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The structure and transformation of the nanomineral schwertmannite: a synthetic analogue representative of
field samples
Rebecca A. Frencha,b*, Niven Monseguec, Mitsuhiro Murayamac,d, Michael F. Hochella, Jr.a,b
a
Environmental Nanoscience and Technology Laboratory, ICTAS, Virginia Tech, Blacksburg, VA 24061, U.S.A.
b
c
Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, U.S.A.
Department of Materials Science, Virginia Tech, Blacksburg, VA 24061, U.S.A.
d
Nanoscale Characterization and Fabrication Laboratory, ICTAS, Virginia Tech, Blacksburg, VA 24061, U.S.A.
Electronic Supplementary Material
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Fig. ESM_1 Needles, on the surface of a schwertmannite sphere produced at room temperature
with 24 hrs of reaction time, after ~7 minutes of exposure to the electron beam on an FEI Titan
operating at 300 kV while focusing and alignment were occurring. The distinct faceting and
increase in crystallinity, as seen from the lattice fringes, was not observed on these needles
before the prolonged exposure to the beam and nowhere else where images were taken for this
study. We did not observe changes to needles that were used for analysis over the course of the
time that we collected images, making us confident that changes to the schwertmannite
morphology and crystallinity only occur after prolonged exposure to the beam and not under the
time (<1 minute) that we needed to collect our images used for analysis.
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Fig. ESM_2 Schwertmannite produced at room temperature with 24 hrs of reaction time (upper
and lower left images). Micro-beam electron diffraction patterns (top right) of the circled needles
shows two diffuse rings covering d-spacings over the range of 53° to 70° 2θ and 24° to 39° 2θ as
pictured by the grey boxes overlying the XRD pattern for the schwertmannite sample. Black
lines on the XRD pattern are from a goethite standard and grey dotted lines are the
schwertmannite literature values (Cornell and Schwertmann 2003).
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(101)
0.255
Fig. ESM_3 D-spacings corresponding to Figure 3 (main text) measured using a Fast Fourier
Transform (FFT) analysis. Schwertmannite produced at room temperature for 24 hours of
reaction time. The d-spacing measured from Fast Fourier Transform (FFT) of lattice fringes in
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image (A) corresponds to the solid and dotted red lines with error bars on the x-ray diffraction
pattern (A). The red solid line with error bar corresponds to d-spacings measured from the lattice
fringes in image (A) upper right and the dotted line corresponds to the lattice fringes from image
(A) lower right. In image (B) the lattice fringes seen in the inset have a d-spacing located at the
solid red line on the XRD pattern (B). The black lines on the horizontal axis are the goethite
standard d-spacings and the grey dotted lines are the schwertmannite literature values (Cornell
and Schwertmann 2003). The (101) reflection for goethite and the 0.255 nm d-spacing for
schwertmannite are labeled in XRD pattern (A) for reference.
The error bars on the d-spacings on the XRD patterns are calculated based on the pixel
size of the Fast Fourier Transform (FFT) of the HRTEM images. D-spacings are measured by
converting the number of pixels from an FFT image to the equivalent distance in nanometers,
therefore, the error in the measurements is the °2θ equivalent of plus or minus 1 pixel length of
the measured d-spacing.
For example, if the measured distance between the center of an FFT image and a spot is
35 pixels and 1 pixel = 0.18 nm-1, then the d-spacing (d) would be
1
𝑑 = 35×0.18
(1)
and the measured d-spacing, if the actual length was plus one pixel, would be
1
𝑑 = 36×0.18
(2)
and similarly for minus one pixel the d-spacing would be
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1
𝑑 = 34×0.18
(3)
The FFT of the image inset in Fig. ESM_3 (A) upper right is shown below for explanation of the
process. In the FFT image the brightest spots (circled in red) were used to calculate the d-spacing
detectable using FFT. Unlike the SAED images in Fig. ESM_2, the FFT images do not have
broad rings. The center of the bright spots may be covered by one pixel at most, therefore the
size of the pixel is the limiting factor for error in the measurement and using plus or minus one
pixel length is appropriate for calculating the error.
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0.255
Fig. ESM_4 Continued on next page
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Fig. ESM_4 D-spacings measured from area where Figure 4a (main text) was taken. Fast Fourier
Transform (FFT) analysis of areas A, B, C in image (upper left) of schwertmannite needles
formed in schwertmannite synthesis run at 75° C for 1 hr gave d-spacings shown in red with
error bars. The black lines on the horizontal axis are the goethite standard d-spacings and the
grey dotted lines are the schwertmannite literature values (Cornell and Schwertmann 2003). The
(101) reflection for goethite and the 0.255 nm d-spacing for schwertmannite are labeled in XRD
pattern (A) for reference. (See Fig. ESM_3 for explanation of error bars).
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Fig. ESM_5 Continued on next page
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0.255
Fig. ESM_5 Continued on next page
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Fig. ESM_5 D-spacings measured from area where Figure 4b (main text) was taken. Fast
Fourier Transform (FFT) analysis of areas A, B, C, D and E in image of schwertmannite needles
formed in schwertmannite synthesis run at 75° C for 1 hr gave d-spacings shown in red with
error bars. The black lines on the horizontal axis are the goethite standard d-spacings and the
grey dotted lines are the schwertmannite literature values (Cornell and Schwertmann 2003). The
(101) reflection for goethite and the 0.255 nm d-spacing for schwertmannite are labeled in XRD
pattern (A) for reference. (See Fig. ESM_3 for explanation of error bars).
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Fig. ESM_6 Schwertmannite produced at 75° C after 1 hr of reaction time (upper and lower left
images). Micro-beam electron diffraction patterns (top right) of the circled needle shows three
diffuse rings with spots covering d-spacings over the range of 60° to 70° 2θ and 26° to 40° 2θ as
pictured by the red boxes overlying the XRD pattern for the schwertmannite sample. The outer
edge of the partial ring (C) had a d-spacing at ~18° 2θ. Black lines on the XRD pattern are from
a goethite standard and grey dotted lines are the schwertmannite literature values (Cornell and
Schwertmann 2003).
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(101)
(111)
Fig. ESM_7 Replicate synthesis of the transformation from schwertmannite to goethite at 75° C
for 24 hrs. XRD patterns were collected on precipitate extracted from the synthesis after 1 hr (A),
3 hrs (B), 6 hrs (C) and 24 hrs (D) of reaction time. Black lines on the XRD pattern are from a
goethite standard and grey dotted lines are the schwertmannite literature values (Cornell and
Schwertmann 2003). The (101) and the (111) reflection for goethite are labeled for reference.
XRD analysis was performed on a PANalytical X-pert PRO diffractometer with a Cu K-alpha
(45kV-40mA) radiation. Analysis was performed from 10.004° 2θ to 79.976° 2θ with step size
0.017°.
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A
B
C
Fig. ESM_8 Powder-XRD patterns showing the transformation from schwertmannite to goethite
at 75° C. These are the same diffraction patterns as seen in Fig. 6. Unlike Fig. 6, the vertical axis
for each pattern is not on the same scale to better see the detail of each diffraction pattern. The
diffraction pattern are shown from precipitate collected after A 1 hr, B 3 hrs, and C 6 hrs of
reaction time. Schwertmannite literature XRD reflection locations (Bigham et al. 1994) (dashed
grey lines) and goethite standard peak locations (solid grey lines) are shown for reference
(Cornell and Schwertmann 2003).
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(101)
Fig. ESM_9 Selected area electron diffraction (SAED) pattern taken on Philips EM420 at 120
kV of precipitate collected after 24 hours of reaction time at 75° C (A). The XRD pattern in (B)
was collected from the same precipitate. The red lines on the XRD patterns are the d-spacing of
the outermost measured point of the rings on the SAED pattern in (A). The transparent red boxes
represent the width of a ring from the SAED, between the two outermost measured d-spacings.
Grey lines on the XRD pattern are from a goethite standard and grey dotted lines are the
schwertmannite literature values (Cornell and Schwertmann 2003). The d-spacings measured
from the SAED show reasonable agreement with the XRD pattern. Preferred orientation of the
particles, highly diffuse spots and rings or spots and rings that are too close to the primary
electron beam to be observable in the SAED pattern may account for ‘missing’ d-spacings –
XRD pattern peaks that do not have a corresponding d-spacing measured from the SAED. For
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example, there is no measureable spot or ring in the SAED pattern for the peak at 21.4° 2θ,
which is the (101) reflection for goethite (labeled in (B)).
References
Bigham JM, Carlson L, Murad E (1994) Schwertmannite, a new iron oxyhydroxysulphate from
Pyhasalmi, Finland, and other localities. Mineralogical Magazine 58 (393):641-648
Cornell RM, Schwertmann U (2003) The Iron Oxides. 2nd Ed. edn. Wiley-VCH, Weinheim
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