War, captives, and human sacrifice were parts of Late Postclassic (AD 1250–1524) Maya culture in highland Guatemala. Las Casas (1958:152) wrote that the supreme lord “put the heads of the sacrificed on some poles on a certain altar dedicated only to this, where they had these for some time, after which they buried them.” These cultural aspects show up in human remains excavated at Iximche’, the Kaqchikel Maya capital. Here, we integrate previously published and unpublished results of stable isotope analyses and explore their implications for diets and the geographic origins of individuals who were buried at the site on the eve of the Spanish conquest. Data from Iximche’ are compared with available results from other ancient Maya sites.

In recent times, forensic science has increasingly relied on methods that use stable and radioactive isotope analysis to identify human remains. The use of 14C-AMS dating of dental enamel and dentine of an individual allows the estimation of the year of birth, while that of stable isotopes of carbon and oxygen can provide information on their geographical origin. Isotopic analysis of a tissue complements existing identification techniques, enhancing the capacity to refine, exclude, and affirm investigative approaches directed towards individual identification.

The primary aim of this exploratory study is to amalgamate diverse isotopic methodologies conducive to the prompt and accurate identification of a deceased individual. In this manuscript, we elucidate the application of a rapid processing technique for whole molars from individuals with documented ages, employed to ascertain age through 14C-AMS dating. Furthermore, an investigation was undertaken to assess the capacity of carbon and oxygen-stable isotopes in distinguishing regional disparities. To achieve this, we conducted a comparative analysis of tooth samples sourced from individuals residing in three cities within the Mexican Republic: Mexico City in the central region, Oaxaca City in the southern region, and Tepic Nayarit on the western Pacific coast. The age of dental piece formation, as estimated through 14C-AMS, exhibited a precise correlation with the actual age. By means of the stable isotope outcomes, the data disclosed substantial disparities in 13C and 18O abundances among teeth from individuals residing in the three cities.

Este trabajo aborda el estudio de la localidad arqueológica Cerro de los Gatos (Tierra del Fuego, Argentina), emplazada en un sector formado luego de la transgresión marina del Holoceno medio. Se exploran las transformaciones naturales y culturales del paisaje a nivel local y regional. Los aspectos presentados y evaluados aquí incluyen (1) tafonomía y evolución geomorfológica de loci diferentes, (2) disponibilidad de recursos y ecología isotópica, (3) tecnología lítica y ósea, (4) registro bioarqueológico humano y faunístico, y (5) cronología y reconstrucción paleoambiental del paisaje costero. Nuestra investigación sugiere que el registro de las ocupaciones humanas y el devenir de los paisajes naturales y culturales de la Bahía San Sebastián son el resultado combinado de la segregación espacial de las actividades (de subsistencia, tecnológicas, rituales) y sus trayectorias tafonómicas durante el Holoceno tardío.

A 350 14C yr discrepancy was found between dates on postcranial remains and mandibular teeth on what was thought to be the same individual from the Early Neolithic cemetery of Shamanka II, Lake Baikal. Stable nitrogen isotope results suggested a major shift in diet between childhood (when the teeth formed) and adulthood (represented by the postcrania), which could have resulted in different 14C ages through a freshwater reservoir effect. Subsequent additional dating on the mandible and postcranial elements, however, indicated that the mandible actually belonged to a different individual. More subtle reservoir effects can be seen on the sequentially forming teeth and mandible. The practice by prehistoric hunter-gatherers of Lake Baikal of re-opening graves and removing cranial elements has long been known, but this is the first evidence for the inclusion of a mandible from a separate individual, though whether it was intentional or incidental is uncertain. As well as providing new insights into mid-Holocene mortuary practices in the region, our findings raise a cautionary note for the examination of disturbed graves.

A new model for the interpretation of radiocarbon (14C) dates of Holocene marine shells is presented. For the Netherlands, the size of reservoir effect is difficult to assess, as these shells often lived in an environment of mixed marine- and river waters. Both stable isotopes 13C and 18O of the shell carbonate give insight in the environmental conditions the shells lived in. River water occurs in two main categories, distinguished by 18O: the Rhine which is dominant, and other rivers. This leads to two estuary mixing lines between the North Sea and rivers. The stable isotopes of the shell carbonate are also indicative for additional processes, such as uptake of secondary carbonate from the soil by shells, and exchange of C isotopes between atmosphere and water. Extensions of the main model deal with special cases such as pools of stagnant water and lakes. The model leads to an assessment of the recent 14C activities of the system the shells lived in, called 14aSYS. The measured 14C activities relative to these 14aSYS values determine the 14C age of the shells and include the reservoir effect. This way we circumvent normalizing to δ13C = –25‰, i.e. the terrestrial timescale and the subsequent correction for reservoir effects. The model is applied to a large legacy dataset of marine shells from the Netherlands, obtained during the last 7 decades. It contains 1116 14C dated shells; for the majority of these, the 3 isotopes 13C, 14C and 18O are measured.

As historic drought conditions become more common in western North America, Late Quaternary hydroclimate records become vital for putting present anthropogenic conditions into a longer-term context. Here, we establish a high-resolution record of drought for the eastern Sierra Nevada (California) using lacustrine carbonates from well-dated sediment cores. We used oxygen and carbon stable-isotope ratios, combined with high-resolution scanning X-ray fluorescence counts of calcium (Ca) and titanium (Ti), to reconstruct the drought record over the last 4600 years in June Lake. We found elevated δ18O and δ13C carbonate isotope values coinciding with peaks in both total inorganic carbon and Ca/Ti, suggesting enhanced carbonate precipitation in response to evaporative concentration of lake water. At least six intervals of prolonged (centennial-scale) carbonate deposition were identified, including three pulses during the Late Holocene Dry Period (LHDP; ~3500–2000 cal yr BP), the Medieval Climate Anomaly (~1200–800 cal yr BP), and the Current Warm Period, which began around 100 cal yr BP. This record highlights the complexities of the LHDP, an interval that was more variable at June Lake than has been previously described in regional records.

The Paleoproterozoic Stollberg Zn-Pb-Ag plus magnetite ore field contains SVALS-type stratabound, limestone-skarn hosted sulphide deposits within volcanic (bimodal felsic and mafic rocks)/volcaniclastic rocks metamorphosed to the amphibolite facies. The sulphide ores consist of semi-massive to disseminated to vein-network sphalerite-galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite). Thermochemical considerations and stabilities of minerals in the systems K-Al-Si-O-H and Fe-S-O and sulphur isotope values for sulphides of δ34SVCDT = +1.12 to +5.71 ‰ suggest that sulphur most likely formed by inorganic reduction of seawater sulphate that was carried in hydrothermally modified seawater fluid under the following approximate physicochemical conditions: T = 250o–350 oC, δ34SΣS = +3 ‰, I = ∼1 m NaCl and a total dissolved S content of ∼0.01 to 0.1 moles/kg H2O. However, a magmatic contribution of sulphur cannot be discounted. Carbon and oxygen isotope compositions of calcite in altered rocks spatially associated with mineralisation show values of δ13CVPDB = −2.3 to −0.8 ‰ and δ18OVSMOW = +9.5 to +11.2 ‰, with one anomalous sample exhibiting values of δ13CVPDB = −0.1 ‰ and δ18OVSMOW = +10.9 ‰. Most carbonates in ore show lighter C and O isotope values than those of Proterozoic (Orosirian) limestones and are likely the result of premetamorphic hydrothermal alteration involving modified seawater followed by decarbonation during regional metamorphism. The isotopically light C and O isotope values are consistent with those for carbonates spatially associated with other SVALS-type deposits in the Bergslagen ore district and suggest that such values may be used for exploration purposes.

After St James the Apostle, Bishop Teodomiro of Iria-Flavia is the most important figure associated with the pilgrimage to Santiago de Compostela. He supposedly discovered the apostolic tomb after a divine revelation between AD 820 and 830 yet, until the discovery, in 1955, of a tombstone inscribed with his name, his very existence was a matter of some debate. Here, the authors employ a multi-stranded analytical approach, combining osteoarchaeology, radiocarbon dating, stable isotope and ancient DNA analyses to demonstrate that human bones associated with the tombstone, in all likelihood, represent the earthly remains of Bishop Teodomiro.

The automatic graphitization system (AGE3) by IonPlus is very popular among radiocarbon dating laboratories. Usually, solid samples are burnt in an elemental analyzer (EA), and the gaseous CO2 is transferred for graphitization. Our system is coupled also with an isotope ratio mass spectrometer (IRMS), which measures the δ13C and δ15N of that gas. Some less routine pretreatment protocols require the production of gaseous samples and prevent the possibility of using the EA-AGE3 system, as the EA is used for solid samples only. In order to use that system, including the measurements of stable isotopes, we developed a glass tube cracker that connects to the EA. The device is routinely used in our laboratory and is mainly built from Swaglok catalog parts. We show that the background (blank) levels of a marble standard are indistinguishable between using the cracker and burning solid marble using the EA. We further demonstrate that the δ13C values are consistent and that the extraction efficiency when using the device is above 93%. Full descriptions, drawings, and working protocol are supplied.

We present new stable oxygen and carbon isotope composite records (δ18O, δ13C) of speleothems from Sandkraal Cave 1 (SK1) on the South African south coast for the time interval between 104 and 18 ka (with a hiatus between 48 and 41 ka). Statistical comparisons using kernel-based correlation analyses and semblance analyses based on continuous wavelet transforms inform the relationships of the new speleothem records to other proxies and their changes through time. Between 105 and ~70 ka, changes of speleothem δ18O values at SK1 are likely related to rainfall seasonality. Variations of δ13C values are associated with changes of vegetation density, prior carbonate precipitation (PCP), CO2 degassing in the cave, and possibly variations of the abundance of C3 and C4 grasses in the vegetation. The relationships of δ18O with other proxies shift between ~70 and 48 ka (Marine Isotope Stages 4–3) so that both stable isotope records now reflect CO2 degassing, evaporation, and PCP. Similar relationships also continue after the hiatus for the deposition phase between 42 and 18 ka. Our findings support modeling results suggesting drier conditions in the study area when the Southern Hemisphere westerlies are shifted north and the paleo–Agulhas Plain is exposed.

The oxygen and hydrogen isotope compositions of volcanic ash layers from coastal outcrops of the Miocene Monterey Formation elucidate the progressive burial diagenesis of glass to bentonite and metabentonite. Volcanic glass that texturally appears unaltered is found to have δl8O and δD values that are significantly higher than fresh volcanic glasses. The positive shift in δ18O and δD values is proportional to the amount of hydration and chemical alteration of the glass samples. Initial hydration of the glass results in the exchange of Na for H; later hydration results in the loss of K and Fe in exchange for H and Na. The δ18O values of the most hydrated glass, and clay minerals from bentonite and metabentonite layers are approximately in equilibrium with slightly modified seawater. The δD values of the hydrated glass, and clay minerals from bentonite and metabentonite layers, are significantly depleted in D relative to seawater and suggest meteoric exchange associated with tectonic uplift and erosion in the Pliocene and Pleistocene. Smectite from bentonite layers has δ18O and δD values similar to the most hydrated glass samples, suggesting similar glass-water and smectite-water fractionation factors. Kaolinite and mixed-layered illite-smectite (I-S) altered from smectite have lower δ18O and higher δD values than their precursor. The δ18O and δD of nonvolcanic siliceous mudstones from the Pt. Arguello oil field show an unusual decrease in δD with increasing burial depth that probably reflects the presence of organic hydrogen in the analyzed samples as well as possible D-depleted formation waters from detrital illite-water and hydrocarbon-water exchange.

Authigenic kaolinite and illite are important diagenetic minerals in the Magnus Sandstone, a giant oil reservoir in the northern North Sea. These clay minerals, separated from three wells, show considerable ranges in their oxygen isotopic composition (δ8OSMOW = +9 to + 16%) and hydrogen isotopic composition (δDSMOW = - 55 to - 105%). The variations in δ18O and δD are positively linearly correlated with a high degree of statistical significance for both kaolinite and illite:

$\begin{array}{c}\mathrm{K}\mathrm{a}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}:\mathrm{n}=12;\delta \mathrm{D}=6.1{\delta }^{18}\mathrm{O}-169;\mathrm{r}=0.66(>95\mathrm{\%})\\ \mathrm{I}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e}:\mathrm{n}=11;\delta \mathrm{D}=5.9{\delta }^{18}\mathrm{O}-159;\mathrm{r}=0.78(>99\mathrm{\%}).\end{array}$

Formation of the clays in a pore fluid of uniform isotopic composition over a range of temperatures appears unlikely. It is suggested that the observed relationships between clay mineral δ18O and δD are perhaps best explained by a model of precipitation at more or less constant temperature from pore fluids which varied isotopically across the oilfield. The isotopic composition of the formation waters would then lie along the line: δDw = 6.2 δl8Ow - 50. This is most plausibly interpreted as a mixing line with suggested minimal endmembers at (δ18O, δD) values of (+4, -24) and (-4, -76). The first of these represents reasonable isotopic values for Magnus Sandstone formation waters. Although δ18O of the second is compatible with an evolved Cretaceous meteoric water, its δD value is difficult to understand in the context of the model.

The Falkland Shelf is a highly productive ecosystem in the Southwest Atlantic Ocean. It is characterized by upwelling oceanographic dynamics and displays a wasp-waist structure, with few intermediate trophic-level species and many top predators that migrate on the shelf for feeding. One of these resident intermediate trophic-level species, the Patagonian longfin-squid Doryteuthis gahi, is abundant and plays an important role in the ecosystem. We used two methods to estimate the trophic structure of the Falkland Shelf food web, focusing on the trophic niche of D. gahi and its impacts on other species and functional groups to highlight the importance of D. gahi in the ecosystem. First, stable isotope measurements served to calculate trophic levels based on an established nitrogen baseline. Second, an Ecopath model was built to corroborate trophic levels derived from stable isotopes and inform about trophic interactions of D. gahi with other functional groups. The results of both methods placed D. gahi in the centre of the ecosystem with a trophic level of ~ 3. The Ecopath model predicted high impacts and therefore a high keystoneness for both seasonal cohorts of D. gahi. Our results show that the Falkland Shelf is not only controlled by species feeding at the top and the bottom of the trophic chain. The importance of species feeding at the third trophic level (e.g. D. gahi and Patagonotothen ramsayi) and observed architecture of energy flows confirm the ecosystem's wasp-waist structure with middle-out control mechanisms at play.

Seven accelerator mass spectrometry radiocarbon (AMS 14C) dates (7260±106∼7607±95 BP averaged 7444±103 BP) on a giant oyster shell, collected from an ancient shore of the Taipei Basin, are similar to the LSC (liquid scintillation counting) 14C age (7260±46 BP) of a grass sample inside the shell. The calibrated 14C ages of the C. gigas by Marine20 are 7490±240∼7805±230 cal BP (average 7660±96 cal BP), generally agreed with the calibrated LSC 14C ages of the grass and the oyster shell. Combined with other 14C ages of shoreline samples in the Taipei Basin, it is evident that sea level rose from 8600 to 7600 cal BP and reached a stand higher than modern sea level. During this marine transgression, the sedimentation rate along the shoreline was very high because 14C dating was not able to detect age differences for 4–5 m thick sediment sequences. Sixty-nine analyses of δ18O and δ13C from the oldest part of the shell exhibit clear seasonal cycles, with a 4-year period of growth in the 5.5-cm section. According to the δ18O values, the ancient oyster grew in a warmer-than-present shoreline environment, suggesting that the current absence of the giant oyster in Taiwan is not due to warming conditions.

Argillaceous rocks are considered to be a suitable geological barrier for the long-term containment of wastes. Their efficiency at retarding contaminant migration is assessed using reactive-transport experiments and modeling, the latter requiring a sound understanding of pore-water chemistry. The building of a pore-water model, which is mandatory for laboratory experiments mimicking in situ conditions, requires a detailed knowledge of the rock mineralogy and of minerals at equilibrium with present-day pore waters. Using a combination of petrological, mineralogical, and isotopic studies, the present study focused on the reduced Opalinus Clay formation (Fm) of the Benken borehole (30 km north of Zurich) which is intended for nuclear-waste disposal in Switzerland. A diagenetic sequence is proposed, which serves as a basis for determining the minerals stable in the formation and their textural relationships. Early cementation of dominant calcite, rare dolomite, and pyrite formed by bacterial sulfate reduction, was followed by formation of iron-rich calcite, ankerite, siderite, glauconite, (Ba, Sr) sulfates, and traces of sphalerite and galena. The distribution and abundance of siderite depends heavily on the depositional environment (and consequently on the water column). Benken sediment deposition during Aalenian times corresponds to an offshore environment with the early formation of siderite concretions at the water/sediment interface at the fluctuating boundary between the suboxic iron reduction and the sulfate reduction zones. Diagenetic minerals (carbonates except dolomite, sulfates, silicates) remained stable from their formation to the present. Based on these mineralogical and geochemical data, the mineral assemblage previously used for the geochemical model of the pore waters at Mont Terri may be applied to Benken without significant changes. These further investigations demonstrate the need for detailed mineralogical and geochemical study to refine the model of pore-water chemistry in a clay formation.

Widespread alteration in the Upper Cretaceous pyroclastic units of the Tirebolu (Giresun) area, NE Turkey, has resulted in significant occurrences of bentonite with economic potential. No detailed geological, mineralogical, or geochemical characterization of these occurrences has been carried out to date. The aim of this study was to describe the geological background, the mineralogical, chemical, and stable-isotope characteristics of the bentonite, and major aspects of their formation, e.g. type and source of low-temperature alteration, mass balance, chemical evolution of the smectites, and geochemistry of major and trace elements. The bentonite contains abundant smectite with occasional kaolinite and mordenite, volcanogenic feldspar, quartz, biotite, hornblende, glass shards, and pumice fragments, along with the diagenetic minerals, opal-CT, and, in some locations, calcite. X-ray diffraction patterns of the clay fractions exhibit characteristics of pure montmorillonite and beidellite-type smectite. Micromorphologically, the smectite exhibits a honeycomb texture, the kaolinite occurs in both vermiform and irregular platy forms, and the mordenite occurs in fibrous form. All of these minerals are edged with devitrified volcanic glass and resorbed feldspar. Chemically, the smectites are Ca-smectite. Geochemical data indicate that alteration of the pyroclastic units took place under suboxic and anoxic environmental conditions during bentonite formation. Field observations and mineralogical, geochemical, oxygen, and hydrogen isotopic data indicate that the alteration of feldspar and volcanic glass in the pyroclastics by mixed meteoric and sea water in a shallow marine environment under alkaline and acidic conditions, respectively, controlled by environmental Al, Ca, and Na concentrations, resulted in the formation of authigenic smectite, mordenite, and kaolinite. A large Ca content in the smectite originated from surrounding units, which resulted in high alkalinity; Mg originated from seawater.

Hydrothermal solutions related to magmatic intrusions that occurred during the Oligo-Miocene resulted in advanced multi-stage alterations and, to varying degrees, kaolinization over a wide area of the Biga Peninsula. The most important formations in these kaolin deposits occurred along the NE–SW-trending Çan-Etili-Bayramiç fault zone. The Bahadırlı, Duman, and Çaltıkara quarries are well preserved kaolin deposits throughout these fault zones located within the Çan Volcanics. Mineralogical, geochemical, and isotopic analyses were performed to identify the environments of formation of kaolins and the origin of the hydrothermal solution that led to the formation of these deposits.

The mineralogical assemblages of each of the quarries differ from each other. The Bahadırlı kaolin quarry comprises kaolinite + alunite ± quartz ± smectite ± plagioclase ± K-feldspar. The Çaltıkara kaolin deposits consist of kaolinite + quartz + alunite ± iron-oxide and ore minerals. The Duman kaolin quarry was considered in two different slopes as a hanging wall block and a footwall block, and the blocks contain kaolinite + quartz + plagioclase ± smectite and kaolinite ± alunite ± smectite ± quartz ± plagioclase ± K-Feldspar ± gypsum, respectively. During petrographic investigations, it was observed that kaolinization occurred generally in K-feldspar and plagioclase phenocrysts and partially in the matrix. Mineralogical and micromorphological investigations revealed that the kaolin group includes the dioctahedral minerals kaolinite and halloysite. According to the trace element contents of the kaolinites, the Çaltıkara and Bahadırlı deposits had a hypogene origin, while the Duman deposit had a supergene origin. δ18O isotopic values of kaolinites ranged from +10.3 to +18.3‰. δ34S isotopic values ranged from –17.2 to +20.2‰ of alunite, galena, and pyrite minerals, which indicate different formation environments for the kaolin quarries. 40Ar/39Ar dating of alunite revealed that the timing of acid-sulfate alteration was compatible with the magmatic intrusions. All these data revealed the mineralogical, chemical, and isotopic differences caused by the effect of different hydrothermal processes in three kaolin deposits located on the same fault zone, close to each other and similar in age.

The origins of dolocrete and associated palygorskite in the Çanakkale region of Turkey have been little studied, but are of fundamental importance for a more complete understanding of the mineralogy of this region. The present study was undertaken in order to narrow this gap. Siliciclastic red mudstones within alluvial-fan deposits of the Middle Miocene Sariyer Formation locally contain dolocretes in various forms (powdery, nodular, and fracture-filling) and scarce matte-brown, authigenic clay lenses. The mineralogical characteristics of dolocrete and authigenic clay lenses were examined using polarized-light microscopy, X-ray diffraction, differential thermal analysis and thermal gravimetry, scanning-electron microscopy, and infrared spectroscopy, as well as by chemical and isotopic methods. These analyses indicate that the dolocretes are indeed predominantly dolomite, coexisting with variable amounts of palygorskite. The authigenic clay lenses are composed mainly of palygorskite. Dolomite appears as euhedral crystals, whereas palygorskite developed authigenically as interwoven fibers on and between resorbed dolomite crystals, rimming euhedral crystals, and as fiber bundles (where dolomite ± magnesite is absent). The stable-isotope values and some petrographic features, such as alveolar texture and dolomite needles, support a pedogenic origin for the dolocretes. In the initial stage, dolomite formed by replacement of siliciclastic red mudstones and/or by precipitation from percolating soil-derived water in a near-surface setting. Subsequently, palygorskite either precipitated on the dolomite crystals from relatively more evaporative water, replaced the host-rock mudstone in the presence of Al + Fe, or formed directly from solution where the Ca/Mg ratio decreased and the Al + Fe increased. In view of the large Cr and Ni contents of the bulk-rock samples, the elements required for the crystallization of dolomite and palygorskite (namely Mg, Ca, Si, Al, and Fe) may have been supplied by weathering of ophiolitic rocks that crop out in the area.

Pliocene volcaniclastic lacustrine rocks of the Doğanbey formation in the Beyşehir region (central Anatolia) are composed of organic-matter-bearing claystone, clastic units and dolomite interbeds, suggesting an anaerobic, shallow swampy, lacustrine depositional environment. The depositional environment was subjected to periodic climatic change during which diagenesis occurred, and smectite, and locally palygorskite, were precipitated. Smectite flakes formed authigenically on feldspar and palygorskite fibers between dolomite rhombs and at the edges of smectite flakes. Increases in leaching of Na, K, Sr, Ba and Rb, increasing Al/Si ratios, and Fe with increasing degree of alteration reveal that hydration of volcaniclastic grains (feldspar, glass) by meteoric water — determined from O and H isotopic values — was the main cause of precipitation of beidellite and montmorillonite based on the tetrahedral charge/octahedral charge ratio, with average structural formulae of (Si7.72Al0.28)(Al3.20Fe0.53Mg0.25 Mn0.02Ti0.04) (Ca0.11Na0.09K0.11), and (Si7.88Al0.13)(Al3.18Fe0.53Mg0.18Mn0.02Ti0.05)(Ca0.11Na0.11K0.09), respectively. Therefore, the Doğanbey-area smectite is presumed to have formed by chemical weathering and dissolution-precipitation from feldspar and glass during diagenesis; palygorskite formed by direct precipitation from Mg-rich solutions during dolomitization, and by transformation from smectite in an alkaline lacustrine environment.

The Söğüt kaolinite deposits at Çaltı, İnhisar, and Küre, Turkey, are an important source of raw materials for the ceramics industry of that country, but no detailed mineralogical or geochemical characterizations of these economically important materials have been carried out to date. The purpose of this study was to fill this gap by performing mineralogical, geochemical, and isotopic characterizations of these kaolinite deposits which occur within Paleozoic granite-granodiorite complexes that are crosscut by aplite and pegmatite dikes, and overlain by Neogene sedimentary units. These units are dominated by quartz veins and networks of subvertical fractures and weak zones that were invaded by hydrothermal fluids, resulting in their kaolinization and silicification. Altered units and related host rocks were examined using polarized-light microscopy, X-ray diffractometry, scanning electron microscopy, infrared spectroscopy, and chemical and isotopic methods. Feldspar crystals are either sericitized or kaolinized, and mica exhibits partial chloritization; Fe-Ti-Mn oxides occur within fractures. Kaolinite crystals occur in authigenic vermiform or plate-like stacked forms, having contacts with resorbed feldspar crystals which locally exhibit thick, platy, and subparallel orientations relative to microfractures, the pathways for hydrothermal-fluid injection. Altered feldspar relicts are associated mainly with kaolinite, smectite, quartz crystals, and illite/mica. Increase in (Al+Fe)/Si in the kaolinized units (relative to host-rock granite and granodiorite complexes and silicification), depletion of Ba+Rb, and a negative Eu anomaly reveal that the alteration of feldspar by hydrothermal fluid, the character of which was determined from O- and H-isotopic values, resulted in the precipitation of kaolinite. Thus, the Söğüt kaolinite deposit possibly formed by hydrothermal alteration and a feldspar dissolution-precipitation mechanism in both the granite-granodiorites complexes and related overlying sedimentary units under acidic environmental conditions, which developed via depletion of the soluble elements Na and Ca.