An abstract submitted for poster presentation in the 32nd IGC, Interlaken, Switzerland,
13-17 July 2011
Bastnäsite from Pakistan coloured by Rare Earth Elements
(REE), exhibiting a colour-change
Franz Herzog , Michael S. Krzemnicki SSEF Swiss Gemmological Institute, Basel, Switzerland, [email protected]
Objectives of study
- role of REE for colour and colour change
behaviour
- colorimetry for dichroism and colourchange behaviour
- detailed chemical composition of
samples
- allocation of REE to spectroscopic
features
Colour-change from yellowish-green (daylight, left) to brownish-red
(incandescent light, right) of Bastnäsite, sample A, 8.824ct.
sample
Samples
- Three Bastnäsite samples (sample 1 – sample 3) were analysed. For comparison reasons a REE-doped glass (sample 4) was
included in the study, too.
-The transparent Bastnäsite samples 1 and 2 showed a more or
less obvious colour-change. Sample 3 showed only a minor
colour shift. The colour-change behaviour of the REE-doped
glass is similar to the one of the minerals.
sample 1
(A)
Bastnäsite
sample 2
(B)
Bastnäsite
sample 3
(C)
Bastnäsite
sample 4
(D)
glass
weight
(ct)
8.828
3.990
SG
Methods and procedures
- quantify colour perception: dichroism and
colour change behaviour
- LA-ICP-MS data for samples
- spectrometry of the Rare Earth Elements (REE)
to link UV-Vis spectra with chemical data
- analyse width of absorptivity bands and
windows
transparency
RI
RI(o)
RI(e)
colour in
daylight
colour in
incandescent light
origin
brownishred
Zagi Mountain,
Pakistan
brownishyellow
Pamir (?)
brownish
brownish
Zagi Mountain,
Pakistan
orangy
reddish
artificial product
5.12
transparent
1.722
>1.78
greenishyellow
5.07
transparent,
somewhat
cloudy
1.723
>1.78
greenishyellow
1.174
5.20
transparent
1.709
2.88
transparent
1.722
>1.78
1.552
UV-Vis spectra and colorimetry
- UV-Vis spectra taken for all samples (unpolarized), in the range
360nm – 800nm, mode absorptivity, resolution 0.350nm.
- CIELab 1976: L*, a*, b* calculated to quantify colour perception
of colour / colour-change. Derived quantities: hue and chroma.
sample
illuminant D65, day-light equiv.
CIELab 1976
L*
Bastnäsite, A
Bastnäsite, B
Bastnäsite, C
glass, D
43.38
39.38
32.77
54.50
a*
17.36
12.45
25.65
14.86
b*
63.24
52.06
60.60
11.16
hab
74.65
76.55
67.06
36.91
illuminant A, incandescent light equiv.
CIELab 1976
Cab
65.58
53.52
65.80
18.59
L*
46.91
42.23
36.96
56.71
a*
27.56
22.46
32.91
23.35
b*
62.77
51.64
62.84
17.36
hab
66.29
66.49
62.36
36.62
Cab
68.55
56.31
70.93
29.09
Chemistry: LA-ICP-MS data
- data measured at the University of Bern.
- system: pulsed 193 nm ArF Excimer laser with an energy-homogenized beam profile coupled with an ELAN DRC-e quadrupole
mass spectrometer.
- standard: NIST 610
- data analysis performed with SILLS
- in the table aside only the REE-oxides (plus ThO2) are given.
- transition element concentrations where below detection limit.
Conclusions
The results gotten so far, plus some spectroscopic considerations, lead to:
- The colour change behaviour of all the samples is linked to a relatively
high concentration of neodymium (Nd) (up to 14% Nd2O3). The important
absorption band at ~580 nm can be attributed to this element.
- Praseodymium (Pr) is present at a low concentrations (up to 3wt% Pr2O3);
its influence is in the range 400 – 500 nm (Dieke et al. 1963) resulting in
higher absorptivity in this part of the spectrum.
Nd has also a few absorption bands in the same range and thus a clear distinction between absorption due to Pr and Nd is difficult.
- The Nd-doped glass sample, containing La and Ce in concentrations 100times lower than in the studied minerals shows a similar colour and colourchange behaviour. Thus, lanthanum(La) and cerium (Ce) do not play an
important role for the colour of the Bastnäsites.
The behaviour of the Nd-doped glass and the Bastnäsites can be compared,
because matrix/coordination effects play a minor role for the well shielded
trivalent REE-ions.
- The colour-change behaviour of all the samples has been quantified by
colorimetric methods, using the CIELab colour-space. The variation in
colour-change can be traced to slight differences in the absorption spectra
of the samples. Based on our preliminary data, the strength of the transmission window at 480 nm may explain the difference in perception.
!
References
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Acknowledgments
Thanks go to G.Bosshart and to Prof. Dr. H.A.Hänni, who both provided samples for the present study.
The nice picture, as shown on top of this poster, has been made by Luc Phan of SSEF. Thank you.
The LA-ICP-MS measurements were done by Prof. T. Pettke of the University of Bern and he also did review the SILLS-analysis. A great thank you to him as well.