Samuilov Kamak”
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Geoarchaeology and Archaeomineralogy (Eds. R. I. Kostov, B. Gaydarska, M. Gurova). 2008. Proceedings of the International Conference, 29-30 October 2008 Sofia, Publishing House “St. Ivan Rilski”, Sofia, 126-129. DETERMINATION OF THE PROVENANCE OF THE ARCHAEOLOGICAL MONUMENT “SAMUILOV KAMAK” Lubomira Macheva Central Laboratory of Mineralogy and Crystallography “Acad. I. Kostov”, Bulgarian Academy of Sciences, 1113 Sofia; [email protected] АBSTRACT . A stone slab bearing an inscription “Samuil – tsar and samodrzhets vsem blgarom” was found on 25 March 2004 in the vicinities of the village Samuilovo, Petrich municipality. At present it is kept at the Historical Museum in Blagoevgrad. On request of the latter, a detailed mineralogical and petrological investigation was carried out at CLMC-BAS to determine its provenance. A small piece of the slab was sampled and studied using polarizing petrographic microscope, scanning electron microscope and X-ray powder diffractometer. The petrographic investigation shows that the rock is serpentinised peridotite, consisting mainly of olivine and tremolitic amphibole. Metamorphosed ultrabasic rocks, analogous to the studied sample build up lenticular and lens-like bodies on the Northern slope of the Belassitsa Mountain (to the South of the Samuilovo village, between Kliuch and Yavornitsa villages, as well as to the South of the Kolarovo village). Based on the accomplished investigations we suggest that the inscribed stone slab which, according to the archeologists served as a border stone of the Mediaeval Kingdom of Tsar Samuil, is most likely to originate from the metamorphosed ultrabasic bodies in the immediate vicinity. Introduction For more precise determination of the rock mineral A stone slab bearing an inscription “Samuil – tsar i composition X-ray powder diffractometer Dron-3M with filtered samodrzhets vsem blgarom” (Fig. 1) was found on 25 March Co radiation was used. 2004 in the vicinities of the Samuilovo village. At present it is kept at the Historical Museum in Blagoevgrad. On request of Petrographic description the latter, a detailed petrological and mineralogical Macroscopically the rock is fine grained, dark green to investigation was carried out at the Central Laboratory of greyish-green in color with a schistose structure. The surface Mineralogy and Crystallography, Bulgarian Academy of of the studied slab is affected by weak weathering processes Sciences, in order to determine its provenance. A series of and precipitation of iron hydroxides (goethite) can be observed analyses – petrological, mineralogical and X-ray powder (Fig. 1). diffraction analysis were carried out to clarify the rock type and its origin. Material and methods A small piece of the slab was extracted and studied using polarizing petrographic microscope, scanning electron microscope and X-ray powder diffractometer. From the sampled study piece cut from the stone slab two thin sections were prepared for petrographic analyses, one double side polished slice for electron microprobe analyze and one sample for qualitative powder diffraction analysis. The petrological identification of the rock was made with polarizing microscope in transmitted light, the identification of the ore minerals was made with polarizing microscope in reflected light and the chemical composition of the minerals was determined with scanning electron microscope Philips SEM-515, equipped with X-ray spectrometer microanalyzer. Fig. 1. Stone slab bearing an inscription “Samuil – tsar i samodrzhets vsem blgarom” 126 The texture of the rock is pseudomorphic to relict-granular, The replacement initiated along the crystal boundaries and formed after olivine and amphibole, mesh texture in relation to progressed inward. In the peripheral parts of the grains and the net-vine-like pattern of the serpentine minerals cutting off after fractures within them a fine fibrous serpentine mineral the olivine aggregates, nematoblastic after amphibole and (chrysotile) is developed. Olivine compositions only cover a hetero-granular after serpentine and chlorite minerals. small range and fall into chrysolite-forstertite group (Table 1). Its forsterite content is close to 90%. The crystallochemical Mineral composition: The rock under study contains 40-50% formulae is (Mg 1,78 Fe 0,20 Ni 0,01 )SiO 4. olivine, 20-30% tremolitic amphibole, up to 20% serpentine- group minerals, up to 2% chlorite and talc and magnetite <1%. Amphibole forms randomly oriented stubby-prismatic to long- The rocks consists mainly of olivine and tremolitic amphibole, prismatic, colourless porphyroblasts, which after their optical forming coarse-grained relics included in a fine grained peculiarities, chemical composition (Table 1) and X-ray chlorite-serpentine matrix, i.e. it could betoken development of diffraction data (Fig. 3), correspond to tremolite. The several superimposed metamorphic processes, by which a crystallochemical formula is (K,Na)(Na 0,09 Ca 1,89 Fe 2+ 0,02 ) magmatic olivine-bearing rock has undergone several stages (Fe 2+ 0,18 Mg 4,69 Ni 0,10 Cr 0,03 ) (Al 0,03 Si 7,97 )O 22 (OH) 2. The elongated of metamorphic transformations. porphyroblasts show intensive transverse fracturing (Fig. 2a). The cracks between individual fragments are sealed with Olivine is presented both by small-sized and by quite coarse, fibrous pile greenish in color serpentine mineral. The tremolitic subrounded to rounded, colourless dismembered relics (Fig. amphibole is formed probably at the expense of primary 2a, b). The coarse grains show sometimes well developed clinopyroxene as a result of superimposed regional individuals with pronounced cleavage. Most grains are metamorphic processes. replaced pseudomorphically by meshwork isotropic serpentine (serpophite). Serpentine group minerals occurred in two of the three known polymorphs: lizardite and chrysotile, determined on the basis of X-ray diffraction analysis (Fig. 3). The detailed petrographic investigations reveal that the serpentinization proceeds in two main stages, marked by different microstructural positions of both serpentine minerals. The products of the first one, called pseudomorphic serpentinization, could be seen in various stages of advancement in different portions of one thin section. Colourless to pile green lizardite replaces the olivine and forms polygonal mesh texture arranged as an irregular anastomousing network between the olivine relics. Sometimes structureless serpentine cores are observed. Small magnetite grains, formed by the process of serpentinization, are scattered all over the rock. The second stage is marked by the production of another serpentine polymorph – chrysotile. It is pile-greenish in color, fibrous, with very low (grey) interference colors and fills cracks in olivine and tremolite porphyroclasts. a Both serpentine polymorphs are indistinguishable in their chemical composition. The summarized formulae, obtained on the basis of three analyses is (Mg 2,62-2,77 Fe0 0,21-0,37 Cr 0,01 Ni 0,01- 0,02 )(Si 1,95-1,98Al 0-0,03 )(OH) 4O5. Тable 1 Chemical composition of the minerals from serpentinized peridotites building up the stone slab with the inscription "Samuil..." Olivine Amphibole Chlorite Serpentine Magnetite An. N Аn. 1 Аn. 2 Аn. 4 Аn.5 Аn.6 Аn. 7 An. 9 SiO 2 40,69 57,93 39,2 38,26 32,95 32,16 0,37 TiO 2 * * * * * * 1,36 Al 2O3 0,36 0,81 0,4 * 14,02 13,12 0,45 FeOt 9,82 1,73 7,76 5,31 3,68 3,76 64,08 MnO 0,09 * 0,15 * * * 0,41 MgO 48,91 22,85 36,49 39,17 34,38 33,7 1,32 CaO * 12,81 * * * * * Na 2O * 0,32 * * * * * b NiO 0,27 0,17 0,43 0,24 0,24 0,24 0,32 Cr O * 0,24 0,28 0,32 2,41 2,69 25,76 Fig. 2. Microphotographs of: above (a) – olivine partially replaced by 2 3 VO meshwork serpentine and tremolitic amphibole crosscutting olivine 4 * * ** * * 0,7 grains and serpentine pseudomorphs after olivine (black plates are of H2O* * 3,13 15,28 16,71 12,33 14,33 * Total 100,14 99,99 99,99 100,01 100,01 100 94,77 isotropic serpentine); below (b) – olivine grain crosscutted by serpentine * - element not determined minerals 127 rock under study had an early stage of hydratation, because it contains relic magmatic olivine grains, being partially serpentinised and on account of which a pseudomorphic mesh texture originates. Experimental studies on the system MgO- SiO 2-H2O have indicated that serpentines cannot be formed at temperatures above 500 0C, and that formation of serpentine by the action of water on forsterite can occur only below 400 0 C (Bowen, Tuttle, 1949). We consider that at a latter stage the rock has undergone regional metamorphism under amphibolite facies conditions at elevated temperatures (above 400 0 and below 600 0C) and medium pressure, during which tremolitic amphibole has been formed, but serpentine minerals remain stable. On the peridotites already metamorphosed in amphibolite facies a low temperature greenschist facies metamorphism is imposed, with which a new episode of partial Fig. 3. X-ray powder diffraction of metaperidotite slab bearing an serpentinization is associated. During this retrograde inscription (abbreviations: Tc – talc; Ol – olivine; Trem – tremolite; Chl – metamorphic stage the crack-filled replacement of tremolite chlorite; Serp – serpentine) and partially of olivine by fibrous chrysotile is realized. The lack of carbonate minerals in the studied rock indicates low activity Chlorite forms small-sized, colourless and irregular in form of CO 2 during the process of regional metamorphism. Identical flakes having anomalous brown interference colours, usually evolution for the regional metamorphism has been proposed arranged in close spatial association with the serpentine for the Lozen metagranites (Macheva et al., 2006), exposed minerals. It is rich in Mg (X Mg -0.94) and the chemical further to the West in the Belassitsa Mountain. composition can be defined as pennine. Its crystallochemical formulae, calculated on the basis of 3 analysis is (Мg 4,85- 4.89 Al 0,63-0,68 Fe 2+ 0,29-0,31 Cr 0,18-0,21 )(Si 3,12-3,13 Al 0,84-0,88 )O 10 (OH) 2 Possible provenance of the archaeological (Table 1).