ORIGINAL PAPER

Multi-methodological investigation of kunzite, hiddenite,alexandrite, elbaite and topaz, based on laser-induced breakdownspectroscopy and conventional analytical techniquesfor supporting mineralogical characterization

Manuela Rossi • Marcella Dell’Aglio • Alessandro De Giacomo •

Rosalba Gaudiuso • Giorgio Saverio Senesi • Olga De Pascale •

Francesco Capitelli • Fabrizio Nestola • Maria Rosaria Ghiara

Received: 6 May 2013 / Accepted: 27 September 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Gem-quality alexandrite, hiddenite and kunzite,

elbaite and topaz minerals were characterized through a

multi-methodological investigation based on EMPA-WDS,

LA-ICP-MS, and laser-induced breakdown spectroscopy

(LIBS). With respect to the others, the latter technique

enables a simultaneous multi-elemental composition

without any sample preparation and the detection of light

elements, such as Li, Be and B. The criteria for the choice

of minerals were: (a) the presence of chromophore ele-

ments in minor contents and/or as traces; (b) the presence

of light lithophile elements (Li, Be and B); (c) different

crystal chemistry complexity. The results show that LIBS

can be employed in mineralogical studies for the identifi-

cation and characterization of minerals, and as a fast

screening method to determine the chemical composition,

including the chromophore and light lithophile elements.

Keywords Laser-induced breakdown spectroscopy �Chemical analysis � Light lithophile elements �Chromophore elements

Introduction

In allochromatic minerals, minor and trace chemical

components are commonly associated with the presence of

color, and, from the gemological point of view, they are

also responsible for the difference between a common

mineral and a gemstone. Metal ions from the first row of

transition elements in the periodic table, especially Ti, V,

Cr, Mn, Fe, and Cu, are the most important causes of color

in oxides and silicate gemstones (Mattson and Rossman

1987a; Fritsch and Rossman 1988). In particular, V3?,

Cr3?, Mn3?, and Cu2? can produce strong colorations

when present at concentrations of tenths of wt%. Color is

due to electronic transitions involving only the electrons in

the d-orbitals. When present by themselves, Fe2?, Fe3?,

and Mn2? must have high concentrations to be able to

cause significant color (Rossman 2009). Instead, interva-

lence charge transfer (IVCT) interactions, involving an

electron exchange between two cations with different

valences (for example, Fe2? and Fe3?, or Fe2? and Ti4?),

are a major source of color in gems and require only a

small amount of the interacting couple to produce intense

color (Mattson and Rossman 1987b; Rossman 2009). For

instance, most tourmaline gems owe their color to Fe2?

(blue tourmalines), Fe3?, Fe2?–Ti4? IVCT (green), Mn3?

M. Rossi (&) � M. R. Ghiara

Department of Earth Sciences, University of Naples ‘‘Federico

II’’, Via Mezzocannone 8, 80134 Naples, Italy

e-mail: manuela.rossi@unina.it

M. Dell’Aglio � A. De Giacomo � R. Gaudiuso �G. S. Senesi � O. De Pascale

Institute of Inorganic Methodologies and Plasmas-CNR,

U.O.S Bari, Via Amendola 122/D, 70126 Bari, Italy

A. De Giacomo � R. Gaudiuso

Department of Chemistry, University of Bari, Via Orabona 4,

70126 Bari, Italy

F. Capitelli

Institute of Crystallography-CNR, Via Salaria Km 29.300,

00016 Monterotondo, Rome, Italy

F. Nestola

Department of Geosciences, University of Padua,

Via Gradenigo 6, 35131 Padua, Italy

M. R. Ghiara

Royal Mineralogical Museum, Museum Centre of Natural

and Physical Sciences, University of Naples ‘‘Federico II’’,

Via Mezzocannone 8, 80134 Naples, Italy

123

Phys Chem Minerals

DOI 10.1007/s00269-013-0631-3

(pink), Mn2?–Ti4? IVCT (yellow), or a combination of

these factors (Pezzotta and Laurs 2011). In addition, such

elements can also provide a ‘‘fingerprint’’ for determining

the provenance of the gemstone. The amount of minor and

trace elements that are incorporated will depend on local

geologic conditions, such as temperature, redox conditions,

and, especially, chemistry (Rossman 2009).

It is also well-known that data about trace elements are

essential for modeling processes such as crystal fractionation,

assimilation, or hydrothermal reworking. In particular, small

differences in element partitioning within minerals allow for

the identification of different geological processes. For

instance, during fractionation processes (London 1986, 1992;

Evensen and London 2002), Li and Be are routinely used as

tracers for magmatic processes (Seitz and Woodland 2000).

Li, Be and B are also considered as important geochemical

tracers for fluids (Henley et al. 1984; Fabre et al. 2002;

Hinsberg et al. 2011). The presence of trace and light litho-

phile elements in gemstones can also be used to discriminate

between natural and synthetic samples, as well as to obtain

information on the methodology of synthesis (Schmetzer

1989, 2008; Shigley and McClure 2009).

In this context, the use of different analytical techniques to

characterize and identify the minerals through chemical

analysis plays a crucial role. The minor and trace elements

(including Li, Be and B) are usually determined using dif-

ferent conventional analytical (destructive and micro-

destructive) techniques, such as electron microprobe ana-

lysis–wavelength-dispersive spectrometry (EMPA-WDS),

laser-ablation inductively-coupled plasma mass spectrome-

try (LA-ICP-MS) or secondary ion mass spectrometry

(SIMS), but to obtain a complete quantitative analysis of all

the elements composing the minerals, in many cases the

contemporary use of more than one technique becomes

necessary. The complete characterization of the mineral

sample can then become time-consuming and expensive. A

simple tool for a fast elemental analysis and preliminary

classification of samples would be extremely useful for

planning further analytical strategies, as well as for sup-

porting crystallographic study. In this frame, laser-induced

breakdown spectroscopy (LIBS) can play an important role

(Beesley 2009; De Giacomo et al. 2012). LIBS is the optical

emission spectroscopy of the plasma induced by laser-matter

interaction (Cremers and Radziemski 2006; Miziolek et al.

2006; Hahn and Omenetto 2010). The advantages of LIBS,

from the analytical point of view, were demonstrated in

several fields, including cultural heritage (Giakoumaki et al.

2007; De Giacomo et al. 2008), soils (Capitelli et al. 2002;

Senesi et al. 2009; Dell’Aglio et al. 2011), rocks (Colao et al.

2004; Salle et al. 2006; Lazic et al. 2007; Tucker et al. 2010),

meteorites (Dell’Aglio et al. 2010), minerals (McMillan

et al. 2006; McManus et al. 2008; Death et al. 2009; Diaz

Pace et al. 2011; Breeding et al. 2011), and space exploration

(Knight et al. 2000; Whitehouse et al. 2001; Salle et al.

2005a; Maurice et al. 2012; Wiens et al. 2012). These

advantages are: no sample treatment required; almost com-

plete non-destructiveness (few tens of ng ablated per shot);

possibility of simultaneous determination of all the elements

(including light atoms); fast response and low-cost set-up;

detection limits in the order of or lower than ppm. Moreover,

when trace element quantification is not crucial, analytical

LIBS measurements can be performed without standards,

with the so-called calibration-free methods (Ciucci et al.

1999), which rely on the following two assumptions: (1) that

the laser-ablated material is in a plasma phase in near-equi-

librium conditions; and (2) that the plasma itself has the same

composition as the irradiated sample. The calibration-free

approaches do not enable the same accuracy of classical

methods with calibration curves, but they can provide

valuable preliminary and fast information about the com-

position when matrix-matched standards are not readily

available, as in the case of minerals.

In this work, LIBS in the calibration-free approach was

applied to crystals of alexandrite (BeAl2O4), hiddenite and

kunzite (LiAlSi2O6), elbaite (Na[Li,Al]3Al6[BO3]3-

Si6O18[OH]4) and topaz (Al2SiO4[F,OH]2), in order to test the

feasibility of the technique in mineralogy. LIBS results were

compared with those obtained with conventional character-

ization techniques, i.e. EMPA-WDS and LA-ICP-MS.

Experimental methods

Materials

All the samples are located at the collection of the Royal

Mineralogical Museum (RMM) from the Centro Musei

delle Scienze Naturali, University of Naples Federico II,

Naples (Italy). The alexandrite sample, a variety of

chrysoberyl, is reported in the RMM catalogue from 1885

(catalogue number: c.n. 17843-E6374) and comes from the

Tokowaya River mines in Ekatherinburg, in the Ural

mountains, a historical locality where the first ‘‘alexandrite

gemstone’’ was collected in 1833. As for the two varieties

of spodumenes, the hiddenite specimen (c.n. 24309) was

collected at Stony Point, North Carolina, USA, whereas the

kunzite (c.n. 2229) comes from Teofilo, Otono, Brazil. The

elbaite (c.n. 22255), the tourmaline group minerals

(XY3Z6[T6O18][BO3]3V3W), was collected in Minas Ge-

rais, Brasil. The topaz sample (c.n. C3693-9070) comes

from Aduncilon, Siberia.

Chemistry: conventional analytical techniques

Quantitative chemical analyses of major and minor ele-

ments were performed with an electron microprobe in

Phys Chem Minerals

123

wavelength dispersive mode (EPMA-WDS) Cameca SX50

(IGAG-CNR, Rome), operating at 15 kV and 15 nA with a

beam diameter 10 lm. For topaz, chrysoberyl, spodumene

and tourmaline samples, the following natural and syn-

thetic materials were used as standards: phlogopite for F

(TAP crystal; Ka); wollastonite for Ca (PET; Ka) and Si

(TAP; Ka); jadeite for Na (TAP; Ka); magnetite for Fe

(LIF; Ka); pericline for Mg (TAP; Ka); orthoclase for

K (PET; Ka); corundum for Al (TAP; Ka); Mn (metallic)

for Mn (LIF; Ka); rutile for Ti (PET; Ka). For alexandrite

and spodumene, Cr (metallic) for Cr (PET; Ka) was also

used. The data were corrected using the PAP program

(Pouchou and Pichoir 1991). Analytical measurements are

affected by a relative uncertainty of 1 % for major ele-

ments, and 5 % for minor elements. For hydrated samples,

the H2O concentration was calculated by the stoichiometry

balance. At least ten analysis points were measured for

each sample.

Quantitative chemical analyses of trace elements and Li,

B and Be were carried out at the Institute of Geosciences

and Georesources of CNR in Pavia by LA-ICP-MS, with a

magnetic sector double-focusing mass spectrometer

equipped with a plasma source-type ELEMENT I (Ther-

moFinnigan) and a laser microprobe developed at the

Memorial University of Newfoundland (Canada) with a

wavelength of 266 or 213 nm. The mass spectrometer is a

Model Element II ICP for torch (CD-2), magnet and

magnetic field controller (option ‘‘fast scanning’’). Tiepolo

et al. (2005) indicated that with a spot size of 40 lm, the

agreement between measured and reference values of Li,

Be and B is generally better than 10 % for NIST SRM 612

(utilized in this work). Tiepolo et al. (2003) demonstrated

that the ELEMENT sector-field mass spectrometer, cou-

pled with a 266-nm laser source, allows trace-element

determination on geological samples to a precision and

accuracy generally better than 10 %. Ten-point analyses

for trace elements and Li, B and Be were performed.

LIBS technique

LIBS experimental setup

The experimental setup (Fig. 1) consists of a pulsed

Nd:YAG (Quantasystem, PILS-GIANT) laser operating at

the third harmonic (355 nm), with a repetition rate of

10 Hz and a laser pulse duration of 6 ns. The laser was

focused on the targets with a 20-cm quartz lens. A 7.5-cm

biconvex quartz lens and an optical fiber were used to

collect the plasma emission and to focus a 1:1 image of the

plasma on the spectroscopic system. The latter comprises a

monochromator (Jobin–Yvon TRIAX 550) coupled with an

intensified charge-coupled device (ICCD) (i3000 Jobin–

Yvon) and a pulse generator (Standford DG 535) for the

triggering system of the emission spectra acquisition. A

computer with dedicated software controlled the time

acquisition parameters, i.e. the number of spectra acquisi-

tion, starting delay time, td, and gate width of ICCD

aperture with respect to laser pulse, tg (De Giacomo et al.

2012).

In this work, the emission spectra were acquired with

the following parameters: td = 300 ns, tg = 3 ls, pulse

energy, E = 12 mJ. The laser energy value was chosen so

as to obtain the best compromise between the smallest

possible laser-induced crater and emission spectra with the

best signal-to-noise ratio. An appropriate number of spec-

tral windows (each covering a spectral range of 16 nm)

were chosen based on the emission peaks of the elements to

analyze. Every emission spectrum was acquired after a

number of accumulations and averaged to optimize the

signal-to-noise ratio. The estimated laser spot diameter in

this experiment was around 100 lm and the crater depth

was on the order of a few hundreds of lm. The dimensions

of the analyzed gemstone slabs were in the range of

1–2 mm3 and they were kept fixed during the measure-

ments. In case of smaller samples, the laser beam dimen-

sion can be properly decreased by adjusting the focal

length of the lens or using microscope optics. The conse-

quential irradiance increase can be compensated by

decreasing the laser energy, so as to avoid significant

sample damage.

Theoretical background

When a nanosecond laser pulse is focused on a sample, if

the irradiance is beyond a certain threshold (in the order of

109 W/cm2), a plasma can be induced at the target surface,

which holds the stoichiometry of the original sample. The

ablated material is completely atomized and ionized, and

Fig. 1 LIBS experimental setup

Phys Chem Minerals

123

through the optical emission spectroscopy of laser-induced

plasma (LIP) it is possible to quantify the concentration of

elements contained in the sample (without the element spe-

ciation). Different analytical methodologies can be applied

to retrieve chemical composition from LIP emission spectra:

(1) the classical calibration curve method using matrix-

matched standards (Salle et al. 2005b); (2) the calibration-

free method based on the local thermodynamic equilibrium

(LTE) assumption, which does not require the use of any

standards (Tognoni et al. 2010); and (3) multivariate analy-

ses (Clegg et al. 2009). In this work, a calibration-free

method was applied and, since no preliminary sample

treatment was required and all the data were collected by a

single measurement, it was possible to perform a simulta-

neous multi-elemental quantitative analysis. LIBS analytical

methodology and principles have been widely discussed in

several papers (Cremers and Radziemski 2006; Hahn and

Omenetto 2010; Salle et al. 2006) and in particular, the

analytical procedure used in this work was discussed in De

Giacomo et al. (2007) and in Dell’Aglio et al (2010).

Whereas LIBS can be really accurate when calibration

curves are used, in this paper a calibration-free method was

selected in order to eliminate the need of matrix-matched

standards, as well as to establish a fast and general pro-

cedure for supporting mineralogical analysis. The calibra-

tion-free methodology was first proposed by Ciucci et al.

(1999) and then adapted for different applications by sev-

eral authors. The basic equations are briefly reported in the

following.

After the emission spectra acquisition, elements of the

sample can be qualitatively identified (by the use of atomic

and ionic emission database) and all the intensities of the

selected emission lines (Table 1) are determined. Then, for

each measured emission line, the number density of atoms

N0,a is given by the following equation:

N0;a ¼Iul

4pGhmulAulgu

Z Tð ÞexpEu

kT

� �;

where T is the experimental temperature measured from

Boltzmann plot and all other terms have the usual mean-

ings as described in references by De Giacomo et al. (2007)

and Dell’Aglio et al (2010).

The obtained average values of the number density of

atoms of each element are inserted in the Saha equation

(De Giacomo et al. 2007):

N0;i

N0;a¼ 2

Zi Tð ÞZa Tð ÞN

�1e

mekT

2ph�2

� �3=2

exp �EIon

kT

� �:

where Ne is the electron number density obtained with the

Stark broadening method, N0,i is the number density of

ions, and all other terms have the usual meaning (De

Giacomo et al. 2007). In this way, the ionized fraction of

elements in the plasma can be evaluated and added to N0,a

to determine the total relative element number density,

NiTOT = N0,a ? N0,i, of the species j present in the sample.

If the sample contains n elements, it is possible to deter-

mine NiTOT for each element and then, by applying a nor-

malization procedure, to retrieve the weight percentage of

each element present in the sample. In this normalization,

all the determined values of NjTOT are multiplied by their

atomic weight and the corresponding sum is forced to be

100 in order to get the weight percentage of each element.

As for the oxygen content, which cannot be directly mea-

sured when experiments are performed in air, in this work

it was calculated based on the stoichiometric relation in the

mineral oxide of the detected metal. This approach requires

that the mineral constituents of the sample are known.

However, based on the ratios of the number density of the

element under investigation to that of major constituents

(generally Al or Si), it is possible to also obtain the ele-

mental analysis (except oxygen) for unknown minerals.

Moreover, by operating in a controlled gas background

environment (i.e., a chamber filled with Ar or N2), it is

anyway possible to measure oxygen content with LIBS (see

for example Dell’Aglio et al. 2010).

For mineralogical applications, knowledge of the num-

ber of atoms of a given element is often required, and this

can be determined as well by the number density ratios

mentioned above. All this relies on the assumption of a

direct correlation existing between the measured emission

intensity and the number density of atoms and ions, which

holds only if the plasma is optically thin, i.e., if self-

absorption is negligible. The most intense lines in the

emission spectra are those involving low-energy levels, but

Table 1 Selected spectral lines of the investigated elements

Species Wavelength (nm)

Si I 250.69, 251.43, 251.61, 251.92, 252.41, 252.85, 288.16,

390.55

Al I 305.46, 305.71, 308.21, 309.27, 305.01, 305.90, 306.43,

306.61, 394.40, 396.15

Li I 610.354

B I 249.67, 249.77

Be II 313.04, 313.11

Ca I 422.67, 526.42,526.55

Fe I 370.10, 370.55, 370.92, 371.99, 372.25, 372.76, 373.04,

373.24, 373.33, 373.48, 373.71, 376.38

376.55, 376.72

Mn I 279.82, 280.11, 407.92, 408.29, 408.36

Mg I 285.21, 277.98, 516.73, 517.28, 518.36

Na I 588.99, 589.59, 568.26, 568.82

Ti I 398.97, 521.039

Cr I 520.45, 520.6, 520.84

V I 410.51, 410.97, 411.18, 411.51, 411.64

Phys Chem Minerals

123

Ta

ble

2L

IBS

rela

tiv

eer

rors

,as

wei

gh

tp

erce

nta

ge

(wt%

)o

fea

chel

emen

t,co

mp

ared

wit

hco

rres

po

nd

ent

EM

PA

-WD

San

dL

A-I

CP

-MS

val

ues

(C.T

.)

Ele

men

tsW

eig

ht

per

cen

tag

e(w

t%)

Ale

xan

dri

teH

idd

enit

eK

un

zite

Elb

aite

To

paz

C.T

.L

IBS

C.T

.L

IBS

C.T

.L

IBS

C.T

.L

IBS

C.T

.L

IBS

Si

0.0

30

±0

.00

23

0.1

±0

.33

0±

33

0.0

±0

.32

9±

31

8.1

±0

.21

9±

21

5.3

±0

.11

3±

1

Al

41

.5±

0.4

41

±6

14

.1±

0.1

15

±1

14

.6±

0.1

14

±1

20

.7±

0.2

22

±2

29

.5±

0.3

30

±3

Li

3.6

±0

.4a

2.2

±0

.33

.7±

0.4

a4

.6±

0.7

0.8

8±

0.0

9a

0.3

1±

0.0

6

B3

.0±

0.3

a3

.4±

0.5

Be

7.1

±0

.7a

7±

1

Ca

0.0

09

±0

.00

10

.00

20

±0

.00

01

0.0

01

7±

0.0

00

30

.00

30

±0

.00

02

n.r

.0

.17

2±

0.0

09

0.0

9±

0.0

1

K0

.00

6±

0.0

00

30

.00

70

±0

.00

04

n.d

.0

.00

70

±0

.00

04

n.d

.0

.01

3±

0.0

01

n.d

.0

.00

9±

0.0

01

n.d

.

Fe

1.1

4±

0.0

11

.1±

0.1

0.5

7±

0.0

30

.52

±0

.05

0.0

30

±0

.00

20

.09

6±

0.0

09

1.9

3±

0.0

21

.8±

0.2

0.0

35

±0

.00

20

.01

9±

0.0

02

Mn

0.0

05

0±

0.0

00

30

.01

3±

0.0

01

0.0

74

±0

.00

40

.09

1±

0.0

09

0.8

8±

0.0

40

.34

±0

.03

0.0

23

±0

.00

10

.01

9±

0.0

02

Mg

0.0

07

0±

0.0

00

40

.01

4±

0.0

01

0.0

03

0±

0.0

00

20

.01

2±

0.0

01

0.1

82

±0

.00

90

.22

±0

.03

0.0

05

0±

0.0

00

3n

.r.

Na

0.0

05

±0

.00

03

0.1

11

±0

.00

61

.0±

0.2

0.0

95

±0

.00

50

.14

±0

.02

1.6

8±

0.0

20

.64

±0

.09

0.0

21

±0

.00

1n

.d.

Ti

0.0

25

±0

.00

10

.01

1±

0.0

01

0.0

02

2±

0.0

00

30

.00

50

±0

.00

03

0.0

18

±0

.00

20

.04

0±

0.0

02

0.0

25

±0

.00

40

.01

5±

0.0

01

n.d

.

Cr

0.2

1±

0.0

10

.22

±0

.03

0.1

31

±0

.00

70

.34

±0

.03

V0

.02

3±

0.0

01

a0

.02

2±

0.0

02

Ocalc

50

±7

51

±1

51

±5

50

±5

42

±4

n.d

.=

no

td

etec

ted

;n

.r.

=n

ot

rev

eale

d;

aL

A-I

CP

-MS

dat

a

Phys Chem Minerals

123

they are in turn those most affected by self-absorption. This

observation clearly suggests that the selection of spectral

lines to be used for the analysis plays a crucial role. In this

work, some elements, such as Li and Na, were clearly

detected, but they could not be exactly quantified, because

most of their detectable peaks within the spectral range

covered by our spectrometer were affected by significant

self-absorption.

A final note about the employed calibration-free ana-

lytical method concerns the experimental uncertainty of the

measurements. The relative error on the calculated con-

centrations depends on the analyzed sample and on the

element under investigation, and generally it is around

10–15 %. Exhaustive discussions on relative error in LIBS

measurements with calibration-free methods are reported

by Tognoni et al. (2007) and Dell’Aglio et al (2010). It

should be noted that in the tables where results of the

analysis are reported, the experimental uncertainties of

conventional analytical techniques are expressed in terms

of standard deviation, which estimates the measurement

precision. Instead, since the number of LIBS replica does

not allow the use of standard deviation, in this case data are

given in terms of relative error (Table 2), which estimates

the measurement accuracy.

Results

Mineralogy1

The alexandrite specimen has a size of 3.5 9 2.5 9 2 cm

and is a 6-peak twin made up by three different interpen-

etrating twins, each clearly showing and the cross streaks.

The specimen’s color is deep green in daylight and deep

raspberry red in incandescent light. Besides, under UV

light (365 nm), it usually becomes transparent, while under

short UV frequencies (254 nm) it is opaque.

The mineral is also characterized by a strong pleoch-

roism: in particular, under incandescent light it shows a red

carmine color along the na optical direction, becoming

yellow along nb and green along nc. In daylight, it shows a

purple violet color along the na optical direction, becoming

pale yellow-green along nb and bluish-green along nc. In

the crystals, twinning planes are visible, as well as primary

and secondary fluid and solid inclusions with micrometric

sizes. On the other hand, some iron oxide impurities were

detected very rarely. Spodumene: the hiddenite sample has

a size of 1.5 9 0.5 9 0.7 cm, is transparent and is emerald

green in color. The crystal habit is prismatic elongated on

c, flattened on (100) with deep vertical striations. The

mineral is also characterized by pleochroism, showing a

green color along the na optical direction and pale yellow

along nc. The hiddenite presents a deep green color under

short UV frequencies (254 nm). The crystal also presents

fluid and solid inclusions.

The kunzite sample is transparent and shows a pale pink

color. This crystal is characterized by habit flattened on

(100) and by the presence of a very pronounced cleavage

plane and fracture. It is weakly pleochroic and has a size of

13 9 2.5 9 4 cm.

The elbaite sample is translucent and shows a dark green

color. This crystal is predominantly prismatic and the prism

Table 3 Conventional (EMPA-WDS, LA-ICP-MS) and LIBS anal-

yses carried out on alexandrite sample

Oxides

wt%

EMPA-

WDS

Avg

LIBS

1

LIBS

2

Elements

ppm

LA-ICP-

MSb

Avg

SiO2 0.064 (25) – Li 1.9 (3)

TiO2 0.041 (5) – B 64 (5)

Al2O3 78.39 (25) 77.47 Mg 38 (5)

Cr2O3 0.299 (78) 0.322 Sc 2.4 (2)

FeO 1.47 (10) 1.41 V 79 (3)

BeO 19.70 (20)a 19.43 Mn 1.2 (0)

CaO 0.012 (12) – Co 0.1 (1)

Na2O 0.007 (8) – Ni 2.2 (9)

K2O 0.007 (5) – Zn 2.8 (4)

Tot. 99.995 98.632 Rb 0.1 (0)

Number of ions on the basis of 4 O Zr 0.1 (0)

Si 0.001 – – Nb 0.3 (2)

Al 1.969 1.972 2.000 Ba 0.1 (0)

Cr 0.011 0.012 0.006 Nd 0.1 (0)

Ti 0.001 – – Sm 0.1 (0)

Fe2? 0.026 0.026 0.027 Yb 0.1 (0)

Ca 0.000 – – Hf 0.1 (1)

Na 0.000 – – Ta 4 (1)

K 0.000 – – Pb 0.1 (0)

Be 1.009 1.008 1.060

Octahedral

site

2.008 2.010 2.032

Berillium

site

1.009 1.008 1.060

Standard deviations are given in parentheses. Avg is the meana LA-ICP-MS datab Sr, Y, Cs, La, Ce, Pr, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Th,

U \ 0.1 ppm

1 All the samples were previously subjected to single crystal X-ray

structure investigation: details can be obtained from the Fachinforma-

tionszentrum Karlsruhe (crysdata@fiz-karlsruhe.de), 76344 Eggen-

stein-Leopoldshafen, Germany, (fax: (49) 7247-808-666; e-mail:

crysdata@fiz-karlsruhe.de) by quoting the depository numbers CSD-

426602 (alexandrite), -426603 (hiddenite), -426604 (kunzite),

-426605 (elbaite) and -426606 (topaz). The list of Fo/Fc data is

available from the authors up to 1 year after the publication

has appeared.

Phys Chem Minerals

123

faces are grooved parallel to the c axis. Its dimensions are

5 9 3 cm. The topaz sample is transparent and light green

to pale yellow-colored. The crystal shows short prismatic

morphology and has a size of 2 9 1 9 1 cm. Under UV

light (365 nm), it is yellow.

Chemistry: conventional analytical techniques

Quantitative chemical analysis (Table 3) of major and

minor elements in alexandrite shows that the average

content of Be is 1.009 atoms per formula unit (apfu), while

Fe2? and Cr3? are, respectively, 0.026 and 0.011 apfu; Ti,

V, and B are detected in traces.

The chemical composition of hiddenite (Table 4) shows

no replacement of Si by Al, and minor substitution of Al by

Fe3? and Cr3?, with tenors, respectively, of 0.0015 and 0.005

apfu, while lower amounts of Fe2?, Na and Mg replace Li.

Moreover, we detected trace elements like V, ranging from

192 up to 248 ppm, and Ti, ranging from 41 up to 60 ppm.

The chemical composition of kunzite (Table 5) shows

very limited replacement of Si by Al (0.006 apfu), and no

replacement of Al by Fe3? and Cr3?. Na, Mn and (minor)

Fe2? replace Li.

The chemical composition of elbaite (Table 6) shows Si

in the tetrahedral site (T site) and B in the B site. As for the

two octahedral sites, we found Al at the Z site and Al, Li,

Fe2?, Mn, and Mg at the Y site. Na, Ca, and K are located

at a [9]-coordinated X-site, with respective concentrations

of 0.701, 0.041 and 0.003 apfu. OH and F are located at the

V and W sites. The trace elements are Zn and Pb, their

content in tourmaline range respectively between 276 and

999 ppm, and between 165 and 233.

Table 4 Conventional (EMPA-

WDS, LA-ICP-MS) and LIBS

analyses carried out on the

hiddenite sample

Standard deviations are given in

parentheses. Avg is the meana Fe3? and Fe2? were

calculated on the basis of the

occupancy of siteb LA-ICP-MS datac Rb, Y, Nb, Cs, Eu, Zr, Gd, Tb,

Dy, Ho, Er, Tm, Yb, Lu, Ta,

Hf < 0.1 ppm

Oxides

wt%

EMPA-WDS

Avg

LIBS 1 LIBS 2 Elements

ppm

LA-ICP-MSc

Avg

SiO2 64.47 (22) 65.18 Be 1.6 (9)

TiO2 0.019 (13) 0.004 B 2.0 (6)

Al2O3 26.74 (16) 28.34 Sc 50 (8)

Cr2O3 0.191 (18) 0.497 V 228 (24)

FeO 0.74 (10) 0.67 Co 0.1 (0.1)

MnO 0.006 (5) 0.017 Ni 0.5 (2)

MgO 0.011 (11) 0.023 Zn 3 (1)

CaO 0.003 (5) 0.002 Sr 0.5 (4)

Na2O 0.150 (31) 1.348 Ba 0.1 (0)

K2O 0.009 (8) – La 0.3 (2)

Li2O 7.79 (30)b 4.74 Ce 0.5 (6)

F 0.005 (10) – Pr 0.1 (1)

(F�Cl)/O 0.002 – Nd 0.2 (2)

Tot. 100.134 100.821 Sm 0.1 (1)

Number of ions on the basis of 6 O Pb 0.1 (0)

Si 2.005 2.021 2.000 Th 0.1 (1)IVAl – – – U 0.1 (1)VIAl 0.980 1.052 1.011

Cr 0.005 0.012 0.011

Fe3?a 0.015 – –

Ti 0.000 0.000 0.000

Fe2?a 0.005 0.018 0.017

Mg 0.001 0.001 0.002

Ca 0.000 0.000 0.000

Na 0.009 0.082 0.082

K 0.000 – –

Li 0.974 0.600 0.589

Mn 0.000 0.000 0.000

F 0.001 – –

Tetrahedralsite 2.005 2.021 2.000

Octahedralsite 1.000 1.065 1.022

Litiumsites 0.990 0.702 0.690

Phys Chem Minerals

123

The composition of topaz is characterized by the presence

of Na, Fe and Mn with, respectively, 0.002, 0.001 and 0.001

apfu, and minor amounts of Ti, Mg and K (Table 7).

LIBS analyses

In this work, the results of the LIBS analyses were pro-

cessed with the procedures described in the following, in

order to obtain apfu data (Tables 3, 4, 5, 6, and 7): first, the

weight percentage (wt%) of each single element was

changed into oxide wt%, and hence the oxide wt% was

Table 5 Conventional (EMPA-WDS, LA-ICP-MS) and LIBS anal-

yses carried out on the kunzite sample

Oxides

wt%

EMPA-

WDS

Avg

LIBS

1

LIBS

2

Elements

ppm

LA-ICP-

MSc

Avg

SiO2 64.25 (42) 62.04 Be 2.0 (5)

TiO2 0.008 (12) 0.030 B 12 (2)

Al2O3 27.54 (11) 26.47 Sc 1.6 (2)

Cr2O3 – – V 0.0 (1)

FeO 0.039 (40) 0.124 Cr 4 (3)

MnO 0.096 (42) 0.117 Co 0.1 (0)

MgO 0.005 (5) 0.020 Ni 0.3 (1)

CaO 0.004 (6) – Zn 6 (4)

Na2O 0.128 (24) 0.189 Sr 0.4 (4)

K2O 0.008 (14) – Zr 0.2 (0)

Li2O 7.95 (24)b 9.90 Ba 0.1 (0)

F 0.019 (37) – La 0.6 (9)

(F�Cl)/O 0.008 – Ce 1 (2)

Tot. 100.039 98.890 Pr 0.5 (4)

Number of ions on the basis of 6 O Nd 0.1 (1)

Si 1.994 1.946 2.000 Sm 0.1 (0)IVAl 0.006 0.054 – Hf 0.1 (0)VIAl 1.001 0.925 1.009 Pb 0.1 (0)

Cr – – –

Fe3?a – – –

Ti 0.000 0.001 0.001

Fe2?a 0.001 0.003 0.003

Mg 0.000 0.001 0.001

Ca 0.000 0.000 0.000

Na 0.008 0.011 0.012

K 0.000 0.000 0.000

Li 0.985 1.249 1.272

Mn 0.003 0.003 0.004

F 0.002 – –

Tetrahedralsite 2 2 2

Octahedralsite 1.001 0.925 1.000

Litiumsites 0.999 1.269 1.292

Standard deviations are given in parentheses. Avg is the meana Fe3? and Fe2? were calculated on the basis of the occupancy of the

siteb LA-ICP-MS datac Rb, Y, Nb, Cs, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ta, Th,

U < 0.1 ppm

Table 6 Conventional (EMPA-WDS, LA-ICP-MS) and LIBS anal-

yses carried out on the tourmaline sample

Oxides

wt%

EMPA-

WDS

Avg

LIBS 1 LIBS

2

Elements

ppm

LA-ICP-

MSd

Avg

SiO2 38.78 (46) 41.93 Be 4 (2)

TiO2 0.067 (15) 0.042 Sc 2.7 (9)

Al2O3 39.20 (57) 41.57 V 4 (1)

B2O3 9.67 (58)c 10.95 Cr 2.5 (7)

FeO 2.490 (77) 2.316 Co 0.9 (7)

MnO 1.132 (63) 0.439 Ni 0.6 (7)

MgO 0.301 (17) 0.365 Zn 433 (279)

CaO 0.241 (18) 0.126 Sr 15 (6)

Na2O 2.266 (34) 0.863 Nb 5 (2)

K2O 0.016 (11) n.d Ba 0.1 (0)

Li2O 1.90 (20)c 0.67 La 0.3 (1)

F 0.89 (18) 0.89b Ce 0.4 (2)

H2O? 3.335 (97)a 3.525a Nd 0.1 (0)

(F�Cl)/O 0.375 0.375 Sm 0.1 (0)

Tot. 99.908 103.311 Ta 3 (1)

Number of ions on the basis of 31

O

Pb 186 (27)

Si 6.187 6.368 6.000 Th 0.1 (0)

B 2.662 2.871 2.689IVAl 6.000 6.000 6.000VIAl 1.372 1.442 1.056

Ti 0.008 0.005 0.005

Fe2? 0.332 0.294 0.278

Mg 0.072 0.083 0.080

Ca 0.041 0.021 0.021

Na 0.701 0.254 0.239

K 0.003 – –

Li 1.217 0.407 0.390

Mn 0.153 0.056 0.053

F 0.450 0.428 –

OH 3.550 3.572 –

Tsite 6.187 6.368 6.000

X ? Ysites 3.899 2.562 2.117

Zsite 6.000 6.000 6.000

B 2.662 2.871 2.689

W ? Vsites 4.000 4.000 –

Standard deviations are given in parentheses, Avg is the meana H2O calculated on basis of the occupancy of the V and W siteb F imposed by EMPA-WDS analysesc LA-ICP-MS datad Rb, Y, Zr, Cs, Pr, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf,

U < 0.1 ppm

Phys Chem Minerals

123

routinely processed to obtain the apfu amount. Results

obtained with this procedure are shown in the tables under

the column heading ‘‘LIBS 1’’. Second, the number density

ratios of each element NjTOT, to the number density of the

major element NxTOT (i.e., Al for alexandrite and Si for

other gemstones) were multiplied by the theoretical stoi-

chiometry (X) of the major element constituent, as pro-

posed by De Giacomo et al (2012). These results are shown

in the tables under the column heading ‘‘LIBS 2’’.

For alexandrite, results for the Be content are 1,008 apfu

(LIBS 1) and 1.060 apfu (LIBS 2), while those of Fe2? and

Cr3? are, respectively, 0.026 apfu (LIBS 1), 0.027 apfu

(LIBS 2), and 0.012 apfu (LIBS 1), 0.006 apfu (LIBS 2).

The chemical composition of hiddenite shows no

replacement of Si by Al, and minor substitution of Al by

Cr3?, whose amount is 0.012 apfu (LIBS 1), 0.011 apfu

(LIBS 2). Moreover, there are low amounts of Fe2?, Na and

Mg replacing Li. Some trace elements are also noteworthy,

such as V (220 ppm, LIBS 1), and Ti (22 ppm, LIBS 1).

The chemical composition of kunzite shows a non-

negligible replacement of Si by Al (0.054 apfu), and no

replacement of Al by Fe3? and Cr3?, while Na, Mn and

(minor) Fe2? replace Li.

The chemical composition of tourmaline shows, in the

Y site, Al and Li, whose contents are, respectively, 1.442

apfu (LIBS 1), 1.056 apfu (LIBS 2), and 0.407 apfu (LIBS

Table 7 Conventional (EMPA-

WDS, LA-ICP-MS) and LIBS

analyses carried out on the topaz

sample

Standard deviations are given in

parentheses, Avg is the meana H2O calculated on basis of the

occupancy of the F and OH siteb F imposed by EMPA-WDS

analysesc Lu \ 0.1 ppm

Oxides

wt%

EMPA-WDS

Avg

LIBS 1 LIBS 2 Elements

ppm

LA-ICP-MSc

Avg

SiO2 32.67 (27) 27.81 Li 4 (7)

TiO2 0.025 (23) – Be 9 (16)

Al2O3 55.79 (40) 56.68 B 26 (26)

FeO 0.045 (41) 0.024 Sc 4 (6)

MnO 0.030 (29) 0.025 V 6 (6)

MgO 0.009 (9) – Cr 120 (177)

CaO 0.011 (9) – Co 0.5 (6)

Na2O 0.028 (24) – Ni 4 (4)

K2O 0.011 (10) – Zn 18 (23)

F 14.64 (72) 14.64 (72)b Rb 0.3 (3)

H2O? 2.897a 2.441a Sr 65 (145)

(F�Cl)/O 6.163 6.163 Y 0.2 (2)

Tot. 99.999 95.456 Zr 0.2 (3)

Number of ions on the basis of 6 O Nb 0.3 (3)

Si 0.995 0.892 1.000 Cs 0.1 (2)IVAl 0.005 0.108 0.000 Ba 6 (12)VIAl 1.999 2.035 2.350 La 13 (29)

Ti 0.001 – – Ce 21 (41)

Fe2? 0.001 0.001 0.001 Pr 8 (12)

Mg 0.000 – – Nd 4 (9)

Ca 0.000 – – Sm 0.8 (9)

Na 0.002 – – Eu 0.1 (3)

K 0.000 – – Gd 0.2 (5)

Mn 0.001 0.001 0.007 Tb 0.1 (1)

F 1.410 1.485 – Dy 0.3 (4)

OH 0.589 0.515 – Ho 0.1 (1)

Tetrahedral site 1.000 1.000 1.000 Er 0.3 (4)

Octahedral site 2.004 2.037 2.358 Tm 0.1 (1)

F, OH 1.999 2.000 – Yb 0.5 (9)

Hf 0.1 (1)

Ta 0.1 (1)

Pb 3 (3)

Th 1 (3)

U 5 (9)

Phys Chem Minerals

123

1), 0.390 apfu (LIBS 2), and lesser amounts of Fe2?, Mg

and Mn (Table 6). In the X site, there are Na and Ca,

respectively with concentrations of 0.254 apfu (LIBS 1),

0.239 apfu (LIBS 2), and of 0.021 apfu (LIBS 1), 0.021

apfu (LIBS 2).

The composition of topaz is characterized by the pre-

sence of Mn and Fe, respectively with concentrations of

0.001 apfu (LIBS 1), 0.007 apfu (LIBS 2), and 0.001 apfu

(LIBS 1, LIBS 2).

Discussion

For the alexandrite sample, Cr3? and Fe3? are mainly

responsible for the chromatic change under irradiation with

different light sources (Scalvi et al. 2003). In particular,

Ahn et al. (2009) demonstrated that this ‘‘alexandrite

effect’’ (Weber et al. 2007) is due to Cr3? ions in the

crystal, which replace the Al3? ions within the Al2 sites.

The correct chemical composition of alexandrite is of

primary importance to distinguish the variety of chryso-

beryl under investigation, as well as for crystallographic

analyses. The LIBS 1 data for chromium are in good

agreement with WDS values, but LIBS 2 data show,

rounded to 3 decimals, half the Cr content of LIBS 1 data.

The correlation between WDS and LIBS data for the Fetot

amount is very good, as well as the one between LA-ICP-

MS and LIBS for the content of beryllium (Fig. 2).

The crystal chemical formulas of alexandrite are:

Al1:969Cr0:011Fe0:026ð ÞP2:006

Be1:009O4

WDSþ LA�ICP�MSð ÞAl1:972Cr0:012Fe0:026ð ÞP

2:010Be1:008O4 LIBS 1ð Þ

Al2:000Cr0:006Fe0:027ð ÞP2:033

Be1:060O4 LIBS 2ð Þ

:

Hiddenite and kunzite minerals are two varieties of

spodumene characterized by their color, due to the

presence of trace elements such as Mn, Ti, V, Cr and Fe.

In particular, the green color of hiddenite has been

attributed to the presence of small amounts of Cr3? and

Fe3? (Deer et al. 1997). Moreover, many studies show

that the presence of V3? with Cr3? can be associated

with green color in minerals. For example, small amounts

of V3? with some Cr3? turn grossular into the green

tsavorite variety (Rossman 2009) and beryl into the

emerald variety (Groat and Laurs 2009). The analytical

data are consistent with the literature, with WDS data

showing the presence of Cr3? and Fe3?, LA-ICP-MS

showing the small amount of V3?, and LIBS showing

Cr3? and V3?. Moreover, there is a very good correlation

between the V content found with LA-ICP-MS and LIBS

(Fig. 3).

The crystal chemical formulas are reported below:

Li0:974Na0:009Fe2þ0:005

� �P0:988

Al0:980Fe3þ0:015Cr3þ

0:005

� �P1:000

Si2:005O6½ � WDSþ LA�ICP�MSð ÞLi0:600Na0:082Fe2þ

0:018

� �P0:700

Al1:052Cr3þ0:012

� �P1:064

Si2:021O6½ � LIBS 1ð ÞLi0:589Na0:082Fe2þ

0:017

� �P0:688

Al1:011Cr3þ0:011

� �P1:022

Si2:000O6½ � LIBS 2ð Þ

:

As for kunzite, its color is due to a relatively high

concentration of manganese (Deer et al. 1997), and

especially to a low Fe:Mn ratio (Claffy 1953). Moreover,

many studies on the color of gemstones report that low

manganese content leads to a pink color in the mineral

(Rossman 2009; Pezzotta and Laurs 2011). The

investigated kunzite samples are characterized by a pale

pink color, and chemical analyses (WDS and LIBS) are

consistent with this observation, revealing low amounts of

Mn (Fig. 4).

The crystal chemical formulas are:

Fig. 2 Alexandrite

(A) composition (apfu) by

EMPA-WDS and LIBS

techniques

Phys Chem Minerals

123

Li0:985Na0:008Mn0:003ð ÞP0:996

Al1:001 Si1:994Al0:006O6½ �

WDSþ LA�ICP�MSð ÞLi1:249Na0:011Mn0:003ð ÞP

1:263Al0:925 Si1:946Al0:054O6½ �

LIBS 1ð ÞLi1:272Na0:012Mn0:004ð ÞP

1:288Al1:009 Si2:00O6½ � LIBS 2ð Þ:

Tourmaline is the sample with the most complex crystal

chemistry of the set studied in this work, and its LIBS

analysis can be considered representative of a general

application for the identification of minerals containing

light lithophile elements. Indeed, tourmaline contains

several elements that can occupy a variety of sites in the

mineral structure. According to the chemical classification

Fig. 3 Hiddenite

(H) composition (apfu) by

EMPA-WDS and LIBS

techniques

Fig. 4 Kunzite

(K) composition (apfu) by

EMPA-WDS and LIBS

techniques

Fig. 5 Tourmaline (Tur)

composition (apfu) by EMPA-

WDS and LIBS techniques

Phys Chem Minerals

123

proposed by Henry et al. (2011), and based on WDS and

LA-ICP-MS analyses, the analyzed sample is classified as

an elbaite. This result is confirmed by cell parameters and

by c/a ratio, according to Deer et al (1997). According to

Pezzotta and Laurs (2011), the green color of tourmaline

(included elbaite) is due to Fe2?, Fe3? and to Fe2?-Ti4?

IVTC, whereas the black color is due to high

concentrations of Fe2?, Mn2? and/or Ti4?. The elbaite

analyzed in this work is characterized by a dark green color

and, consistently, the chemical composition shows the

presence of Fe2?, Mn2? and very small amount of Ti4?. The

correlation between WDS ? LA-ICP-MS and LIBS data

for the tourmaline is highly variable, due to the complex

crystal chemistry. In the LIBS 1 data, the F content was

imposed based on WDS analyses and the H2O? content was

established by imposing V3 ? W = 4 (Henry et al. 2011).

The B content obtained with LIBS is in good agreement

with that obtained with LA-ICP-MS (Fig. 5).

The crystal chemical formulas are:

Ca0:041Na0:701ð ÞP0:742

Li1:217Al1:372Fe2þ0:332Mg0:072Ti0:008Mn0:153

� �P3:154

Al6

Si6:187O18ð Þ BO3ð Þ2:662 F0:450OH3:550ð Þ4WDSþ LA�ICP�MSð Þ

Ca0:021Na0:254ð ÞP0:275

Li0:407Al1:442Fe2þ0:294Mg0:083Ti0:005Mn0:056

� �P2:287

Al6

Si6:368O18ð Þ BO3ð Þ2:871 F0:428;OH3:572ð Þ4 LIBS 1ð ÞCa0:021Na0:239ð ÞP

0:260

Li0:390Al1:056Fe2þ0:278Mg0:080Mn0:053

� �P1:857

Al6

Si6O18ð Þ BO3ð Þ2:689 F0:450OH3:550ð Þ4 LIBS 2ð Þ:

The topaz sample is characterized by pale yellow and

pale green colors, and the chemical analyses show the

presence of small amounts of Fe and Mn (Fig. 6). As in the

previous case, after careful evaluation of chemical data

obtained by WDS analyses and by bibliography review

(Deer et al. 1997), in the LIBS 1 procedure we assumed the

F content obtained by the WDS and imposed F ? OH = 2

to establish the H2O? content. In this case, though the

concentration of F in topaz is very high, LIBS 1 is in good

agreement with the WDS data.

Conclusions

In this work, alexandrite, hiddenite, kunzite, elbaite and

topaz were characterized with different conventional ana-

lytical techniques (i.e., EMPA-WDS, LA-ICP-MS) and

with LIBS in the calibration-free mode. In particular, this

study demonstrated that LIBS can be employed in miner-

alogical studies, for minerals with simple and complex

crystal chemistries. The results showed a good agreement

between LIBS, EMPA, and LA-ICP-MS data for major

elements such as Si, Al, Be and B. Promising results were

also achieved for chromophore elements, i.e., Fe, Cr, V,

and Mn. On the other hand, the concentrations found by

LIBS for Li and Na did not agree with those found via

conventional analysis methods, due to self-absorption of

the emission lines of these elements. Thus, further work is

planned to improve the technique on elements whose

quantitative analysis is still not feasible, and to perform the

technique on other minerals. Moreover, using a portable

LIBS instrument, valid and fast in situ analyses could be

performed, which would be particularly important for

outcrop minerals or gemstones located in museums, but

would also represent a suitable choice for applicative lab-

oratories, such as those with a gemological focus.

Acknowledgments We would like to thank M. Serracino (IGAG

CNR, Rome, Italy) and M. Tiepolo (IGG CNR, Pavia, Italy) for their

skillful technical assistance during, respectively, WDS and LA-IPC-

MS analyses, and G. Chita (IC-CNR, Bari, Italy) for preliminary

Fig. 6 Topaz (Tpz)

composition (apfu) by EMPA-

WDS and LIBS techniques

Phys Chem Minerals

123

X-ray data collections. Finally, we would like to gratefully

acknowledge the editor M. Rieder, S. Mills and another anonymous

reviewer for their careful revision of the manuscript.

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