G. Parthasarathy

FERROUS SAPONITE FROM THE DECCAN TRAP, INDIA, AND ITS APPLICATION IN ADSORPTION AND REDUCTION OF HEXAVALENT CHROMIUM

Abstract A green-colored clay mineral occurring on the walls of amygdaloidal cavities and along fractures in the Deccan flood basalts at Killari, Maharashtra, India has been identified as iron-rich saponite, with chemical composition (Na0.60K0.04Ca0.47)(Mg2.05Fe2+3.95)(Si6.45Al1.55)O20(OH)4. The XRD data of the Killari sample with d (001) = 1.70 nm and b0 = 0.9275(2) nm conform to that of ferrous saponite. The FTIR spectra show vibrational bands typical of trioctahedral smectites. Differential thermal and thermogravimetric analyses show strong endothermic peaks due to dehydroxylation at 390-420 and 1070-1130 K accompanied by weight losses of 17 and 2.3 wt%. A weak exothermic peak at about 1220 K is also observed. These peaks are characteristic of smectites. 29Si and 27Al MAS NMR studies show that silicon and most of the aluminum in the clay mineral are in fourfold coordination. We have demonstrated for the first time the usefulness of ferrous-saponite in reduction of hexavalent chromium. X-ray photon electron spectroscopic (XPS) studies of the fine clay sample treated with dichromate solution show that this ferrous saponite has the capability of adsorbing and reducing hexavalent chromium to the trivalent state.

Occurrence of natural fullerenes in low grade metamorphosed Proterozoic shungite from Karelia, Russia

Abstract We report on the occurrence of fullerenes in Proterozoic shungite (∼2 Ga) from the shungite mine, Kondopoga, Karelia, Russia (62.12°N 34.17°E). The presence of fullerenes has been confirmed by mass spectrometry, with peaks at 360 and 720 amu (atomic mass unit), powder X-ray diffraction showing ten diffraction peaks corresponding to the fullerite structure with a = 1.4201(5) nm, and 13 C nuclear magnetic resonance (NMR) spectroscopic studies, showing a peak at 143.2 ppm. In the Kondopoga shungite mine, fullerenes occur in silty shales that have experienced greenshist facies metamorphism.

Thermal stability and spectroscopic studies of zemkorite: A carbonate from the Venkatampalle kimberlite of southern India

Abstract While characterizing the mineralogy of the kimberlite at Venkatampalle, Andhra Pradesh, India (Lat. 14° 56′ 00″ N, Long. 77° 22′ 15″ E), we found the second occurrence of zemkorite (Na,K)2Ca(CO3)2, which was first reported in 1989 in the Udachnaya kimberlite in Siberia. We also report the first Fourier-transform infrared spectroscopic (FTIR), scanning electron microscopic, thermo-gravimetric, and high-temperature heat capacity measurements on this rare carbonate. Powder X-ray diffraction (XRD) patterns show 24 well-resolved diffraction lines, all of which can be indexed with a hexagonal cell with a = 10.038(5) Å and c = 12.726(5) Å. The vibrational spectrum of zemkorite at room temperature exhibits 13 distinct absorption bands in the frequency range 2000 to 400 cm-1. The IR bands of zemkorite indicate a structural similarity with shortite but are quite distinct from other alkali carbonates such as nyerereite, butschiite, and fairchildite. The heat capacity of zemkorite has been measured up to 700 K by differential scanning calorimetric techniques. The temperature dependence of the heat capacity of zemkorite was fitted with the polynomial Cp = 140.2 + 8.584 × 10-2 T - 2.458 × 106 T-2. The upper thermal stability of zemkorite is ~700 K, which is similar to shortite. Zemkorite may have formed during the late stages of kimberlite genesis, possibly as a result of metasomatism or by the breakdown of metasomatic natrocarbonatitic minerals or glass that segregated on decompression melting in the upper mantle.

57Fe Mössbauer spectroscopy and electrical resistivity studies on naturally occurring native iron under high pressures up to 9.1 GPa

Abstract We report the pressure dependence of the Mössbauer spectra and the electrical resistivity up to 9.1 GPa at room temperature for a native iron sample collected from the Precambrian Chaibasa shales, Singhbhum Craton, Eastern India. The Mössbauer spectroscopy of the sample at ambient conditions yields isomer shifts and magnetic hyperfine field values that confirm the presence of Fe0 oxidation state. Many theories have been put forward to explain the origin of this native iron including a Precambrian meteoritic impact. High-pressure Mössbauer spectroscopic measurements using diamond anvil cell (DAC) showed a constant isomer shift up to 5.6 GPa with a subtle variation of -1.07 × 10-3 mm/s/GPa followed by sharper change -4.3 × 10-3 mm/s/GPa above 6.3 GPa, a pressure much lower than the usual value reported for metallic iron. Further increase of pressure to 9.1 GPa results in the emergence of a tiny peak at ~0 isomer shift indicating the onset of the martensitic phase transition of iron from the body-centered-cubic (bcc) to hexagonal-close-packed (hcp) transition 4 GPa lower than the transition pressure normally observed for pure iron. This phase transition in the native iron is confirmed by high-pressure electrical resistivity study. Lowering of the transition pressure could be due to nucleation of hcp by stacking faults caused by shock metamorphism resulting from the Precambrian impact in the region.

High-pressure electrical resistivity behaviour of nanocrystalline perovskite (La, Sr)(Mn, Fe)O3

We report here the electrical resistivity of nanocrystalline perovskite-structured La–Sr manganites as a function of pressures up to 8 GPa, at room temperature. The nanocrystalline perovskite manganites were prepared by the sol–gel technique and found to have crystallite sizes of 12–18 nm. The pressure dependence of the electrical resistivity shows a first-order phase transition at 0.66(2) GPa and a subtle phase transition between 3.5 and 3.8 GPa. The first-order transition at 0.66 GPa can be related to the transition from localized-electron to band magnetism.

High-pressure electrical resistivity and Mössbauer spectroscopic studies on narrow band Co0.8Fe0.2S2 nanoparticles up to 8 GPa

Pyrite-structured Co0.8Fe0.2S2 colloid nanoparticles, synthesized using solution method with particle distribution around 3–4 nm determined by transmission electron microscope, were studied under high pressure up to 8 GPa using 57Fe Mössbauer spectroscopy and electrical resistivity techniques. Drastic decrease in TC by magnetic measurements indicated nanosize of the particle. Higher quadrupole splitting (QS) at ambient condition was due to large lattice strain and electric field gradient generated by higher surface to volume ratio of these nanosized particles. Under pressure, Mössbauer parameters – isomer shift and QS – showed an expected trend up to ∼5.6 GPa. Above 6.4 GPa, the QS remained constant up to 8 GPa with decreased value. Even after decompression, the high-pressure phase is retained. The variation of pressure coefficient of electrical resistivity from −0.021/GPa to −0.151/GPa across 6.8 GPa suggested a second-order phase transition. The coincidence of observed value with that of the bulk suggested that the particle size does not impart much influence on transition pressure. This is the first report on nanoparticles of Co0.8Fe0.2S2 under pressure.

Meteorite Fall at Sadiya, India: A Raman Spectroscopic Classification

This report demonstrates compositional, spectroscopic and mineralogical analysis of a new meteorite that fell at Natun Balijan village of Sunpura, Sadiya, India on June 5, 2017. The olivine and pyroxene composition (Fa28.97; Fo71.03; Fs24.47; En74.03 and Wo1.5) of the meteorite are determined. The measured Raman band positions are consistent with chemical composition for olivine and pyroxene. The compositional and spectroscopic analysis of the Sadiya meteorite sample show that the meteorite belongs to LL-5 type chondrite.

High-pressure electrical resistivity studies on FeSe2 and FeTe2

We report here for the first time the pressure dependence of the electrical resistivity of ferroselite (FeSe2) and frohbergite FeTe2 up to 8 GPa. The synthetic ferroselite shows a pressure induced marcasite-to-NiAs type structural phase transition at 6.8 GPa and frohbergite shows the transition at 5.7 GPa. The transition was observed with a discontinuous resistivity decrease by 0.9 times. We also present here the XRD results on the FeSe2 and FeTe2. The relevance of the phase transition to Martian mineral chemistry is discussed.

High‐Pressure Studies on Synthetic Orthorhombic Cubanite (CuFe2S3)

We report here the successful synthesis of orthorhombic cubanite using microwave heating and characterization for its orthorhombic phase by powder X‐ray diffraction, Micro‐Raman, Thermal and high pressure electrical resistivity measurements. The unique physico‐chemical conditions prevent successful synthesis under laboratory conditions, however the mineral occurs in nature in its orthorhombic form. A distinct endothermic dip in Differential Thermal Analysis (DTA) at 280 °C is in agreement with the DTA data reported for natural cubanite. An irreversible first order phase transformation from orthorhombic to NiAs structure is seen at 4 GPa/200 °C. High pressure electrical resistivity and micro‐Raman spectroscopic measurements on the synthetic sample exhibit transformation to isocubanite.

High-pressure studies on nanocrystalline borderline Co1–xFexS2 (x = 0.4 and 0.5) using Mössbauer spectroscopic and electrical resistivity techniques up to 8 GPa

ABSTRACT Like bulk, Co1–xFexS2 nanoparticles also display an anomaly at x = 0.5. The borderline contiguous Co1–xFexS2 (x = 0.4 and 0.5) nanoparticles were synthesized with colloidal method and characterized for pyrite structure using various techniques, viz., X-ray diffraction, energy dispersive X-ray analysis (EDAX), S K-edge X-ray absorption near edge spectra, transmission electron microscopy (TEM) and Fourier transformed infra-red spectroscopy. The report presents the effect of high pressure on the borderline compositions using the Mössbauer spectroscopic and electrical resistivity techniques. Magnetic measurements on the system showed drastic lowering of Tc due to nanosize of the particles. With increased pressure, quadrupole splitting showed an expected trend of increase to attain a peak representing a second-order phase transition between 4 and 5 GPa for both the compositions. The pressure coefficient of electrical resistivity varied from –0.02 GPa to –0.06 GPa across transition pressure indicating a sluggish nature of transition. This is the first report of pressure effect on nanosized borderline compositions.