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Introduction

The aim of the study: To identify the complex mineral composition of a siliceous nodular chert from Rona (Salaj distr., Romania), polished surface (5 x 7 cm). The studied sample is of gemological value.

Selection of the analytical technique: Micro-Raman spectroscopy was employed as a handy, non-destructive technique in order to monitor the characteristic vibrational structure of this coloured natural SiO₂ (quartz) variety, which represents a heterogeneous mixture of microcrystalline to nanocrystalline mineral phases. The conventional, bulk mineralogical methods (XRD, optical microscopy) cannot be used due to their lower phase resolution.

Experimental

A micro-Raman set-up of the Dylor LabRam integrated system with the 514.5 nm excitation line (200 mW) from an Ar ion laser was employed for the measurements. The geological sample was placed on the microscopic plate for optical microscopy and Raman alignment. Ten overlaps /2 times were set for the spectra collection. A 10x microscope objective was used. A Peltier cooled CCD detection was performed with a resolution of 2 cm-1.

Using Origin for graphing and data analysis

Origin software was selected for illustrating the acquisition of Raman data due to its versatility in plotting various value ranges of the data sets and in comparing several sets of measured data. Origin is also useful for comparing our own measured data with reference data from international Raman databases (such as RRUFF) for fingerprinting the specific bands of each phase.

We have imported the measured data as single or multiple ASCII files (*.txt), then we used the “Plot: Line” function for representing the spectra. The spectral range can be flexibly chosen. In the present work the 200-1200 cm-1 range was used for the Wavelength scale (x, abscissa) from the whole range of measured data (100-3800 cm-1) due to the presence of significant phase bands in this interval. Additionally, performing arithmetic operations, the spectra could be arranged in a convenient manner. Some weak signal intensity data could be multiplied; others, too high, could be divided by an appropriate factor, in order to arrange as many spectra as relevant within a single graph.

Relevant peak positions can be also plotted individually and additional text editing is possible within the graph, due to the flexible menu bar of Origin.

Results

The Raman signal from different regions of the sample as depicted in Fig. 1 (normal photography of the sample) has been collected. Selected spectra (marked from 1 – 14) are presented in clockwise display around Fig. 1 (the location, in cm-1, of the dominant bands is given in brackets in the explanation next to each spectrum). Macroscopically, five different areas were considered: (A) the rim of the nodule (spectra 10, 11); (B) the external zone, dark-green to black in colour (spectra 4, 8); (C) the median zone, yellow-orange (spectrum 5), or red (spectra 7, 9, 12) in  colour; (D) the internal zone, whitish-grey in colour (spectra 1, 2 3); and (E) network (“septaria”-like) of white veins and veinlets (spectra 6, 13, 14). The yellow areas (zone C) revealed the highest Raman intensity signal without any fluorescence background, whereas the green regions (zone B) showed different spectral patterns with different intensities of the 461 cm^-1 band and high fluorescence background.

Fig. 1 – Museum sample under study (siliceous nodular chert from Rona, Salaj distr., Romania) Download a PDF of the image above

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