Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba)

Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl), in the agates confirm their contemporaneous supply with SiO2 and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO2 with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm−1 and moganite at 502 cm−1, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply.

Experimental Constraints on the Crystallization of Silica Phases in Silicic Magmas

Low-pressure silica polymorphs, e.g. quartz (Qtz), tridymite (Trd), and cristobalite (Crs), are common in silicic magmas, but the conditions of their formation are still unclear. The stability fields of these polymorphs have been determined in the SiO2, SiO2-H2O, and haplogranite systems, but these simple systems are not directly applicable to silica polymorph crystallization in natural silicic magmas. The present study compiles an experimental database of new and previously-published data documenting the crystallization of silica phases in natural silicic magmas and simple synthetic systems. Silica polymorphs are identified using Raman spectroscopy and their pressure-temperature domains of occurrence and chemical compositions are determined at pressures between 0.1 and 200 MPa, temperatures between 685 to 1200 °C, and under H2O-saturated and H2O-undersaturated conditions. Qtz is the stable silica polymorph at pressures higher than 25-30 MPa, temperatures between ∼700 and 950 °C, and occurs for a narrow range of melt SiO2 contents (∼79-81 wt%). Constraints on Qtz stability derived from simple systems are mutually compatible with, and thus applicable to natural compositions. This is consistent with Qtz compositions being close to ‘pure’ SiO2, both in experiments and nature. In volcanic systems, Qtz crystallization may occur in magmatic reservoirs and deep volcanic conduits. Trd did not crystallized in the experiments conducted as part of this study despite several experiments having been performed in the Trd stability field. This is consistent with results from the literature which show that Trd crystallization is kinetically inhibited in particular relative to Crs. Natural Trd have compositions deviating substantially from ‘pure’ SiO2, so that stability limits determined in simple systems should not be applied directly to natural cases. Crs was encountered at pressures below 20-30 MPa (or H2O contents < ∼1.5 wt%), from sub-solidus (∼800 °C) to near-liquidus (up to 1040 °C), and coexisting with melts having a large range of SiO2 contents (70-81 wt%). The Crs stability field is much larger in natural magmas compared to pure SiO2 systems. Crs is a metastable phase stabilized by components (Al, Na, K; about 3 wt%) present in the silicic melt. In volcanic systems, Crs crystallization may thus be restricted to subsurface manifestations such as lava domes.

Quantifying mass changes during hydrothermal alteration in listwaenite-type mercury mineralization, Tavreh area, northwestern Iran

The Tavreh mercury prospect, a listwaenite-type alteration/mineralization system, is located c. 90 km west of Khoy in northwestern Iran. Tavreh is hosted within the Khoy ophiolite zone. Three types of listwaenites have been recognized in the Khoy ophiolite: silica, silica-carbonate and carbonate. Of these three, Hg mineralization at Tavreh is spatially and genetically associated with the silica-type listwaenite, also known as berberite. Alteration and mineralization at Tavreh are restricted to a faulted contact between shale and serpentinite. The Tavreh listwaenite is inferred to form from the hydrothermal alteration of brecciated serpentinite. Major mineralogical changes resulting from this alteration include the decomposition of serpentine-group minerals and the formation of silica phases. In this study, the mass changes of 18 listwaenites from Tavreh were assessed relative to the least altered serpentinites. To illustrate these changes quantitatively a comparative analysis of three different methods of calculating mass change was undertaken using Grant's isocon analysis, MacLean's equation and Gresens’ equation. Results from the three methods are similar. Listwaenite alteration was associated with a large increase in SiO2 (44.4, 36.2, 63.9%, respectively). MgO and loss on ignition were depleted (−34.8, −36.9, −36.6; −8.5, −9.3, −8.3%, respectively) and Al2O3 was relatively unchanged (0.7, 0.6, 0.9%). Mercury is the most enriched rare element in altered rock (375.1, 346.8, 474.6 ppm). Arsenic, Pb, Au and Sb were also enriched. The intensity of mass changes of the various alteration components increases significantly from the serpentinite wall rock towards the listwaenite alteration and the ore-bearing zone. Therefore, the mass balance method can probably be used to locate mineral deposits from a few hundred metres and to explore for blind mineral deposits.

Analysis of nonsteroidal anti-inflammatory drugs by using microfluidic techniques: a review

: In this review paper, miniaturized techniques, including both electromigration and liquid chromatographic ones, have been considered discussing their main features in the analytical field for the separation and analysis of nonsteroidal anti-inflammatory drugs (NSAIDs). In capillary electrophoresis (CE) and nano-liquid chromatography (nano-LC), separation are performed in capillaries with internal diameter (I.D.) lower than 100 m and therefore flow rates in the range 100- 1000 nL/min are applied. Therefore due to the low flow rate, high mass sensitivity can be obtained. Usually conventional UV detectors are used on-line; however these techniques can be coupled with mass spectrometry (MS). CE and nano-LC have been also applied to the separation of NSAIDs using silica stationary phases (SP) modified with C18 promoting interaction with analytes mainly based on hydrophobic interaction. In addition the use of chiral SP resulted effective for the chiral resolution of these compounds. In addition to silica phases, monolithic (both organic and inorganic) material has also been used. Although most of the presented studies aimed to demonstrate the usefulness of the considered microfluidic techniques, some applications to real samples have also been reported.

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The first two indicate that the formation of agates was likely marked by recrystallization of metastable silica phases (i.e., opaline silica or massive chalcedony). The presence of lattice-bladed (after barite and calcite) quartz may be, in turn, ascribed to the boiling-related conditions that could have triggered the formation of abundant copper and iron sulfides found within silica matrix. Additionally, the local occurrence of growth lines (so-called Bambauer quartz) and intergrowth of length-slow and length-fast chalcedony are linked to the variations of physico-chemical conditions during rock formation (alkaline-acidic). According to Raman spectroscopy, silica matrix of the agates is made of α-quartz with a local admixture of moganite (from 0.0 up to 78 wt.%), but also contains numerous solid inclusions of hematite, celadonite, as well as poorly-organized carbonaceous material and rutile. These phases were likely emplaced during low-temperature hydrothermal activity of SiO2-bearing fluids that originated from post-magmatic hydrothermal activity developed within host rocks and/or meteoric waters.

The impact of calcination on changes in the physical and mechanical properties of the diatomites of the Leszczawka Member (the Outer Carpathians, Poland)

The work concerned the effects of the thermal treatment of diatomites from the Jawornik deposit (an example of the diatomites of the Leszczawka Member of the Polish Outer Carpathians). Five distinct lithological varieties were subjected to calcination at 600°C in ambient air.The thermal impact induced the following changes to the rocks. Their overall rock porosity increased, most distinctly in the initially softer varieties, and the internal pores of the siliceous frustules themselves usually became larger due to the initial melting of the silica phases. Most of the diatoms, quartz and feldspars cracked as a result of their brittle fracturing under compressive strain resulting from substantial and differing size changes of growing grains. Clay minerals were thermally transferred, changing their volume. The organic matter dispersed through-out the diatomites was partly oxidized and removed. At the same time, the structure of the rocks was strengthened, as confirmed by an increase in their microhardness. The microhardness of soft and porous diatomite varieties increased considerably on heating, but that of the hard and compact variety changed to a smaller degree. The increase is directly related to the content of the clay minerals. The impact of other mineral components was not detected. The calcination of lithologically diversified diatomites provided the mineral with raw material with deicing and antisliding properties. The technology of its production has been determined by the authors and submitted as a patent.

Electrical conductivity studies on silica phases and the effects of phase transformation

Starting with the same sample, the electrical conductivities of quartz and coesite have been measured at pressures of 1, 6, and 8.7 GPa, respectively, over a temperature range of 373–1273 K in a multi-anvil high-pressure system. Results indicate that the electrical conductivity in quartz increases with pressure as well as when the phase change from quartz to coesite occurs, while the activation enthalpy decreases with increasing pressure. Activation enthalpies of 0.89, 0.56, and 0.46 eV, were determined at 1, 6, and 8.7 GPa, respectively, giving an activation volume of –0.052 ± 0.006 cm3/mol. FTIR and composition analysis indicate that the electrical conductivities in silica polymorphs is controlled by substitution of silicon by aluminum with hydrogen charge compensation. Comparing with electrical conductivity measurements in stishovite, reported by Yoshino et al. (2014), our results fall within the aluminum and water content extremes measured in stishovite at 12 GPa. The resulting electrical conductivity model is mapped over the magnetotelluric profile obtained through the tectonically stable Northern Australian Craton. Given their relative abundances, these results imply potentially high electrical conductivities in the crust and mantle from contributions of silica polymorphs. The main results of this paper are as follows:The electrical conductivity of silica polymorphs is determined by impedance spectroscopy up to 8.7 GPa.The activation enthalpy decreases with increasing pressure indicating a negative activation volume across the silica polymorphs.The electrical conductivity results are consistent with measurements observed in stishovite at 12 GPa.