MINERALOGICAL CHARACTERIZATION OF AMAZONITES FROM …

95

Estudos Geológicos vol.27(1) 2017 www.ufpe.br/estudosgeologicos

MINERALOGICAL CHARACTERIZATION OF AMAZONITES FROM SERRA DO

PINHEIRO, SERTÂNIA (PE), BRAZIL

Glenda Lira Santos¹

Eduardo Toshiyuki Fagundes Watanabe¹

José Ferreira de Araújo Neto¹

Igor Manoel Belo de Albuquerque e Souza¹

Sandra de Brito Barreto¹ 10.18190/1980-8208/estudosgeologicos.v27n1p95-107

¹Departamento de Geologia DGEO/CTG/UFPE, [email protected];

[email protected]; [email protected]; [email protected]

[email protected]

RESUMO

A amazonita da Serra do Pinheiro trata-se da única ocorrência deste mineral

cadastrada no estado de Pernambuco. Esta mineralização chama atenção pelas especificidades

de ambiente geológico de ocorrência e possíveis usos como material gemológico. Este

trabalho reúne dados espectroscópicos e químicos deste mineral e modo de ocorrência. A

amazonita estudada encontra-se dispersa em veios pegmatítitcos encaixados no granodiorito

peralcalino da Serra do Pinheiro. Apresenta cor verde clara, densidade 2,5 g/cm3 e hábito

prismático. Os cristais deste mineral foram investigados por difração de raios-X, fluorescência

de raios-X, espectroscopia no infravermelho por refletância total atenuada, análise

termodiferencial e termogravimétrica, espectroscopia de absorção na faixa do infravermelho,

e espectroscopia de refletância. Esta amazonita se caracteriza por ser uma microclina de

baixo ordenamento de Si/Al, com conteúdos de chumbo de 325 ppm e conteúdos de ferro de

307 ppm, com bandas de absorção relacionadas a estes elementos cromóforos, os quais são

responsáveis pela coloração verde clara.

Palavras chave: amazonita, Serra do Pinheiro, caracterização química e espectroscópica,

coloração verde clara.

ABSTRACT

The amazonite from Serra do Pinheiro is the only occurrence of this mineral found

registered in the Pernambuco state, Brazil. This mineralization stands out for its specific

geological environment and potential use as a gemmological specimen. This paper includes

the spectroscopic and chemical data from this mineral and how it presents itself. The studied

amazonite is found scattered in pegmatite veins, hosted in a peralkaline granodiorite from

Serra do Pinheiro. It presents pale green colour, density of 2.5 g/cm³ and prismatic habit. This

mineral’s crystals were investigated by x-ray diffraction, x-ray fluorescence, infrared

spectroscopy by attenuated total reflectance, differential thermal and thermogravimetric

analysis, infrared absorption spectroscopy and reflectance spectroscopy. This amazonite is

characterized by a microcline with a low degree of Al/Si order, containing approximately 325

ppm of lead and 307 ppm of iron, presenting absorption bands related to these elements which

are chromophores and responsible for its pale green colour.

Keywords: amazonite, Serra do Pinheiro, chemical and spectroscopic characterization, pale

green colour.

http://www.ufpe.br/estudosgeologicos

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INTRODUTION

The amazonite is a variety of

potassic feldspar that stands out for its

blue-green colour, geological environment

of occurrence and gemmological use. This

mineral has been focus of several types of

research which aimed to define the cause

of its coloration (eg. Hofmeister &

Rossman, 1985; Ostrooumov et al., 1989),

however the lead ions as agents of colour

is the most recognized hypothesis

(Hofmeister & Rossman, 1985; Petrov et

al., 1993). On the other hand, other studies

suggest that the presence of iron, structural

water, degree of order/disorder of Al/Si in

the crystalline structure act as mechanisms

of colour generation (Ostrooumov &

Banerjee, 2005; Ostrooumov, 2012).

The amazonite’s mineralization

from Serra do Pinheiro occurs in metric to

centimetric pegmatites veins and it is

inserted in the geological context of the

Transversal Domain of Borborema

Province (Santos et al., 2015). This

occurrence is part of new pegmatites

which have been systematically mapped

out of the Seridó Pegmatite Province, such

as Itapiúna, Cristais-Russas, Solonópole-

Quixeramobim (Santos et al., 2014) and

the Vierópolis Pegmatite District in the

sertão region of Paraíba state, Brazil

(Barreto et al., 2016; Bezerra, 2016).

The purpose of this paper is the

characterization of the amazonite from

Serra do Pinheiro with focus on the

identification of its typomorphic,

gemmological, spectroscopic and

geochemical features, still pursuing,

through these analyses, the recognition of

the colouring agents of its green colour.

GEOLOGICAL SETTINGS

The Borborema Province (BP)

defined by Almeida et al. (1977), located

in Northeastern Brazil, consists of a

complex orogenic system formed by

agglutination of crustal fragments during

late Neoproterozoic. The BP configuration

includes a Paleoproterozoic basement, an

Archean nucleus and a Meso- to

Neoproterozoic supracrustal, all deformed

by the Brasiliano-Pan-African Cycle (650-

500 Ma) (Brito Neves et al. 2000; Van

Schmus et al. 2008). These authors

subdivided the province into five domains:

Médio Coreaú, Ceará Central, Rio Grande

do Norte, Transversal and Meridional (Fig.

1A).

Within the Zona Transversal

Domain, the Rio Capibaribe Terrain

includes a set of meta-vulcanic-

sedimentary rocks of Tonian age and meta-

plutonic rocks of Orosiriane age

comprising the Vila Moderna Intrusive

Suite, which includes the Serra do

Pinheiro. This suite is characterized by an

expressive alkaline to peralkaline

magmatism and contains the Serra do

Pinheiro facies composed of fine-grained

granodiorites (Santos, 2012) which host

the amazonite pegmatites of the studied

area (Fig. 1B).

Glenda Lira Santos et al.


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Figure 1. Geological settings. A) Tectonostratigraphy compartmentalization of Borborema

Province, adapted from Santos et al. (2014); B) Map showing the occurrence of Serra do

Pinheiro’s amazonite (Vila Moderna Suite), clipped from Sertânia map (SC.24-X-B-I) (Santos

et al., 2016).

Amazonite from Serra do Pinheiro

The pegmatites from Serra do

Pinheiro (Fig.2) comprises homogeneous

bodies, without the development of

specifics mineralogies zones (London,

2008). These bodies occur as centimetric

veins with diverse directions and as a

principal body of sub-vertical direction and

metric size, which fills fractures of the host

granodiorite. These pegmatites are

essentially composed of microcline, smoky

quartz, amazonite and albite-oligoclase

plagioclase, presenting biotite, garnet,

kozulite and metallic minerals as

accessories minerals.

The microcline has centimetric size,

with anhedral form and pale pink colour,

composing the majority of the pegmatite

veins. It presents green portions which

characterizes the amazonite, with

subhedral form and prismatic habit

reaching up to 2.5 cm in length. The quartz

appears in smaller proportion, with

millimetric to centimetric size, smoky

colour, granular habit, anhedral form,

emplaced in the interstices among the

major minerals. The plagioclase consists of

albite-oligoclase type and exhibits a pale

green colour composing the fine matrix

constituting a main part of the pegmatites.

MINERALOGICAL CHARACTERIZATION OF AMAZONITES...


98


Figure 2. Sketches of the principal pegmatites body and veins. A) The quarry located in Serra

do Pinheiro with a general context of the pegmatite; B) Homogeneous pegmatite with metric

and centimetric sizes.

The biotite appears as the main

accessory mineral and found randomly

scattered associated to millimetric euhedral

crystals of red-brownish garnet.

Additionally, the kozulite exhibits

aglomerates with prismatic form, striated

and subhedral habit with centimetric size.

Millimetrics metallic minerals were

seldonly found.

The pegmatites bodies are hosted in

a peralkaline granodiorite, fine grained

with a gray colour, hypidiomorphic, sub-

phaneritic texture and presenting

equigranular crystals. The mineralogy of

this granodiorite consists of plagioclase,

quartz, microcline, aegirine-augite,

riebeckite, allanite, apatite and opaque

minerals (Fig. 3).

Figure 3. Contact between the host rock and the pegmatite. A) Contact between the host rock

and the pegmatite in field scale; B) Microphotography from the contact between the host rock

and a phenocryst of microcline (Kfs) from the pegmatite.



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SAMPLING AND ANALYTICAL

METHODS

Amazonite samples were collected

from the pegmatite veins from the

principal occurrence of Serra do Pinheiro

to perform the spectroscopic and chemical

analysis. These samples were cut in order

to obtain faceted specimens (e.g. square,

rectangular and cabochon form) and small

slabs. The faceted ones were used for

gemmological characterization and the

slabs were used to obtain parallel polished

sections with approximate thickness of 1

mm for spectroscopic analyses. Amazonite

samples were also grounded to 200 mesh

in order to embrace other characterization

techniques.

The characterization techniques to

the grounded samples were performed

according to the following parameters:

- X-ray diffraction (XRD) (Bruker

diffractometer, D2-Phaser,

monochromatic radiation of Cu K=

1.5406 Å, operated to 30 kV and 10

mA, with a counting time per step of

1s and a minimum goniometer step

size of 0.0202 °/s);

- X-ray fluorescence (XRF) (Rigaku

model ZSX Primus II x-ray

fluorescence spectrometer, equipped

with Rh tube and 7 analytical

crystals, by the method of calibration

curves, that were constructed with

international references materials);

- Infrared Spectroscopy by Attenuated

Total Reflectance (ATR) (Bruker

spectrometer, Vertex 70, resolution

of 4 cm-1, analysis interval from

6000 to 400 cm-1 and 64 scans);

- Thermogravimetric and Differential

thermal analysis (TG-DTA)

(Shimadzu, DTG 60H, thermic

analyzer, heating rate of 10°C/min,

maximum temperature of 1000°C,

N2 flow of 50ml/min, alumina as

reference);

The methods and parameters used on the

parallel polished section are presented

below:

- Infrared Absorption

Spectroscopy (IR) (Bruker

spectrometer, Vertex 70, resolution

of 4 cm-1, analysis interval from

6000 to 400 cm-1 and 64 scans);

- Reflectance Spectroscopy

(FieldSpec 4 Standard-Res

spectroradiometer which covers from

visible to short waves infrared

wavelength (350– 2500 nm). Contain

3 independent detectors with spectral

resolution of 1.4 nm to the interval

350 – 1000 nm e 1.1 nm to interval

1001 – 2500 nm);

From the faceted specimens, it was

possible to identify some physical and

optical properties through the following

methods and equipment:

- Gemmological Properties (density

and weight: Shimadzu model

AUY220 analytical balance,

sensibility of 0.1 mg, field of tare

200 g and stability time of 3 seconds;

refractive index: Schneider

refractometer RF2 with polarizing

filter, Anderson Solution n=1.79 and

built-in interference-filter 589.3 nm.

Special high-transparent glass prism

and base S1 with transformer,

adjustable, and universal lamp UL2

with iris diaphragm).



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RESULTS

X-ray diffraction (XRD)

This fast and simple method is

widely used to identify mineral phases by

comparing the reference diffractogram and

the obtained diffractogram (Hulkins,

1981).

As shown in Figure 4, in the

amazonite’s diffractogram were identified

microcline and albite (sodic plagioclase)

mineral phases, corroborating the perthitic

exsolution texture presented by microcline

in general.

Figure 4. Diffractogram of the amazonite in study with interpretations of the mineral phases

microcline (M) and albite (A).

X-ray fluorescence (XFR) and Infrared

Spectrometry by Attenuated Total

Reflectance (ATR)

The x-ray fluorescence was

used to identify major and minor elements

of amazonite (Table 1) and it was

fundamental to determine the presence of

lead, element responsible for the amazonite

colour which differentiates this mineral

from others potassic feldspars (Hofmeister

& Rossman, 1985; Ostrooumov, 2012).

Ostrooumov & Benerjee (2005)

and Ostrooumov (2012) mention blue

amazonite with 0.09 %wt of PbO from

Kola Peninsula pegmatites, Russia, and

blue-green amazonite with 0.03 %wt of

PbO from Chihuahua pegmatite, Mexico.

The amazonite samples from Serra do

Pinheiro resemble in lead content to the

Chihuahua amazonites, comprising

concentrations of 0.04 %wt of PbO

(~325ppm Pb).

Table 1. Results from the semi-quantitatives chemical analysis in %wt. BaO, TiO2, ZnO2, Cl e

ZnO were omitted from the table because it wasn’t detected.



101


The infrared spectroscopy by

attenuated total reflectance (ATR) resulted

in an absorption spectrum (Fig. 5) that

agree with the identification of this mineral

as microcline (Vahur et al., 2016).

The link between the x-ray

fluorescence and infrared spectroscopy by

ATR (Fig. 6) was named in Hofmeister &

Rossman (1985) as a method for

determination of degree of Al/Si order

from the separation of the two overlapping

peaks near 768 and 728 cm-1 (Hafner &

Laves, 1957). Thus, as a measure of this

separation it was used the ratio of the depth

of the trough (d) to the height of the

overlap (m), resulting in the ratio d/m

shown in Figure 5.

The studied amazonite presents 325

ppm of lead, ratio d/m of 1.3 and degree of

Al/Si order of 0.26 in a range of 0 to 1. As

shown in the graphic Figure 6B, the

feldspar fits in the blue type, presenting

small concentration of lead and low values

of degree of Al/Si order.

Figure 5. Absorption spectra in the infrared region. A) Amazonite studied spectrum with the

absorption peaks identified; B) Microcline spectrum with absorption peaks identified by

Vahur et al. (2016).



102


Figure 6. Identification of degree Al/Si order. A) ATR spectrum with the d and m parameters

located; B) Identification graphic of the studied amazonite inside the ordination crystalline

field from the parameters and lead concentration, modified from Hofmeister & Rossman

(1985).

Thermogravimetric and Differential

thermal analysis (TG-DTA)

The thermogravimetric and

differential thermal analysis are similar

methods, which the studied samples are

subject to a thermal cycle from 25°C to

1000°C. The thermogravimetry evaluate

the occurrence of mass loss and the

differential thermal analysis focus on the

physical or chemical changes when heating

the sample.

The differential thermal curve

shows an endothermic reaction, which has,

at the beginning, a slight deviation from

linearity, indicating a small transformation

in the sample by the temperature of 70°C

(Fig. 7A). The thermogravimetric curve

(Fig. 7B) does not show an expressive

mass loss, since the graphic keeps constant

during all process.

Figure 7. Differential thermal and thermogravimetric analysis. A) Differential thermal with

an endothermic reaction, with a slightly transformation in 70°C; B) Thermogravimetric

analysis showing none significant mass loss.



103


Infrared Absorption Spectroscopy (IR)

The infrared absorption

spectroscopy (IR) helps the identification

of molecular bonds through the

interconnected atoms vibration or rotation

(Smith et al.,1988). In this paper the IR

assists the determination of water presence

in the potassic feldspars, whether it is

amazonite or not (Hofmeister & Rossman,

1985; Beran, 1986; Chorrecher & Garcia-

Guinea, 2011).

It was identified absorption features

in the studied samples related to the

presence of H-bonded OH in 3403 cm-1

(2938.58 nm) and features due to OH-

stretching vibration in 3644 cm-1(2744.24

nm) (Fig. 8) (Ostrooumov & Benerjee,

2005).

Figure 8. Absorption spectra in the infrared region of the studied amazonite showing OH-

stretching vibration in 3644 cm-1 (2744.24 nm) and H bonded OH in 3403 cm-1(2938.58 nm).

Reflectance Spectroscopy

The reflectance spectroscopy is an

analytical technique that uses the

electromagnetic energy reflected by

materials in the visible-near infrared

spectrum (VNIR) and short wave infrared

spectrum (SWIR) (Clark, 1999). This

analysis has a purpose to obtain

information about the mineralogical and

chemical composition of the minerals.

Hofmeister & Rossman (1985),

Ostrooumov & Rossman (2005) and

Ostrooumov (2012) indicate absorption

features in amazonite related to Pb+

electron center in 625 nm and to finely

dispersed oxides and hydroxides of Fe3+ in

380 nm.

As a result, the studied amazonites

demonstrate absorption features in the

VNIR spectrum responsible for the

presence of Fe3+ oxides and hydroxides in

377 nm and also the presence of Pb+

electron center in 636 nm. In the SWIR

spectrum, the amazonites show OH in

1415-1420 nm, H2O in 1925-1930 nm and

Al-OH in 2200 nm absorption features

(Figure 9), also presented in Pontual et al.

(1997).



104


Figure 9. Reflectance spectra with identification of the bands responsible for iron (377 nm),

lead (636 nm), hydroxyl (1415-1420 nm), water (1925-1930 nm) and aluminum-hydroxyl

(2200 nm) features in the studied amazonites.

Gemmological Properties

Some physical and optical pro-

perties were defined and made part in the

qualification of this mineral as a gem-

mological material; these are density,

transparency, refraction index and bire-

fringence that can be observed in Table 2.

The amazonite mineralization of

Serra do Pinheiro presents samples with

light hues, however applicable to

gemmological usage and jewelery. The

gems A and B (Figs. 10A and 10B) show

typical characteristics for amazonite

according to the references quoted in

gemdat, presenting index of 1.520 to

1.527, birefringence of 0.005 to 0.007 and

density of 2.54 to 2.55 g/cm³. The gem C

(Fig. 10C) has shown some interference in

its gemmological properties due to the

presence of associated quartz.

Table 2. Identification table of physics and optics properties of the studied gems.

Gemas A B C

Cut Cabochon Square Rectangular

Size (mm) 15.88 x 12.86 x

8.46

8.69 x 6.53 17.77x12.83 x

7.93

Colour Pale green Pale green Pale green

Weight (ct) 14.38 3.88 14.05

Density 2.553 g/cm³ 2.544 g/cm³ - *

Transparency Opaque Opaque Opaque

Lustre Vitreous Vitreous Vitreous

Hardness 6.5-7 6.5-7 6.5-7

Reffraction

index Birrefrigence

1.520 – 1.525

0.005

1.520 – 1.527

0.007

1.516 – 1.520

0.004

*Density not calculated due to quartz association



105


Figure 10. Amazonite gems from Serra do Pinheiro. A) Gem A in cabochon; B) Gem B in

square form; C) Gem C in rectangular form associated with quartz.

FINAL CONSIDERATIONS

The analyzed amazonites from the

pegmatite veins of Serra do Pinheiro can

be characterized by RTA spectroscopy and

XRD analysis as a microcline with albite

exsolution (perthite). In this context, the X-

ray fluorescence shows few concentrations

of lead (325 ppm), which allied to the RTA

technique demonstrate low values of

degree of Al/Si order. The studied crystals

by IV spectroscopy reveal the presence of

structural water through the molecular

vibration of OH and H-OH. The presence

of these molecules is corroborated by the

reflectance spectroscopy and it adds the

presence of Fe3+ and Pb+.

Finally, it was found that the pale

green colour is originated due to the

presence of iron and lead, both responsible

for the absorption features of 377 nm and

636 nm and concentrations of 307 ppm and

325 ppm, respectively. The presence of

lead makes the amazonite different from

others potassic feldspars. In addition,

because of its beautiful colour and high

hardness it can be used in the production of

gemmological pieces to the jewellery

industry.

Acknowledgments

The authors would like to thank

Facepe, for the concession of the PIBIC

scholarship and the Gemmological

Laboratory (LABGEM-UFPE) for the

support on the preparation and the

gemmological identification of the

samples. They also express their thanks to

Dr. Carlos Alberto Santos (CPRM-PE) for

the indication of the studied area to the

Mineralogical Technology Laboratory

(LTM-UFPE) and to Professor Dr. Pedro

Guzzo (DEMINAS-UFPE) for the

absorption spectroscopic analysis, to NEG-

LABISE (UFPE) for the X-ray

fluorescence, professor Dr. Thais Carrino

(DGEO-UFPE) for the reflectance

spectroscopy and professor Dr. Ricardo

Scholz (UFOP) for the X-ray diffraction.

At last, the authors appreciate the final

revision of this work from master degree

student Iuri Lira (West Virginia

University).

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