Search

Dehydration of silanol and molecular water in 60 agates from 12 hosts with ages between 23 to 2717 Ma has been investigated using desiccators and high-temperature furnace heating. There are wide differences in the water data obtained under uncontrolled and fixed atmospheric water vapour pressure conditions. After agate acclimatization at 20°C and 46% relative humidity, the total water (silanol and molecular) was determined in powders and mini-cuboids by heating samples at 1200°C. Agates from hosts < 180 Ma all showed a greater mass loss using powders and demonstrate that after prolonged high-temperature heating, silanol water is partially-retained by the mini-cuboids. Desiccator dehydration of powders and slabs shows that powder preparation can produce water losses; this is particularly relevant in agates from hosts < 180 Ma. The identified problems have consequences for water quantification in agate and chalcedony using infrared or thermogravimetric techniques. Mobile and total water in agate is considered in relation to host-rock age, mogánite content and crystallite size. Links are observed between the various identified water contents allowing comment on quartz development and agate genesis. The water data also supports previous claims that agates from New Zealand and Brazil were formed long after their host.

Understanding the retention mechanism of U by amorphous silica (i.e., opal) in the environment is of great importance to nuclear-waste disposal because opals can retain U for millions of years. Uraniferous opals from Spor Mountain and the Thomas Range, Utah, U.S.A., are examined in terms of their mineralogical, chemical, and isotopic compositions. Uranium-rich zones composed of most likely vorlanite, CaUO 4 , occur in fibrous opal-CT (termed lussatite) along the interface of the phase with microcrystalline quartz. Red- to black-colored precipitates of vorlanite also occur in the interstices between fibers and grains in lussatite and massy opals, respectively. The high abundance of vorlanite in certain growth zones can be explained by the diffusion of Ca and U along boundaries of layers, grains, and fibers and by the release of Ca and U through the transformation of opal-A into opal-CT and from opal-CT into microcrystalline quartz. Similar O-isotope compositions of opal-CT and associated microcrystalline quartz indicate that crystallization processes and deposition of subsequent layers of opal occurred from fluids of similar origin and T . Differences in the isotope and chemical composition between uraniferous opals/microcrystalline quartz, the SiO 2 polymorph moganite, and pyrolusite indicate the past occurrence of various alteration processes involving fluids of different composition and T . The results of this study indicate that assemblages of opal and microcrystalline quartz can retain U for millions of years as long as confined pore spaces between different generations of opal and quartz or between growth features of opal provide space for the nucleation and adsorption of U-bearing phases and species.

This study presents Raman spectroscopic analyses of melt inclusions in tourmaline from tourmaline-quartz-(muscovite) assemblages of common pegmatites of the Gatumba-Gitarama area (Rwanda). The melt inclusions show a main mineralogy composed of muscovite, α-quartz, moganite, dickite, and minor feldspars which demonstrate, in combination with the observation of dawsonite, nahcolite, jeremejevite, and childrenite daughter minerals, a CO 2 -, H 2 O-, B 2 O 3 -, and P 3 O 4 -enriched peraluminous boro-aluminosilicate composition for the trapped melt. The variable amount of acidic interstitial fluids inside the melt inclusions resulted mainly from heterogeneous trapping of omnipresent, exsolved aqueous fluids during melt inclusion entrapment. Aliquots of this exsolved alkali-rich aqueous fluid phase are preserved in the numerous coexisting fluid inclusions in tourmaline. The observed mineralogy and composition of the melt inclusions deviates strongly from a bulk single-phase melt crystallization model for pegmatite formation. Based on reported experimental, theoretical, and natural constraints, an alternative hypothesis can offer an explanation for the formation of the anomalous but omnipresent residual melts trapped in tourmaline: i.e ., the immiscibility of a hydrous fluid and a boro-aluminosilicate melt from the residual bulk aluminosilicate melt. The chemically anomalous composition of the immiscible boro-aluminosilicate melt trapped inside the melt inclusions can explain the mineralogical transition from a granitic mineral mode towards a schorl-quartz-(muscovite) assemblage within the common mineralogical zonation of the pegmatite dike.

Silica polymorphs, such as quartz, tridymite, cristobalite, coesite, stishovite, seifertite, baddeleyite-type SiO2, high-pressure silica glass, moganite, and opal, have been found in lunar and/or martian rocks by macro-microanalyses of the samples and remote-sensing observations on the celestial bodies. Because each silica polymorph is stable or metastable at different pressure and temperature conditions, its appearance is variable depending on the occurrence of the lunar and martian rocks. In other words, types of silica polymorphs provide valuable information on the igneous process (e.g., crystallization temperature and cooling rate), shock metamorphism (e.g., shock pressure and temperature), and hydrothermal fluid activity (e.g., pH and water content), implying their importance in planetary science. Therefore, this article focused on reviewing and summarizing the representative and important investigations of lunar and martian silica from the viewpoints of its discovery from lunar and martian materials, the formation processes, the implications for planetary science, and the future prospects in the field of “micro-mineralogy”.

In 2008, excavations were conducted by the Northern Ceramic Society at the site of the former Lancaster delftware potworks, which operated between 1754 and about 1790. The recovered sherds have been non-destructively examined by Raman and electron microscopies and the iron phases in the biscuit by Mossbauer spectroscopy. These methods have provided a new understanding of the mineralogy of the delftware produced at Lancaster using clay imported from Carrickfergus in Ireland and blended with the local ferruginous-aluminous clays. This has implications for the attribution of delftware produced at Liverpool, Bristol, Scotland and Ireland. The Carrickfergus clay has been found to be highly dolomitic resulting in the body of the delftware forming diopside and the magnesian olivine forsterite, when fired. Brookite had not converted to rutile, nor had tridymite, nor cristobalite formed; the K-feldspars did not undergo further chemical reactions, and the observation of metakaolin would suggest that the biscuit firing temperature was in the order of 800–900 °C. Chalcedony containing moganite was established as the silica source. A cobalt-containing lead-tin glaze was applied to the biscuit body and, after decorating, was fired. The mineralogy of the pigments used to decorate the objects indicate the yellow to be the PbSbSn triple oxide, the green to be a copper silicate mixed with lead-tin yellow, the purple and browns to be manganese silicates and the blue to contain cobalt spinels and cobalt pyroxenes. Diopside crystals together with recrystallised tin agglomerates have been observed floating within the glaze. Copyright © 2015 John Wiley & Sons, Ltd.

Quartz microcrystals from the El Indio Au-Ag-Cu deposit (Chile) preserve a rare glimpse into the high-temperature evolution of silica. Here, we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates “opal” and moganite. We propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase. Evidence for sequential dehydration to from silica hydrate to quartz (silica hydrate⌫opal⌫moganite⌫quartz) is provided by SHRIMP 18O microanalytical data that show oscillatory isotopic zoning from 3.6‰ to 16.2‰ δ18O (± 0.5‰) coupled with K and Al variations. We estimate that the precursor silica hydrate deposited between ~ 480–680 °C and contained 32–63 wt% H2O. Silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation, a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the El Indio copper-gold deposit. Modern understanding of ore-forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions. The discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids. Quartz matured from silica hydrate would record “pseudo-primary” fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while δ18O data may bias fluid provenance interpretation within sub-volcanic systems.

References in the ancient sources indicate that the Libyan desert was a source of ‘carbunculi’: semi-precious red stones and gemstones variously interpreted as ruby, garnet and spinel, amongst others. While gemstones are not attested in the geological strata of Fazzan (south-west Libya), a range of silica-based stones including chert, chalcedony, agate and carnelian are known to originate in this area, linked to an early civilisation known as the Garamantes. It has been long proposed that the geochemical signature and the variations in the relative proportions of quartz:moganite phases can be used to distinguish between groups of stones of different origin. The proposed methodology was tested on a number of archaeological samples from the Garamantian sites of Jarma (ancient Garama) and Saniat Jibril, in Fazzan. Fragments of chert, carnelian and amazonite found at the two sites have been identified as raw materials associated with beadmaking. Trace elemental data obtained by LA-ICP-MS were combined with mineralogical data obtained by X-ray powder diffraction and Raman spectroscopy on the same samples and a group of reference samples. The dataset has been compared with the available literature and data from other localities around the world. To this purpose a preliminary database of silica-based materials was established for provenance work. Based on the scarce data available in the literature, the importation of these stones from Eastern localities such as India may be ruled out. The measured data on archaeological samples and debitage allow us to define a reliable reference group of parameters for materials from Fazzan, which are likely to be derived from a unique geological source. The methodology should be extended and compared with cherts and carnelians from a range of Mediterranean and Sub-Saharan sites. This characterisation work is a tool of high potential utility for a new investigation of ancient contact and trade across the Trans-Saharan zone.

Pre-ceramic polymers have previously been known as polymeric precursors that can be converted to silicon carbide, diamond and diamond-like carbon upon heating at ambient inert atmosphere. Here, we report to demonstrate a novel and simple method for the production of crystalline SiO2 ceramic species using a polymeric precursor, which is poly(silyne-co-hydridocarbyne), upon heating under an ambient air atmosphere. The synthesis of both the polymer and the resulting crystalline ceramic is relatively straightforward and unique. All characterization methods such as Raman and X-ray analysis were showed that SiO2 with a different crystal structure is successfully produced at 1000, 750, and 500 A degrees C under an ambient air atmosphere. In addition, the type of produced SiO2 strictly depends on process temperature. The results also showed that the materials are polycrystalline which is evaluated from the comprehensive XRD analysis. SiO2 produced is the mixture of Tridymite-M, syn and Moganite at 1000 A degrees C, the mixture of Coesite and Tridymite at 750 A degrees C, and the mixture of Coesite and amorphous SiO2 at 500 A degrees C.

Chert from the limestones and marly limestones of Castelltallat Formation (Ebro Basin) was widely used throughout prehistoric times in north-eastern Iberia to produce stone tools due to its properties and accessibility. A rough estimate indicates that this rock—either as raw material or as lithic products—was distributed mainly to the north of the outcrops, over an area of 6000–8000 km2. However, other rocks in the area have similar characteristics which can lead to confusion in the interpretation of its prehistoric use and distribution. In order to establish useful archaeometric criteria for differentiating this chert from other similar, the Castelltallat chert is characterised in petrographic, mineralogical and geochemical terms. The chert nodules are found to be homogenous at the macroscopic and microscopic level, with a significant presence of bioclasts, thus indicating they might be formed in a freshwater lake environment by the early diagenetic replacement of carbonates in shallow waters. The mineralogical composition is homogeneously uniform and of a flint type, characterised by an almost exclusive predominance of quartz, without any opaline phases, and a variable proportion of moganite. The iron oxide content is very low, whereas its chemical composition is unusually high in uranium which correlates positively with carbonate content and negatively with silica. Archaeometrical parameters are provided to reach a proper identification of tools knapped with this chert. This way, chert from Castelltallat Formation turns out to be a valuable lithological marker to evaluate the range of mobility of the human groups who lived in the north-eastern Iberia and their contacts with neighbouring areas., The fieldwork to obtain rock samples and the analysis of the samples was cofunded by the Direcció General del Patrimoni Cultural de la Generalitat de Catalunya and IMF-CSIC, within the framework of an interinstitutional collaboration agreement for the development of the Catalan Lithotheque of siliceous rocks (LITOcat project). Specific studies on the petrological characterisation of Castelltallat chert and its prehistoric use were carried out within the research project ‘Aprofitament prehistòric i històric del sílex a Catalunya: contextos extractius i de primera transformació’, funded by the Department of Culture of the Government of Catalonia (ref. núm. 2014/100778). The significance of human use and scope of distribution of Castelltallat chert are being approached within the project ‘Productions, technical variability and technological innovation in the Neolithic’, funded by the Spanish Ministry of Economy and Competitiveness (Grant HAR2016–76534-C2–2-R).

This study presents the geology and mineralogy of the agate nodules that occur in the Pliocene polymictic conglomerates, northwest of Dereyalak village, Eskisehir, northwest Turkey, and their relationship to sepiolite nodules. The host-rock of the agates is a layer 5–25 m thick with an outcrop length of more than 1.5 km. This host rock consists of well-rounded pebbles, mainly composed of opal, ultramafics, sandstones, limestones and volcanics. The conglomerates unconformable overlie an Upper Cretaceous ophiolite complex. This tectonic contact, which trends E–W, is accompanied by another NE–SW-trending major fault. Most of the agate nodules are present along these fault zones. The mainly white and black agate nodules range from 5 to 30 cm in diameter and show a zoned macrostructure. The transparent core is surrounded by a translucent white zone, which, in turn, is surrounded by a zone rich in black dendrites. An opal-rich zone is present nearest the outermost rim. XRD analyses revealed alpha quartz (chalcedony), opal-CT (pseudocrystalline cristobalite), opal-C (pseudocrystalline tridymite) and moganite as the main mineral constituents of the agates. Under polarised light, the predominance of microcrystalline, fibrous quartz (“chalcedony”) is observed. The black colour and fabric of the third, outer zone is due to the presence of black pyrolusite and manganite dendrites. SEM images also show different textures in agates that confirm the presence of a zoned structure in the dendritic agates. Nodular sepiolite (Meerschaum) is well known and has been exploited for centuries in Eskisehir region. Major sepiolite nodule deposits are mainly observed northeast and east of the Dereyalak region. The field observations and the mineralogical data, including comparison between the internal texture of agates and sepiolite nodules and additional geochemical analyses, suggest that the Dereyalak dendritic agates could have formed by sepiolite replacement by low-T silica-rich hydrothermal solutions that most likely circulated along the fracture systems.

Luminescent metal–organic frameworks (MOFs) are appealing. In this regard, the dye molecule 1,2,4,5-tetra(4-pyridyl)benzene (bztpy), a tetradentate ligand, can be rationally incorporated for the modulation of structures and photochemical properties as a building unit. By reacting bztpy and K[Ag(CN)2] in the presence of CuCl in a solution of NH3·H2O/EtOH at room temperature, we managed to synthesize two novel MOFs exhibiting good luminescent behavior. In the reaction, CuCl was proven to be important as a catalyst, which enables the reactant to generate reactive intermediate [Ag(NH3)2]Cl with the help of ammonia. Thus, Ag+ ions released from intermediate promote the formation of products 1 and 2. Transformation between two compounds can be realized by simple operations. Topology studies reveal that 1 belongs to a 4-fold interpenetrating “moganite” net, but for 2 a new-found (3,4)-connected net occurred with a Schlafli symbol of {62.104}{62.10}2. Photoluminescent behaviors of solid 1 and 2 exhibit intense em...

In the search for newer distributed phases that can be used in Ni-composite coatings, inexpensive and naturally available pumice has been identified as a potential candidate material. The composition of the pumice mineral as determined by Rietveld analysis shows the presence of corundum, quartz, mulllite, moganite and coesite phases. Pumice stone is crushed, ball-milled, dried and dispersed in a nickel sulfamate bath and Ni-pumice coatings are electrodeposited at different current densities and magnetic agitation speeds. Pumice particles are uniformly incorporated in the nickel matrix and Ni-pumice composite coatings with microhardness as high as 540 HK are obtained at the lowest applied current density. In the electrodeposited Ni-pumice coatings, the grain size of Ni increases with the applied current density. The overall intensity of texture development is slightly stronger for the Ni-pumice composite coating compared to plain Ni coating and the texture evolution is possibly not the strongest deciding factor for the enhanced properties of Ni-pumice coatings. The wear and oxidation resistances of Ni-pumice coating are commensurate with that of Ni-SiC coating electrodeposited under similar conditions. (C) 2014 Elsevier B.V. All rights reserved.

Moganite, a newly approved mineral, is microcrystalline silica. Samples of microcrystalline silica varieties containing variable amounts of moganite have been analyzed using absorption infrared spectroscopy (IR). The main spectral differences between moganite and α-quartz occur in the wavenumber region below 650 cm−1. Above this wavenumber, the frequencies of Si-O stretching vibrations of moganite are almost identical to those of quartz. Additional moganite bands were recorded near 165, 207, 296, 343, 419, 576, and 612 cm−1, and several of these extra IR bands have been identified for the first time in moganite. The results indicate that moganite and quartz have different crystal structures and symmetries in terms of different tetrahedral linkages. Infrared spectra obtained from samples with different moganite contents cannot be simply explained by mechanical mixing of the two moganite and quartz end phases. The change in moganite content leads to not only a variation of spectral intensity, but also a systematic modification in band position and full-width at half maximum. This unusual behavior is attributed to grain size, strains, and stacking faults in moganite and the intergrowth of moganite with fine-grained quartz. The close correlation between band width and moganite content is indicative of an improved crystallinity with decreasing in moganite concentration that has been identified in natural quartz variations. The results imply that moganite may play a role in the formation or crystallization of microcrystalline quartz. The present IR application offers a new method to estimate the moganite content in microcrystalline silica varieties.

Reported here is a novel Cd(II) coordination polymer (namely [Cd(L1)0.5(ntd)]n (1), L1 = N,N′-di(4-pyridyl)adipoamide, H2ntd2 = 1,4-naphthalenedicarboxylic acid). Single-crystal X-ray diffraction reveals that the framework can be represented as metal/organic-binodal 4-connected Moganite net with three-fold interpenetration. Importantly, for the first time, an unusual coordination mode is observed for the acylamide ligand in such manner that every L1 ligand links four Cd(II) ions through two nitrogen atoms from two pyridine groups and two oxygen atoms from two acylamide groups.

Two forms of silica mineralization, i.e.: (1) vein agates and (2) moss agates, were encountered for the first time within spilitized (albite-rich) alkali-basalts from the upper parts of Borowno quarry (Intra-Sudetic Basin). Vein agates consist predominately of length-fast chalcedony evolving into strongly-zoned prismatic quartz with a distinctive blue cathodoluminescence. They exhibit the abundance of microtextures typical of epithermal vein systems (i.e. pseudo-bladed, flamboyant/feathery, jigsaw puzzle/mosaic, and Bambauer quartz), but also comprise numerous solid inclusions such as sulphides (pyrite, marcasite, chalcopyrite, and chalcocite), sulphates (barite), carbonates (calcite, dolomite), carbonaceous material, and hematite. The formation of vein agates was marked by strong variations in physicochemical conditions (pH, silica content) of SiO2-bearing fluids and recrystallization of metastable silica phases (i.e. moganite), coupled with the local replacement of pre-existing sulphates and/or carbonates by silica during boiling-related conditions. Their LREE-enriched rare-earth patterns with positive Eu anomaly invoke the close relationship between host rocks alterations (spilitization) and origin of SiO2-rich fluids. Conversely, moss agates have formed as a result of a spherical growth of siliceous phases within vesicles and contain numerous mineral inclusions of hematite, goethite, anatase, as well as ore-related phases (mottramite and dickite). The examination of their surface micro-morphology showed a strong age-related maturation marked by the development of stacked silica plates (plate-edge-like structure). The botryoidal-shaped, white-coloured, and Al-enriched outermost layer of these agates possibly reflects a rapid deposition of the initial silica gel onto the cavity walls. The HREE-enriched rare-earth patterns of the moss agates, combined with the presence of positive Eu and Ce anomalies, suggest their parental SiO2-rich fluids could be derived from multiple sources (e.g. volcanics mesostasis and/or meteoric waters). The fluids responsible for the formation of agate mineralization within alkali-basalts in Borowno could be derived from syn- or post- alterations of their host rocks.

Blue chalcedony nodules have been mined from the Sundikendagi deposit in the Mayislar–Saricakaya (Eskisehir) region of north-central Turkey since ancient times; however, no modern geological study of this deposit has yet been published. Although ancient and current mining production have both taken place in an area of complex geology, our study and analyses of the deposit suggests a simple model of sedimentary deposition for its origin. The repeated episodes of tectonic activity, accompanied by brittle deformation, metamorphism, and hydrothermal activity, which characterize this part of the Anatolian Peninsula with its complex junction of tectonic plates, appear to have had little influence on the blue chalcedony nodules that make the deposit valuable other than perhaps to influence their trace-element composition. The physical nature of the nodules as revealed by polarized-light microscopy and XRD—they are composed only of fibrous length-fast quartz (chalcedony) and fibrous length-slow quartz (moganite), but contain neither platy opal-CT nor opal-C—is consistent with a sedimentary origin as are their overall shape and strata-bound occurrence in a sandstone (arkose). The relatively high concentrations of some trace elements in the nodules revealed by ICP-AES, suggest involvement of hydrothermal fluids during the direct epigenetic formation of chalcedony concretions during diagenesis of the enclosing sandstone or by alteration of diagenetic concretions of another composition. Sources could include upwardly moving hydrothermal fluids entering the sedimentary basin from underlying older Saricakaya intrusive rocks or sea-floor hydrothermal vents in the vicinity during diagenesis in the Palaeocene and Eocene (65–37.8 Ma) periods. Oxygen isotope analyses (SMOW) (using EA-IRMS) of both the blue chalcedony nodules (δ 18 O = + 28.2‰ to + 30.8‰) and the enclosing sandstone (δ 18 O = + 11.3‰ to + 13.2‰) suggest that the nodules formed during diagenesis at a low temperature of around 55 °C, although they are encased in sandstone whose grains came from rocks that formed at significantly higher temperature, perhaps above 100 °C. The unbanded Sundikendagi chalcedony nodules are similar in occurrence to the banded Fairburn agates of South Dakota, USA and the Dryhead agates of Montana, USA, which formed in Palaeozoic limestones, except that the blue chalcedony is hosted in sandstone. Other sedimentary agates are generally believed to have formed by the alteration of diagenetic concretions from the outside, inward. No other agates or chalcedonies hosted in sandstone are known for comparison with this deposit. Thus, the deposit appears to be unique. It is possible that the Sundikendagi unbanded blue chalcedony formed as epigenetic concretions during diagenesis of the sandstone—a mechanism previously shown for large crystals of other minerals found in sandstones.

In order to characterize ornamental stones and gemstones from the monstrance ‘Tabernaculo de la Colegiata de S. Pedro’ in Lerma, Burgos, a Raman spectroscopy in situ non-destructive study has been performed on these materials. The Raman spectra obtained correspond to cornalline-like chalcedony, nephrite jade, quartz veinlets, agates with moganite and jasper, together with goethite showing several degrees of alteration. Various types of marble and lapis lazuli were identified. The lapis lazuli samples show different Raman spectra depending on the blue–white chromatic gradient, due to the variations in the sulfur concentration. Raman spectroscopy allows for the correlation of the presence of radical molecular ions S2−, S3− and SO42−, with the colour centres and colouration of lapis lazuli. Two varieties of marble are present, one of brown tones with greyish-white incrustations and another with alternating white and black veinlets with yellow inclusions. In the former, white masses of microcrystalline calcite appear together with quartz inclusions, around which brown halos of goethite develop as an alteration product. In the latter, they show calcite in the white veinlets and calcite with graphite in the black ones. The mineralogical analysis of ornamentals stones on art objects allows determining their degree of conservation and the extension of the deterioration, in order to establish strategies for the cleaning and restoration. These results are an important factor to date the object and for shedding light about its authorship. Furthermore, it could eventually help to establish connections between the tabernacle-monstrance and other tabernacles in Castilla y Leon. Copyright © 2013 John Wiley & Sons, Ltd.

Transitions in petrified wood samples from several Thailand–Myanmar sites included opal-A to opal-CT, opal-A to metastable β -crystobalite and reordered opal-CT → quartz. The moganite in the samples indicates a potential volcanic silica source for the wood petrifactions. Silica phase transitions evident under SEM include opal-A → opal-CT, opal-A → cuboid quartz (pseudocubic) and opal-CT → quartz. Certain stages of silica transformation and transition processes seem to overlap. Such overlapping results when lepisphere blades transform to the high crystallinity of prismatic quartz through a solid-state mechanism. The results provide some insights into the petrified wood genesis in the region. Thermal inputs into the silicification process may have pre-dated final burial of woods in flood deposits.

Characterization of Brazilian agates containing a lower horizontally banded section and an upper chamber with bands parallel to the walls shows that these agates formed much later than the 135 Ma Paraná basalt host rock. Age differentiation between the two types of banding shows that the horizontal bands formed between 43 to 63 Ma ago with a final infill of wall-lining bands between 7 and 27 Ma later. The horizontal bands have a higher Al3+ concentration and a greater crystallite size than the wall-lining layers; they have a lower mogánite content and defect-site water content. The formation of these agates appears to be the result of a three-stage process. After the separate formation of horizontally banded and wall-lining agate, a silica infill seals the gap between the agate and the cavity wall. The detection of cristobalite in some specimens indicates that genesis of both the horizontally banded and wall-lining deposits in the Brazilian samples proceeds along an amorphous silica → opal-CT → opal-C → chalcedony pathway.

In order to tune the Zn(II) coordination polymer architectures, three position isomers, 1,4- (pbtmb), 1,3- (mbtmb), and 1,2-bis(1,2,4-triazol-1-ylmethyl)benzene (obtmb), and flexible tetracarboxylate ligand, 5,5′-(1,4-phenylenebis(methylene))bis(oxy)diisophthalic acid (H4L1), were used to assemble compounds with different kinds of entangled networks, namely, {[Zn2(L1)(pbtmb)2]·4H2O}n (1), {[Zn2(L1)(mbtmb)2]·4H2O}n (2), and [Zn2(L1)(obtmb)2]n (3). Flexible bis(triazole) ligands present diverse conformations in these compounds. Compound 1 shows a 4-fold interpenetrating three-dimensional architecture with 4-connected mog (moganite) topology. Compound 2 displays a 2D + 2D → 3D polycatenation network simplified as new 4-connected topology with the Schlafli notation of {4,64;8}2{42,64}. Compound 3 is a 2D polythreading structure, in which two layed networks interpenetrate in a (2D/2D) parallel fashion. The powder X-ray diffraction, thermal stabilities, and the photoluminescence of 1–3 have also been investigated.

The thermal behaviour of silica rocks upon heat treatment is dependent on the constituent minerals and petrographic texture types. These constituents can be shown to be mainly quartz in the form of two types of chalcedony (Length-fast (LF) chalcedony and Length-slow (LS) chalcedony, the latter also being termed quartzine) and moganite. Even though the thermal behaviour of LF- chalcedony is well understood, major uncertainties persist concerning the high-temperature behaviour of LS-chalce- dony and moganite. We present here a comparative study of these three constituents of common silica rocks. Our results show that the chemical reaction is the same in all three, Si-OH ? HO-Si ? Si-O-Si ? H2O, but that the reaction kinetics and activation temperatures are very dif- ferent. LS-chalcedony begins to react from 200 C upwards, that is at temperatures 50 C below the ones observed in LF-chalcedony, and shows the fastest reaction kinetics of this 'water' loss. Chemically bound water (SiOH) in moganite is more stable at high temperatures and no specific activation temperature is necessary for trig- gering the temperature-induced 'water' loss. Moganite is also found to act as a stabilizer in silica rocks preventing them from temperature-induced fracturing. These findings have implications for the study of potential heat treatment temperatures of silica rocks (in industry and heritage studies), but they also shed light on the different structures of SiO2 minerals and the role of OH impurities therein.

We present the first thermal infrared reflectance spectral characterization of moganite and mixtures of moganite with microcrystalline quartz. We find that for relatively high (>50%) abundances of moganite, the absolute reflectance for samples is significantly reduced. Using microscopic-Raman (~1 μm/pixel) measurements, we estimate the moganite content for various samples. We then compare Raman-derived moganite abundances with microscopic infrared reflectance (25 μm/pixel) spectra to determine the effects of increasing moganite abundance on thermal infrared spectra. We find that moganite is broadly spectrally similar to quartz with major reflectance maxima located between ~1030 and 1280 cm −1 and ~400 and 600 cm −1 ; but there are characteristic differences in the peak shapes, peak center positions, and especially the relative peak reflectance magnitudes. For regions with high (>50%) moganite content, the relative magntitudes of the reflectance maxima at 1157 and 1095 cm −1 (R 1095 /R 1157 band ratio) can be used to estimate the moganite content. This work demonstrates the utility of thermal infrared microspectroscopy in isolating phases that are intimately mixed in a sample, and has applications in planetary science, where constituent phases of quartz-rich sedimentary rocks can be identified using remote or in situ thermal infrared spectroscopy.

Agate nodules from Kerrouchen (Khenifra Province, Meknes-Tafilalet Region) in Morocco occur in Triassic basalts and reach up to 30 cm in diameter. Monocentric, banded agates—mainly in pastel pink, grey, white and yellow—with infiltration canals (osculum) are observed. Raman microspectroscopy revealed that the agates mainly consist of low quartz and subordinately moganite with distinctive 460 and 501 cm−1 marker bands, respectively. Linear mapping indicated that moganite mainly concentrates in grey zones of the monocentric agate nodules. The other types, polycentric and pseudostalactitic agates, are usually brown and red and contain minerals such as hematite and goethite. They form both regular and irregular mosaics rich in ornamentation. Occasionally, aggregates of copper sulphides or titanium oxides (rutile) can also be observed. These minerals are sometimes accompanied by carbonaceous material marked by 1320 and 1585 cm−1 Raman bands. It seems that formation of agates from Kerrouchen was induced by Si-rich and Fe-moderate fluids. Copper sulphides, rutile, and carbonates (possibly calcite) were most likely incorporated during post-magmatic processes. The origin of solid bitumen can be the result of hydrothermal or hypergenic processes.

The thermal stabilities of OH and H2O in moganite from the Mogan D formation, Gran Canaria have been investigated during heating and cooling using in situ Fourier transform infrared spectroscopy. The spectra have also been analysed in terms of CO2 incorporated in moganite. The results show that while the thermal responses of H2O and OH in moganite display similarities to those reported in agate, the spectra are not completely identical. Absorptions in the O-H stretching region reveal that dehydration and dehydroxylation is a multi-stage process. Although hydrogen loss starts below 400-500 K, hydrous species may well remain in moganite even at 1060 K. After cooling, the spectra of multiphonon bands from lattice vibrations in a moganite sample that had been heated up to 1060 K became similar to those of agate implying a moganite to α-quartz transformation can be thermally induced. CO2 was first detected at temperatures ∼460 K. However, a further increase in CO2 absorption has been identified around 940 K, and this is probably associated with the decomposition of CO3.

Results are presented for the cathodoluminescence (CL), X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and simultaneously two thermal analyses of (DTA/TGA) spectroscopy of dendritic agate which occurs in the Pliocene polymictic conglomerates of the Hoyuklu Formation, North West of the Dereyalak village (Eskisehir, Turkey). Micro-Raman measurements were performed on dendritic agate and then strong quartz and moganite peaks were identified at 465 and 501 cm −1 , respectively. Thermal analysis shows the loss of water and hydroxyl units occurs in 2 distinct stages; at 796 and 808 °C. Spatially resolved CL results at room temperature were recorded for chosen 3 different areas. Grey area (100% SiO 2 ) displays the lowest CL emission. Brown area (99.7% SiO 2 and 0.3% Fe 2 O 3 ) contains exsolved non-detected ironed phases such as goethite-lepidochrocite to explain the brown colour and the iron point substitutional defects attributed to the 643 nm CL emission. White outer (98.7% SiO 2 and 1.3% Al 2 O 3 ) would be strongly disordered as observed in the “amorphous” Raman spectrum containing as inferred from the spectrum CL on the outer areas, particularly non-bridging oxygen hole centres (NBOHC) (317 nm) and [AlO 4 ]°/H + (380 nm) centres produced by large amounts of aluminium in the lattice (1.33% Al 2 O 3 ). When it comes to collect the data in the time resolved CL spectrum, at least three broad emission bands were detected in: a green band of low intensity at about 496 nm, intense orange band at about 600 nm, and a red band at 670 nm. The CL emission at 670 nm shows some relationships between the hydroxyl or alkali content and the abundance of O 2 (super 3-) centres and E ′ 1 centres. Another conspicuous observed feature in the CL spectra of agates is the existence of an orange emission band centred at around 600 nm. The predominance of the yellow CL emission band and the high concentration of E ′ 1 centres are typical for agates formed by acidic volcanism processes.

Amethyst crystals on matrix specimens from the Dursunbey-Balikesir region in Turkey have five representative purple color zonings: dark purple, light purple, lilac, orchid, and violet. The purple color zonings have been analyzed with optical absorption spectra in the visible wavelength region, chemical full trace element analyses (inductively coupled plasma-atomic emission spectroscopy and inductively coupled plasma-mass spectroscopy), and scanning electron microscopic images with high magnification. It can be proposed that the production of the purple color in amethyst crystals is due to three dominant absorption bands centered at 375, 530, and 675 nm, respectively. In addition, the purple color zonings are also due to four minor absorption bands centered at 435, 480, 620, and 760 nm. X-ray diffraction graphics of the investigated amethyst crystals indicate that these crystals are composed of a nearly pure alpha-quartz phase and do not include any moganite silica phase and/or other mineral implications. Trace element analyses of the amethyst crystals show five representative purple color zonings, suggesting that the absorption bands can be mainly attributed to extrinsic defects (chemical impurities). However, another important factor that influences all structural defects in amethyst is likely to be the gamma irradiation that exists during amethyst crystallization and its inclusion in host materials. This gamma irradiation originates from the large underlying intrusive granitoid body in the region of amethyst formation. Irradiation modifies the valence values of the impurity elements in the amethyst crystals. It is observed that the violet-colored amethyst crystals have the most stable and the least reversible coloration when exposed to strong light sources. This situation can be related to the higher impurity content of Fe (2.50 ppm), Co (3.1 ppm), Ni (38 ppm), Cu (17.9 ppm), Zn (10 ppm), Zr (3.9 ppm), and Mo (21.8 ppm).