Spectrochimica Acta Part A 73 (2009) 456–459
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Surface-enhanced Raman spectroscopy for identifying rock composition
Maurizio Muniz-Miranda
a,∗
, Cristina Gellini
a
, Luca Bindi
b
a
Dipartimento di Chimica, Università di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino, Italy
b
Museo di Storia Naturale, Sezione di Mineralogia, Università di Firenze, via La Pira 4, 50121 Firenze, Italy
article info
Article history:
Received 5 May 2008
Accepted 18 December 2008
Keywords:
SERS
Rock composition
Minerals
abstract
Since long time Raman scattering revealed to be a very sensitive and easy technique to deal with the
detection of mineral compounds, due to their vibrational fingerprint. However, presence of small inclu-
sions or identification of grains with very small dimensions in rocks may still remain problematic for
normal Raman detection apparatus. In this work, we propose to apply the SERS (surface-enhancement
Raman scattering) effect to micro-Raman measurements for the identification and characterization of
pyroxene grains embedded in polymorphous rock matrices.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Spectroscopic methods provide fundamental information in the
mineralogical analysis on rocks and soils. Actually, these investiga-
tions are expected to allow identifying: (a) the different minerals
constituting the rock matrix; (b) the chemical and structural com-
position of these minerals. All these results can be obtained by
means of the Raman spectroscopy [1–4], which provides a detailed
investigation on lattice and molecular vibrations, along with the
possibility to detect inorganic and organic impurities. Among many
other spectroscopic techniques, the Raman scattering presents the
additional advantage of examining the samples in a non-destructive
way, preserving their structure and morphology, by using a simple
measuring apparatus which does require neither vacuum-chamber
sample holder nor particular manipulation of the sample.
The development of laser Raman microprobes has also allowed
in situ analysis of mineralogical phases inside rock matrices, with
dimensions smaller than 1 m. In such a way the presence of tiny
inclusions or grains could be easily detected [1–4]. The Raman tech-
nique, however, has some limitations due to two main factors: first,
the low sensitivity owing to the scarce scattering cross-section of
the analyte; for this reason the signal-to-noise ratio is often inad-
equate for detecting molecules in low concentrations. The second
limiting factor is the fluorescence emission induced by the laser
excitation, which could be much more intense than the Raman
scattering and hide relevant spectral features.
These problems can be successfully overcome by the use of
the SERS (surface-enhanced Raman scattering) spectroscopy [5–7],
which is commonly adopted in trace analysis of compounds with
∗
Corresponding author. Tel.: +39 0554573091; fax: +39 0554573077.
E-mail address: muniz@unifi.it (M. Muniz-Miranda).
low detection limits. Actually, molecules located in proximity of
metallic nanoparticles, suchasAg, Auor Cu atomic clusters, undergo
huge Raman enhancements, along with a drastic quenching of flu-
orescence.
In this work we propose the use of the SERS effect obtained with
a Raman micro spectrometer in order to identify and characterize
very small mineral grains embedded in polymorphous rock matri-
ces. As a test to verify the sensitivity of this procedure, pyroxene
crystals have been chosen as the mineral to be detected in the poly-
morphous rock, by considering the importance of pyroxene group
as rock-forming mineral on Earth as well as on other planets. In
fact, it is well known that determining the composition and struc-
ture of pyroxenes in rocks is very relevant for understanding their
petrogenesis [8].
2. Experimental
2.1. Sample characterization
The rock sample used in this study was collected at the Monte
Vulture Volcano, the easternmost volcanic district of the Roman
Comagmatic Province. The volcano lies over upper Triassic to
Pliocene sedimentary terrains belonging to the Irpinian, Ariano
and Lagonegro sedimentary Units. It is close to the buried front
marking the overthrust of Southern Appennine nappes on the Apu-
lian foreland, in correspondence with the western border of the
Adriatic–Bradanic foretrough [9].
The rock sample is a basanite, a silica undersaturated rock, char-
acterized by higher volatile concentrations and a lower K
2
O/Na
2
O
value with respect to typical Roman High Potassium Series (HKS)
rocks. Leucite and haüyne are the dominant undersaturated phases.
The rock exhibits a seriate porphyritic texture with phenocrysts of
high-Mg olivine, enclosing minute euhedral chrome spinel inclu-
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.12.044