GEMOLOGIC CHARACTERIZATION AND MINERAL
CHEMISTRY OF BLUE TOURMALINES FROM THE PEGMATITIC BODY OF BOM JESUS, BANABUIÚ - CEARÁ,
NORTHEAST BRAZIL
Gabriela Meireles Rosa1
Glória Maria Silva Hamelak2
Martha Noélia Lima3
Andressa de Araujo Carneiro4
Débora Macedo do Nascimento5
Maria Lucenilda Fortunato6
Eulivana Assunção Livalter de Moura7
José de Araújo Nogueira Neto8
Tereza Falcão de Oliveira Neri9
Francisco Marques Junior10
Otaciel de Oliveira Melo11
Jose Airton Paiva12
Department of Geology, Federal University of Ceará (UFC) 1,2,3,4,5,8,9,11;
Department of Physics, Federal University of Ceará (UFC)7,12;
Department of the Environment of Ceará6;
Master Teacher In Memorium10 [email protected]
Keywords: Blue Tourmalines, Gemologic Characterization, Mineral Chemistry.
INTRODUCTION
Pegmatites represent one of the main
sources of minerals of both gems and
industrial applications in the northeastern
Brazil. More specifically the pegmatitic
body of Bom Jesus is located within the
municipal limits of Banabuiú, in the central
region of the state of Ceará. It corresponds to
a field with significant production of green
and blue tourmalines with gemological
quality. The work under review presents
the results of both gemological laboratory
tests and mineral chemistry analysis of
blue tourmalines, given the relevance of
these for local production. Gemological
characterization was carried out using
refractometer, spectroscope, polariscope,
immersion by dense liquids and gemological
microscope. Mineral chemistry analyses
Estudos Geológicos v. 19 (2), 2009
were performed by electron microprobe
and, additionally, by Raman spectrometry
in the frequency range between 100 to 1200
cm-1.
GEOLOGICAL FRAMEWORK
The Bom Jesus pegmatite belongs
to the Sub-Pegmatitic Province of Ceará
(SSPCE), located in Ceará Central
(DCC), northern portion of the Province
Borborema (PB) (Fig. 1). The pegmatitic
bodies of the Banabuiú region - Ceará,
are associated with granites, migmatites,
and gneisses from the basement and
show intrusive contacts with the whole
set of lithotypes. These pegmatites are
allochtonous relative to the granites, but
their distribution is more frequent in
gneisses than in the granites.
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GEMOLOGIC CHARACTERIZATION AND MINERAL CHEMISTRY OF BLUE TOURMALINES FROM THE
PEGMATITIC BODY OF BOM JESUS, BANABUIÚ - CEARÁ, NORTHEAST BRAZIL
The Senador Pompeu and Orós
shear zones limit the BanabuiúSolonópole Pegmatitic Field and seem
to have influenced the regional structure,
not only the rocks from the basement
(“paleoproterozoic teranes”), but also the
plutonic rocks (Marques Jr. et al. 1988,
Marques Jr. 1992, Marques Jr and Nogueira
Neto 1992). The factors that control the
positioning and the mineralization of the
pegmatites in the region are: tectonics,
lithology, size, and internal arrangement
of the bodies (Marques Jr. 1992). These
occur as vein filling discontinuities
(faults, fractures, and foliation) or as more
expressive bodies whose thickness and
length reach tens of meters.
Regarding lithological control, the
complex heterogeneous pegmatites are
generally restricted to the intrusions in the
gneisses, while the simple homogeneous
pegmatites occur both in gneisses as in
granites. The homogeneous pegmatites
show a simple mineralogy consisting of
feldspars, quartz, and muscovite, but may
be mineralized in beryl and tourmaline.
Some more simple heterogeneous terms
show minor zoning with predominance of
aplitic textures in the rims and coarse texture
in the cores of the bodies, typically with
tourmaline, beryl, apatite, and amblygonite
(Marques Jr et al. 1988, Marques Jr. 1992,
Marques Jr and Nogueira Neto 1992).
The Bom Jesus pegmatite is intruded
into a biotite-muscovite granite, along
a normal fault. It is a tabular-shaped,
heterogeneous complex with sometimes
incomplete zoning, being characterized by
“body replacement” with albite, muscovite,
and lepidolite. In the rims of the body,
aplitic texture (Zone I) predominates and
coarse in more central portions. Zone II is
marked by the presence of quartz-feldspar
graphic texture and by the association
K-feldspar + quartz + mica. Zone III has
small “concentrations” of the remains
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of K-feldspar in contact with albite and
tourmaline are found in these areas. The
association tourmaline + albite is close to
the core of quartz (zone IV).
GEMOLOGIC
CHARACTERIZATION
Three samples of blue tourmalines
were analyzed (Fig. 2), two of which
intensely fractured and the other free
of fractures. There were no inclusions.
Refraction indexes show that the blue
tourmalines are uniaxial and have negative
birefringence around 0.010. Spectroscope
analyses show highest absorption values in
the bands 650nm (Red) / 550nm (Green)
/ 450nm (Purple), corresponding to a
spectrum characteristic of tourmalines.
The density considered, estimated by
means of dense liquid was ρ> 2.9 g/cm3,
with specific weight (among samples) up
to γ = 0.63 g. As most of the tourmaline
has a relative density ranging from 2.823.32 g/cm3, they are within expectations.
The blue tourmalines showed strong
pleochroism ranging from dark to light
blue. The gems did not show fluorescence.
MINERAL CHEMISTRY
The samples showed proportions in
Fe +2 and Na + with values ranging from
0.8238 to 0.9087 and from 0.7952 to
0.8532, with Al content of between 7.242
and 7.414, respectively, per formula unit.
The chemical composition obtained for the
extreme values of the elements, calculated
on the basis of 29 oxygen corresponds to:
Na(0,7952 – 0,8532) Fe+2(0,9087 – 0,8238) Al(7,2415
B3 Si(6,0648 – 6,1440) O27 (OH)4
– 7,3373)
This composition indicates predominantly schorlitic alkaline tourmalines.
Regarding the formation environment such
minerals are arranged in Li-rich pegmatites
Estudos Geológicos v. 19 (2), 2009
Gabriela Meireles Rosa et al.
and aplitic granites (Hawthorne and Henry
1999).
Relative to the Raman spectrum, two
dominant absorption bands were recognized,
respectively in 1050, and 700 nm, and two
secondary at 380 and 210 nm. The former
are attributed to atomic displacements
located within the BO3 triangles, in the SiO4
tetrahedrons, and in the AlO6 octahedrons
(McKeown 2008). The subordinate bands
are attributed to optical transitions of Fe2
+
and Mn2 +, as well as a higher degree of
disorder in Fe-Mn-tourmaline can cause the
blue color (Castañeda 2002).
REFERENCES
Castañeda, C. 2002. Caracterização
Mineralógica de Amostras Naturais e
Tratadas de Turmalinas e Morganitas
do Distrito Pegmatítico de Araçuaí,
Minas Gerais, Tese de Doutorado Nº
049, Universidade de Brasília/ Instituto
de Geociências, 230 p.
Hawthorne, F.C., Henry, D.J., 1999.
Classification of the minerals of the
tourmaline group. European Journal of
Mineralogy, 11 (2): 201–215.
Estudos Geológicos v. 19 (2), 2009
Marques JR., F., Nogueira Neto, J. A.,
Néri, T. F. O. 1988. Contribuição à
Geologia do Campo Pegmatítico de
Berilândia, Ceará. In: Anais do XXXV
Congresso Brasileiro de Geologia,
Belém, 1: 329–337.
Marques JR., F. 1992. Geologia do
Campo Pegmatítico de Berilândia-CE.
Instituto de Geociências. Universidade
de São Paulo. São Paulo. Dissertação
de Mestrado, 152 p.
Marques JR., F. & Nogueira Neto, J. A.
1992. Considerações Petrogenéticas
do Campo Pegmatítico de Berilândia
(CE). In: 37º Congresso Brasileiro
de Geologia, São Paulo. Anais do 37º
Congresso Brasileiro de Geologia.
São Paulo: Sociedade Brasileira de
Geologia, 2: 53-54.
McKeown, D. A. 2008. Raman
Spectroscopy, vibrational analysis, and
heating of buergerite tourmaline. Phys.
Chem. Minerals, 35: 259-270.
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GEMOLOGIC CHARACTERIZATION AND MINERAL CHEMISTRY OF BLUE TOURMALINES FROM THE
PEGMATITIC BODY OF BOM JESUS, BANABUIÚ - CEARÁ, NORTHEAST BRAZIL
Figure 1 - Location of studied area (Field of Pegmatitic Banabuiú-Solonópole) for Litostructure
main areas of the State of Ceará.
Figure 2 - Photograph of blue tourmaline studied.
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