Spectroscopy studies of natural andalusite from Brazil
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Українське мінералогічне товариство
2011
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| Цитувати: | Spectroscopy studies of natural andalusite from Brazil / M. Taran, M. Koch-Müller // Записки Українського мінералогічного товариства. — 2011. — Т. 8. — С. 200-203. — Бібліогр.: 3 назв. — англ. |
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irk-123456789-573642014-03-08T03:01:45Z Spectroscopy studies of natural andalusite from Brazil Taran, M. Koch-Müller, M. Матеріали VIII з’їзду Українського мінералогічного товариства 2011 Article Spectroscopy studies of natural andalusite from Brazil / M. Taran, M. Koch-Müller // Записки Українського мінералогічного товариства. — 2011. — Т. 8. — С. 200-203. — Бібліогр.: 3 назв. — англ. 2218-7472 http://dspace.nbuv.gov.ua/handle/123456789/57364 549.08 uk Записки Українського мінералогічного товариства Українське мінералогічне товариство |
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Матеріали VIII з’їзду Українського мінералогічного товариства Матеріали VIII з’їзду Українського мінералогічного товариства |
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Матеріали VIII з’їзду Українського мінералогічного товариства Матеріали VIII з’їзду Українського мінералогічного товариства Taran, M. Koch-Müller, M. Spectroscopy studies of natural andalusite from Brazil Записки Українського мінералогічного товариства |
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Article |
| author |
Taran, M. Koch-Müller, M. |
| author_facet |
Taran, M. Koch-Müller, M. |
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Taran, M. |
| title |
Spectroscopy studies of natural andalusite from Brazil |
| title_short |
Spectroscopy studies of natural andalusite from Brazil |
| title_full |
Spectroscopy studies of natural andalusite from Brazil |
| title_fullStr |
Spectroscopy studies of natural andalusite from Brazil |
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Spectroscopy studies of natural andalusite from Brazil |
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spectroscopy studies of natural andalusite from brazil |
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Українське мінералогічне товариство |
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2011 |
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Матеріали VIII з’їзду Українського мінералогічного товариства |
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http://dspace.nbuv.gov.ua/handle/123456789/57364 |
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Spectroscopy studies of natural andalusite from Brazil / M. Taran, M. Koch-Müller // Записки Українського мінералогічного товариства. — 2011. — Т. 8. — С. 200-203. — Бібліогр.: 3 назв. — англ. |
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Записки Українського мінералогічного товариства |
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AT taranm spectroscopystudiesofnaturalandalusitefrombrazil AT kochmullerm spectroscopystudiesofnaturalandalusitefrombrazil |
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2025-07-05T08:35:53Z |
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2025-07-05T08:35:53Z |
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1836795758000472064 |
| fulltext |
200 ISSN 2218-7472. Записки Українського мінералогічного товариства. 2011, том 8
Elongated prismatic gem-quality zone-colored andalusite
crystals from an unspecified location in Brazil were studied
by polarized optical absorption and infrared FTIR spectros-
copy. In unpolarized transmitted light, the sample displayed
a strange hard-to-described color, distributed as irregular in-
termittent darker and lighter zones. Aside the main oxides,
SiO2 = 36.22 % and Al2O3 = 63.31 %, in all differently-colo-
red zones the microprobe analyses revealed an admixture of
iron of around 0.35 wt. % when calculated as FeO and traces
of titanium. Concentrations of all other elements typical natu-
ral andalusite, Cr, V and Mn, were found to be lower than the
detection limits. The results of the Fe and Ti quantification by
ICP-MS in four points are shown in Table. As seen, the darker
zones are obviously enriched in both Fe and Ti compared to
the light ones.
The main feature in polarized optical absorption spectra
of the light and dark zones (Fig. 1, a, b, respectively) appears
in E||c-polarization as a broad and intense absorption band
with a maximum at around 20500 cm–1, which together with
the high-energy edge causes a dusk red color at E||c. By ener-
gy, width, polarization and some other properties this band is
attributed to electronic Fe2+/Ti4+ intervalence charge-transfer
(IVCT) transition. The Fe- and Ti-contents of variously-colo-
red zones of the andalusite sample studied (see Table) well
quantitatively proves this assignment [3].
Thermal behavior of the Fe2+/Ti4+ IVCT band is essentially
different under oxidizing and reducing conditions. At oxidizing
conditions, the thermal behavior of the band is also different in
the light and dark zones. Thus, the band is stable at temperatures
lower than ca. 700 °C. At higher temperatures the intensity de-
creases until it vanishes at 1000 °C in lightly-colored zones and
1100 °C in darkly-colored ones. Under annealing in reducing
conditions at 700 °C and 800 °C, the band slightly increases and
maintains its intensity when treated at higher temperatures up to
1000 °C. These results demonstrate undoubtedly that the weak-
ening and disappearance of the Fe2+/Ti4+ IVCT band in spectra of
andalusite under annealing in air is caused by oxidization of Fe2+
to Fe3+ in IVCT Fe2+/Ti4+-pairs. The different thermal stability
of the band in lightly- and darkly-colored zones of the samples
evidence a certain self-stabilization effect over an interaction be-
tween Fe2+/Ti4+-pairs involved in IVCT process.
© M.N. TARAN,
M. KOCH-MÜLLER,
2011
UDK 549.08
M.N. TARAN 1, M. KOCH-MÜLLER 2
1 Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Science of Ukraine
Palladin Ave., 34, 03680, Kyiv-142, Ukraine
2 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 3.3
Telegrafenberg, D 327, D-14473 Potsdam, Germany
SPECTROSCOPY STUDIES
OF NATURAL ANDALUSITE FROM BRAzIL
201ISSN 2218-7472. Записки Українського мінералогічного товариства. 2011, том 8
Spectroscopy studies of natural andalusite from Brazil
Polarized FTIR spectra in the range 3200—3650 cm–1 of light and dark zones
are shown in Fig. 2, a, b, respectively. They consist of a series of sharp absorption
lines E||b ≈ E||a >> E||c attributed by Burt et al. [1] to stretching OH-vibrations.
The spectra measured on different zones show distinct differences: the harp lines at
3481 cm–1 and 3437 cm–1 (denoted by arrows), which in the light zones are distinct
in E||a and E||b polarizations, in the E||b-polarized spectrum of the dark zone are
absent. On the whole, the integrated absorbance in the spectral range in question,
3800 cm–1 — 3200 cm–1, is somewhat higher in the light zone than in the dark one.
ICP content of iron and titanium measured in four points of differently-colored zones
of the andalusite crystal studied, ppm
Element
Zones
Light, point 1 Dark, point 2 Light, point 3 Dark, point 4
Ti 13 31 13 37
Fe 2206 2390 2038 2772
Fig. 2. Polarized FTIR spectra of natural andalusite: a — lightly-colored zone; b — darkly-
colored zone
Fig. 1. Polarized optical absorption spectra of natural andalusite: a — lightly-colored zone
(point 3, see table 2); b — darkly-colored zone (point 4)
202 ISSN 2218-7472. Записки Українського мінералогічного товариства. 2011, том 8
M. Taran, M. Koch-Müller
Using a calibration proposed by Burt et al. [1], one can evaluate the respective water
content (in ppm by weight) as ~95 and ~76. The infrared hydroxyl absorption lines
transform under annealing at T > ca. 800 °C. At higher temperatures the intensities
of the split doublet at 3527 cm–1, 3517 cm–1 and, to a somewhat lesser extent, the
line 3438 cm–1 continually decreases whilst that of the line at 3462 cm–1 significantly
increases in all three polarizations, E||a, E||b and E||c, probably, at the expense of
the former three bands. Under annealing at 1100 °C during 2 hours these processes
facilitate, especially, the doublet at 3527 cm–1 and 3517 cm–1 drastically decreased.
Further heating at 1100 °C leads to a continual decrease of all absorption lines in
the range 3800—3200 cm–1. The line at 3437 cm–1, though significantly decreased,
still has a distinct E||b-polarized component in the light zone and none at all in the
dark one.
From the data obtained it is obvious that there are several types of OH-groups
in the andalusite structure and that the red color of andalusite, caused by the elec-
tronic Fe2+/Ti4+ IVCT transition between Fe2+ and Ti4+ in the adjacent octahedra sites,
relates to a certain type of these hydroxyl groups. The main difference between the
spectra of the light and dark zones, differing in intensity of Fe2+/Ti4+ IVCT absorption
band, consists, as mentioned above, of the presence or absence of the E||b-polarized
components of the two distinct lines at 3481 cm–1 and 3437 cm–1. This can only be, if
some portion of hydroxyls, causing these absorption lines, is differently oriented in
the two zones. Therefore, one can discriminate between at least three types of OH-
groups in the structure: first, those, which cause all other absorption lines, aside the
lines at 3481 cm–1 and 3437 cm–1. Their predominant orientation in the both zones is
O—H-vector lying in the ab-plane since all absorption lines are strong in E||a- and
E||b-polarization, but very weak in E||c one. Theoretical calculation of critical points,
potential for protonation in andalusite, and polarization properties of the infrared
absorption lines in question suggests that these may be O1 and O2 [1]. Second, there
are two types of hydroxyl groups, both causing the lines at 3481 cm–1 and 3437 cm–1,
which are differently oriented in the structure of the two zones: those in the light
zones having O—H-vector again laying within the ab-plane similarly to all others
(see above), and those in the dark zones, which O—H-vector is strictly aligned along
a-axis of the structure.
Our data clearly show that in the dark zones, enriched by Fe and Ti, a part of
OH groups, causing the absorption lines at 3481 cm–1 and 3437 cm–1, both strictly
E||a-polarized, should neighbor Fe2+-Ti4+-pairs or other combinations of these ions.
Their O—H-vector aligns parallel to a-axis thus causing zero intensity of the lines in
question at E||b and E||c. Most probably, this is due to a redistribution of electronic
density along the bond between Fe2+ and Ti4+ ions in IVCT pairs or complexes, which
concentration in the dark zones are estimated to be nearly four times higher than in
the light zones (cf. Figs. 1, a, b). Summarizing, we can assume that in the light zones
a major part of OH-groups is related to isolated iron ions, most probably, Fe2+, which
charge balance at Al3+ to Fe2+ substitution maintains by O2– → OH substitution, pro-
bably, on neighboring O1 and O2. Note, that the total amount of OH, evaluated from
the FTIR spectra, ~95 wt. ppm of water (see above) or 1717 mole ppm of protons,
is commensurable with the amount of Fe2+: in light zones the total concentration of
iron is nearly 2000 ppm (see Table), from which, according to Meisel et al. [2], the
Fe2+/Fe3+ ratio is of the order of one, i. e. around 1000 wt. ppm, or 2785 mole ppm of
Fe2+. It is also essential that Meisel et al. (1990) assume “…that Fe2+ content in anda-
lusite is limited — e. g. by the presence of Ti4+ or other cations providing the charge
compensation…”. Therefore, it may as well be provided by protons incorporating
into neighboring oxygen sites.
20�ISSN 2218-7472. Записки Українського мінералогічного товариства. 2011, том 8
Spectroscopy studies of natural andalusite from Brazil
As we assumed, in the dark zones the Fe2+/Ti4+ IVCT pairs, formed by ferrous
and titanium ions substituting Al3+ in adjacent octahedral sites of the structure are
self-stabilized due to an interaction between them. Also, there may be complexes
like Fe2+-Ti4+-Fe2+ or more complicate Fe- and Ti-aggregates elongated along c-axis,
which thermally are more stable than isolated Fe2+/Ti4+ pairs. For charge balance a
part of the OH-groups in the dark zones, which are seen in the spectra as the strictly
E||a-polarized lines at 3481 cm–1 and 3437 cm–1 (see above), is adjacent to such Fe2+-
and Ti4+-aggregates.
REFERENCES
1. Burt J.B., Ross N.L., Gibbs G.V., Rossman G.R., Rosso K.M. Potential protonation sites in the
Al2SiO5 polymorphs based on polarized FTIR spectroscopy and properties of the electron
density distribution // Phys Cnem Minerals. — 2007. — 34. — Р. 295—306.
2. Meisel W., Schnellrath J., Griesbach P., Gutlich P. The state of iron in andalusite // Hyperfine
Interactions. — 1990. — 57. — Р. 2261—2268.
3. Taran M.N., Koch-Müller M. Optical absorption of electronic Fe—Ti charge-transfer
transition in natural andalusite: the thermal stability of the charge-transfer band // Phys. Chem.
Minerals. — 2011. — 38. — Р. 215—222.
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