Inclusions occurring in leucites from the Oscar and Zirkel Mesa leucite lamproites consist of high (>10 wt.%) and low (<4 wt.%) K2O discrete glass phases which are interpreted to have resulted from silicate liquid immiscibility. Both glasses are quartz-normative and rich in Ti, Fe, Mg, Ca and Ba and poor in Al. The high-K glass is strongly peralkaline. Neither of the glasses have compositions representative of any known lamproites. These inclusions represent the first described examples of silicate-silicate immiscibility shown by glass in alkaline rocks.

Pressures of 10 GPa and above can bring about phase transformations in many oxides, an effect of great interest to geochemists and geophysicists. We can interpret such behaviour as due to the differential compressibility of 'anion' and 'cation' leading to a progressive rise in radius ratio with pressure, and hence, on the classic crystallochemical picture, eventually to an increase in co-ordination number (though with complications which make prediction difficult). More generally, pressure affects Gibbs free energy G directly; for oxides a pressure of 5 GPa gives, very roughly, the same contribution to G as 100°C in temperature (though with opposite sign). Thus high pressure significantly affects the shape and structure of phase diagrams, showing increasingly important effects above, say, 10 GPa—but again prediction can be difficult. However these two complementary approaches to the effects of pressure, helpful though they can be conceptually, are 'crystal-based' and totally neglect another rather littleknown but potentially important effect--the formation of amorphous solids; 'polymers' and glasses. Since amorphous materials are 'non-equilibrium' they are not readily dealt with theoretically; also, since they are difficult to detect by standard crystallographic techniques, they can be overlooked experimentally. The pressure-induced formation of amorphous solids could have significant implications for both geochemistry and geophysics.

We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes.

Common silicate glasses are among the most brittle of the materials. However, on warming beyond the glass transition temperature Tg glass transforms into one of the most plastic known materials. Bulk metallic glasses exhibit similar phenomenology, indicating that it rests on the disordered structure instead on the nature of the chemical bonds. The micromechanics of a solid with bulk amorphous structure is examined in order to determine the most basic conditions the system must satisfy to be able of plastic flow. The equations for the macroscopic flow, consistent with the constrictions imposed at the atomic scale, prove that a randomly structured bulk material must be either a brittle solid or a liquid, but not a ductile solid. The theory permits to identify a single parameter determining the difference between the brittle solid and the liquid. However, the system is able of perfect ductility if the plastic flow proceeds in two dimensional plane layers that concentrate the strain. Insight is gained on the nature of the glass transition, and the phase occurring between glass transition and melting.

The maximum value at equilibrium of the relaxed enthalpy of some glasses is viewed as alinear function of the annealing temperature from the Kauzmann temperatureTK up to a vitreous transition temperature T*g which is not time dependent. The frozen enthalpy and entropy at T*gare determined from the specific heat difference between glass andundercooled melt which is constant between TKandT*g. The Gibbs free energy change at T*g is equal to zero. The vitreous transition is a thermodynamic transitionwithout latent heat. A model is used to describe this phenomenon. A volume energy saving εv equivalent to a complementary Laplace pressure has beenadded to the classical Gibbs free energy change for a crystal formation in a melt. Thereis a change of the Vogel–Fulcher–Tammann (VFT) temperature at T*g corresponding to a decrease of the free volume disappearancetemperature. Scaling laws linking the crystal homogeneous nucleation temperatures to T*g are used to predict the two VFT temperatures, the thermodynamic vitreoustransition induced by vitreous (super)-clusters and the frozen enthalpy and entropy at T*g only knowing T*g, the melting temperature Tmand the fusion heat ΔHm of any fragileglass-forming melt.

This article reviews recent advances in understanding amorphous glassy states in dense colloidal suspensions with or without short-range interparticle attractions. Experiments, theory, and simulation show that two kinds of glassy states are possible, dominated respectively by repulsion and attraction. Under suitable conditions, a small change in the interparticle potential can lead to a transition between these two kinds of colloidal glasses that entails sharp changes in material properties such as the shear modulus. This may provide novel routes for fine-tuning the properties of industrial pastes and slurries.

During several hundred years of burial in the soil, glass objects, especially those with the high potassium content of medieval compositions, develop heavily corroded surfaces, showing phenomena such as local pitting, laminated layers, and browning effects. The long-term behavior of glass in the soil or in contact with groundwater is not only of interest to glass scientists with a background in archaeology, but also for those with a special interest in nuclear-waste management. Several attempts have been made to propose the decomposition of ancient glasses as an indicator for the performance of buried nuclear-waste glasses. In spite of differences in glass composition and exposure conditions, the development of alteration or corrosion layers with time is a common concern, as shown in this article. Laboratory experiments, representing a simplified model for real conditions, offer the possibility for systematic investigations.

An overview of key materials for optical communications, including semiconductors, dielectrics, glasses, and organics, is presented in this issue of MRS Bulletin. Materials quality is in all cases crucial for advanced device and system performance. Materials properties and important problems are reviewed, and their impact on the performance of state-of-the-art optical devices is assessed and demonstrated by means of selected examples.

Europium oxide has been doped into sodium borosilicate (NBS) glasses (8Na2O-32B2O3- 60SiO2- xEu2O3) at various concentration levels. The glass raw materials were calcined at 800°C and melted at 1380°C and transparent glasses with slightly red color were obtained by quenching the melt in air. Thermal analyses and other experimental results showed excellent glass forming capability with Tg around 480 to 520°C at various doping concentrations. A typical deformation temperature of the europium-doped NBS glasses is around 570-660°C with densities around 2.45 to 2.6 g/cm3. The doped NBS glasses were further heat-treated at 560°C for phase-separation. Scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), thermomechanical analysis (TMA), optical absorption spectrometer and fluorescence emission spectrometer were used to explore properties of the glasses.

The Miocene-Pliocene Macusani ash-flow tuffs and glasses from SE Peru are a rare example of two-mica felsic peraluminous volcanic rocks. They outcrop in three separate tectonic basins of the Cordillera Oriental in the Central Andes. In the Macusani field, the rocks are characterised by andalusite and muscovite phenocrysts. Compositions are both very felsic and peraluminous, similar to two-mica granites. Field relations, age differences and isotopic heterogeneities suggest that several distinct magma batches were involved. Two separate magmatic stages are recognised: (1) partial melting and evolution at or near the source region, and (2) shallow-level crystallisation and eruption. Magma genesis involved partial melting of metapelitic materials, with internally controlled. High heat flux, rapid heating, elevated temperatures and F-rich compositions were essential for the production of these mobile, H2O-undersaturated magmas. Chemical variations between the erupted products can be attributed to different degrees either of partial melting in somewhat variable source materials and (or) of fractional crystallisation at shallow levels. We discuss some important differences between the magmatic evolution of the Macusani Volcanics and of Hercynian and Himalayan two-mica granites.