Electron paramagnetic resonance, NIR studies on zoisite, clinozoisite and chrom-zoisite minerals

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Abstract

A zoisite group of mineral samples from different localities are used in the present study. An EPR study on powdered samples confirms the presence of Mn(II), Fe(III) and Cr(III) in the minerals. NIR studies confirm the presence of these ions in the minerals.

Introduction

The epidote group comprises a widespread and chemically complex family of rock forming silicates whose composition can be represented as

The members of the family are

Zoisite, a rock-forming mineral, is a basic calcium and aluminium silicate with orthorhombic structure. The important substitutents are Mn, Cr, and rare earth elements [1]. Literature survey reveals that the relative concentrations of di-, tri- and tetravalent cations are identical in zoisite and epidote but that the Fe content in zoisite (about 2–3.5 wt% Fe2O3) is generally less than in epidote (about 9–12 wt% Fe2O3) [2]. Zoisite containing some iron replacing aluminium may be identical in composition with an epidote (“clinozoisite”) poor in iron [3]. A similar situation occurs in clinozoisite in that the manganese is replaced by calcium in the distorted octahedral position with oxygen ligands. It is in agreement with ionic radius and charge considerations [3], [4]. Also Mn(II) ions in zoisite occupy the Ca sites because of ionic radius and charge considerations [4]. Zoisite is again classified into zoisite, clinozoisite and chrom-zoisite. Manganese rich variety of the mineral zoisite is called thulite or rosaline. Clinozoisite is poor in iron and its composition is identical with epidote mineral. Chromium containing zoisite is called as chrom-zoisite (Thulite was first discovered in Lom, Norway in 1820. It is named after the mythical island of Thule in the belief that the island is Norway.) The cell parameters of zoisite are a = 16.212, b = 5.559 and c = 10.036 A.U. where as for the clinozoisite cell dimensions are a = 8.879, b = 5.583 and c = 10.155 A.U. [5]. Crystal chemistry of synthetic strontium containing solid-solution series of zoisite and clinozoisite was studied by Dörsam et al. [6]. Local relaxations around Fe(III) and Cr(III) in Al sites in minerals were studied [7]. Thulites from different localities were investigated by X-ray and by electron microprobe analysis. It is suggested that low MnO values in the range 0.05–0.9 wt% cause pink colour in thulites, regardless of symmetry [8]. Fe content in zoisite is about 2–3.5 wt% of Fe2O3 [2] The chemical analysis of zoisite is reported and reveal that FeO is present 0.196 wt% and MnO = 0.37 wt% [9], where as the optical absorption and EPR spectral were studied on clinozoisite [10], [11]. During hydrothermal alteration, V and Cr were scavenged by fluids from the graphitic gneisses, and fixed by the oxide and silicates [12].

Section snippets

Experimental

Mineral samples of zoisite (LOM, OPPLAND, Norway), clinozoisite (Gavilanes, Baja California, Mexico) and chrom-zoisite (Merkerstein, Tanzania, East Africa) supplied by MCR USA are broken from deeply colored crystals and used in the present work.

EPR spectra of the samples in powder form are recorded at room temperature (RT) on a JEOL JES-TE100 ESR spectrometer operating at X-band frequencies (υ = 9.37978 GHz for zoisite, υ = 9.38376 GHz for clinozoisite and υ = 9. GHz for chrom-zoisite respectively). For

EPR spectral analysis

The studied zoisites from different regions are different in colour and in chemical compositions. The comparison between EPR spectra of all the mineral samples reveals some characteristic features. Fig. 1 shows the EPR spectrum of zoisite, recorded at room temperature (RT) from 0 to 400 mT (in Fig. 1A), 250 to 370 mT (in Fig. 1B). The EPR spectra of clinozoisite recorded at RT at different ranges [from 0 to 500 mT (in Fig. 2A), 75 to 225 mT (in Fig. 2B) and 275 to 425 mT (in Fig. 2C)] are shown in

Conclusions

Zoisite group of minerals used in the present study contains ferric iron, manganese and chromium transition metal ions in the minerals with low concentrations.

The EPR studies indicate the presence of Fe(III) and Mn(II) centers, having g values 6.30, 4.35 and 2.96 in zoisite and 6.70, 4.30 in clino-zoisite for Fe(III) and 6.3, 4.10 and 2.95 in chrome-zoisite. This also indicates that Fe(III) is in octahedral structure. Even at low temperature no changes are observed in the EPR spectrum. The

Acknowledgements

The authors (SLR & RR) are thankful to UGC, New Delhi for financial assistance (Major research project no: 38-188/2009).

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