raman tremolite actinolite

Danilo Bersani1, Sergio Andò2, Paolo Gentile2, Laura Scrocco1 and Pier Paolo Lottici1

1University of Parma, Department of Physics and Earth Sciences , Parco Area delle Scienze 7/a, 43124 Parma (Italy)2University of Milano-Bicocca, Department of Geological Sciences and Geotechnologies, Piazzale della Scienza 4, 20126 Milano (Italy)

The aim of the work is to find a non-destructive method for the identification of the composition of nephritic jades Ca2(Mg2+,Fe2+)5Si8O22(OH)2

based on Raman spectroscopy.

We started from the fundamental work on the cation distribution on amphiboles of Wang et al. [3] and from the application of Raman micro-spectroscopy on the study of jades proposed by Chen et al. [2] to develop a quick method for the identification of nephrite using both low-wavenumber and high wavenumber parts of the spectrum.

In this work we analyzed by micro-Raman spectroscopy twenty samples of minerals in the tremolite-actinolite series, coming from different localities in the Alps, to study the relationship between the Raman spectra and the composition, obtained by SEM-EDXS, and the influence of different cations (Table 1).

The ratio X= Mg2+ /(Mg2+ +Fe2+) discriminates the members of the series: X≥0.9 = tremolite0.5<X<0.9 = actinolite X<0.5 = ferro-actinolite.

Raman spectra were then measured on the same spots used for SEM-EDS analysis and a correlation between the X ratio with the Raman peak positions was obtained. The increase of X = Mg2+/(Mg2++Fe2+) corresponds to a linear increase of the main peak wavenumber.

Raman spectra at high wavenumbers (symmetric OH stretching) show:

a single peak (A) ~ 3674 cm-1 for substitution MgMgMg, a second peak (B) ~ 3657 cm-1 for substotution MgMgFe or MgFeMg, a third peak (C) ~ 3646 cm-1 for substitution FeFeMg or FeMgFe a fourth peak (D) ~3620 cm-1 for substitution FeFeFe.

This behavior is due to the fact that in nephrites the OH group vibrations are influenced by the population of 3 metal sites where Fe2+ or Mg2+ could be present [2].

Campolongo, Switzerland

From a simple statistical model based on the population of the 3 sites, we obtain another relation useful to estimate the X value from the Raman spectrum: X=(A12)/(1/3+A12), where A12 is the ratio between the areas of A

and B Raman bands, often the most visible ones.In this way we obtained 2 simple and independent methods to evaluate the composition of nephritic minerals from their Raman spectrum.

1. 2.

3.

Table 1: analyzed samples, position of the main Raman peak and X=Mg/(Mg+ +Fe2+) ratio obtained by SEM-EDXS.

Micro-photo of a zoned actinolite crystal: the green colour, increasing from left to right, is due to Fe ions.

Study of the composition of amphiboles in the tremolite-actinolite series by micro-Raman

and SEM-EDXS

 N. Wavenumber (cm-1)

Mg2+ /(Mg2+ +Fe2+) (from EDXS)

#1 671,41 0,81

#2 672,94 0,88

#3 672,95 0,87

#4 672,47 0,83

#5 669,30 0,76

#6 671,41 0,76

#7 672,76 0,85

#8 673,85 0,94

#9 674,05 0,96

#10 671,60 0,89

#11 673,50 0,94

#12 673,04 0,92

#13 673,87 0,95

#14 672,68 0,87

#16 674,75 1,00

#17 665,64 0,44

#18 672,54 0,85

#19 669,50 0,67

#20 671,94 0,85

#21 674,23 0,99

1-7 16-218-15

Malenco Valley, Italy

0

100

200

300

400

500

600

Inten

sity (

cnt)

200 300 400 500 600 700 800 900 1 000Raman Shift (cm-1)

751.1

179.2 394.5

674.4

1030

.2 223.8

233.5

251.3

370.1

416.4

351.4 33

3.0

1071

.5 10

61.3

931.9

0.0

0

100

200

300

400

500

600

700

Inten

sity (

cnt)

200 300 400 500 600 700 800 900 1 000 1 100Raman Shift (cm-1)

436.1

223.5

514.7

671.8

1060

.0

174.7

156.9

349.1

371.1

391.5

413.3

742.1

929.6

1059

.8 1028

.4

947.5

OH

12

3

We verified the shift at low wavenumbers of the main peak with the increase of iron, performing a micrometric Raman map on a zoned needle-shaped actinolite crystal coming from Alpe Rosso (Vigezzo Valley, Italy).

References1.Rong Wang and Wei-Shan Zhang, Application of Raman spectroscopy in the nondestructive analyses of ancient Chinese jades, J. Raman Spectrosc. 42, 1324 (2011)2.T.-H. Chen, T. Calligaro, S. Pagès-Camagna, M. Menu, Investigation of Chinese archaic jade by PIXE and µ-Raman spectrometry, Appl. Phys. A 79, 177 (2004)3.A. Wang, P. Dhamelincourt, G. Turrell, Raman Microspectroscopic Study of the Cation Distribution in Amphiboles, Appl. Spectrosc. 42, 1441 (1988)

Map of the position of the Peak 1: light = higher wavenumber (Fe-poor) dark = lower wavenumber (Fe-rich)

Wavenumber (cm-1)

X r

atio

(fr

om S

EM

-ED

XS

)

X r

atio

(fr

om R

aman

– O

H b

ands

)

X ratio (from SEM-EDXS)

Correlation between X= Mg2+ /(Mg2+ +Fe2+) and the wavenumber of the main Raman peak.

Comparison between the values of X= Mg2+ /(Mg2+

+Fe2+) measured by SEM-EDXS and estimated by Raman.