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54ª Reunião Anual da SBPC – Goiânia, GO – Julho/2002
GEOCIÊNCIAS
CHARACTERIZATION OF A FERRIC IRON TOURMALINE FROM GERMANY
Warde A. da Fonseca-Zang1 wabf@cefetgo.br, Joachim W. Zang1, Wolfgang Hofmeister2
1 - Área de Química do CEFET-GO, Rua 75, 46, Setor Central, Goiânia, GO, Brazil;
2 - Institute for Gemstone Research, Department of Geosciences, University of Mainz, D-55099 Mainz, Germany.
INTRODUCTION: The tourmaline group is a solid solution series of complex boron-silicates, normally described by proportions of end-member constituents and with the general formula as:
X[9]Y[6]3Z[6]6(O,OH)3(OH,F)(B[3]O3)3[Si[4]6O18]
with X
=Na, Ca, [ ], K, Bi etc.; Y=Al, Li, Mg, Fe2+, Fe3+, Mn2+, Mn3+, Cr, V, Cu2+ etc.; Z=Al, Fe3+, Cr, V, Mg, Fe2+, Ti etc.;
B=B3+; Si=Si4+, Al, B.
The end-member buergerite is defined by the occupation of the Y-site with Fe3+ and the Z-site with Al3+, the end-member schorl is
defined by the occupation of the Y-site with Fe2+ and the Z-site with Al3+.
METHODS: Chemical analyses of tourmaline normally are performed with electron microprobe (EMPA). With EMPA most significant elements can be determined, accept Li+, B3+, OH- and the Fe2+/Fe3+ ratio, for which following general assumptions are made: B3+ = 3 pfu (per formula unit), Li = 3-sumY (sum of cations at the Y-site) pfu, all iron as Fe2+.
Samples of tourmaline from Saar-Nahe-Basin, Germany, have been chemically investigated by EMPA (Cameca Camebax, 15 kV, 10nA). The cell parameters were determined by Guinier X-Ray-method and the structural refinement
was performed with a Enraf Nonius CAD4 automated four-cycle-diffractometer (graphite-monochromatized Mo-Kα X-radiation, SHELXL93-Software, starting parameters from the buergerite of Barton, 1969).
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Fig. 1: Buergerite from Germany,
length 160 micrometer
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RESULTS:
Structural formula:
(Na0,5[]0,5)[9](Fe3+1,75Fe2+0,49Al0,75)[6]Al[6]6O3,17 OH0,83 (BO3)3 [Si[4]6O18]
;
I(min): 2s(I); Refined variables: 94; Data(N): 1124; Calculated density: 3.216 g/cm3; Z/unit cell: 3; absorption factor: 24.1cm-1; F(000): 1488.1; Space group:R3m; a0=15.951(5) Å; c0=7.1502(3) Å; V=1575.48 Å; R
(anisotropic): 4.98% for 998 Fo>2s(Fo); R (anisotropic): 4.4% for 568 reflexes.
Table: Refined positional parameters and structure data for a sample from Saar-Nahe-Basin:
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X-site |
Y- site |
Z- site |
B- site |
Si- site
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Coordination
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9
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6
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6
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3
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4
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Occupation
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Na0.5[]0.5
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(Fe3+1.75Fe2+0.49 Al0.75)
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Al6
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B3
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Si6
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Point symmetry
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3m
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m
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1
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m
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1
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Atomic coordinates
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x
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1.0
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0.87605(12)
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0.70196(11)
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0.88960(3)
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0.80830(10)
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y
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1.0
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0.93802(6)
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0.73858(11)
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0.7793(7)
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0.81041(10)
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z
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0.7764(21)
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0.3707(2)
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0.3905(2)
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0.5464(13)
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0.9995(2)
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Beq
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0.0260(5)
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0.0100(5)
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0.0073(4)
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0.008(2)
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0.0055(5)
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Interatomic dist. [Å]
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O1
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2.014(7)
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O2
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(x3) 2.545(13)
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(x2) 1.978(4)
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1.377(11)
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O3
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2.139(7)
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1.987(3)
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O4
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(x3) 2.808(8)
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1.621(2)
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O5
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(x3) 2.759(8)
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1.629(3)
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O6
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(x2) 2.011(4)
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1.871(4)
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1.621(4)
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O7
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1.886(4)
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1.616(4)
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O7'
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1.950(4)
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O8
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1.891(4)
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(x2)1.375(6)
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O8'
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1.919(4)
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Mean Me-O [Å]
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2.704(10)
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2.022(5)
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1.917(4)
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1.376(8)
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1.622(3)
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s
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0.140
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0.060
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0.044
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0.001
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0.005
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Withthe sum of ionic radiicalculated after the ionic radii from Shannon(1976), i.e. Fe2+[6] = 0.645 Å, Fe3+[6] = 0.780 Å, Al3+[6] = 0.535 Å and O2- = 1.36 Å, the values for distances of
O2-[3] <->Fe2+[6] = 2.140 Å, O2-[3] <->Fe3+[6] = 2.005 Å and O2-[3] <->Al3+[6] = 1.895 Å may be estimated.
The mean octahedral bond-lengths and the corresponding cation radii in published iron-bearing tourmaline were correlated together with the data from this work. This shows the following decreasing values for Fe3+-containing tourmalines at the Y-site
: 2.006(3) Å (buergerite, Barton, 1969) <2.022(5) Å (this work) <2.032(2) Å (schorl – Foit, 1989) <2.060 (2) Å (schorl
- Fortier & Donnay, 1975).
CONCLUSIONS:The observed mean bond-lengths and the results of the site-population refinement lead to the
suggestion that most of the observed iron at the Y-site of the tourmaline from Germany is in trivalent state. These results were essential for the definition of this tourmaline as buergerite.
Agência Financiadora: CNPq
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