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Sen et al. Reply:We agree with Brazhkin [1] that indeed the most conclusive proof for a first-order phase transition in glassy materials is the direct observation of two coexisting macroscopic phases of identical composition but with different structure and properties. In this regard, the observation of two glass-transition endotherms in the differential scanning calorimetry (DSC) curves of Ge 2:5 As 51:25 S 46:25 glass samples quenched from pressures in the transformation range as shown in our Letter [2] provides strong, albeit indirect, evidence in favor of the coexistence of two macroscopic phases. Such distinct endotherms cannot correspond to a single, partially polymerized glass phase. Moreover, our recent DSC studies at slow heating rates (20 K= min) show two partially overlapping crystallization exotherms in samples quenched from intermediate pressures, further corroborating this conclusion.The density of the molecular phase is 3:39 g cm ÿ3 while that of the predominantly network phase recovered from the highest pressure of 14.4 GPa is 3:60 g cm ÿ3 . This is a relatively small change in density (6%) compared to those observed in the case of high-and lowdensity phases of liquid and fluid P (40%) and of amorphous H 2 O (20%) [3,4]. Such small difference in density in combination with strong absorption of the chalcogenide glass in the visible wavelengths make direct observation of two coexisting phases impossible with regular optical microscopy. X-ray radiography experiments in situ at high pressure are underway to address this issue.Brazhkin has also argued that the Raman spectroscopic observation of the molecular-to-network phase transition over a finite range of pressures as reported in [2] cannot be ascribed to slow transformation kinetics associated with an underlying first-order transition. On the contrary, our recent experiments at 10 GPa have shown that the degree of transformation from the molecular to the network phase is strongly time dependent at least up to 50 h at pressures in the transformation range. This result indicates that sluggish transformation kinetics is indeed responsible for the observed broadening of the pressure interval associated with the transformation. This result is consistent with the scenario of nucleation and growth of a network phase within the matrix of the molecular phase with increasing pressure, which is the hallmark of first-order transition. However, smeared transitions can also show time dependent degree of transformation, which typically follows a logarithmic kinetics [5]. Further experiments are currently underway to completely rule out this possibility in our case.Finally, and most interestingly, we have recently been able to find a temperature region slightly above the T g of the molecular phase where the pressure-quenched samples can be annealed over a reasonably long period of time without crystallization. Glass samples quenched from 12 GPa at ambient temperature, when annealed for 3 h at 313 K, showed significant increase in the intensity of the intramolecula...

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International audiencemicrostructural evolution of a two-phase composite (olivine + Fe-Ni-S) during large shear deformation, using a newly developed high-pressure X-ray tomography microscope. Two samples were examined: a load-bearing framework-type texture, where the alloy phase (Fe-Ni-S) was present as isolated spherical inclusions, and an interconnected network-type texture, where the alloy phase was concentrated along the silicate grain boundaries and tended to form an interconnected network. The samples, both containing ~10 vol% alloy inclusions, were compressed to 6 GPa, followed by shear deformation at temperatures up to 800 K. Shear strains were introduced by twisting the samples at high pressure and high temperature. At each imposed shear strain, samples were cooled to ambient temperature and tomographic images collected. The three-dimensional tomographic images were analyzed for textural evolution. We found that in both samples, Fe-Ni-S, which is the weaker phase in the composite, underwent signifi cant deformation. The resulting lens-shaped alloy phase is subparallel to the shear plane and has a laminated, highly anisotropic interconnected weak layer texture. Scanning electron microscopy showed that many alloy inclusions became fi lm-like, with thicknesses <1 m, suggesting that Fe- Ni-S was highly mobile under nonhydrostatic stress, migrated into silicate grain bound aries, and propagated in a manner similar to melt inclusions in a deforming solid matrix. The grain size of the silicate matrix was signifi cantly reduced under large strain deformation. The strong shape-preferred orientation thus developed can profoundly influence a composite's bulk elastic and rheological properties. High-pressure-high temperature tomography not only provides quantitative observations on textural evolution, but also can be compared with simulation results to derive more rigorous models of the mechanical properties of composite materials relevant to Earth's deep mantle