Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Raman spectra of organic compounds kladnoite (C6H4(CO)2NH) and hoelite (C14H8O2)—Rare sublimation products crystallising on self-ignited coal heaps
Introduction
Few organic compounds crystallise in the geological record and are recognised as minerals. Few of them, as whewellite (Ca(C2O4)·H2O) are relatively well-known from sedimentary and coal series from long time and were studied extensively [1], [2]. Crystallised whewellite typically occurs in carbonate concretions in the frame of coals as well as in hydrothermal veins, see for example the review published by Hoffmann and Bernasconi [3].
Some of organic compounds occur crystalline at superficial parts of self-ignited coal heaps or strata [4], [5], [6]. The well-known self-ignition of coal is considered as a process in which radical formation, weathering of finely dispersed pyrite and bacterial activity participate [7]. These organic phases are recently formed products of accumulation of the sublimation and condensation of gases liberated during coal ignition in situ. Only several compounds formed in the frame of this process are actually accepted as minerals by the International Mineralogical Association (IMA): kratochvilite (C13H10, fluorene) and kladnoite (C6H4(CO)2NH, phthalimide) for example, which were described from burning coal waste heaps at Kladno (Central Bohemia) [5] and ravatite, pure phenanthrene (C14H10) in naturally burning coal seams at Ravat (Tadjikistan) [8].
Rich associations of neoformed minerals were described from several sites of burning coal strata and heaps in Pensylvania [9], Bohemia (Kladno, Bílina, Radvanice) [3], [5], [10], and Germany [11]. Žáček [10] proposed a classification of these minerals based on the formation process. It includes minerals formed through sublimation (elemental sulphur, selene, salmiac, organic minerals) and by interaction of solid rock with aggressive gaseous phase (numerous simple and complex sulphates especially of Al, Fe and NH4). Specific phases are represented by high-temperature neoformed minerals occurring in central parts of the heaps (cordierite, hematite, anortite). Other secondary phases are, on the other hand, formed through low-temperature weathering on superficial parts of coal and claystones (epsomite, gypsum, natrojarsite). Organic compounds occur in the form of black solid bituminous mass and also as several relatively stable minerals crystallising as fine lumps or needles (kladnoite, hoelite, kratochvilite). Unstable platy white crystals were observed only twice during winter 1988 (natural naphthalene) [5].
Kladnoite, C6H4(CO)2NH, natural equivalent of phthalimide, the only mineral containing nitrogen as NH-group, was discovered on the burnt bituminous coal dumps of Kladno, Czech Republic in 1937 [4]. Since, several structural data were obtained on similar material sampled during the period 1982–1988 [5]. This mineral was detected also in the frame of spontaneous fires of lignite at Bílina (North Bohemian Tertiary Basin, Miocene) and at Radvanice (Intrasudetic Basin, Eastern Bohemia, Stephan). In 1985 the mineral was found on the burnt waste heaps of Kopeisk (Chelyabinsk coal basin, Russia). Here, kladnoite forms the small friable accumulations of light silvery flakes with perfect cleavage and size to 2–5 mm. Density varies from 1.43 to 1.50 g/cm3 [6].
Hoelite, C14H8O2, natural analogue of 9,10-anthraquinone, was discovered in 1922, in the Pyramid Mine (Spitsbergen) where it occurs as a result of ignition of underground coal benchs [12]. Later the mineral was found on the burning waste heaps of coal mines of Freital (Germany) and Kladno (Czech Republic) [13]. The mineral forms microscopic brightly yellow transparent crystals with length of 1–10 mm and thickness not more than 0.01 mm or radiated accumulations of these crystals on the fractures or cleavage planes of the burnt rocks.
Infrared and Raman spectra of natural oxalates from different environments have been obtained and interpreted [14], [15]. Terpenoid hydrocarbons, which form pure mineral phases (fichtelite, hartite) and are frequently only minor compounds in natural resins and fossil resins were investigated by Raman spectroscopy by Jehlička et al. [16]. Jehlička et al. [17], [18] obtained and interpreted Raman spectra of mellite and idrialite, examples of organic crystalline minerals.
Phthalimide is a planar molecule with C2v symmetry. Hase et al. [19] and Bigotto and Galasso [20] assigned the vibrational spectra of phthalimide. For a phthalimide molecule there are 29 in-plane vibrations (15A1 + 14B2) and 13 out-of-plane vibrations (6A2 + 7B1). A detailed assignment of most of the observed frequencies in Raman spectra of synthetic phthalimide polycrystalline and single crystal samples has been previously reported [21], [22]. For monoclinic phthalimide crystals symmetry, space group , Z = 4, all vibrational Ag and Bg fundamentals are Raman active.
The 9,10-anthraquinone molecule belongs to D2h point group. The 66 internal modes of vibrations can be divided as 11b1u + 11b2u + 6b3u + 12ag + 4b1g + 6b2g + 11b3g + 5au, where all the g vibrations are Raman active, u vibrations are infrared active and au vibrations are inactive in both [23].
This study brings a Raman spectral characterisation of kladnoite and hoelite from coal environments for the first time. Raman microspectrometry helps to detect these phases non-destructively, directly in the frame of rare samples.
Section snippets
Samples
Three samples of organic minerals (two samples of kladnoite, one sample of hoelite) were investigated in this study. More detailed description of their occurrence in Kladno heaps is given by Žáček [5], [13]. Details about hoelite from Kopeisk are described by Chesnokov and Shcherbakova [6]. The rare specimens from Kladno and Kopeisk originate from the private collections of the authors and from the Mineralogical Collection of the Charles University in Prague.
Raman microspectroscopy (RMS)
Micro-Raman analyses of crystals
Kladnoite
Characteristic Raman spectra obtained for kladnoite samples are presented in Fig. 1, Fig. 2. The wavenumber positions of the bands and their approximate assignments are given in Table 1. Phthalimide vibrational modes are known and Raman spectra obtained in the frame of this study are in agreement with published spectra. Main phthalimide vibrational modes are observed by conventional Raman microspectroscopy as well as using FT-Raman (1064 nm excitation) spectroscopy in the case of kladnoite from
Conclusions
Raman spectral signatures have been identified and assigned for natural samples of kladnoite and hoelite for the first time. Identification of these materials in the geological record is indicative of previous biological activity as well as medium temperature transformation. Raman microspectroscopy is suitable to investigate the character of these organic minerals. Raman spectroscopic features obtained on organic minerals in rocks and coals can be successfully used for the non-destructive
Acknowledgements
This work was partly supported by a grant IAA3111306 from the Grant Agency of the Academy of Science of the Czech Republic, by MSM0021620855 of the Ministry of Education, Youth and Sport of the Czech Republic and by a grant 06-05-64845 by Rusian Foundation of Basic Resarch. The participation of JJ in Georaman 2006 was partly supported by Renishaw. We are grateful for these supports.
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