Physicochemical structural characterization of ambers from deposits in Poland

The physical and chemical properties of 8 samples of amber from different localities in Poland (Baltic Coast, Belchatow Tertiary brown coal, and Jaroszow clay mine) were investigated by N-2 sorption at 77 K, positron annihilation spectroscopy for chemical analysis (PASCA) and by organic geochemical methods (FT-IR, H-1 and C-13 NMR, GC, and GC-MS). The porosity of the ambers as determined by PASCA consists of narrow micropores with diameters ranging from 0.8 to 0.9 nm and a volume 0.025 cm(3) g(-1). In the external eroded part of the amber samples (rind) the concentration of pores where positronium atoms can form is lower and consists of approximately 1/2 the concentration as in their interior. Values of pore parameters determined from sorption of N-2 are comparable with those found by the PASCA method. The average diameter of pores ranges from 2 to 12 nm, while their volume varies from 0.018 to 0.048 cm(3) g(-1). The chemical character of the ambers is similar based on FT-IR spectroscopy. However, noticeable differences in concentrations of ester and hydroxyl groups are observed in both exterior and interior regions, where the abundances of the ester groups are lower in the exterior rind. The proportion of organic material extractable with chloroform-methanol (1:1, v/v) ranges from 15 to 50% and correlates inversely to the average reflectances (R(r)) of polished amber surfaces which range from 1.7 to 0.1%. These variations are attributed to differing concentrations of oxygenated groups in the respective amber samples. The FT-IR spectra of the non-polar fractions (NP) from the extracts resemble the spectra of the source ambers. However, the intensities of the absorbance for the hydroxyl group are much lower, while absorbances for exomethylene groups are not present. The H-1 and C-13 NMR data of NP fractions showed a complex diversity of components in mixtures with different relative concentrations but predominantly aliphatic in character for the respective samples. GC and GC-MS analyses of these fractions revealed that they are comprised of a mixture of compounds typical for Baltic amber but with variable relative concentrations. Two groups of compounds are found to be common to all NP fractions. The first is a minor concentration of homologous n-alkanes with a characteristic Gaussian distribution in the range from C-22 to C-32 and maximizing at C-26-C-27. In addition C-22 is characteristically slightly higher in concentration compared to C-23. The second group of compounds is comprised of succinates with methyl, fenchyl, bornyl and isobornyl alcohols. The composition of these diesters revealed the same equilibrium ratio between compounds with fenchyl, bornyl and isobornyl alcohols in all NP fractions. We suggest an early enzymatically controlled (bacterial) process in the formation of succinates during resin diagenesis from the biotic precursors, yielding the same characteristic ratio of the respective succinates in these ambers. These results show that all the ambers analyzed here fall into a common class of fossil resin, succinite (class Ia) independent of the sample location in Poland. Copyright (C) 1996 Elsevier Science Ltd.