Brazilian Journal of Geology
DOI: 10.1590/2317-4889201920190014
G
OLO IA
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BJGEO
ARTICLE
SOCIEDA
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SDE 1946
Mineralogical and gemological characterization
of emerald crystals from Paraná deposit, NE Brazil:
a study of mineral chemistry, absorption and
reflectance spectroscopy and thermal analysis
José Ferreira de Araújo Neto1* , Sandra de Brito Barreto1
Axel Müller2 , Lauro Cézar Montefalco de Lira Santos1
, Thais Andressa Carrino1
,
Abstract
The Paraná deposit, located at Southwestern Rio Grande do Norte state, in Brazil, is one of the few emerald deposits found at Borborema
Province. The mineralization occurs in phlogopite schists and actinolite-phlogopite schists associated with pegmatites and albitites within
the Portalegre Shear Zone. Unlike other well-known Brazilian emerald deposits, the mineralogy of Paraná emeralds has remained poorly
investigated for the last 40 years. In this study, we conducted mineralogical characterization of theses emeralds through gemological testing,
mineral chemistry, absorption and reflectance spectroscopy, and thermal analysis. The Paraná emeralds are bluish-green colored, characterized by high refractive index, several two-phase fluid inclusions and mica is the main mineral inclusion. Electron probe microanalysis
and laser ablation-inductively coupled plasma-mass spectrometry analyses detected the presence of Fe2+ (0.43–1.94 wt.% FeO) and Cr3+
(0.04–0.14 wt.% Cr2O3) as the main chromophores replacing octahedral Al3+ in the crystal structure. In addition, substantial amounts of
MgO (0.40–2.72 wt.%), Na2O (0.50–1.81 wt.%), and Cs2O (0.07–0.44 wt.%) were also identified. The main causes for its coloration were attributed to Cr3+ absorption features in visible spectral range, which were corroborated by absorption and reflectance spectra. The presence of
types I and II H2O at channel-sites was recorded in Fourier-transform infrared spectra and demonstrated by dehydration processes observed
in different thermal and thermogravimetric analyses.
KEYWORDS: Paraná emerald; mineralogy; gemology; geochemistry; spectroscopy.
INTRODUCTION
According to Wood and Nassau (1968), these channels can
allow the entrance of alkali ions, such as Na, Cs, Rb and K,
as well as two types of water molecules: type I H2O, with no
association to alkali ions, and type II H2O, coordinated with
alkali ions. Type II H2O can be singly (H2O IIs) or doubly
coordinated to an alkali ion (H2O IId). The latter occurs in
highly hydrated systems with two water molecules surrounding an alkali ion.
At the Borborema Province, Northeastern Brazil, emerald mineralizations are particularly uncommon. Punctual well
known deposits are Tauá (Ceará state), which is recognized by
low-quality emerald crystals (Schwarz 1987), and the recently
discovered Fazenda Bonfim occurrence (Cavalcanti Neto and
Barbosa 2007), within the Seridó Mobile Belt (Rio Grande
do Norte state), which presents gem-quality crystals that
were targeted for prospective activities and scientific research
in the last decade (i.e., Scholz et al. 2010, Zwaan et al. 2012,
Santiago et al. 2018).
The Paraná emerald deposit is located at the extreme
Southwest of Rio Grande do Norte state and has been known
since the 1980s. It has been recognized by considerable amounts
of gem-quality crystals found in quartz and pegmatite veins
hosted in biotite/phlogopite schists (Vasconcelos 1984, Moraes
1999). During such period, emeralds crystals were explored by
local miners until almost exhaustion. Brief descriptions of the
Emerald is a green gem variety of beryl (Be3Al2Si6O18).
Its coloration is due to amounts of Cr3+, V3+ or Fe replacing
Al3+ at the Y site in the crystal structure, and it needs a specific
geological environment to bring together such chromophore
elements, which are commonly concentrated in mafic and
ultramafic rocks, whereas Be is originated from Be-bearing
pegmatites or, in some cases, brine fluids (Walton 2004).
The beryl structure was firstly reported by Bragg and West
(1926) and it consists of an Al or Y site octahedrally coordinated by six oxygen atoms and Be and Si sites both tetrahedrally coordinated by four oxygen atoms. The arrangement
of SiO4 tetrahedra forms stacked six-membered rings, which
results in channel sites parallel to the crystallographic c axis.
1
Departamento de Geologia, Universidade Federal de Pernambuco –
Recife (PE), Brazil. E-mails: araujoneto.geo@gmail.com;
sandradebritobarreto@gmail.com; thais.carrino@gmail.com;
lauromontefalco@gmail.com
2
Natural History Museum – Oslo, Norway and Natural History
Museum – London, United Kingdom. E-mail: a.b.muller@nhm.uio.no
*Corresponding author.
© 2019 The autors. This is an open access article distributed under
the terms of the Creative Commons license.
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Braz. J. Geol. (2019), 49(3): e20190014
geological framework at the Paraná region were performed by
the Geological Survey of Brazil in regional geological mapping
(Medeiros 2008, Souza 2017), but the emerald mineralogical
characterization remains uninvestigated. Recent mining activities in the region have been retaken by Mineração Limeira
Comércio, Exportação e Importação Ltda, by opening a new
commercial possibility for emerald from the Paraná deposit.
Since the Paraná emerald deposit is poorly known and may
present important clues on the gemological-geological aspects
of Borborema Province, this paper combines several analytical techniques, such as gemological study, mineral chemistry,
absorption and reflectance spectroscopy, as well as thermal
analysis to introduce and compose the mineralogical characterization of Paraná emerald crystals. Our aim is to investigate internal features and gemological properties, as well as
the spectral signature and chemical composition of Paraná
emerald, identifying chromophore elements responsible for
the green coloration and the main substitutions at the Y and
channel sites, generating suitable data that allow comparison
with other emerald crystals found worldwide.
zones that are probable Ediacaran-Cambrian in age (Brito Neves
et al. 2000, Medeiros 2008, Viegas et al. 2014). Emerald mineralization occurs along the transcurrent Portalegre Shear Zone,
in a discontinuous NE-SW trend with, at least, 20 km length
(Moraes 1999). This regional-scale structure represents the
contact between meta-granitoids of Jaguaretama Complex
and both meta-plutonic and metavolcanossedimentary units of
Caicó Complex (Medeiros 2008). The Paraná region is characterized by occurrences of phlogopite and actinolite-phlogopite
schists that are part of the metavolcanossedimentary sequence
of Caicó Complex (Fig. 1B), which also comprises spatially
associated mylonites, gneisses, amphibolite, and quartzites.
The phlogopite and actinolite-phlogopite schists occur
as vertical to subvertical lenses along mylonitic domains of
Portalegre Shear Zone (Fig. 2A). They host several recrystallized veins and veinlets with granitic composition and aplitic
to pegmatitic texture. These granitic bodies occur as boudins
constituted mostly by quartz and/or feldspar and minor biotite
(Fig. 2B). The emerald crystals have grown into the schists, as
well as in these small granitic bodies or at the contact between
both rocks (Fig. 2C).
Several granitic pegmatites have been reported within
the Portalegre and Vieirópolis shear zones (Araújo Neto et al.
2018). At the Paraná deposit, centimeter- to meter-sized granitic
pegmatite lenses and veins are found in association with the
emerald host rocks. These pegmatites often present a simple
GEOLOGICAL SETTING
Paraná deposit is situated in the Rio Grande do Norte
Sub-province of the Borborema Province (Fig. 1A).
This sub-province is characterized by several major ductile shear
A
B
MCS: Médio Coreaú Sub-province; CCS: Ceará Central Sub-province; RNS: Rio Grande do Norte Sub-province; TRS: Transversal Sub-province; MES:
Meridional Sub-province.
Figure 1. Regional geological setting of Paraná emerald deposit. (A) Main emerald deposits within the tectonic subdivisions of Borborema
Province. Modified from Santos et al. (2014). (B) Geologic map of Paraná region with the localization of Aroeira and Pitombeiras mines.
WGS 84 horizontal projected datum UTM zone 24S. Modified from Araújo Neto et al. (2018).
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Braz. J. Geol. (2019), 49(3): e20190014
composition that is made up of potassium feldspar with minor
quartz and muscovite.
At least two main mines are recognized at the Paraná
deposit: the Pitombeiras and the Aroeira mines. In these
mine shafts, meter-sized dykes of sodium-rich plagioclase
(albitite) can be found adjacent to the basement gneisses and
host schists. These albitites occur as tabular sheared pegmatite bodies (Fig. 2D), which are mostly composed by albite
but with minor light green muscovite.
Internal features were observed through transmitted light
(TL), polarized light (PL), and dark field illumination (DFI)
using a Schneider gemological microscope coupled with the
Zeiss-Stemi (Stemi 2000-C) optical system and a Schneider
horizontal oil-immersed gem microscope also coupled with the
Zeiss-Stemi (Stemi 2000-C) optical system, both with maximum magnification of 50x. A calcite dichroscope was used for
pleochroism observation. The Chelsea filter was employed for
color variation test and short- and long-wave ultraviolet (UV)
lamps were used to observe fluorescence.
Major and minor element chemical analysis was carried
out on 15 emerald crystals (99 spots) in eight thin sections at
the Electron Microprobe Laboratory of the Universidade de
Brasília (LASON – UnB). The thin sections were previously
carbon-coated in an Edwards Auto 306 vacuum chamber.
Chemical data were obtained by JEOL JXA-8230 electron
microprobe, equipped with a scanning electron microscope
(SEM), five wavelength dispersive X-ray spectrometers (WDS),
and one energy dispersive X-ray spectrometer (EDS). The system was operated for standard silicate analysis, using LASON’s
internal standards for multi-standard calibration: albite (Na),
forsterite (Mg), topaz (F), microcline (Al, Si and K), andradite (Ca and Fe), vanadinite (Cl and V), MnTiO3 (Ti and
Mn), Cr2O3 (Cr), NiO (Ni), and pollucite (Cs). The following parameters were used: acceleration voltage of 15 kV, with
bean current of 10 nA, bean diameter of 1 µm, and a 10-second
SAMPLING AND ANALYTICAL METHODS
Emerald samples were collected in Pitombeiras and
Aroeira mines or provided by Mineração Limeira Comércio
Exportação e Importação Ltda. We examined three faceted
emerald gemstones ranging from 0.98 to 3.55 ct, three polished parallel plates, three samples of pulverized emerald,
and 20 thin sections made from emerald crystals and emerald-bearing rocks.
Standard gemological study was carried out at the Gemology
Laboratory of Universidade Federal de Pernambuco (LABGEM
– UFPE). A Schneider RF2 refractometer with polarizing filter and a refractive index fluid (n = 1.79) were employed to
measure the refractive index and birefringence. Weight and
specific gravity were obtained by a Shimadzu AUY220 hydrostatic digital analytical balance, with a sensibility of 0.1 mg.
A
B
C
D
Figure 2. Geological features of Paraná emerald deposit. (A) Vertical contacts between phlogopite schist and mylonitic gneiss.
(B) Quartz-feldspar boudins inserted in the phlogopite schist on a gallery wall at Pitombeiras mine. (C) Light-colored emerald prisms in a
quartz vein hosted in the phlogopite schist. (D) Contact between albite pegmatite (albitite) and mylonitic gneiss at Pitombeiras mining front.
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Braz. J. Geol. (2019), 49(3): e20190014
count time for peak. Optical microscope images from CCD
video camera and backscattered electron images were used
for selecting analysis spots and avoiding mineral inclusions.
Minor and trace elements were determined by laser
ablation-inductively coupled plasma-mass spectrometry
(LA-ICP-MS) conducted in 12 thin sections of emerald crystals. The analysis was performed at the Geological Survey of
Norway, using a double-focusing sector field mass spectrometer, model ELEMENT XR, from Thermo Scientific, which is
combined with a NewWave UP193FX laser probe. The 193-nm
laser had a repetition rate of 20 Hz, a spot size of 75 µm, and
an energy fluence of 5.5 to 6.5 mJ/cm2 on the sample surface.
A continuous raster ablation with laser speed of 15 µm/s on an
area of approximately 300 × 150 µm was applied in the emerald crystals. The isotope 29Si was used as internal standard, by
applying Si concentration obtained through electron microprobe or stoichiometric concentration of Si in Be3Al2SiO6 for
emerald crystals without microprobe chemical data. External
multi-standard calibration was performed using the reference
materials NIST SRM 610, 612, and 614 and 1830, BAM No.1
amorphous SiO2 glass from the Federal Institute for Material
Research and Testing in Germany, the Qz-Tu synthetic pure
quartz monocrystal provided by Andreas Kronz from the
Geowissenschaftliches Zentrum Göttingen (GZG), Germany.
Certified, recommended, and proposed values for these reference materials were taken from Jochum et al. (2011) and from
the analysis certificates, where available. Each measurement
comprised 15 scans of each isotope. An Ar blank was run before
each reference material and sample measurement to determine the background signal. The background was subtracted
from the instrumental response of the reference material/
sample before normalization against the internal standard, in
order to avoid instrumental drift effects. This was carried out
to avoid memory effects between samples. A weighted least
squares regression model, including several measurements of
the six reference materials, was used to define the calibration
curve for each element.
Visible-near infrared absorption spectra were obtained for
three double-sided polished emerald plates using a PerkinElmer
spectrophotometer, Lambda 35 model, at the Ionizing Radiation
Metrology Laboratory of Universidade Federal de Pernambuco.
The spectrophotometer operated in the 400–1,100 nm spectral
range, with 4 nm slit, data interval of 1 nm and scanning speed of
120 nm/min. Short-wave and mid-wave infrared spectra of the
same three samples were obtained at the Mineral Technology
Laboratory of Universidade Federal de Pernambuco, using a
Bruker Fourier-transform infrared (FT-IR) spectrometer, model
Vertex 70, operating in a range from 7,500 to 1,500 cm-1 with
an 128x scanning and a resolution of 4 cm-1.
Reflectance spectroscopy was conducted in 10 rough
and faceted emerald crystals, using FieldSpec4TM Standard
Resolution spectroradiometer (Analytical Spectral Devices)
at the Institute of Geoscience of the Universidade Estadual
de Campinas (UNICAMP). The spectroradiometer records
spectra in 2,151 channels, with wavelengths ranging from 350
to 2,500 nm that comprise the visible to near infrared range
(VNIR, 350–1,200 nm) and the short-wave infrared range
(SWIR, 1,200–2,500 nm). The spectral resolution is 1.4 nm
for the 350–1,000 nm range, and 1.1 nm for 1,001–2,500 nm
(Malvern Panalytical 2018). A contact probe with internal light
source and ~20 mm spot size was used. Data were calibrated
using a Spectralon® white plate. The samples were measured
at least three times, and an average reflectance curve was calculated for each sample.
Differential thermal and thermal gravimetric analyses were
performed simultaneously in three emerald powder samples at
the Mineral Technology Laboratory of Universidade Federal
de Pernambuco, using a Shimadzu thermal analyzer, DTG
60H model, with a heating rate of 10ºC/min and maximum
temperature of 1,150ºC. The atmosphere used for the analyses was nitrogen (N2). Calcined alumina was used as a reference material and an alumina cylindrical crucible was used at
0.5 mm diameter and 0.25 mm height.
GEMOLOGICAL PROPERTIES
Emerald crystals are translucid to transparent and occur as
hexagonal prisms with irregular basal termination (Fig. 3A).
They show green to bluish green colors, varying from weak to
strong saturations (Fig. 3B). Crystals with strong color saturation present expressive greenish blue to yellowish green
dichroism under the dichroscope and polarized light microscope (Fig. 3C). Growth and color zoning concentric to the c
axis is evidenced in emerald plates parallel to the basal plane
(Fig. 3D).
Refractive indexes measurements of the ordinary
(no = 1.590) and extraordinary (ne = 1.580–1.583) rays yield
a birefringence from 0.007 to 0.010. Average specific gravity
is 2.74, and the examined crystals are inert to short- and longwave UV radiation, besides they do not show a reaction under
the Chelsea filter.
The dominant group of inclusions is the two-phase liquid-gas type fluid inclusions (Fig. 4A). Several inclusions
exhibit a preferential orientation that corresponds to a crystallographic direction of the emerald crystal (possibly the
c axis), thus being considered primary inclusions contemporaneous with the crystal growth. The cavities containing fluid
inclusions are parallel to fine growth tubes and have an irregular or rectangular shape. They usually occur as elongated rectangles with straight or cuneiform termination or as negative
crystals (crystal-like cavities). Two-phase inclusions were also
observed in partially healed fissures (Fig. 4B). These inclusions
are considered pseudo-secondary because they are hosted in
late-formed fissures, but still during crystal growth. The entangling of numerous fractures cracks and healing fissures confers
a Jardin aspect to these emeralds (Fig. 4C), due to the resemblance of shrubs in a garden.
Few mineral solid inclusions were encountered. Brownish and
greenish mica plates with hexagonal sub-idiomorphic to idiomorphic forms are the main mineral inclusions. These mica
crystals occur parallel to the orientation of growth tubes; therefore, they are considered syngenetic solid inclusions (Fig. 4D).
Color-zoned emerald crystals, consisting of hexagonal, concentric growth zones vary from different shades of
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Braz. J. Geol. (2019), 49(3): e20190014
A
B
C
D
Figure 3. Emerald from Paraná deposit. (A) Rough emerald prism in quartz-feldspar matrix from Pitombeiras Mine. (B) Faceted bluish-green
emerald from Aroeira Mine. (C) Dichroism in faceted emerald under transmitted light (TL) and polarized light (PL) in immersion horizontal
microscope. (D) Emerald basal section with concentric color zoning and associated feldspar and phlogopite along fractures.
A
B
C
D
Figure 4. Interior features of a representative Paraná emerald crystal (ML06). (A) Two-phase fluid inclusions parallel to growth lines direction
(50x PL). (B) Fluid inclusions in partially healed fissures (50x TL). (C) Jardin pattern generated by several fluid inclusions, cracks, and fissures
(12.5x DFI). (D) Mica lamellae inclusions parallel to growth lines direction (50x PL).
5
Braz. J. Geol. (2019), 49(3): e20190014
green to almost colorless. The crystals have two-phase fluid
inclusions arranged along the color striations and concentrated in an intermediate zone, immediately external to
the central colorless core (Fig. 5A). The distinct boundary
between the colorless core and oscillatory zoned green overgrowth indicates a change in the crystallization environment.
The oscillatory overgrowth may reflect extrinsic changes of
the crystallization environment (P-T, tectonics?) or intrinsic diffusion-controlled growth, whereby the coloring agents
become depleted due to fast crystal growth rate or enriched
during slow growth.
Zones with high inclusion content were identified with
SEM. Qualitative EDS analysis revealed inclusions of quartz,
potassium feldspar, and sericitized plagioclase. They delimited
two different zones in a hexagonal basal section. The presence
of these two zones suggests that the Paraná emeralds have
gone through at least two stages of crystal growth (Fig. 5B).
Ga, Ge, and B are present in very low concentrations, averaging lower than 50 ppm. Germanium and B have specifically
low concentrations of < 5 ppm.
The calculated values for atoms per formula unit (apfu)
in Table 1 show deficiency of Al3+ at the octahedral coordinated Y structural site (Al < 2 apfu), with mean values varying
from 1.460 to 1.855 apfu. This Al3+ deficiency is commonly
accompanied by accommodation of Cr, V, Fe, Mn, Mg, Ni,
and Ti. Figure 6A shows the relationship between the Al
content versus the sum of other Y-site cations, exhibiting a
strong negative linear correlation, with a Pearson correlation
coefficient (r) equal to -0.989. On the other hand, the substitution of Al3+ by bivalent cations (e.g., Fe2+, Mg2+, Mn2+, and
Ni2+) at the Y-site is associated with coupled substitutions
of monovalent cations (e.g., Na+, K+, Cs+) in structural channels to obtain charge balance. As expected, Paraná emeralds
have a strong positive linear correlation between the sum of
Y-site bivalent cations versus the sum of monovalent cations
of the structural channels, with r equal to 0.982 (Fig. 6B).
Samples that plot at the right side of the 1:1 straight line
suggest that part of the analyzed Fe in emerald is present as
Fe3+ (Groat et al. 2008).
CHEMICAL COMPOSITION
Representative emerald electron microprobe chemical
data are presented in Table 1. Paraná emeralds show moderate
concentrations of Fe (average 0.94 wt.% FeO) and relatively
low Cr (average 0.08 wt.% Cr2O3) and V (average 0.02 wt.%
V2O3). Intense bluish green samples contain these chromophore elements in concentrations, which are 1.4 to 2.6 times
higher than those of crystals with less intense color (e.g. ML05
sample). The FeO/(Cr2O3+V2O3) ratio ranges from 5.0 to 21.6.
Concentrations of Mg are moderate to high (average
1.15 wt.% MgO), ranging from 0.40 to 2.72 wt.%. Sodium
and Cs are also present in relevant content of 1.00 wt.% Na2O
and 0.16 wt.% Cs2O. The H2O content was calculated through
the empirical equation of Marshall et al. (2016), which established: H2O = 0.5401 ln Na2O + 2.1867, leading to an average
content of 2.15 wt.% for the Paraná emeralds.
TheLi, B, Mn, Ge, Rb, K, Ca, Sc, Ti, and Ga trace elements
were detected by LA-ICP-MS (Tab. 2). Average K concentrations range from 210 to 1,811 ppm, while Li is found in relatively low concentrations of 86 to 381 ppm. Manganese, Ti,
Chemical zoning
EPMA element profiling was carried out on the ML05
crystal, in order to detect which coloring agents are responsible for the observed zoning. Figure 7 illustrates the chemical
variation along the sampling profile.
The colorless core has a lower average Al2O3 content than
the greenish margin (Fig. 7B), which allows an increased uptake
of other Y-site cations. MgO is mainly higher in the core, suggesting that Mg2+ plays an important role as Al3+-substituent
Y-site cation in the core zone. This is accompanied by the uptake
of compensating monovalent cations in the structural channels, which is represented by the increase in Na2O and K2O
concentrations (Fig. 7D). The FeO content shows an erratic
behavior in the crystal core. While V2O3 behaves erratically
across the entire crystal, the Cr2O3 content decreases considerably in this central colorless region (Fig. 7E). Thus, elevated
A
B
Figure 5. Zoned emerald crystals. (A) Oscillatory concentric color zoning. The red dashed line indicates the boundary between the colorless
core and the outer, green growth zones (16x PL). (B) Backscattered electron image shows mineral inclusions of quartz (Qtz), K-feldspar
(Kfs), and sercitized (Ser) plagioclase (Pl) delimiting two stages of crystal growth in an emerald crystal (Em).
6
Table 1. Electron probe microanalysis average chemical composition of Paraná emerald samples.
Sample
ML01A1
(3)
63.91
0.00
15.44
0.86
0.01
1.23
0.09
0.03
0.00
0.03
1.05
0.03
0.10
0.02
13.23
2.20
98.22
6.033
1.718
0.068
0.001
0.173
0.006
0.002
0.000
0.000
1.969
0.003
0.191
0.004
0.004
0.202
3.000
ML01A2
(3)
64.84
0.00
16.10
0.73
0.03
0.96
0.05
0.02
0.01
0.02
0.81
0.03
0.08
0.00
13.41
2.07
99.15
6.036
1.767
0.057
0.002
0.133
0.003
0.001
0.001
0.000
1.965
0.002
0.145
0.003
0.003
0.153
3.000
ML01A3
(3)
64.49
0.02
15.91
0.70
0.04
0.84
0.06
0.01
0.03
0.02
0.73
0.08
0.07
0.04
13.31
2.01
98.36
6.049
1.760
0.055
0.003
0.117
0.004
0.001
0.002
0.002
1.943
0.002
0.132
0.010
0.003
0.147
3.000
ML01A4
(3)
64.57
0.01
15.25
0.89
0.03
1.27
0.14
0.04
0.02
0.01
1.04
0.06
0.16
0.01
13.33
2.21
99.02
6.051
1.684
0.070
0.002
0.177
0.010
0.003
0.001
0.001
1.948
0.001
0.188
0.007
0.006
0.203
3.000
ML01A5
(3)
64.32
0.09
15.36
0.91
0.01
1.20
0.10
0.02
0.02
0.01
1.03
0.05
0.14
0.01
13.30
2.19
98.76
6.041
1.700
0.071
0.001
0.168
0.008
0.001
0.001
0.006
1.957
0.001
0.187
0.007
0.006
0.200
3.000
ML01B1
(5)
65.85
0.04
15.80
0.86
0.02
1.26
0.12
0.02
0.01
0.02
1.11
0.03
0.12
0.01
13.62
2.24
101.13
6.037
1.707
0.066
0.002
0.172
0.009
0.001
0.001
0.003
1.960
0.002
0.197
0.004
0.005
0.208
3.000
ML01B2
(4)
66.05
0.02
15.98
0.82
0.03
1.08
0.10
0.01
0.02
0.02
1.08
0.04
0.12
0.01
13.65
2.22
101.25
6.042
1.723
0.063
0.002
0.147
0.007
0.001
0.002
0.001
1.946
0.002
0.191
0.005
0.005
0.202
3.000
ML01B3
(6)
65.82
0.05
15.36
0.97
0.03
1.46
0.10
0.01
0.00
0.02
1.29
0.05
0.11
0.01
13.60
2.32
101.20
6.045
1.662
0.075
0.002
0.200
0.007
0.001
0.000
0.003
1.950
0.002
0.230
0.005
0.004
0.242
3.000
ML04
(15)
65.06
0.05
16.95
0.50
0.03
0.43
0.08
0.02
0.01
0.01
0.51
0.04
0.16
0.01
13.48
1.82
99.15
6.027
1.850
0.038
0.002
0.059
0.006
0.001
0.001
0.004
1.962
0.001
0.091
0.004
0.006
0.102
3.000
ML05
(20)
63.28
0.07
13.92
1.94
0.02
1.70
0.11
0.04
0.02
0.01
0.66
0.13
0.44
0.01
13.08
2.34
98.42
6.043
1.567
0.155
0.001
0.242
0.008
0.002
0.001
0.003
1.980
0.003
0.247
0.016
0.018
0.284
3.000
ML07
(18)
65.01
0.07
16.55
0.70
0.03
0.62
0.04
0.02
0.02
0.01
0.66
0.03
0.27
0.00
13.47
1.96
99.46
6.029
1.809
0.054
0.002
0.086
0.003
0.001
0.002
0.005
1.963
0.001
0.118
0.004
0.011
0.133
3.000
ML08A
(3)
63.62
0.05
13.03
1.41
0.03
2.72
0.06
0.01
0.03
0.09
1.81
0.18
0.24
0.01
13.13
2.51
98.90
6.051
1.460
0.112
0.002
0.386
0.004
0.001
0.002
0.003
1.971
0.009
0.333
0.021
0.010
0.373
3.000
ML09A1
(4)
64.93
0.03
16.29
0.75
0.01
0.62
0.05
0.02
0.02
0.01
0.65
0.03
0.15
0.01
13.41
1.95
98.95
6.047
1.788
0.059
0.001
0.086
0.004
0.001
0.002
0.002
1.942
0.001
0.118
0.004
0.006
0.129
3.000
ML09A2
(3)
65.48
0.12
17.10
0.43
0.01
0.40
0.07
0.00
0.00
0.01
0.50
0.02
0.18
0.00
13.57
1.81
99.71
6.027
1.855
0.033
0.001
0.055
0.005
0.000
0.000
0.008
1.958
0.001
0.089
0.002
0.007
0.100
3.000
The number of analyzed spots per sample is given in parenthesis. *Total iron presented as FeO; **determined by stoichiometry based on 3 Be and 18 O apfu; †calculated using the equation H2O = 0.5401 ln Na2O + 2.1867 (Marshall et al., 2016).
Braz. J. Geol. (2019), 49(3): e20190014
7
SiO2 (wt.%)
TiO2
Al2O3
FeO*
MnO
MgO
Cr2O3
V2O3
NiO
CaO
Na2O
K2O
Cs2O
Cl
BeO**
H2O†
Total
Si (apfu)
Al
Fe2+
Mn
Mg
Cr
V
Ni
Ti
Y-site (total)
Ca
Na
K
Cs
R (total)
Be
EM82
(6)
65.03
0.03
14.70
1.57
0.02
1.50
0.06
0.02
0.02
0.01
1.35
0.04
0.12
0.02
13.41
2.35
100.23
6.055
1.612
0.122
0.001
0.208
0.004
0.001
0.002
0.002
1.952
0.001
0.243
0.005
0.005
0.254
3.000
Braz. J. Geol. (2019), 49(3): e20190014
Cr3+ concentrations might be responsible for high green color
intensities of Paraná emeralds.
A second broad absorption occurs in the range of 829–836
nm and it is attributed to the 5T2 → 5E transition in Fe2+ at the
octahedral Y-site (cf. Wood and Nassau 1968, Rondeau et al.
2008, Zwaan et al. 2012). Characteristic absorption VNIR
spectra for Paraná emeralds are shown in Figure 8.
Spectra obtained by FT-IR spectroscopy applied on
the same three emerald samples are characteristically for
alkali-bearing emeralds (Fig. 9), with types I and II H 2O
absorption features, besides features related to the presence of CO2 molecules (cf. Wood and Nassau 1967, 1968,
Schmetzer et al. 1997, Rondeau et al. 2008, Zwaan et al.
2012). The features centered at 7,098 and 5,275 cm-1 are
SPECTROSCOPY
The absorption spectra of three double polished plates of
emerald samples in the visible-near infrared range exhibit a broad
absorption band with maximum absorption at 603–608 nm,
derived from electronic transitions in Cr3+ at the octahedral Y-site
(transition 4A2 → 4T2, Wood and Nassau 1968). Subtle absorption features are present at 425–430, 682–684 and 637 nm.
They all indicate the presence of Cr3+ in the crystal structure.
Table 2. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) minor and trace element composition of Paraná
emerald samples.
Sample
EM21
ML01A ML01B ML01C
ML02
ML03
ML04*
ML05*
ML07*
ML08
ML09A ML09B
Li (ppm)
125.87
195.83
183.74
217.32
160.98
98.79
381.17
171.02
366.58
86.54
325.22
353.15
1.61
2.58
1.62
3.24
2.83
4.41
1.95
2.31
2.79
2.90
2.47
1.96
B
Ge
0.84
0.58
0.55
0.66
0.48
0.59
0.83
0.85
0.77
0.72
0.59
0.71
Rb
32.76
63.22
78.61
68.78
133.00
140.36
64.99
104.02
107.47
142.14
62.48
72.87
Cs
1,331.27 1,553.11 1,875.86 1,619.03 2,035.79 5,110.79 2,959.10 5,595.66 5,452.48 5,970.31 1,708.37 2,466.92
Na
3,900.05 8,393.88 9,146.13 8,631.43 13,807.80 14,480.79 4,015.59 11,346.26 5,510.30 14,717.93 4,814.85 5,095.99
Mg
3,309.87 8,637.71 9,501.11 8,833.70 16,705.98 19,791.55 2,971.31 12,801.91 4,787.15 19,964.64 3,690.90 3,958.18
P
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
< LOD
K
250.23
441.88
527.90
470.46
1,415.91 1,811.07
254.03
1,350.40
363.01
1,701.44
209.93
213.00
Ca
23.75
69.84
76.60
75.27
342.88
23.16
229.71
20.63
570.72
21.88
24.89
493.74
Sc
23.53
73.99
71.51
82.22
140.23
85.82
23.96
341.94
66.88
121.91
34.33
35.99
Ti
10.03
3.88
3.08
3.48
3.78
44.31
10.08
58.75
14.60
73.47
5.17
9.49
V
52.23
92.80
110.67
106.15
204.07
174.51
44.82
155.49
73.13
169.06
66.06
59.20
Cr
373.55
567.08
616.88
622.99
574.35
149.72
270.17
649.51
275.79
124.75
399.00
501.77
Mn
11.78
30.59
33.02
34.66
50.34
28.41
54.79
86.79
65.18
33.96
54.81
58.12
Fe
Ga
3,790.10 5,909.42 6,230.95 6,629.29 11,123.88 12,953.17 3,973.98 14,608.05 6,102.39 12,891.85 4,404.76 4,701.20
12.18
10.07
10.52
11.47
10.56
11.39
19.99
14.11
27.55
12.69
23.71
24.47
Average of six analyses per sample at different spots; LOD: limit of detection; *crystal also chemically analyzed by electron probe microanalysis.
Figure 6. Correlation among different element concentrations for 99 analyzed spots on 15 emerald samples from Paraná. The number of
analyzed spots per sample is given in parentheses. (A) Al versus the sum of other Y-site cations. (B) Bivalent Y-site cations versus monovalent
channel-site cations. Diagrams after Groat et al. (2008).
8
Braz. J. Geol. (2019), 49(3): e20190014
A
B
C
D
E
Mc: microcline; Phl: phlogopite.
Figure 7. Electron probe microanalysis analysis along a transversal profile in a color-zoned emerald crystal (sample ML05). (A) Basal section
of sample ML05 showing the locations of the analytical EPMA spots. Crossed nicols. The diagrams show concentrations of (B) Al2O3, (C) FeO,
MgO, (D) Na2O, Cs2O, K2O, (E) Cr2O3 and V2O3 along the profile. The green columns mark the analyses located in the colorless crystal core.
Figure 8. Visible and near infrared absorption spectra of Paraná emeralds. The spectra were obtained with incident beam parallel to the c axis
(Beam || c) and perpendicular to the c axis (Beam ⊥ c).
9
Braz. J. Geol. (2019), 49(3): e20190014
attributed to H2O II molecules (i.e., water molecules associated with alkali ions in the beryl structural channels; Wood
and Nassau 1968). Subtle absorption features at 7,142, 5,448
and 5,100 cm-1 are associated with the presence of H2O I, and
a large band between 4,000 and 3,350 cm-1 is related to the
presence of both types of water molecules. The presence of
CO2 is marked by a sharp absorption feature at 2,359 cm-1.
Rondeau et al. (2008) also attribute the subtle absorption at
the 2,376–2,378 cm-1 range to the CO2 molecule; therefore,
these features may be derived from the isotopic effect of the
combination of 13C and 18O.
Reflectance spectroscopy was applied in three faceted
emeralds, since it is a non-destructive method that does not
need further sample preparation (Fig. 10). The Paraná emeralds show absorption features in the VNIR range at 428 and
611–629 nm related to electronic transitions in Cr3+, and a broad
band centered at 848 nm derived from electronic transitions in
Fe2+ at the octahedral Y-site (Wood and Nassau 1968, Rondeau
et al. 2008, Zwaan et al. 2012). In the SWIR range, the emeralds show weak absorption features at 1,149 nm, three sharp
features between 1,375–1,460 nm (center at 1,410 nm) and
other three sharp features between 1,785–2,022 nm (center
at 1,894 nm), all attributed to vibrational processes of types I
and II water molecules (cf. Wood and Nassau 1967, 1968,
Clark et al. 1990, Schmetzer et al. 1997, Rondeau et al. 2008).
Four other absorption features centered at 2,072, 2,158, 2,205
and 2,329 nm were observed, but not identified.
THERMAL ANALYSIS
Differential thermal and thermal gravimetric analyses performed on three pulverized emerald samples demonstrate an
endothermic reaction curve with maximum peak at 921–933°C
(Fig. 11). At least two mass loss events by dehydration are registered at the temperatures of 300–500°C and 800–1,050°C.
The total water loss varies from 1.85 to 2.13 wt.% at 1,050°C.
Sample ML05 registered the higher water loss (2.13%), which
concurs with the high H2O content calculated by the empirical equation of Marshall et al. (2016).
The possible dehydration process starts with the loss
of H2O I and/or H2O IId (doubly coordinated water), and
then, at temperatures higher than 800ºC, there is the release
of H2O IIs (singly coordinated water). This sequence is a
result of the bond strength of each type of water molecule
and the balance of forces at the structural channels, based
on topology calculation and bond valence (cf. Fukuda and
Shinoda 2008, Fridrichová et al. 2016). Therefore, the mass
loss at 300–550°C is attributed to the release of water types
IId and/or I. Further mass loss at 800–1,050°C is associated
with the loss of water type IIs.
Figure 9. Fourier transform infrared (FTIR) absorption spectra
of Paraná emeralds. The spectra were obtained with incident beam
parallel to the c axis (Beam || c) and perpendicular to the c axis
(Beam ⊥ c).
Figure 10. Staked reflectance spectra of faceted emerald samples.
Average of five, four, and three spectral analyses for samples ML03,
ML06 and ML02, respectively.
10
Braz. J. Geol. (2019), 49(3): e20190014
DISCUSSION
Walton 2004). These so-called desilicated pegmatites or albitites are common to several Brazilian metasomatic emerald
deposits, such as Carnaíba and Socotó — Bahia state (Giuliani
et al. 1990) and Fazenda Bonfim — Rio Grande do Norte state
(Zwaan et al. 2012).
The refractive indices for Paraná crystals are relatively high,
but fall within the range expected for most natural emeralds,
which is between 1.570 and 1.600 for the ordinary ray, and
1.564–1.593 for the extraordinary ray (Schwarz 1987). It is
difficult to establish individual influences of the factors that
have an increasing effect on refractive indices, but the presence of structural water and alkaline ions (specially Na and
Cs) and the high concentration of Fe and Mg are probably the
main characteristics of Paraná emeralds. These high refractive
indices, according to Zwaan et al. (2012), are common for
schist-related emerald deposits.
The absence of reaction through Chelsea filter observation
is typical for emeralds with high Fe content (e.g., South Africa
and India emeralds; Webster 1975). Two-phase fluid inclusion
is the main group of inclusions found in emerald crystals, but
mineral inclusions such as mica (possibly phlogopite), feldspar
and quartz were also identified. The non-appearance of pyrite
inclusions can distinguish Paraná emeralds from Bahia ones,
including those from Socotó and Carnaíba (cf. Schwarz 1987).
Furthermore, the absence of amphibole inclusions is characteristic, since it is a common mineral inclusion in emeralds of other
schist-related deposits of Brazil and around the globe (Schwarz
1987, 1998, Zwaan et al. 2005, 2012). Additionally, the color
zoning accompanied by two-phase fluid inclusions concentration zones suggest an abrupt change in the physicochemical conditions of the environment at crystal growth (Schwarz
1987). Nevertheless, these internal features are very similar
to those of Bonfim emeralds (Zwaan et al. 2012). Thus, additional chemical characterization should be performed for source
mine identification.
According to the Schwarz (1987) empirical subdivision
for Cr2O3, FeO, MgO and Na2O concentration ranges in emeralds, the Paraná emerald presents low Cr, medium to high Fe,
medium to low Mg and Na. The main trivalent cations replacing Al3+ at the octahedral Y-site are Cr3+, V3+ and possibly Fe3+.
Heterovalent substitution plays a major role at the octahedral Y-site, with the entry of Fe2+ and Mg2+ coupled with Na+
and other monovalent cations entrance at the channel sites.
Figure 12A shows the relative concentration of the main substituents of octahedral Al for Paraná emerald and other deposits around the world. Magnesium is the main substituent for
most of the samples, but Paraná emerald also contains Fe in
similar proportions. In Figure 12B, the relative concentration
of chromophore elements for Paraná emerald evidences that
it is one of the highest Fe-rich emeralds among several other
deposits. The comparison with other Brazilian emeralds is
presented in Figures 13A and 13B, and the high Fe content
allows the individualization of Paraná deposit from the others.
Zwaan et al. (2012) proposed several diagrams for discriminating Fazenda Bonfim deposit and other different schist-related emerald deposits, by plotting LA-ICP-MS chemical data. The Fazenda
Bonfim deposit presents strong geological similarities with the
Emerald mineralization in the Paraná region is a schist-type
deposit controlled by a major shear zone. Chromium and Be
sources are probably linked to spatially associated mafic rocks
(metamorphosed into amphibolite) and Be-bearing pegmatites, respectively. Although beryl has not been found yet in
meter-sized pegmatites lenses associated with the host rocks,
emerald crystals can be commonly found in small pegmatite
and aplite veins within the schist foliation. In addition, beryl-bearing pegmatites within regional shear zones are reported
few kilometers east of the Paraná deposit (Barreto 1991).
The presence of Na-rich plagioclase bodies associated with the
emerald mineralization suggests metasomatic modification of
granitic pegmatites through desilication (Giuliani et al. 1990,
Figure 11. Thermogravimetric analysis/differential thermal
analysis (TGA/DTA) curves of pulverized emerald samples. The
green rectangles delimit the zones of the corresponding water
molecule loss from the channel-sites.
11
Braz. J. Geol. (2019), 49(3): e20190014
Paraná deposit, including association with phlogopite schists and
granitic intrusions, as well as being affected by transcurrent shear
zones within the Rio Grande do Norte Sub-province (Zwaan et al.
2012, Santiago et al. 2018). In the K-(Li+Cs)-Rb ternary diagram,
Paraná emeralds can be individualized by their higher amount of
Li+Cs, especially for their high Cs content (Fig. 14).
Despite the low content of Cr3+, both absorption and reflectance spectra of the Paraná emeralds are typical of Cr-emeralds
(Schmetzer et al. 1974, Zwaan 2006). They have absorption
features at 428, 608, 637, and 683 nm associated with the presence of Cr3+. Therefore, the absorption features derived from
electronic transitions in Cr3+ and the weak absorption in the
green range are responsible for its green coloration. This statement is in consonance with the decrease of Cr2O3 at the colorless core zones in zoned crystals (Fig. 7E). The presence of
Fe2+ broad absorption feature at ~830 nm does not influence
its coloration, since it is situated outside the visible range
(cf. Zwaan 2006). The spectra obtained for the ML05 sample cut
Figure 12. Ternary composition diagrams for Paraná emeralds and worldwide deposits. (A) Composition in terms of main Y-site
Al-substituents (MgO-FeO-Cr2O3). (B) Composition in terms of main chromophore elements in emerald (FeO-Cr2O3-V2O3). Results shown
for 210 analyses from literature and 99 analyses of Paraná emerald. The number of analyses per country is given in parenthesis. Symbols in black
were compiled from Groat et al. (2008), which has as source of data: Kovaloff (1928), Zambonini and Caglioto (1928), Leitmeier (1937),
Otero Muñoz and Barriga Villalba (1948), Simpson (1948), Gübelin (1958), Vlasov and Kutakova (1960), Martin (1962), Petrusenko et al.
(1966), Beus and Mineev (1972), Garstone (1981), Hanni and Klein (1982), Graziani et al. (1983), Kozlowski et al. (1988), Hammarstrom
(1989), Ottaway (1991), Schwarz (1991), Abdallah and Mohamed (1999), Gavrilenko and Pérez (1999), Alexandrov et al. (2001), Groat
et al. (2002), Marshall et al. (2004), Vapnik et al. (2005, 2006), Zwaan et al. (2005), Gavrilenko et al. (2006) and Zwaan (2006). Symbols in
grey were compiled from Aurisicchio et al. (2018). The diagrams are after Hammarstrom (1989).
Figure 13. Ternary composition diagrams for Brazilian emeralds. (A) Composition in terms of main Y-site Al-substituents (MgO-FeO-Cr2O3).
(B) Composition in terms of the main chromophore elements in emerald (FeO-Cr2O3-V2O3). Results shown for 52 analyses from literature
and 99 of the Paraná emerald. The number of analyses per deposit is given in parenthesis. Fazenda Bonfim data compiled from Zwaan et al.
(2012). Other chemical data compiled from Schwarz (1987), which has as source of data: Graziani and Lucchesi (1979), Hanni (1982),
Hanni and Kerez (1983), Eidt and Schwarz (1987), Hanni et al. (1987) and Schwarz (1987). The diagrams are after Hammarstrom (1989).
12
Braz. J. Geol. (2019), 49(3): e20190014
in different positions (parallel and perpendicular to the c axis)
show variations in the maximum absorption centralization,
as well as a variation of the relative absorbance values, which
reflects the strong dichroism of Paraná emeralds (Fig. 3C).
FTIR spectra are consistent with alkali-rich emeralds
(cf. Zwaan et al. 2012), with absorption features related to
the presence of types I and II water (Fig. 8). The high content of sodium and the large water loss associated with type
II water molecule suggest that the presence of type II H2O in
the structural channels is more expressive than type I H2O.
Additionally, some CO2 molecules can also be expected, as
evidenced by an absorption feature at 2,359 cm-1.
greenish blue to yellowish green. These crystals are colored mostly
by Cr, as evidenced by its characteristic absorption spectra and
color zoning chemistry. However, due to its elevated Fe content,
they exhibit broad absorption features at ~830 nm and do not show
reaction through Chelsea filter, which can be a diagnostic criterion
to distinguish Paraná emeralds from several other schist-related
deposits. Additionally, the high refractive indices, the presence of
numerous two-phase fluid inclusions and partially healed fissures,
and the absence of pyrite and amphibole as mineral inclusions can
also be used as criteria for distinction from other Brazilian emeralds, such as those from Santa Terezinha (GO), Carnaíba (BA),
Socotó (BA), Itabira (MG), and Tauá (CE).
When comparing our results with available data from
Fazenda Bonfim emerald deposit, also located in Rio Grande
do Norte Sub-province, there is some overlap when it comes
to geological setting and internal features, but one can be easily differentiated from the other by its chemical composition.
The Paraná emeralds show higher FeO content and lower
Cr2O3. The alkali concentration can also be used for discriminating purposes, especially due to the elevated Cs content in
Paraná emeralds, although spectroscopically they both show
characteristics for alkali emeralds with strong absorption features related to the presence of type II water.
Future research at the Paraná deposit could focus on structural analysis, whole-rock/mineral geochemistry and isotope
geology aiming to define mineralization controls and evolution,
contributing to better understand the Borborema Province
metallogenetic scenario.
CONCLUSIONS
The Paraná emerald deposit, located at Rio Grande do Norte
Sub-province of the Borborema Province, NE Brazil, is characterized by emerald-bearing granitic aplite and pegmatite veins
within phlogopite and actinolite phlogopite schists associated
with granite pegmatite bodies and desilicated pegmatites (albitites). These rocks form a mylonitic melange along the Portalegre
Shear Zone. Interaction between such rocks at high temperature shearing most certainly allowed the mobilization of Be and
chromophore elements, such as Cr, Fe and V to form emerald.
Our study has shown that Paraná emeralds have a main bluish green color, although they are typically dichroic, varying from
ACKNOWLEDGMENTS
The authors are grateful to Mr. Luis Amorim and all the
crew of Mineração Limeira Comércio, Exportação e Importação
for providing support to our research in field studies and
for supplying emerald samples. We thank Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
for the scholarship granted to José Ferreira de Araújo Neto.
We would like to express our gratitude to Professor Dr. Nilson
Botelho (Universidade de Brasília, Brazil) for the EPMA facilities. We thank Professor Dr. Lee Groat (University of British
Columbia, Canada) for his assistance with emerald chemical
data. We also thank Professor Dr. Pedro Guzzo and Fania Mateus
(Universidade Federal de Pernambuco, Brazil) for spectroscopic
and thermic analysis facilities. The reflectance spectroscopy
was done in the Geoscience Institute of UNICAMP thanks to
Professor Dr. Carlos de Souza Filho, Dr. Rebecca Scafutto, and
Dr. Rosa Pabón. We appreciate Igor Souza and Glenda Santos
of the Gemological Laboratory of the Universidade Federal
de Pernambuco for their general assistance with this research.
Figure 14. Ternary K-(Li+Cs)-Rb diagram for emeralds from Rio
Grande do Norte sub-province. Average values in ppm obtained
by laser ablation-inductively coupled plasma-mass spectrometry
(LA-ICP-MS) for 12 samples from Paraná and 15 from Fazenda
Bonfim. Data from Fazenda Bonfim compiled from Zwaan et al.
(2012). Diagram after Zwaan et al. (2012).
ARTICLE INFORMATION
Manuscript ID: 20190014. Received on: 02/25/2019. Approved on: 06/19/2019.
Author J. N. wrote the first draft of the manuscript, prepared Figures 1 to 14 and produced Tables 1 and 2; author S. B. provided support
and advisement regarding mineral chemistry, aided the field studies and gemological testing, improved the text through corrections and
suggestions; author T. C. performed reflectance spectroscopy in emerald samples, aided the spectral interpretations, revised and improved
the manuscript; author A. M. provided LA-ICP-MS data for emerald samples, revised and improved the manuscript; and author L. S. aided
in field studies and geological characterization, revised, and improved the manuscript.
Competing interests: The authors declare no competing interests.
13
Braz. J. Geol. (2019), 49(3): e20190014
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