O czarnym bursztynie i gagacie

Barbara Kosmowska-Ceranowicz, Zdzisław Migaszewski

Abstrakt


ON BLACK AMBER AND JET

Summary
The preliminary mineralogic and petrographic investigation performed on natural fossil black resins, jets and black amber jewellery (often defined as "black ambers") indicated the presence of numerous differences in mineral (structural composition, physicochemical properties etc.).
l . Natural fossil black resins are represented by three types, as follows:
1.1. The first type includes 2 samples of stantienite (black amber sensu stricto) derived from Palmniki deposite (near Kaliningrad - USRR). They have a velvet black colour (Fig. 2), a brown to light brown streak (the latter from Simon's collection) and a strong lustar on polished surface. Under microscope the resin is a structureless groundmass of dark gray colour containing yellow orange and blood red inner reflections (Fig. 3). The both varieties reveal the same shape of the infra-red curve (refer to Fig. 4) and almost identical mean values of microhardness (230 and 240 MPa).
1.2. The second type is represented by 2 samples (Fig. 6) of soft fossil resin from the Miocene deposits of Ukraine (USRR) and Bitterfeld (in the vicinity of Halle, GDR). They display unique similarity in mineral (structural) composition, partly depicted by the shape of the IR curve (see Fig. 4), as well as by identical, through exceptionally low mean values of microhardness (70 MPa each). Macroscopically, it is a structureless, porous cracked and lusterless resin, of black color yielding a dark grayish brown streak. Under microscope the resin is a homogenous, through somewhat porous, groundmass of gray colour with numerous yellow orange and blood red inner reflections. In addition, it contains scattered fragments of crushed plant tissue (Fig. 7).
1.3. The third type represented a small lump of resin found within the Miocene deposite in Bitterfeld, where it was found along with the second type mentioned above. The resin has a velvet black colour, a brown streak and a very luster. Under microscope the resin is a homogenous groundmass of gray colour, deprived of inner reflections. It has the highest mean value of microhardness, which is 280 MPa, exceeding the analogous value for all fossil black resins. Though resembling stantienit sensu stricto in many ways it displays quite a different shape of the IR curve (see Fig. 4)
1.4. To the same type belongs a sample (Fig. 8), from Bytów (some 80 km southwest of Gdańsk in Poland). It reveals black colour and black streak, as well as good luster on polished surface. Microscopically, the resin is a gray homogeneous groundmass, somewhat porous, containing yellow orange and blood red inner reflections. The mean microhardness amounts to 210 MPa. The shape of the IR curve is almost identical of that for the type from Bitterfeld (variety).
2. Jet from Odrowąż (some 25 km north of Kielce, Poland) assigned to the lower Jurassic is the resin-bearing variety of brown-humic coal (Fig. 9). The jet in question is represented at least by three petrographic varieties of different mineral (structural) composition. Macroscopically, it is pitchblack, semi-gloss, sometimes lusterless, displaying earthy, occasionally conchoictal fracture and massive, sporadically streaky structure. Under microscope, lightgray - gray coBinite (primarily "mottled"), sometimes lightgray - darkgray telinite impregnated with resinite are predominant. Mineral substance composed primarily of pyrite, sporadically of syderite is encountered in various proportions (Fig. l 0). The mean values of microhardness range from 210 to 260 MPa. The IR curve (refer to Fig. 4) reveals the lack of the carboxyl group within a 1700 cm-1 absorption band, which is characteristic of that for the fossil resins.
3. Black amber jewellary is represented on Polish market (Fig. 11 ), by fine-grained Baltic amber (succinite) pressed under high pressure and temperature. Such prefabricated resin reveals black colour and black streak, and good luster on polished surface. Under microscope, it resembles the breccia composed of dark gray particles cemented by crypto/micro-grained, light gray substance (Fig. 12). The IR curve is characteristic of that for succinite (see Fig. 5). The mean value of microhardness amounts to 270 MPa, being within the values for typical ambers (240 through 280 MPa).
As indicated by preliminary mineralogic and petrografic examinations the most interesting and mysterious group are fossil black resins (black ambers sensu stricto). In the authors' view, the genesis of the black ambers seems to be connected with the strokes of thunderbolts. The investigation performed did not explain the main cause of such differentiation within the group mentioned above (various entry cheroical composition of resins, influence of different physico-chemical conditions on polimeryzation, oxidation etc.). New data in this respect may be obtained from fluorescence, cheroical and reflection testings. Considering this, the problem of the genesis of black ambers still remains an open question.

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