Remote Raman Spectroscopy of Various Mixed and Composite Mineral Phases

Introduction: Remote-Raman [e.g.,1-5] and micro-Raman spectroscopy [e.g., 6-10] are being evaluated on geological samples for their potential applications on Mars rover or lander. The Raman lines of minerals are sharp and distinct. The Raman fingerprints of minerals do not shift appreciably but remain distinct even in sub-micron grains and, therefore, can be used for mineral identification in fine-grained rocks [e.g., 4,7]. In this work we have evaluated the capability of a directly coupled remote Raman system (coaxial configuration) for distinguishing the mineralogy of multiple crystals in the exciting laser beam. We have measured the Raman spectra of minerals in the near vicinity of each other and excited with a laser beam (e.g. α-quartz (Qz) and K-feldspar (Feld) plates, each 5 mm thick). The spectra of composite transparent mineral plates of 5 mm thickness of α-quartz and gypsum over calcite crystal were measured with the composite samples perpendicular to the exciting laser beam. The measurements of remote Raman spectra of various bulk minerals, and mixed and composite minerals with our portable UH remote Raman system were carried out at the Langley Research Center in a fully illuminated laboratory. Experimental Set-up and Samples: The pulsed remote Raman system has been described in detail elsewhere [1-3]. In brief, it consists of a Kaiser F/1.8 Holospec spectrometer equipped with a gated thermoelectrically cooled CCD detector from Princeton Instruments (Model I-MAX-1024-E). A 127-mm telescope (Meade ETX-125 Maksutov Cassegrain, 1900 mm focal length), a frequency-doubled small Nd:YAG pulsed laser source (Model Ultra CFR, Big Sky Laser, 532 nm, 35 mJ/pulse, 20 Hz, pulse width 8 ns, central laser spot divergence 0.5 mrad), The telescope is directly coupled to the spectrometer through a 20x (NA = 0.35, long focal length = 20 mm) microscope lens. A 532 nm SuperNotch Plus holographic filter is used in front of the microscope lens to minimize the Rayleigh scattering signal. The laser beam is made coaxial with the telescope optical axis using two small prisms [2, 11]. All Raman spectra were measured in a coaxial mode with 100 micron slit and the intensified CCD operated in the gated mode with gate width of 1.1 μs. The integration time for each of the spectra for mixed mineral phases and composite mineral phases was 1 s and 30 spectra were accumulated, and the individual mineral spectrum was measured with 1 s integration time with 1 accumulation. Neon lines are used in calibrating the spectra and measured remote Raman spectra of benzene and cyclohexane peak positions are within + 2 cmof standard values. The rock-forming mineral samples were purchased from Ward’s Natural Science Establishment, Inc. The 5 mm thick plates were cut from α-quartz crystal from Hot Spring, Arkansas, orthoclase crystal (KAlSi3O8) from Gothic, Colorado, and gypsum, variety Satin spar from Highland Arkansas, USA. Calcite crystal and dolomite samples used as a bulk samples in these experiments were from Chihuahua, Mexico. Results: The remote Raman spectra of α-quartz and feldspar plates both excited by the laser beam are shown in Fig. 1 along with the spectra of α-quartz and K-feldspar measured individually. The Raman lines of α-quartz and K-feldspar are marked on the mixed Raman spectra by Qz and Feld, respectively. In the Raman spectra of K-feldspar and α-quartz the strong fingerprint Raman lines of K-feldspar (472 and 512 cm) corresponding to symmetric stretching of oxygen of four-membered rings of TO4 tetrahedra, where T= Si, Al, νs(T-O-T) and of α-quartz (462 cm) corresponding to symmetric stretching of oxygen of six-membered SiO4 tetrahedra, νs(Si-O-Si) [13-14], are well resolved (Fig. 1). In the mixed spectrum of αquartz and K-feldspar crystals, the 512 cm line of Feld is clearly visible but the 472 cm line of Feld overlaps with the strong Qz line at 462 cm (Fig. 1 top