1,721,082 research outputs found
Prograde metamorphic evolution and development of chloritoid bearing eclogitic assemblages in subcontinental metagabbro (Sesia-Lanzo zone, Italy)
No full textIn the coronitic metagabbroic rocks of the Corio and Monastero metagabbro bodies in the continental Sesia–Lanzo zone of the western Italian Alps, a variety of mineral reactions that testify to prograde conditions from greenschist to eclogite-facies can be recognised. A microstructural and microchemical study of a series of samples characterized by coronitic textures and pseudomorphic replacement of the original igneous minerals has allowed the prograde reactions undergone by the rocks to be established.
In completely eclogitized coronitic samples, paragonite, blue amphibole, garnet, epidote, fine grained jadeite and chloritoid occur in plagioclase microdomains (former igneous plagioclase). The mafic mineral microdomains consist of glaucophane and garnet. Complexly-zoned amphiboles constrain changing metamorphic conditions: cores of pre-Alpine brown hornblende and/or tremolite are preserved inside rims of a sodic–calcic amphibole that are in turn surrounded by a sodic amphibole. The main highpressure mineral assemblage, as seen in mylonites, involves glaucophane, chloritoid, epidote, garnet±phengite, ±paragonite. Some layers within the gabbro contain garnet, omphacite, ±glaucophane, and acid dykes crosscutting the gabbro body contain jadeite, quartz, garnet, epidote and paragonite.
The presence of chloritoid-bearing high-pressure assemblages reflects hydration of the gabbros during their pre-Alpine exhumation prior to subduction, as well as the composition of the microdomains operating during subduction. The pressure and temperature conditions of gabbro transformation during subduction are inferred to be 450–550 °C at up to 2 GPa on the basis of the chloritoid-bearing assemblages. The factors controlling the reaction pathway to form chloritoid-bearing high-pressure assemblages in mafic rocks are inferred from these observations
Alpine Metamorphic Evolution of Corio and Monastero Metagabbros (Sesia-Lanzo Zone): an Example of Prograde Evolution and Development of Chloritoid-bearing Eclogitic Assemblages in Basic Rocks
No full textSteady mineral sequence generated by reaction between olivine and plagioclase during sub-seafloor metamorphism in Al-Mg-gabbros, Northern Apennine ophiolites, Italy
No full textA petrological approach to the study of ancient glass.
No full textIn glass archaeology, "production indicators" are those remains which testify to specific operations carried out during the productive cycle; they allow us to reconstruct the processes and
technological expedients used in the past to produce glass. The intermediate products of glass melting reveal textural and chemical inhomogeneities which may be used to infer parts of the production history. Samples of glass masses belonging to an early stage of glass making were unearthed in the archaeological sites of Lomello (province of Pavia, Italy) (I-IV centuries A.D.) and Val Gargassa (Genova) (late XII Century A.D.). In these materials,
unmelted mineraI phases, banded textures, phase separation, and crystallisation alI produce different textures which supply information on provenance of raw materials; components added and the efficiency of the melting process; liquid immiscibility and
refining processesc; ooling rate of vitreousp roducts.
The composition of glass influences the different arrangements of tetrahedral-fold cations (SiO4-, AIO3-) and determines the various properties and features of the glass itself. The low effective
diffusivity prevents the composition from becoming homogeneous during glass melting, and consequently evidence of early production steps is retained. Mineral relics present in the micro-textures of materials (i. e., fining slags) have great archaeometric value, because they may indicate the provenance of raw materials. The composition of relic mineral phases also supplies important analytical grounds on which to define the recipes followed during glass making, in terrns of vitrifying, stabilising, flushing and additive components
Stained glass windows of Certosa di Pavia: micro-structure and chemical composition of glass panels
No full textA new assessment on yhe chemical compositin of stained gless from the Certosa di Pavia, Italy.
No full textThe samples from three stained glass windows examined bere afe made of Si-Ca-K glass. The exception is the yellow glass sample,
which has anomalous content in alkalis and calcium, yellow glass has the lowest sodium content, but it contains appreciable amounts of manganese. Zoned glass was produced by first making a multi-layered glass. In the twelfth and thirteenth centuries translucent glass was produced by multilayering , because certain colored glasses were so dense in tone that they did not allow sufficient light through. The composition of the Certosa glass is similar to that of Medieval glass. Medieval glass from Northern Europe, has been classified as: i) early wood-ash, ii) late wood-ash and iii) wood-ash-lime glass. In those times
(1000-1400 A.D.), beech-wood ash is considered to have been the raw material used by glassmakers. Differences in ash composition afe expected for beech trees growing in different countries and climates, but the differences in the Certosa glass are certainly due to different batch composition. Close relationships between the
compositions and ages of glass pieces cannot be inferred, owing to the small number of analysed samples. Therefore, replacements of glass pieces in the course of a rather long history of repair works, are far from being identified. In any case, knowledge of the chemical composition of glass is useful not only to speculate about ancient recipes or provenance attributes, but also to provide constraints when evaluating the chemical stability of glass exposed to atmospheric and polluting agents over several centuries
Le analisi mineralogico-petrografiche nello studio del vetro antico
No full textIn questo lavoro è illustrato con esempi, l’approccio interpretativo allo studio delle microstrutture presenti entro un manufatto vetroso e inoltre sono messe a confronto le informazioni che si possono trarre dalle analisi di “bulk” con quelle che si possono ottenere dalle analisi “in situ”. Per fare questo, sono stati scelti due tipi di reperti: un semilavorato ed alcuni frammenti di manufatti, vale a dire dei prodotti finiti .
I manufatti finiti, che sono il risultato di un ciclo di produzione e di lavorazione conservano il minor numero di indicatori tecnologici alla piccola scala di osservazione, poiché l’intento del vetraio era quello di produrre un manufatto molto omogeneo, in tessitura ed in colore. Un manufatto, quindi, è solitamente un vetro più omogeneo rispetto agli indicatori di produzione. In quest’ottica è possibile assumere che piccole porzioni del manufatto siano rappresentative della composizione chimica del manufatto stesso. Il bisogno di un’intrinseca conoscenza dei caratteri tessiturali e composizionali dei manufatti in vetro ha creato la necessità di poter prelevare frammenti, come primo passo per la comprensione delle modalità di manifattura e delle ricette di produzione. Tali dati rappresentano altresì gli indizi primari per la conoscenza di tali materiali e del loro stato di conservazione
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