Türkiye Jeoloji Bülteni

Abstract: Precious and Semi-Precious Stones, / in otherwords "gemstones", occupy a special placeamong other minerals and rocks due to theirrarity in nature, their durability, and some of theirphysical and chemical properties. Gemstoneshave been widely used in different cultureswith different meanings since the beginning ofhumankind . The belief that some stones weremagic in Mesopotamia about 6000 years ago,that amethyst was an antidote in Ancient Greece,that lapis lazuli in Egypt was considered sacred inIndia, and that jadeite was considered sacred inIndia show that various meanings were attributedto gemstones and that they attracted particularinterest from the past to the present.The use of precious and semi-precious stonesin different cultures as a symbol of status, powerand wealth, which is obtained by combining thestones with the art of jewellery and mountingthem on metal, is common not only in Anatolia butalso all over the world. Some precious and semiprecious stones are also used as ornaments andare an indicator of the owners’ aesthetic-artisticunderstanding. In addition, precious and semiprecious stones have the potential to contribute tothe economic resources of the region where theyare mined.

Abstract: The silicified wood samples that we identified in Western Anatolia are located in: the Late MiocenePliocene Çokköy Formation in Tavşanlı (Kütahya) and its surroundings, in Osmancalı (Manisa) and its vicinity inthe Early (?)-Middle Miocene Foça Tuff, and in Banaz (Uşak) around the Middle-Late Miocene Yeniköy Formationin the terrestrial sediments. The formation of wood petrified by silicification is associated with clastic sedimentsformed simultaneously with the Neogene period volcanic activity in the region. In the mineralogical-petrographicexamination of three oriented thin sections, including transverse, radial and tangential, chalcedony polymorphs ofsilica and lesseramounts of quartz and opal were detected in the silicified wood samples. Mineralogical interpretation was also supported by X-Ray Diffractometry (XRD) analysis. As a result of paleobotanical identification made withthin sections in the a botanical microscope; Taxodioxylon Hartig 1848 (Swamp cypress), Pinuxylon Gothan 1906(Pine), Pterocaroxylon Müll.-Stoll et Mädel 1960 (Walnut), Quercoxylon (Kräusel 1939) Gros 1988 (evergreen oak),and Glyptostroboxylon Conwentz 1884 (Swamp cypress) fossil wood species were determined. According to thesefindings, a warmer and more humid climate prevailed than today, and there were cypress forests on the edges ofswamps and streams, with pine and evergreen oak forests on the slopes. Within the scope of gemological examinationof the silicified wood samples, those which are rich in color/pattern were selected, polished by giving them cabochonforms, then mounted in silver. A small amount of facet cutting was carried out from unpatterned and monochromaticsamples. The silicified wood samples, which have high value in terms of hardness, exhibit an unstable character dueto the weak zones they contain, and they do not show the desired performance in terms of pattern/color.

Abstract: Agates in the study area, located around the town of Almus in Tokat province, are found in the form ofnodules along an approximately E-W trending fault within the volcanogenic sandstones of the Almus formation.The Upper Cretaceous aged Artova ophiolite complex, represented by metabasic ,and ultrabasic rocks on top of thePaleozoic aged Tokat metamorphics forming the basis of the study area, stands at the tectonic boundary, ,and theHaydaroğlu formation, which includes the Middle Eocene aged volcanogenic units, cuts ,and covers these older units.Sedimentary rocks such as sandstone, conglomerate ,and marine limestone containing volcanic materials belongingto older units, as well as the Lower Miocene aged Almus formation, which contains the agates, also unconformablyoverlie all units.Almus region agates contain on average 93.4%SiO2, 3.59%Fe2O3, 0.03%MgO, 0.72%CaO, 793 ppm Cr2O3,11.98 ppm Ni, 13.4 ppm Cu, 4.69 ppm Pb, ,and 35.4 ppm Zn. They are associated with the ophiolite complex, whileCu, Pb ,and Zn are associated with the early stages of volcanism. The high Fe2O3 content in agates is associatedwith hematite and goethite which were transported in agates before SiO2 emplacement. The CaO in the agate isthe product of the last phase of the solution forming the agate, and MgO is associated with the presence of dolomite. Field observations and geochemical evaluations of agates show that agates are epigenetically emplaced as aresult of the circulation of SiO2, which is the main component due to hydrothermal activities that occurred after therock formation of the Haydaroğlu formation, within the volcanogenic sandstones.

Abstract: The study area is located within the borders of Tokat province, which is one of the richest provinces inTurkey in terms of gemstone diversity. The region contains various high quality gemstone formations, such as jasper,opal, chalcedony, agate, and chrysoprase, which have both geological and commercial value.Metamorphic, magmatic, and sedimentary units from the Paleozoic to the present day crop out in the studyarea. While Paleozoic-Mesozoic Tokat metamorphics and Artova ophiolite mélange form the basement, the Almusformation, which is represented by Lower Miocene aged volcano-sedimentary rocks from the central parts of thearea to the east, covers these units unconformably. Quaternary slope debris and alluviums in the Yeşilırmak basinare the youngest units in the region. The chalcedony formations are observed along an E-W trending almost vertically dipping fault zone passingthrough the north of Döllük and Korucak villages within the metamorphic units of the Tokat Massif. The chalcedoniesare in the cracks, fractures and cavities of the recrystallized limestones that exhibit a fractured and brecciatedstructure along a line of approximately 25 m in the direction perpendicular to this fault zone.In our mineralogical, petrographic, and gemological studies, quartz, tridymite, cristobalite, calcite and dolomiteswere determined, together with chalcedony. Chalcedonies have an average of 96.3% SiO2, 1.96% Fe2O3, 0.56%MgO, 1.02% CaO, 780 ppm Cr2O3, 6.3 ppm Cu, 20 ppm Hg, 4.2 ppm Pb, 18 ppm Ni and 77 ppm Zn. Chalcedonyformations are the last product of a hydrothermal system related to volcanic activities after the formation of the wallrocks. The movement of chalcedony-generating solutions within the fault zone and their inclusion of carbonate wallrock and secondary calcite formations during chalcedony formation ensured that the CaO and MgO content of thechalcedony was relatively high.

Abstract: The study area is located around Gümüşyurt and Bayırlı villages in the county of Artova in Tokatprovince, Turkey. Permian-aged amphibolite rocks formed the foundation of the region. In the upper part of thisunit, Silurian-aged, grey-beige colored and recrystallized limestone is found. There are mainly metamorphic unitsof Permian metaclastics, metamorphic rocks and partially-silicified limestone blocks at the base. The metamorphicunits of metabasic origin consist of rocks such as chlorideschist, glaucophaneschist, micaschist and amphibolite.The partially-silicified limestone blocks of white, cream, beige, and blackish grey colors are hard and brittle. Theophiolithic rocks from Late Jurassic to Early Cretaceous show a wide distribution in the south part of the study area.Eocene limestone/conglomerate units and Pliocene conglomerates are unconformably overlaid on all previous units.Chalcedony samples, mostly green in color, occur within a small part of the Eocene limestone. Various green tonesof the chalcedony samples can be seen in different parts of the study area. Although some translucent, dark brown togrey color tones of the chalcedony samples are lower gem-quality material, the majority of the samples are of gemquality, good enough to be fashioned as cabochons and beads. Based on field observations, Raman spectroscopy,GemmoFTIR (Fouirer Transform Infrared Spectrophotometer for Gemology), geochemical analyses and standard  gemological tests, the samples were identified as chalcedony in general with different tones of green, specifically"Chromium Chalcedony".

Abstract: The chalcedony formations observed in the village of Nebiköy and its surroundings in Tokat province arefound in Permian-Triassic metamorphic rocks as veins with a thickness a few mm to 10 cm. Chalcedony, which hascolor of light blue-blue tones, is generally banded and sometimes has a kidney-like structure.In thin sections made from chalcedony, fibrous acicular quartz minerals and microcrystalline quartz mineralswere determined in a banded structure. Iron oxide fillings were encountered in the secondary dolomite formationsand micro-fractured cracks that cut these minerals in places. Quartz and dolomite associations were detected inXRD analyzes of the chalcedony samples.The main oxides in the chalcedony sample taken from the study area were determined as 94.08% SiO2, 1.45%CaO, 1.06% MgO, 0.37% Al2O3 and 0.31% Fe2O3; In the host rock (metasandstone) sample, it was determined as 37.5% SiO2, 16% CaO, 8% MgO, 6% Al2O3 and 4% Fe2O3. In the chalcedony sample, enrichments were alsoobserved in trace elements of 140.4 ppm As, 36.97 ppm Co, 5.4 ppm Ni and 1.45 ppm Cu; while in the host rock,enrichments were observed in trace elements of 53.3 ppm V, 28.9 ppm Ni, 7.5 ppm Co, 4.8 ppm As, 4.6 ppm Cu, 2.4ppm Mo, 2 ppm Ga and 1.5 ppm Bi.Cabochon cutting studies were carried out from the chalcedony samples taken from the study area to be usedin jewelry. Considering the light-dark blue color tone, durability, massive structure and workability observed in anarea of approximately 1 km2, it was determined that the chalcedony in the region can be used as gemstones.

Abstract: This study presents the geology and mineralogy of the opal nodules which occur in the Pliocene carbonatedconglomerates, NW of the Yenisofça village (Eskişehir, NW Turkey). The host-rock of opals consist of well-roundedpebbles, mainly opal, ultramafics (mainly serpentinites), and micritic limestones. The mainly white, transparent andgrey colored opal nodules range from 2 to 30 cm in diameter The X-ray diffraction (XRD) analyses revealed opalCT, quartz, dolomite and sepiolite as the main mineral components of opals. Scanning electron microscope (SEM)images show that sphere and fibrous textures are found in opal nodules. The comparison of fibrous textures observedin the opal nodules with those observed in sepiolite nodules based on SEM images showed that there is a similaritybetween them. The major element composition of the opal by SEM and whole rock geochemical analyses reveal thatthe concentration of the MgO is higher than the other oxides in the opals. Field studies, geochemical analyses, andmineralogical data suggest that the Yenisofça opals could have formed by sepiolite replacement by low temperaturesilica-rich hydrothermal solutions that circulated along the fractures systems. This result is similar and coherentwith the properties and formation of opal and cryptocrystalline quartz occurrences outcropping in the Eskişehir andKütahya regions.

Abstract: Interest in gemstones, which have been used as symbols of beauty, wealth and status since prehistoric times,has increased in recent years and taken an important place in many countries. The Menderes Massif is the mostimportant area where bauxite deposits are observed in Turkey. Gemstone quality diaspore crystals are formed only in these deposits. In this study, the mineralogical, geochemical and gemological features of the diaspore formationson the south-western side of the Menderes Massif (Bafa Lake - Milas) were investigated. In field studies carried outin the study area, diaspore crystals were found in the Pınarcık region, located between Bafa Lake and Milas, withinmetabauxite formations together with chloritoid and muscovite, and in some places were encountered in alteredzones close to the metabauxite limestone contact. As a result of petrographic analyses of the wallrock samples,grano-lepidoblastic textured “muscovite-quartzschist”, “quartz-phyllite”, “mica-quartzschist” and lepidoblastictextured “phyllite”, and nematoblastic textured “greenschist-chlorite-epidote fels” rocks were identified. To evaluatethe main oxide, traces and rare earth elements on the 28 collected samples, Al2O3-SiO2-Fe2O3 triangular variationwas used. According to the classification, the bauxite samples were determined to be in the “ferric bauxite” and“bauxite” areas. As a result of SEM imaging and EDS analyses performed on the samples, it was determined thatthe samples mainly contained O, Al and Si. K, Na, Fe, Ca and Mg were also found close to these elements. UsingRaman Spectroscopy, in the light of matching values obtained from the microscope`s own database, it was seenthat the samples taken from Pınarcık (PI) region were diaspore, and brucite (Mg(OH)2)) and ugillite (Ca3(Ti, Al,Zr)9O20) minerals were also detected. In addition, FTIR analysis was performed on 4 samples taken from Pınarcık(PI) region and according to the results of their analysis, it was seen that the samples were compatible with standardwavelength references. It was scientifically proven that the samples collected from the region and subjected to facetprocessing are gemstone quality diaspore crystals with various color variations. This study will make an importantcontribution to understanding the properties of the gemstone quality diaspore mineral, its formation, and findingsimilar formations in the region.