Romanian Reports in Physics, Vol. 59, No. 3, P. 819–824, 2007

NANO-MINERALS IDENTIFICATION BY DIFFERENT PHYSICAL TECHNIQUES

G. UDUBASA1, S. CONSTANTINESCU2, N. POPESCU-POGRION2, P. HIRTOPANU1, S.S. UDUBASA3

1Geological Institute of Romania, Bucharest, Str. Caransebes 1, udubasa@geo.edu.ro; 2 National Institute of Materials Physics, Bucharest-Magurele, Str. Atomistilor 105bis,

sconst@infim.ro 3 University of Bucharest, Bd. N. Balcescu 1, udubasa@geo.edu.ro

(Received October 9, 2006)

Abstract. The combined investigation techniques (EMPA, ME, NGR) evidenced the third level of the inclusions at nano-size scale for different metamorphic deposits. So, NGR technique reveals the contribution of the (Mn, Fe) oxides – small particles at standard (RT) at very low temperature (LHeT) in the 57Fe γ- ray signal. On the other hand the ME techniques confirmed the presence of the nanometric Fe and Mn oxides inclusions in manganoan fayalites (Razoare deposit) and “coral like” aggregates of nanometric Au -“solid- bubbles” in the gold ores of the Southern Carpathians.

Key words: Mössbauer spectroscopy, electrons microscopy, Fe and Au compunds, tephroite-fayalite, pyrite and pyrrhotite.

1. INTRODUCTION

The mineral world includes in present about 4000 valid mineral species of various compositions (from native elements to organic compounds) and size. Some 400 or 500 of the existing mineral species are known only as inclusions in other minerals and their investigation by different and adequate physical and chemical techniques reveals the chemical evolution of the minerals for metamorphic deposits. Depending on their size, such minerals can be classified as follows: (1) micro-minerals, from one millimeter down to 5-10 micrometer in size, which can be identified under the optical microscope; (2) "infra-minerals", i.e. submicroscopic inclusions, some several micrometers to thousands of manometers in size; commonly these minerals can be identified by using EMPA; (3) "nano-minerals", of nanometer size. The adequate physical techniques for the growing and the presence of such nanometer inclusions are electron microscopy (ME: TEM, SEM, SAED, HRTEM etc.) and nuclear gamma resonance (NGR, for Fe,Sn,Au,Eu, etc. species). The aim of this article is to point out the presence of nanometer inclusions in some minerals of interest, using NGR techniques and ME.

G. Udubasa et al. 2 820

-10 -5 0 5 10

2,5x106

2,6x106

2,6x106

2,7x106

2,7x106

2,8x106

2,8x106

2,9x106

2,9x106

T=4.2KR=1/60

-12 -8 -4 0 4 8 12

2x105

3x105

4x105

5x105

6x105

7x105

8x105

9x105

1x106

T=296KR=1/73

-2 -1 0 1 2 3 4 5

2,1x105

2,2x105

2,2x105

2,2x105

2,2x105

2,2x105

2,3x105

2,3x105

2,3x105

Rel

ativ

e Tr

ansm

issi

on

T=296KR=1/3

-10 -5 0 5 10

0,86

0,88

0,90

0,92

0,94

0,96

0,98

1,00

ε(Mn,Fe)2O3

ε088(v)

Energy in velocity units

2. EXPERIMENTAL ASPECTS AND RESULTS

The NGR spectra have obtained on sample powders at RT, using a 57Co:Rh source (intensity of 50mCi, a57Fe= 2%, f (300K)=0.76, Γobs=0.22mm/s metallic iron foil). The spectra are showing superposed signals of 57Fe in different local vicinities, characterizing by electric field gradient (EFG) and/or internal magnetic field (B). They have been analyzed by the χ2-criterium fit procedure. The selected powder samples have been investigated using JEM-200 CX installation at 200KV, with TEM in bright/dark field (BFTEM/DFTEM) and SAED techniques. The minerals of interest are extracted from the Mn-Fe Razoare deposit and gold-ores of Valea lui Stan, Costesti and Jidostita. The NGR analysis of some Razoare manganoan fayalite reveals the presence of low intensity sextet pattern, corresponding to the small concentration of the Mn-Fe oxide in magnetic phases (down arrows).

Fig. 1 – The evidence of Mn/Fe oxide patterns contribution to the RT and LHeT manganoan-faylite spectra.

Moreover, the evidencing distortion (up arrows Fig. 1.) of the doublet patterns (for high-resolution spectra, R=Γobs/δE), one suggests the superpara-magnetic state of oxides, at RT spectra. The LHeT spectrum of the same samples are showing many sextet-patterns and in consequence an apparently growing of magnetic phase weight, Table 1, for details [1, 2]. The RT distortion of the spectrum central doublet and the apparently growing of the magnetic phase at

3 Nano-minerals identification 821

LHeT has suggested the presence of the Mn-Fe oxide in small particles (under ~150Å) on the surface and/or inside of manganoan fayalite grains. In deed, the growing of the sextet patterns with the temperature decreasing corresponds to the increasing both of the volume energy vs. thermal one ratio, and of the magnetic local field’s time flipping at the 57Fe Mössbauer probe.

Table 1

NGR spectral parameters of Mn-Fe oxide extract from a Razoare’s manganoan fayalite spectrum

Mn-Fe OXIDE

PATTERN of

088 sample

δFe

[mm/s]

eQVzz/2

[mm/s]

Bloc

[T]

Relative area

[%]

RT

III1 0.17(5) 0.60(9) – 2.3(1)

32 0.00(3) –0.18(6) 49.0(1) 6.2(2.0)

42 0.56(6) 1.04(12) 46.6(4) 8.0(2.0)

LHeT

C 0.20(7) –0.15(14) 52.0(5) 15.7(2.8)

D 0.90(7) –2.13(14) 45.1(5) 14.6(2.8)

E 1.40(10) –1.27(19) 43.7(5) 5.1(4.2)

On the other hand, TEM analysis (Figs. 2, 3) of Razoare manganoan-fayalite has revealed the presence of pyroxferroite crystals and the wustite mangano-grunerite (formerly dane-morite) and magnetite nano-precipitations [3]. The identification of wustite (as precipitation and/or microinclusions) and pyroxferroite (as single crystals with perfect geometrical shape) in the manganoan fayalite gives a deeper insight into metamorphic evolution of the Razoare Mn-Fe deposits, the largest of this kind in Romania, with a geological age of more than 550My. The PT estimations (corresponding to the mineral assemblages) show highest metamorphic peak to slightly exceed the upper amphibolite metamorphic facies (about 700oC and 700MPa), entering the field of ecologite facies. After primarily crystallization of manganoan orthoferrosilite, the manganoan fayalite has formed on it, being the first retrograde transformation; the second retrograde transformation was that of manganoan fayalite into mangangrunerite. Udubasa et al. [4] supposed that the second retrograde transformation of manganoan fayalite includes mainly mangangrunerite (formerly dannemorite). It is to note that abundant mangangrunerite is accompanied by development of magnetite by increasing oxygen fugacity during the late evolution of the deposit.

G. Udubasa et al. 4 822

Fig. 2 – a) TEM on mangangrunerite (with parallel cleavage) and wustite precipitation/inclusions; b) BFTEM, DFTEM, and SAED on a manganoan fayalite crystals group with precipitations.

The recent analyses have evidenced "infra-minerals" and "nano-minerals" in

the gold ores of Valea lui Stan, Costesti and Jidostita, containing pyrite, arsenopyrite, chalcopyrite and pyrrhotite as major minerals [5]. So, the TEM/SAED analyses carried out on powder samples from Costesti and Valea lui Stan sulfides, have evidenced nanometric “precipitates” of Au or Au-Ag alloy on pyrite or pyrrhotite grain surfaces. Those are showing diverse morphologies, i.e. from isolated nano-particles, some tens of nanometers in size, up to “coral-like” aggregates.

Such nano-grains or aggregates have also been identified (although more rarely) on arsenopyrite and chalcopyrite. Usually these aggregates or precipitates are observed (“built”) at edge or/and corners of the crystals.

3. SUMMARY

Instead of conclusion one remarks the capability of combined investigation techniques (EMPA, ME, NGR) to evidence the third level of the inclusions at nano-size scale. This level, which could appear during the metamorphic processes, has been predicted in Udubasa ([5, 6, 7]). So, NGR technique reveals the contribution of the (Mn, Fe) oxides – small particles at standard (RT) at very low temperature (LHeT) in the 57Fe γ-ray signal. On the other hand the ME techniques

0.2µm

0.4µm

(b)

(a)

5 Nano-minerals identification 823

confirmed the presence of the nanometric Fe and Mn oxides inclusions in manganoan fayalites (Razoare deposit) and “coral like” aggregates of nanometric Au – “solid-bubbles” in the gold ores of the Southern Carpathians.

Fig. 3 – a Pyrite single crystal with Au precipi-tates and Au particle as chains

(“coral”-like); ave-rage dimension ~60nm. Sample 08_pyVS-h.

Fig. 3b – Pyrrhotite single crystal with Au and Ag3AuS precipitates. Au particles (“coral-chains”)

with bimo-dal repartisions of the grain size (~18nm and ~155nm). Sample 10_pyrCN-g-01.

G. Udubasa et al. 6 824

The analyses and interpretation of the NGR, ME data are in progress in order to elucidate the role of the mechano-thermo-chemically processes supported by the above mentioned mineral deposits on the gold trap and nanometric size gold inclusions/precipitates and aggregates.

Acknowledgements. The authors acknowledge to Ministry of Education and Research of

Romania / National Program CERES for the financial support of the scientific activities in the frame of the C4-209/2004-2006 project.

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