American Mineralogist, Volume 95, pages 1515–1522, 2010 Far infrared spectroscopy of carbonate minerals TA T IANA N. BRUSEN T SOVA ,1,* RO B ER T E. PEALE ,1 DOUGLAS MAUKONEN ,1 GEORGE E. HARLOW,2 JOSE PH S. BOESEN B ERG ,2 AN D DEN T ON EB EL 2 1Department of Physics, University of Central Florida, Orlando, Florida 32816, U.S.A. 2Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, U.S.A. AB S T RAC T This study presents far infrared spectra in the range 650–70 cm–1 of 18 common and rare carbon- ate minerals. Mineral samples of known provenance are selected and physically characterized to determine the purity of the crystalline phase and their composition. The fine ground mineral powders are embedded in polyethylene pellets, and their transmittance spectra are collected with a Fourier spectrometer. The far infrared spectra of different carbonate minerals from the same structural group have well-defined similarities. Observed shifts generally manifest the mass effect of the constituent metal cations. Remarkable spectral differences occur for different carbonates in the far IR region and may serve as fingerprints for mineral identification and are more useful identifiers of carbonate spe- cies than those in any other infrared range. For some of the minerals studied here, like kutnohorite, artinite, gaylussite, and trona, no far infrared spectra to that extend (up to 70 cm–1) have been found in literature. Keywords: IR spectroscopy, far infrared, transmittance spectra, carbonate minerals IN T RO D UC T ION and Buseck (1986) by means of laboratory analysis. Kemper –1 The vibrational spectrum of a carbonate mineral consists of et al. (2002a, 2002b) attributed the 109 and 161 cm emission mid-infrared (IR) internal modes due to bending and stretching bands in spectra of planetary nebula NGC 6302 collected by within the carbonate anion and far IR lattice modes. The far IR the Infrared Space Observatory (ISO) to calcite and dolomite. –1 modes probes lattice vibrations and are unique for a specific crys- Similar emission bands between 111 and 91 cm were found by talline structure, differing significantly for different polymorphs Chiavassa et al. (2005) in 17 out of 32 ISO spectra of proto stars of the same chemical compound in different crystalline forms. and were again attributed to the presence of calcite. All these data The spectral region where lattice vibrations occur is below 600 encourage an extensive laboratory infrared studies of terrestrial cm–1 (Nyquist and Kagel 1971; White 1974). The far IR is an carbonate minerals to the far IR range. ideal spectral range when it comes to distinguishing between All information on vibrational spectroscopy of carbonate different minerals. For distinguishing carbonates with IR spectra, minerals up to 1973 has been reviewed in White (1974). Also, far-infrared data are indispensible. One situation where mineral measured and calculated vibration frequencies for all carbon- identification using IR spectroscopy is of greatest use is in the ate groups are presented in Gadsden (1975), although in this astronomical analysis of cosmic dust. Successfully launched in publication only peak frequencies are given, rather then the full 2009, Herschel Space Observatory will collect the spectra from spectra themselves. Within the mid-IR range, the vibrational the interstellar objects in the far IR and sub-millimeter range spectra of naturally occurring carbonate minerals have been –1 (167–15 cm–1). This will substantially enhance astronomical extensively studied down to 625 cm (Huang et al. 1960; Adler observations in this range (Rowan-Robinson 2009). These and Kerr 1962, 1963a, 1963b; Chester et al. 1967; Hunt et al. spectra will provide the information about the distribution of 1950; Weir et al. 1961). IR spectra of many carbonate minerals in –1 different mineral species in space, which in turn allows to deduce the range between 400 to 4000 cm with specified characteristic information on physical conditions and the possible presence of frequencies are given in Moenke (1962, 1966). A study of three water in its different states. Only since the past 10 years has far possible carbonate polymorphs (aragonite, calcite, and vaterite) IR spectroscopy become a usable tool for identifying the mineral is reported in Weir et al. (1961). dust composition around the astronomical objects (Chiavassa et Only a few far IR spectral data on naturally occurring car- al. 2005). Since that time, attempts to identify some common bonate minerals of sufficient quality to be useful for application minerals that possess prominent far IR features observed in dust to astronomical data analysis can be found in the literature. found around various astronomical objects are being undertaken. There is a substantial work on 10 anhydrous normal carbonates –1 Among those minerals the carbonates group is especially interest- (Morandat et al. 1967), where the far IR spectra (400–12 cm ) ing. The presence of carbonate grains within interplanetary dust together with the assignments for all the lattice vibrations are particles was demonstrated by Sandford (1986) and Tomeoka provided. Absorption spectra of 7 anhydrous normal carbonates in 500–30 cm–1 range are presented in Angino (1967). However, * E-mail: [email protected] all these spectra contain gaps around 350 and 140 cm–1, which 0003-004X/10/0010–1515$05.00/DOI: 10.2138/am.2010.3380 1515 1516 BRUSENTSOVA ET AL.: FAR INFRARED SPECTROSCOPY OF THE CARBONATE MINERALS distorts the overall spectral pattern. Far IR spectra between 200 EX P ERI M EN T AL D E T AILS –1 to 50 cm of 3 carbonates (dawsonite, calcite, and dolomite) Carbonate samples (Table 2) were selected from the mineral collection of the are presented in Karr et al. (1969). IR spectra (mass absorption American Museum of Natural History (AMNH). Most samples were extracted from coefficients) of dolomite, calcite, aragonite, magnesite, siderite, specimens as macroscopic crystals by visual inspection under a stereo microscope and ankerite in the range of 5000–50 cm–1 also are presented to manually select those with similar morphology and absence of visible zoning, exsolutions, or inclusions. in Kemper et al. (2002a, 2002b), but with no peak frequencies All samples were examined to establish their identity, major-element com- specified. Moreover, data presented as mass absorption coeffi- position, and chemical homogeneity. X-ray diffraction analyses of macroscopic cient, while suitable for astrophysics, is less useful for mineral grains were performed using a Rigaku DMAX/Rapid microdiffraction system, identification where the raw data are in the form of transmittance. using incident-monochromatized CuKα radiation, operating at 46 kV and 40 mA. Subsequent search/matching was carried out using JADE software (Materials Data, A detailed IR investigation of the Cd- and Mg-carbonate solid Inc.) referenced to the ICDD PDF-2 diffraction database file. The lattice parameters –1 solutions has been performed in the range of 50–2000 cm extracted from powder XRD data are given in Table 2. (Bromiley et al. 2007). In Bessière-Morandat et al. (1970), the Chemical composition and impurity content were analyzed by electron micro- vibrational features (no full spectra) in the range of 5000–67 probe analysis (Cameca SX100) of polished grain mounts. Backscattered electron cm–1 for four hydroxyl-containing carbonates (hydrocerussite, imaging was used for a basic assessment of homogeneity and existence of included phases, and energy dispersive spectrometry was used to confirm the phases pres- hydromagnesite, azurite, and malachite) are tabulated and as- ent as well as to identify any included phases. Both individual point analyses and signed to different virbrational modes. The vibrational modes traverses were carried out on single grains. Traverses were particularly valuable in of azurite and malachite with the assignments in the region evaluating compositionally zoning and were typically ~1 mm long with 10 points. of 5000 to 60 cm–1 (again without presentation of the spectra All samples considered in this study have acceptable purity (near end-member composition) and homogeneity. The impurities determined by electron microprobe themselves) are also reported in Goldsmith and Ross (1968). IR analysis are presented in the Table 2, in the form of molar percentage of the carbon- spectra of three hydrated and hydroxyl-containing carbonates ates of a corresponding metal. Complete data is available from the data website for (nesquehonite, artinite, and hydromagnesite) in the 2000–350 this project: http://research.amnh.org/users/debel/pub/LAP-data/.1 cm–1 range have been reported in White (1971). The spectra of 23 All the carbonates were further ground in a McCrone Micronizer cylindrical carbonate compounds between 3800 and 45 cm–1 are presented mill to powders with intended average particle size of ~5 µm or less. Particle sizes were verified by scanning electron microscopy of dispersed powders. Sufficiently by Nyquist and Kagel (1971). Most are represented by chemical small particle size is important to minimize scattering (Coleman 1993; Brügel 1962), reagents, and no assignments or peak frequencies are specified. whose signature is a sloping transmittance baseline. Also, the effective thickness Also, the 200–45 cm–1 region is poorly resolved, which makes [d = m/(ρ·A), where m is mineral mass per pellet, ρ is mineral mass density, and A the main characteristic features in the far IR range difficult to is the area of the IR sample pellet cross-section] of a dilute dispersion of minerals –1 in a sample matrix should not be less than the individual particle size. identify. Far IR transmittance spectra (4000 to 30 cm ) of various IR transmittance measurements were made on suspensions of mineral particles alkaline earth double carbonates (huntite, alstonite, norsethite, in polyethylene (PE) pellets 3 cm in diameter and 1 mm thick containing ~700 mg baritocalcite, and benstonite) are reported in Scheetz and White of PE powder (Mitsui Chem MIPELON XM-220 PE microparticles with 30 µm (1977), and the assignments for lattice vibrations are provided.