA920; HORIBA, Japan) in the 0.02 ≤ d ≤ 2000 µm range. The particle size distribution of each specimen
was measured at least twice, and their averaged data were used for this study. The characteristic
diameters D10, D50, and D90 were calculated, averaging the median values. The diameters D10, D50, andD90 correspond to the apertures related to 10, 50, and 90 % passing of material, respectively. Preparations
were carried out with ethanol (C2H5OH) and distilled water as dispersion solvent of wet method using ahomogenizer. Their dispersion time of the homogenizer were for 2 minutes. The relative refractive
indexes for wollastonite and diopside were 1.24 and 1.28, respectively.

Scanning electron microscopy (SEM)
The particle size and shape of wollastonite-2h, wollastonite-317h, diopside-2h, and diopside-512h wereobserved by SEM (JSM-7100F; JOEL, Japan) at the Industrial Research Institute of Ishikawa, Japan.
The milled samples were attached to carbon tape and subsequently coated with an Au layer. SEM images
were acquired with 15 kV secondary electrons.

Powder X-ray diffraction (XRD) measurements
Powder XRD patterns were measured with an X-ray powder diffractometer (RINT 2200; Rigaku, Japan)
operated at an acceleration voltage and current of 40 kV and 30 mA, respectively. The initial and milled
samples were placed on a glass sample holder and scanned using Cu Kα monochromatic radiation; thedata sampling interval was 2θ = 0.02°, the scanning speed was 2θ = 2.0°/min, and the scanning range
was 2 ≤ 2θ ≤ 120°.

Calculation of correlation function G(r)To obtain the atomic-scale structure parameter, such as atomic pair distances, the author carried out
another powder XRD measurements for wollastonite-317h, diopside-512h, wollastonite glass, anddiopside glass under the following conditions. Each sample was placed on a brass sample holder and
XRD measurements carried out with Mo Kα monochromatic radiation using a step-scanning method inincrements of 2θ = 0.5° in the range 2 ≤ 2θ ≤ 140° for fixed-time measurements of 1000 s. In addition,
the XRD pattern of α-Al2O3 reagent was measured under the same conditions because of the correction
for small contamination of α-Al2O3 from the vessel. All of the obtained data were corrected forabsorption factor and normalized by the Krogh-Moe and Norman method [17]. Based on these corrected
data, the procedure described by Marumo and Okuno [18] led to the interference function S・i(S), radial
distribution function (RDF), and correlation function G(r) of wollastonite-317h, diopside-512h, thewollastonite glass, and the diopside glass. For these calculations, the densities of wollastonite-317h anddiopside-512h were measured by a gas-displacement pycnometry system (AccuPyc II 1340;
Micromeritics, USA) were used. The measured densities of wollastonite-317h and diopside-512h wereρ = 2.86 and 2.76 g/cm3, respectively. The fused glasses' density data were used those reported by
Shimoda et al. (2005): ρ = 2.91 g/cm3 for CaSiO3 glass and ρ = 2.84 g/cm3 for CaMgSi2O6 glass.

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Amorphization of wollastonite and diopside by ball

Fourier transform infrared (FT-IR) spectroscopy
We recorded infrared (IR) spectra of all of the samples in ATR mode using FT-IR spectrometer(NICOLET iS10; Thermo Fisher Scientific, USA) equipped with a diamond crystal. The spectrum's

wavenumber range was ν = 525–4000 cm−1, and its bandpass was ν = 4 cm−1.

3 Results
Chemical compositions
The chemical compositions obtained by EPMA analyses for the original specimens showed almost ideal
chemical compositions of the wollastonite and diopside (Table 1). Namely, their impurity levels were
low. For example, the MnO and NiO contents in the wollastonite specimen were less than 0.30 mass%.
The Al2O3 content in the diopside specimen was less than 1.0 mass%, and the MnO and Na2O contents
in it were less than 0.2 mass%.
Table 1. Chemical compositions of the original specimens.
(Units: wt.%)
Mineral

Wollastonite(Units: Atomic ratio)

Diopside

CationWollastonite

Diopside

SiO2

51.19

54.83

Si4+

1.981

1.972

TiO2

0.00

0.01

Ti4+

0.000

0.000

Al2O3

0.01

0.81

Al

3+

0.000

0.034

Cr2O3

0.00

0.00

Cr3+

0.000

0.000

FeO

0.00

0.09

3+

Fe

0.000

0.003

MnO

0.30

0.17

Fe2+

0.000

0.000

0.010

0.005

0.005

0.000

NiO

0.16

0.01

Mn2+

MgO

0.01

18.20

Ni2+

CaO

48.29

25.89

Mg2+

0.001

0.976

Na2O

0.00

0.16

Ca2+

2.002

0.998

K2O

0.00

0.00

Na+

0.000

0.011

Total

99.96

100.17

K+

0.000

0.000

Total

4

4

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Yoshitaka MUROUCHI, Masayuki OKUNO, and Naoya SASAKI

Particle size distributions and scanning electron microscopy (SEM)
observations
Figures 1 and 2 show the particle size distributions of the milled wollastonite and diopside, respectively.
Tables 2 and 3 show the diameters D10, D50, and D90 of the milled wollastonite and diopside, respectively.
Both D50 decreased under 10 µm for 2 h of milling. Then their peak shifted to around 30 µm for 317 hof milling. The D50 of the wollastonite increased for 16 h of milling and then re-decreased to D50 = 9.66
µm. By contrast, the D50 of the diopside increased for 317 h of milling and decreased to D50 = 8.48 µm
for 512 h of milling.

Figure 1. The particle size distribution of thewollastonite as a function of milling time.

Figure 2. The particle size distribution of thediopside as a function of milling time.

Table 2. Diameters D10, D50, and D90 of the milledwollastonite as a function of milling time.

Table 3. Diameters D10, D50, and D90 of the milleddiopside as a function of milling time.

(Units: µm)(Units: µm)

Milling time

D10

D50

D90

Milling time

D10

D50

D90

0h

5.42

27.52

106.05

0h

8.81

47.05

119.75

2h

1.50

6.59

25.08

2h

1.26

3.52

7.53

16 h

1.96

12.52

38.50

16 h

1.39

7.57

24.35

67 h

2.58

11.92

33.38

67 h

1.56

8.09

24.47

317 h

3.05

9.66

27.84

317 h

2.14

14.67

38.90

512 h

1.63

8.48

31.84

Figure 3 shows the SEM images of wollastonite-2h, wollastonite-317h, diopside-2h, and diopside512h. Many particles about 1 µm were observed to adhere to a particle over 10 µm size in the imagesof 2 h of milling. By contrast, the aggregation of small grains was observed in the images of wollastonite317h and diopside-512h. The small particle size in the images of 2 h of milling was not much different
from that of isolated small particles, and the particles included in aggregates particles in the images ofwollastonite-317h and diopside-512h. Almost all particles were angular in the images of 2 h of milling.

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Amorphization of wollastonite and diopside by ballFigure 3. The SEM images of wollastonite-2h, wollastonite-317h, diopside-2h, and diopside-512h.

Many uniform spherical particles were observed in the images of wollastonite-317h and diopside-512h.
Since inorganic nanoparticles such as SiO2 are very easy to agglomerate, the aggregation was induced
despite a dispersion process in particle size distribution measurements.

Powder X-ray diffraction (XRD) measurements
Figure 4 shows the XRD patterns of the initial and milled samples of the wollastonite. Although the� 20) increased, other peaks decreased for 2 h of milling. The
sharp peak intensity at 2θ = 30.00° (3intensities of all of the sharp peaks decreased after 2 h of milling. Finally, only a broad peak at 2θ ≈ 30°
was observed in the patterns of wollastonite-317h. Figure 5 shows the patterns of the initial and milled
samples of the diopside. The intensities of almost all of the sharp peaks decreased for 2 h of milling. In
particular, the intensities of the sharp peaks at 2θ angles of about 27° (2� 20), 43° (3� 30), 52° (1� 50), and72° (5� 33) largely decreased. By contrast, the intensities of the sharp peaks at 2θ = 29.84° (2� 21) and 2θ
= 35.62° (221) increased for 2 h of milling. The intensities of all of the sharp peaks decreased after 2 hof milling. Finally, only a broad peak at 2θ ≈ 30° was observed in the patterns of diopside-512h likewollastonite-317h. However, some new weak sharp peaks in the patterns of wollastonite-317h anddiopside-512h were observed. These peaks entirely matched the peak positions of α-Al2O3.

Yoshitaka MUROUCHI, Masayuki OKUNO, and Naoya SASAKI

Figure 4. The XRD patterns of the wollastonite asa function of milling time. Plus (+) marks indicatedthe peak positions of α-Al2O3.

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Figure 5. The XRD patterns of the diopside as afunction of milling time. Plus (+) marks indicatedthe peak positions of α-Al2O3.

Interference function S・i(S) and correlation function G(r) curves
Figure 6 shows the XRD pattern with Mo Kα monochromatic radiation of the wollastonite glass and thepattern which subtracted the effect of α-Al2O3 estimated from measured intensity under the same
conditions for wollastonite-317h (317h - Al2O3), and Figure 7 shows those of the diopside glass and thepattern which subtracted the effect of α-Al2O3 estimated from measured intensity under the same
conditions for diopside-512h (512h - Al2O3). Their patterns showed intense broad peaks at 2θ ≈ 13.50°.
Two weak sharp peaks at 2θ = 22.5° and 26.5° remained in the pattern of diopside-512h corrected for
α-Al2O3.

Figure 6. The XRD patterns of the wollastonite
glass and wollastonite-317h corrected for α-Al2O3.

Figure 7. The XRD patterns of the diopside glassand diopside-512h corrected for α-Al2O3. The
weak sharp peaks showed by asterisk (*) marks
were not that of α-Al2O3.

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Amorphization of wollastonite and diopside by ball

The S・i(S) curves calculated from the XRD patterns for wollastonite-317h corrected for α-Al2O3
contamination, the wollastonite glass, diopside-512h corrected for α-Al2O3 contamination, and thediopside glass are shown in Figures 8 and 9.

Figure 8. The S・i(S) curves of wollastonite-317h
corrected for α-Al2O3 and the wollastonite glass.

Figure 9. The S ・ i(S) curves of diopside-512h
corrected for α-Al2O3 and the diopside glass.

The G(r) curves calculated from these S・i(S) data are shown in Figures 10 and 11. Maxima of G(r)curves show the atomic pair distribution, such as Si–O atomic pairs in the samples. According to theprevious studies on CaSiO3, MgSiO3, and CaMgSi2O6 glasses [19–21] and ionic radii data [22], themaximum at 0.160 ≤ r ≤ 0.164 nm in the G(r) curve was attributable to average Si–O atomic pairs in
SiO4 tetrahedra, that at r ≈ 0.24 nm was an average of overlapped peaks of Ca–O, Mg–O, and O–O
atomic pairs, and the shoulder or maximum at r ≈ 0.32 nm was attributable to average Si–Si pairs
between neighboring SiO4 tetrahedra.

Figure 10. The G(r) curves of wollastonite-317h
corrected for α-Al2O3 and the wollastonite glass.

Figure 11. The G(r) curves of diopside-512h
corrected for α-Al2O3 and the diopside glass.

Yoshitaka MUROUCHI, Masayuki OKUNO, and Naoya SASAKI

25

Infrared (IR) spectroscopic analysis
The IR spectra of the initial and milled wollastonite samples are shown in Figure 12. The diopside ones
are shown in Figure 13. Sharp absorption bands were observed in the ν = 550–600, 600–700, and 800–
1250 cm−1 regions in both initial samples' spectra. Their sharp bands become broader with increasing
milling time. After the prolonged ball milling, only the strong broad band remained at around ν = 918
cm−1 in the spectrum of wollastonite-317h and ν = 926 cm−1 in that of diopside-512h. Furthermore, newbroad bands exhibited at ν = 665 cm−1 in the spectra of wollastonite-317h and diopside-512h. The very
weak broad band at ν = 1400–1550 cm−1 was observed in all milled samples' spectra. Weak sharp bands
and weak broad bands were observed in the ν = 550–600 and 600–800 cm−1 regions, respectively, in the
spectra of both wollastonite and diopside glasses. The strong broad band at around ν = 906 cm−1 wasobserved in the wollastonite glass' spectra, and that at around ν = 903 cm−1 was observed in the diopside
glass' spectra.

Figure 12. The IR spectra of the wollastonite as afunction of milling time and its glass.

Figure 13. The IR spectra of the diopside as afunction of milling time and its glass.

4 Discussion
Atmospheric effect on mechanical milling
Kalinkin et al. [16] reported that wollastonite and diopside were nearly amorphized by pestle-and-mortar
milling. They also described α-quartz crystallization, resulting from atmospheric water vapor's catalytic
effect during milling. However, the H2O absorption bands at ν = 1650 cm−1 and 3300–3700 cm−1 arealmost not detected in this study (Figs. 12 and 13) because we used the sealed vessel for ball milling to
avoid the atmospheric water vapor's catalytic effect and dried its vessel after each sampling. This result
is consistent with the absence of α-quartz in the XRD patterns and IR spectra. In previous studies [23,24],
the CO32− bands appeared in the ν = 1400–1550 cm−1 region in the IR spectrum. Moreover, Kalinkina etal. [14] reported that diopside milled for an extended duration contained more than 10 wt.% of CaCO3

26

Amorphization of wollastonite and diopside by ballfrom its IR spectrum. However, in this study, the CO32− band is very weak in all milled samples. Besides,
we cannot find the peaks attributable to MgCO3, CaCO3, or CaMg(CO3)2 in the XRD patterns.
Consequently, the milled samples absorb almost no atmospheric water vapor and little atmosphericcarbon dioxide in this study.

Structure changes and amorphization by ball milling
As mentioned above, α-quartz and carbonates may not crystallize in the milled wollastonite and diopside.
So, we discussed the amorphization of the wollastonite and diopside crystals during milling.
The IR sharp absorption bands of the initial wollastonite and diopside samples became broader withincreasing milling time. However, the strong absorption band position at around ν = 920 cm−1 (Si–O
stretching vibrations of SiO4 tetrahedra) does not almost change. These facts infer that the chain structureof SiO4 tetrahedra keeps in the narrow range of the strong absorption band despite the prolonged ball
milling. Furthermore, the IR spectra of wollastonite-317h and diopside-512h are similar to those of thewollastonite and diopside glasses, respectively (Figs. 12 and 13). However, there are some differencesin the strong band position around ν = 920 cm−1, and a new weak broad absorption band appeared ataround ν = 665 cm−1 in the spectra of wollastonite-317h and diopside-512h. Its band entirely matches
the band positions of α-Al2O3.
On the other hand, we observed substantial changes between the XRD patterns of the initial samples
and the samples milled for 2 h (Figs. 4 and 5); the changes in the patterns of the diopside wereparticularly pronounced. This result indicates that the powder prepared from monocrystalline diopside
keeps higher crystallinity than the powder prepared from polycrystalline wollastonite and is inagreement with the diopside D90 larger than wollastonite one. Besides, the intensities of the relative
peaks in the patterns of the initial samples substantially differ from those in standard XRD patterns ofwollastonite-2M (Powder Diffraction File (PDF) 43-1460 of International Centre for Diffraction Data
(ICCD)) and diopside (PDF 41-1370). By contrast, the relative peaks’ intensities in the patterns of thesamples milled for 2 h are similar to those listed in the ICCD PDFs. Besides, the particle size distribution
after 2h of milling (Fig. 1) is suitable for powder XRD measurements. Therefore, the peak intensities in
the patterns of the samples milled for 2 h expressed more accurate intensities.
Figure 14 shows the integrated intensity ratio changes in the 2 ≤ 2θ ≤ 120° range of the XRD patterns
for the milled wollastonite and diopside. After 2 h of milling, the integrated intensities in the patterns of
the milled wollastonite decreased faster than those of the milled diopside. Mg–O ionic bond energy is
greater than Ca–O one. Besides, the ionization energy of Mg2+ cations is larger than that of Ca2+ cations.
Ca–O bonds are more friable than Mg–O bonds in inosilicates [25]. So, Mg2+ cations in the diopsidestrongly connect the single chains of SiO4 tetrahedra. This result is consistent with diopside exhibiting
greater hardness (Mohs hardness 5½–6½) than wollastonite (Mohs hardness 4½–5). The XRD patternsof wollastonite-317h and diopside-512h were similar to those of the wollastonite and diopside glasses,
respectively (Figs. 4–7). However, weak sharp peaks on the patterns of wollastonite-317h and diopside512h are observed. We also found the additional band located at the ν = 665 cm−1 in the IR spectra ofwollastonite-317h and diopside-512h. These bands correspond to α-Al2O3 contamination (Figs. 12 and13). This α-Al2O3 contamination may be due to the alumina vessel. Consequently, the XRD integrated
intensities of wollastonite-317h and diopside-512h increased by small contamination of α-Al2O3.

Yoshitaka MUROUCHI, Masayuki OKUNO, and Naoya SASAKI

27

Figure 14. The XRD integrated intensity ratios inthe 2 ≤ 2θ ≤ 120° range. The ratio for 2 h of milling
was set to 1.00.

In summary, wollastonite and diopside crystal almost amorphized by prolonged ball milling without
the α-quartz crystallization, even small contamination of α-Al2O3. Moreover, small differences between
the amorphous milled and glass samples were observed on S・i(S) and IR spectra. The next section will
discuss their atomic-scale structures and the difference between those of the amorphous phases in themilled and glass samples.

Atomic-scale structures of amorphous phases
The S・i(S) curves include structural information for the amorphous phase. In this study, the S・i(S) curveof wollastonite-317h is roughly similar to that of the wollastonite glass. This result indicates thatwollastonite-317h probably includes a small quantity of crystallite-like phase. The S ・ i(S) curve ofdiopside-512h is also similar to that of the diopside glass. However, the amplitudes of their milled
samples are smaller than those of the glass samples. This result may indicate many variations of structure
unites such as linkages of SiO4 tetrahedra in milled samples.
The G(r) curves for the wollastonite and diopside glasses (Figs. 10 and 11) roughly match those reported
in previous studies [20,21,26]. The calculated Si coordination number in wollastonite-317h anddiopside-512h was 4.0–4.1. Hence, we consider that the basic structure unites of their milled samples
are the SiO4 tetrahedra and consistent with the first maximum position (0.160 ≤ rSi–O ≤ 0.164 nm) of
their G(r) curves. The attenuation distance (rS) of the G(r) curve in wollastonite-317h is similar to thatin the wollastonite glass (rS > 8.0 nm) (Fig. 10). The curve’s attenuation distance in diopside-512h issimilar to that in the diopside glass (rS ≈ 7.0 nm) (Fig. 11). rS/rSi–O is known as short-range orderparameters [20]. Those for wollastonite-317h and diopside-512h are 4.9 and 4.4, respectively. Yin et al.
[20] reported that the short-range order parameter in CaSiO3 glass (approximately 5.0) was larger thanthat in MgSiO3 glass (approximately 4.4). The short-range order parameter is inverse proportion to
Coulomb’s force. Coulomb’s force for Mg–O bonds with smaller Mg2+ cation is greater than that for
Ca–O bonds with larger Ca2+ cation. Their results are consistent that the short-range order parameters indiopside-512h and the diopside glass are smaller than those in wollastonite-317h and the wollastonite
glass, respectively. The amorphous phases in the milled wollastonite and diopside retain roughly similar
short-range structures to those of the wollastonite and diopside glasses. However, not a little differencein the atomic-scale structures was found on the S・i(S) and G(r) curves. Namely, the milled samples may
have a similar structure to the crystalline phase.
Generally, the IR bands in the ν = 800–1250 cm−1 region for silicates were assigned to Si–O
asymmetric stretching vibrations within a SiO4 tetrahedron [27,28]. Four bands at approximately ν =870, 920, 990, and 1090 cm−1 in the spectrum of CaO–SiO2 systems were assignable to Q0 (monomer),

28

Amorphization of wollastonite and diopside by ballQ1 (dimer), Q2 (chain), and Q3 (sheet) species of SiO4 tetrahedra, respectively [29]. CaSiO3 and
CaMgSi2O6 glasses had Q2 as dominant species with contributions from Q0 and Q3 species [21,30]. In
the present study, the absorption band at the ν = 918 cm−1 of wollastonite-317h was located at a higher
wavenumber position than the wollastonite glass one (ν = 906 cm−1). Moreover, the band at the ν = 926
cm−1 of diopside-512h was located at a higher wavenumber position than the diopside glass one (ν =903 cm−1). Furthermore, the shapes of the bands of wollastonite-317h and diopside-512h were sharper
than those of the wollastonite and diopside glasses. These results are consistent with the discussion of
the S・i(S) and G(r) curves. Therefore, wollastonite-317h and diopside-512h may include a larger numberof Q1 and Q2 species than the wollastonite and diopside glasses. It means that the single chains of SiO4
tetrahedra may remain even in wollastonite-317h and diopside-512h despite the prolonged ball milling.
The IR bands in the ν = 600–700 cm−1 region were assigned to Si–O–Si bending vibrations within
a SiO4 tetrahedron (Kubicki et al., 1992). In the present study, these sharp bottom bands in its region of
the initial samples for the wollastonite and diopside become broader with increasing milling time andalmost disappear in the spectra of wollastonite-317h and diopside-512h. Its result indicates that the Si–
O–Si angles between neighboring SiO4 tetrahedra in wollastonite-317h and diopside-512h have many
variations, and some Si–O–Si bonds are broken.

5 Conclusions
In this study, based on the XRD measurements and IR spectroscopy, we investigated the amorphization
processes of the wollastonite and diopside by ball milling and the atomic-scale structures of theamorphous phases obtained by prolonged ball milling. Our results showed almost no absorption of water
vapor or carbon dioxide during the milling experiment. Furthermore, we found that the crystal phase of
the wollastonite transformed into an amorphous phase easier than that of the diopside. This is because
Mg–O bonds in the diopside connect stronger than Ca–O bonds. However, comparing the atomic-scale
structures of the amorphized wollastonite and diopside with those of the wollastonite and diopside
glasses, respectively, the single chains of SiO4 tetrahedra remained well even in the milled wollastonite
and diopside. We found that the amorphous wollastonite and diopside are easily obtained by ball milling,
and they are expected as useful materials for 3D printers and bioactive implant use.

Acknowledgments
We express supreme thanks to Dr. M. Hamada of Kanazawa University for EPMA analyses. The authors
are greatly indebted to the editors and two anonymous referees.

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